CHAPTER VI_Bacteria Especially as they are related to the economy of nature, to industrial processes, and to the public health

_BACTERIA IN MILK, MILK PRODUCTS, AND OTHER FOODS_

Injurious micro-organisms in foods are, fortunately for the consumers,

usually killed by cooking. Vast numbers are, as far as we know, of

no harm whatever. Alarming reports of the large numbers of bacteria

which are contained in this or that food are generally as irrelevant

as they are incorrect. Bacteria, as we have seen, are ubiquitous. In

food we have abundance of the chief thing necessary to their life and

multiplication--favourable nutriment. Hence we should expect to find in

uncooked or stale food an ample supply of saprophytic bacteria. There

was much wholesome truth in the assertions made some two years ago by

the late Professor Kanthack, to the effect that good food as well as

bad frequently contained large numbers of bacteria, and often of the

same species. It is well that we should become familiarised with this

idea, for its accuracy cannot be doubted, and its usefulness at the

present time may not be without its beneficial effect.

Nevertheless, it is well we should know the bacterial flora of good and

bad foods for at least two reasons. First, there is no doubt whatever

that a considerable number of cases of poisoning can be traced every

year to food containing harmful bacteria or their products. To several

of the more notorious cases we shall have occasion to refer in passing.

Secondly, we may approach the study of the bacteriology of foods with

some hope that therein light will be found upon some important habits

and effects of microbes. There can be little doubt that food-bacteria

afford an example of association and antagonism of organisms to which

reference has already been made. Any information that can be gleaned to

illumine these abstruse questions would be very welcome at the present

time. But there is a still further, and possibly an equally important,

point to bear in mind, namely, the economic value of microbes in food.

In a short account like the present it will be impossible to enter into

hypotheses of pathology, but we shall at least be able to consider

some of these interesting experiments which have been conducted in the

sphere of beneficial bacteria.

The injurious effects of organisms contained in foods has been

elucidated by the excellent work of the late Dr. Ballard. From the

careful study of a number of epidemics due to food poisoning, this

patient observer was able, without the aid of modern bacteriology,

to arrive at a simple principle which must not be forgotten. Food

poisoning is due either to bacteria themselves or to their products,

which are contained in the substance of the food. In cases of the

first kind, bacteria gaining entrance to the human alimentary canal,

set up their specific changes and produce their toxins, and by so

doing in course of time bring about a diseased condition, with its

consequent symptoms. On the other hand, if the products, sometimes

called _ptomaines_, are ingested as such, the symptoms set up by their

action in the body tissues appear earlier. From these facts Dr. Ballard

deduced the simple principle that if there is no incubation period or,

at all events, a comparatively short space of time between eating the

poisoned food and the advent of disease, the agents of the disease are

products of bacteria. If, on the other hand, there is an incubation

period, the agents are probably bacteria.

It is necessary to mention two other facts. Dr. Cautley[53] has

recently been engaged in isolating from poisoned foods the different

species of bacteria present. It would appear that these are limited,

as a rule, to two or three kinds. As regards disease, the organisms of

suppuration are the most common. Liquefying or fermentative bacteria

are frequently present, the _Proteus_ family being well represented.

In addition there are, according to circumstances, a number of common

saprophytes. Now, as we have pointed out, these organisms may act

injuriously by some kind of cooperation, or they may by themselves

be harmless, and pathological conditions be due to the occasional

introduction of pathogenic species.

The other fact, requiring recognition from anyone who proposes to study

the bacteriology of foods, is that a certain appreciable amount of

the responsibility for food poisoning rests with the tissues of the

individual ingesting the food. There is ample evidence in support of

the fact that not all the persons partaking of infected food suffer

equally, and occasionally some escape altogether. We know little or

nothing of the causes of such modification in the effect produced.

It may be due to other organisms, or chemical substances already in

the alimentary canal of the individual, or it may be due to some

insusceptibility or resistance of the tissues. Be that as it may, it is

a matter which must not be neglected in estimating the effects of food

contaminated with bacteria or their products.

_Milk._ There are few liquids in general use which contain such

enormous numbers of germs as milk. To begin with, milk is in every

physical way admirably adapted to be a favourable medium for bacteria.

It is constituted of all the chief elements of the food upon which

bacteria live. It is frequently at a temperature favourable to their

growth. It is _par excellence_ an absorptive fluid. A dish of ordinary

water and a dish of newly drawn milk laid side by side, and under

similar conditions of temperature, will rapidly demonstrate the

difference in degree of absorptivity between the two fluids. Yet,

whilst this general fact is true, we must emphasise at the outset the

possibility and practicability of securing absolutely pure sterile

milk. Recently some milking was carried out under strict antiseptic

precautions, with the above sterile result. The udder was thoroughly

cleansed, the hands of the milker washed with corrosive sublimate and

then pure water, the vessels which were to receive the milk had been

carefully sterilised, and the whole process was carried out in strict

cleanliness. The result was that the sample of milk remained sweet and

good and contained no germs. It should be stated that the first flow of

milk, washing out the milk-ducts of the udder, was rejected. This fact

of the sterility of cleanly drawn milk is not a new one, and has been

established by many bacteriologists. Milk, then, is normally a sterile

secretion. How does it gain its enormously rich flora of bacteria?

_Sources of Pollution of Milk._ These are various, and depend upon many

minor circumstances and conditions. For all practical purposes there

are three chief opportunities between the cow and the consumer when

milk may become contaminated with bacteria:

1. At the time of milking.

2. During transit to the town, or dairy, or consumer.

3. After its arrival.

_Pollution at the Time of Milking_ arises from the animal, the

milker, or unclean methods of milking. It is now well known that in

tuberculosis of the cow affecting the udder the milk itself shows the

presence of the bacillus of tubercle. In a precisely similar manner all

bacterial diseases of the cow which affect the milk-secreting apparatus

must inevitably add their quota of bacteria to the milk. To this matter

we shall have occasion to refer again. There is a further contamination

from the animal when it is kept unclean, for it happens that the

unclean coat of a cow will more materially influence the number of

micro-organisms in the milk than the popularly supposed fermenting food

which the animal may eat. It is from this external source rather than

from the diet that organisms occur in the milk. The hairy coat offers

many facilities for harbouring dust and dirt. The mud and filth of

every kind that may be habitually seen on the hinder quarters of cattle

all contribute largely to polluted milk. Nor is this surprising. Such

filth at or near the temperature of the blood is an almost perfect

environment for many of the putrefactive bacteria.

The milker is also a source of risk. His hands, as well as the clothes

he is wearing, can and do readily convey both innocent and pathogenic

germs to the milk. Clothed in dust-laden garments, and frequently

characterised by dirty hands, the milker may easily act as an excellent

purveyor of germs. Not a few cases are also on record where it

appears that milkers have conveyed germs of disease from some case of

infectious disease, such as scarlet fever, in their homes. But under

the more efficient registration of such disease which has recently

characterised many dairy companies, the danger of infection from this

source has been reduced to a minimum. The habit of moistening the hands

with a few drops of milk previous to milking is one to be strongly

deprecated.

Professor Russell recounts a simple experiment which clearly

demonstrates these simple but effective sources of pollution:

"A cow that had been pastured in a meadow was taken for the

experiment, and the milking done out of doors, to eliminate

as much as possible the influence of germs in the barn air.

Without any special precaution being taken the cow was

partially milked, and during the operation a covered glass

dish, containing a thin layer of sterile gelatine, was exposed

for sixty seconds underneath the belly of the cow in close

proximity to the milk-pail. The udder, flank, and legs of

the cow were then thoroughly cleaned with water, and all of

the precautions referred to before were carried out, and the

milking then resumed. A second plate was then exposed in the

same place for an equal length of time, a control also being

exposed at the same time at a distance of ten feet from the

animal and six feet from the ground to ascertain the germ

contents of the surrounding air. From this experiment the

following instructive data were gathered. Where the animal was

milked without any special precautions being taken there were

3250 bacterial germs _per minute_ deposited on an area equal

to the exposed top of a ten-inch milk-pail. Where the cow

received the precautionary treatment as suggested above, there

were only 115 germs per minute deposited on the same area.

In the plate that was exposed to the surrounding air at some

distance from the cow there were 65 bacteria. This indicates

that a large number of organisms from the dry coat of the

animal can be kept out of milk if such simple precautions as

these are carried out."[54]

The influence of the barn air, and the cleanliness or otherwise of

the barn, is obviously great in this matter. As we have seen, moist

surfaces retain any bacteria lodged upon them; but in a dry barn,

where molecular disturbance is the rule rather than the exception, it

is not surprising that the air is heavily laden with microbic life.

Here again many improvements have been made by sanitary cleanliness

in various well-known dairies. Still there is much more to be done

in this direction to ensure that the drawn milk is not polluted by a

microbe-impregnated atmosphere.

The risks in _transit_ differ according to many circumstances. Probably

the commonest source of contamination is in the use of unclean utensils

and milk-cans. Any unnecessary delay in transit affords increased

opportunity for multiplication; particularly is this the case in the

summer months, for at such times all the conditions are favourable to

an enormous increase of any extraneous germs which may have gained

admittance at the time of milking. Thus we have (1) the milk itself

affording an excellent medium and supplying ideal pabulum for bacteria,

(2) a more or less lengthened railway journey or period of transit

giving ample time for multiplication, (3) the favourable temperature

of summer heat. We shall refer again to the rate of multiplication of

germs in milk.

Lastly, many are the advantages given to bacteria when milk has reached

its commercial destination. In milk-shops and in the home there are not

a few risks to be added on to the already imposing category. Water is

occasionally, if not frequently, added to milk to increase its volume.

Such water of itself will make its own contribution to the flora of the

milk, unless indeed, which is unlikely, the water has been recently and

thoroughly boiled before addition to the milk. Again, it is impossible

to suppose that in small homes--perhaps of only one room--where the

milk stands for several hours, pollution is avoidable. From a hundred

different sources such milk runs the risk of being polluted.

Before proceeding, a word must be said respecting the first milk

which flows from the udder in the process of milking, and which is

known as the _fore-milk_. This portion of the milk is always rich in

bacterial life on account of the fact that it has remained in the

milk-ducts since the last milking. However thorough the manipulation,

there will always be a residue remaining in the ducts, which will,

and does, afford a suitable nidus and incubator for organisms. The

latter obtain their entrance through the imperfectly closed teat

of the udder, and pass readily into the milk-duct, sometimes even

reaching the udder itself and setting up inflammation (_mastitis_).

Professor Russell states that he has found 2800 germs in the fore-milk

in a sample of which the average was only 330 per cc. Schultz found

83,000 micro-organisms per cc. in the fore-milk, and only 9000 in

the mid-milk. As a matter of fact, most of this large number belong

to the _lactic-acid fermentation_ group, and the fore-milk rarely

contains more than two or three species, and still more rarely any

disease-producing bacteria. Still, they occur in such enormous numbers

that their addition to the ordinary milk very materially alters its

quality. Bolley and Hall, of North Dakota, report sixteen species of

bacteria in the fore-milk, twelve of which produced an acid reaction.

Dr. Veranus Moore, of the United States Department of Agriculture,[55]

concludes from a large mass of data that freshly drawn fore-milk

contains a variable but generally enormous number of bacteria, but

only several species, the last milk containing, as compared with

the fore-milk, very few micro-organisms. The bacteria which become

localised in the milk-ducts, and which are necessarily carried into the

milk, are for the greater part rapidly acid-producing organisms, _i.

e._, they ferment milk-sugar, forming acids. They do not produce gas.

Still their presence renders it necessary to "pasteurise" as soon as

possible. Dr. Moore holds that much of the intestinal trouble occurring

in infants fed with ordinarily "pasteurised" milk arises from acids

produced by these bacteria between the drawing of the milk and the

pasteurisation.

_The Number of Bacteria in Milk._ From all that has been said

respecting the sources of pollution and the favourable nidus which milk

affords for bacteria, it is not surprising that a very large number of

germs are almost always present in milk. The quantitative estimation

of milk appears more alarming than the qualitative. It is true some

diseases are conveyed by bacteria in milk, but on the whole most of the

species are non-pathogenic. Nor need the numbers, though serious, too

greatly alarm us, for, as we shall see at a later stage, disease is a

complicated condition, and due to other agencies and conditions than

merely the bacteria, which may be the _vera causa_. In addition to the

fact that the high numbers have but a limited significance, we must

also remember that there is no uniformity whatever in these numbers.

The conditions which chiefly control them are (1) temperature, (2) time.

_The Influence of Temperature._ We have already noticed, when

considering the general conditions affecting bacteria, how potent

an agent in their growth is the surrounding temperature. Generally

speaking, temperature at or about blood-heat favours bacterial growth.

Freudenreich has drawn up the following table which graphically sets

forth the effect of temperature upon bacteria in milk:

3 hours. 6 hours. 9 hours. 24 hours.

59° F. 1 + 2.5 5 163

77° F. 2 18.5 107 62,100

95° F. 4 1,290 3,800 5,370

This instructive table claims some observations. It will be noticed

that at 59° F. there is very little multiplication. That may be

accepted as a rule. At 77° F. the multiplication, though not

particularly rapid at the outset, results finally, at the end of

the twenty-four hours, in the maximum quantity. These were probably

common species of saprophytic bacteria, which increase readily at a

comparatively low temperature. During the subsequent hours, after the

twenty-four, we should expect a decline rather than an increase in

62,000, owing to the keen competition consequent upon the limitation

of the pabulum. From a consideration of these figures we conclude that

a warm temperature, somewhat below blood-heat, is most favourable

to multiplication of bacteria in milk; that the common saprophytic

organisms multiply the most rapidly; that, in the course of time,

competition kills off a large number.

Let us take another example, from Professor Conn:

77° F. 95° F.

2 hours after milking (liquefied the

plate of gelatine) 1,275,000

6 " " " 14,620,000 45,900,000

9 " " " 36,550,000 57,800,000

24 " " " 13,702,000,000 13,812,500,000

[Bacteria per cub. inch.]

These almost incredibly large figures illustrate much the same points,

particularly the rapid multiplication at blood-heat, and the later rise

at 77° C.

_The Influence of Time_ is not less marked than that of temperature, as

the following table will show:

Milk drawn at 59° C. = 153,000 m.o. per cub. in.

After 1 hour = 616,000 " " " "

" 2 hours = 539,000 " " " "

" 4 " = 680,000 " " " "

" 7 " = 1,020,000 " " " "

" 9 " = 2,040,000 " " " "

" 24 " = 85,000,000 " " " "

(Conn.)

Freudenreich gives another example, as follows:

Milk drawn at 15.5° C. = 27,000 m.o. per cc.

After 4 hours = 34,000 " " "

" 9 " = 100,000 " " "

" 24 " = 4,000,000 " " "

Concerning these figures little comment is necessary. But here again,

we may remember that this rapid multiplication continues only up to a

certain point, after which competition brings about a marked reduction.

The effect of temperature and time has been illustrated by Dr. Buchanan

Young's recent researches, laid before the Royal Society of Edinburgh.

He estimated that in the Edinburgh milk supply three hours after

milking there were 24,000 micro-organisms per cc. _in winter_; 44,000

_in spring_; 173,000 _in late summer and autumn_. Again, he found that

five hours after milking there were 41,000 micro-organisms per cc. in

country milk, and more than 350,000 micro-organisms per cc. in town

milk. Many London milks would exceed 500,000 per cc.[56]

There is no standard or uniformity in the numerical estimation of

bacteria in milk. A host of observers have recorded widely varying

returns due to the widely varying circumstances under which the milk

has been collected, removed, stored, and examined. Nor is it possible

to establish any standard which may be accepted as a _normal_ or

healthy number of bacteria, as is done in water examination. Bitter has

suggested 50,000 micro-organisms per cc. as a maximum limit for milk

intended for human consumption.

But owing to differences of nomenclature and classification, in

addition to differences in mode of examination at present existing

in various countries, it is impossible to state even approximately

how many bacteria and how many species of bacteria have been isolated

from milk. Until some common international standard is established

mathematical computations are practically worthless. They are

needlessly alarming and sensational. And it should be remembered that

great reliance cannot be placed upon these numerical estimations.

They vary from day to day, and even hour to hour. Furthermore, vast

numbers of bacteria are economic in the best sense of the term, and

the bacteria of milk are chiefly those of a fermentative kind, and not

disease-producers.

_Kinds of Bacteria in Milk._ It is clear from the foregoing that the

only valuable estimation of bacteria in milk is a qualitative one. The

kinds commonly found may be classified thus:

1. Non-pathogenic; fermenting and various unclassified micro-organisms.

2. Pathogenic; tuberculosis, typhoid, cholera, scarlet fever,

diphtheria, and suppurative diseases have all been spread by the agency

of milk.

1. _The Fermentation Bacteria_

At the most we can make a merely provisional classification of these

processes. Many of them are intimately related. Of others, again, our

knowledge is at present very limited. It may be advisable, before

proceeding, to consider shortly what are the constituents of milk upon

which living ferments of various kinds exert their action. A tabulation

of the chief constituents would be as follows:

{ (1) Water 87.5 per cent.

Ordinary { (2) Milk-sugar 4.9 " "

fresh milk = { (3) Fat 3.6 " "

100 per cent. { (4) Proteids (casein, etc.) 3.3 " "

{ (5) Mineral matter 0.7 " "

-----

100.0

Another mode of expressing average milk constitution would be thus:

Fat 4.1 per cent.

Solids not fat 8.8 " "

----

12.9 " "

It is probably too obvious to need remark that milks vary in standard,

but the above figures may be taken as authentic averages.

_Milk-sugar, or Lactose_ (C_{12}H_{24}O_{12}). This is an important and

constant constituent of milk. It forms the chief substance in solution

in whey or serum. Milk-sugar approximates to dextrose in its action on

polarised light. By boiling with sulphuric acid it is converted into

dextrose and galactose.

_Fat_ occurs in milk as suspended globules, and by churning may be made

into butter.

_The Proteids_ include casein, albumen, lactoprotein, and a small

quantity of globulin. These are the nitrogenous bodies.

_Mineral Matter._ The ash of milk, obtained by careful ignition of the

solids, contains calcium, magnesium, potassium, sodium, phosphoric

acid, sulphuric acid, chlorine, and iron, phosphoric acid and lime

being present in the largest amounts.

(1) _Lactic Acid Fermentation._ If milk is left undisturbed, it is well

known that eventually it becomes sour. The casein is coagulated, and

falls to the bottom of the vessel; the whey or serum rises to the top.

In fact, a coagulation analogous to the clotting of blood has taken

place. In addition to this, the whole has acquired an acid taste. Now,

this double change is not due to any one of the constituents we have

named above. It is, in short, a fermentation set up by a living ferment

introduced from without. The constituent most affected by the ferment

is the milk-sugar, which is broken down into lactic acid, carbonic acid

gas, and other products.

For many years it has been known that sour milk contained bacteria.

Pasteur first described the _Bacillus acidi lactici_, which Lister

isolated and obtained in pure culture. Hueppe contributed still further

to what was known of this bacillus, and pointed out that there were

a large number of varieties, rather than one species, to be included

under the term _B. acidi lactici_. We have already seen that these

bacilli do not as a rule liquefy gelatine, form spores, are non-motile,

and are easily killed by heat.

When a certain quantity of lactic acid has been formed the fermentation

ceases. It will recommence if the liquid be neutralised with carbonate

of lime, or pepsine added. Since Pasteur's discovery of a causal

bacillus for this fermentation, other investigators have added a

number of bacteria to the lactic acid family. Some of these in pure

culture have been used in dairy industry to add to the butter a pure

sour taste, a more or less aromatic odour, and a higher degree of

preservation.

(2) _Butyric Acid Fermentation._ This form of fermentation is also one

which we have previously considered.

Both in lactic and butyric fermentation we must recognise that in the

decomposition of milk-sugar there are almost always a number of minor

products occurring. Some of the chief of these are gases. Hydrogen,

carbonic acid, nitrogen, and methane occur, and cause a characteristic

effect which is frequently deleterious to the flavour of the milk

and its products. Most of the gas-producing ferments are members

of the lactic acid group, and are sometimes classified in a group

by themselves. In cheese-making the gases create the pin-holes and

air-spaces occasionally seen.

(3) _Curdling Fermentations without Acid Production._ Of these there

are several, caused by different bacteria. What happens is that the

milk coagulates, as we have described, but no acid is produced, the

whey being sweet to the taste rather than otherwise. Digestion of

casein may or may not take place.

We must now mention several fermentations about which little is known.

They are designated by terms denoting the outward condition of the

milk, without giving any information respecting the real physiological

alteration which has occurred.

(4) _Bitter Fermentation._ Some bitter conditions of milk are due to

irregularity of diet in the cow. Similar changes occur in conjunction

with some of the acid fermentations. Weigmann and Conn have, however,

shown that there is a specific bitterness in milk due to bacteria which

appear to produce no other change. Hueppe suggests that it may be

due in part to a proteid decomposition resulting in bitter peptones.

There seems to be some evidence for supposing that the bitter bacteria

produce very resistant spores, which enable them to withstand treatment

under which the lactic acid succumbs.

(5) _Slimy Fermentation._ This graphic but inelegant word is used to

denote an increased viscosity in milk, and its tendency when being

poured to become ropy and fall in strings. Such a condition deprives

the milk of its use in the making of certain cheeses, whilst in

other cases it favours the process. In Holland, for example, in the

manufacture of Edam cheese, this "slimy" fermentation is desired.

_Tættemœlk_, a popular beverage in Norway, is made from milk

that has been infected with the leaves of the common butter wort,

_Pinguicula vulgaris_, from which Weigmann separated a bacillus

possessing the power of setting up slimy fermentation. There are,

perhaps, as many as a dozen species of bacteria which have in a greater

or less degree the power of setting up this kind of fermentation. In

1882 Schmidt isolated the _Micrococcus viscosus_, which occurs in

chains and rosaries, affecting the milk-sugar. It grows at blood-heat,

and is not easily destroyed by cold. Its effect on various sugars is

the same. The _M. Freudenreichii_, the specific micro-organism of

"ropiness" in milk, is a large, non-motile, liquefying coccus, which

can produce its result in milk within five hours. On account of its

resistance to drying, it is difficult to eradicate when once it makes

its appearance in a dairy. The organism used in making Edam cheese

is the _Streptococcus Hollandicus_, and in hot milk it can produce

ropiness in one day. A number of bacilli have been detected by several

observers and classified as slime fermentation bacteria. The _Bacillus

lactis pituitosi_, a slightly curved, non-liquefying rod, which is said

to produce a characteristic odour, in addition to causing ropiness,

brings about some acidity. _B. lactis viscosus_ is slow in starting its

fermentation, but maintains its action for as long as a month. Many

of the above organisms, with others, produce "slimy" fermentation in

alcoholic beverages as well as in milk.

(6) _Soapy Milk._ This is still another form of fermentation, the

etiology of which has been elucidated by Weigmann. The _Bacillus

saponacei_ imparts to milk a peculiar soapy flavour. It was detected in

the straw of the bedding and hay of the fodder, and from such sources

may infect the milk. There is little or no coagulation.

(7) _Chromogenic Changes._ We have already remarked that colour is the

natural and apparently only product of many of the innocent bacteria.

They put out their strength, so to speak, in the production of bright

colours. The chief colours produced by germs in milk are as follows:

_Red Milk._ _Bacillus prodigiosus_, in the presence of oxygen, causes

a redness, particularly on the surface of milk. It was the work of

this bacillus that caused "the bleeding host," which was one of the

superstitions of the Middle Ages. _B. lactis erythrogenes_ produces

a red colour only in the dark, and in milk that is not strongly acid

in reaction. When grown in the light this organism produces a yellow

colour. There is a red sarcina (_Sarcina rosea_) which also has the

faculty of producing red pigment. One of the yeasts is another example.

It must not be forgotten that redness in milk may actually be due to

the presence of blood from the udder of the cow. In such a case the

blood and milk will be inextricably mixed together, and not in patches

or a pellicle.

_Blue Milk_ is due to the growth of _Bacillus cyanogenus_. This is an

actively motile rod, the presence of which does not materially affect

the milk, but causes the milk products to be of poor quality.

_Yellow Milk._ _Bacillus synxanthus_ is held responsible for curdling

the milk, and then at a later stage, in redissolving the curd, produces

a yellow pigment.

_Violet and Green Pigments_ in milk are also the work of various

bacteria.

2. _Various Unclassified Bacteria_

In milk this is a comparatively small group, for it happens that

those bacteria in milk which cannot be classified as fermentative or

pathogenic are few. The almost ubiquitous _Bacillus coli communis_

occurs here as elsewhere, and might be grouped with the gaseous

fermentative organisms on account of its extraordinary power of

producing gas and breaking up the medium (whether agar or cheese)

in which it is growing. What its exact rôle is in milk it would be

difficult to say. It may act, as it frequently does elsewhere, by

_association_ in various fermentations. Some authorities hold that its

presence in excessive numbers may cause epidemic diarrhœa in infants.

Several years ago a commission was appointed by the _British Medical

Journal_ to inquire into the quality of the milk sold in some of the

poorer districts of London. Every sample was found to contain _Bacillus

coli_, and it was declared that this particular microbe constituted 90

per cent. of all the organisms found in the milk.[57] We record this

statement, but accept it with some misgiving. The diagnosis of _B.

coli_ four or five years ago was not such a strict matter as to-day.

Still, undoubtedly, this particular organism is not uncommonly found

in milk, and its source is unclean dairying. In the same investigation

_Proteus vulgaris_, _B. fluorescens_, and many liquefying bacteria

were frequently found. Their presence in milk means contamination with

putrefying matter, surface water, or a foul atmosphere.

A number of water bacteria find their way into milk in the practice

of adulteration, and foul byres afford ample opportunity for aërial

pollution.

Another unclassified group occasionally present in milk is represented

by moulds, particularly _Oidium lactis_, the mould which causes a white

fur, possessing a sour odour. It is allied to the _Mycoderma albicans_

(_O. albicans_), which also occurs in milk, and causes the whitish-grey

patches on the mucous membrane of the mouths of infants (_thrush_).

These and many more are occasionally present in milk.

3. _The Disease-Producing Power of Milk_

The general use of milk as an article of diet, especially by the

younger and least resistant portion of mankind, very much increases

the importance of the question as to how far it acts as a vehicle

of disease. Recently considerable attention has been drawn to the

matter, though it is now a number of years since milk was proved to

be a channel for the conveyance of infectious diseases. During the

last twenty years particular and conclusive evidence has been deduced

to show that milch cows may themselves afford a large measure of

infection. The recent extensive work in tuberculosis by the Royal

Commission has done much to obtain new light on the conveyance of

that disease by milk and meat. The enormous strides in the knowledge

of diphtheria and other germ diseases have also placed us in a better

position respecting their conveyance by milk. Generally speaking, for

reasons already given, milk affords an ideal medium for bacteria, and

its adaptibility therefore for conveying disease is undoubted. We may

now suitably turn to speak shortly of the outstanding facts of the

chief diseases carried by milk.

_Tuberculosis._ It is well known that this disease is not a rare one

amongst cattle. The problem of infective milk is, however, simplified

at the outset by recognising the now well-established fact that the

milk of tuberculous cows is only certainly able to produce tuberculosis

in the consumers when the _tuberculous disease affects the udder_.

This is not necessarily a condition of advanced tuberculosis. The

udder may become affected at a comparatively early stage. But to make

the milk infective the udder must be tubercular, and milk from such

an udder possesses a most extraordinary degree of virulence. When

the udder itself is thus the seat of disease, not only the derived

milk, but the skimmed milk, butter-milk, and even butter, all contain

tuberculous material actively injurious if consumed. Furthermore,

tubercular disease of the udder spreads in extent and degree with

extreme rapidity. From these facts it will be obvious that it is of

first-rate importance to be able to diagnose udder disease. This is not

always possible in the early stage. The signs upon which most reliance

may be placed are the enlargement of the lymph-glands lying above the

posterior region of the udder; the serous, yellowish milk which later

on discharges small coagula; the partial or total lack of milk from one

quarter of the udder (following upon excessive secretion); the hard,

diffuse nodular swelling and induration of a part or the whole wall of

the udder; and the detection in the milk of tubercle bacilli. The whole

organ may increase in weight as well as size, and on _post-mortem_

examination show an increase of connective tissue, a number of large

nodules of tubercle, and a scattering of small granular bodies, known

as "miliary" tubercles. _Tuberculin_ may be used as an additional test.

The udder is affected in about two per cent. of tuberculous cows.

There are a variety of causes in addition to the _vera causa_,

the presence of the bacillus of tubercle, which make the disease

common amongst cattle. Constitution, temperament, age, work, food,

and prolonged lactation are the individual features which act as

predisposing conditions; they may act by favouring the propagation of

the bacillus or by weakening the resistance of the tissues. To this

category must further be added conditions of environment. Bad stabling,

dark, ill-ventilated stalls, high temperature, prolonged and close

contact with other cows, all tend in the same direction.

Though there can be no doubt as to the virulence of tuberculous milk,

it may be remembered with satisfaction that only about two per cent.

of tuberculous cows have unmistakably tubercular milk. Even of this

tubercular milk, unless it is very rich in bacilli and is ingested

in large quantities, the risks are practically small or even absent.

Practically the danger from drinking raw milk exists only for persons

who use it as their sole or principal food, that is to say, young

children and certain invalids. With adults in normal health the danger

is greatly minimised, as the healthy digestive tract is relatively

insusceptible. Moreover, dairy milk is almost invariably _mixed_

milk; that is to say, if there is a tubercular cow in a herd yielding

tubercle bacilli in her milk, the addition of the milk of the rest of

the herd so effectually _dilutes_ the whole as to render it almost

innocuous.

But if for practical purposes we look upon all milk derived from

tubercular udders as highly infective, we may adopt a comparatively

simple and efficient remedy. To avoid all danger it is sufficient to

bring the milk to a boil for a few minutes before it is consumed; in

fact, the temperature of 85° C. (160° F.) prolonged for five minutes

kills all bacilli. The common idea that boiled milk is indigestible,

and that the boiling causes it to lose much of its nutritive value, is

largely groundless.

Milk may become tubercular through the carelessness or dirty habits of

the milker. Such a common practice as moistening the hands with saliva

previously to milking may, in cases of tubercular milkers, effectually

contaminate the milk. Again, it may become polluted by dried tubercular

excreta getting into it. Such conveyances must be of rare occurrence,

yet their possibility should not be forgotten.

An infant suckled by a tuberculous mother would run similarly serious

risks of becoming infected with the disease.

In Liverpool, Dr. E. W. Hope, the Medical Officer of Health, has

organised an admirable system of examination by skilled bacteriologists

to find to what degree the Liverpool milk supply is contaminated

with tubercle. The final result of this pioneer work, which ought

really to be undertaken by every great corporation responsible to the

citizens for a pure water and pure milk supply, is to the effect that

in Liverpool 5.2 per cent. of the samples of milk taken from the city

shippons contains tubercle bacilli. As regards the milk sent in from

the country, the return is that 13.4 per cent. is contaminated with the

bacillus of tubercle.

-------------------------------+--------------------------------+--------

TOWN SHIPPONS. | COUNTRY SHIPPONS. | TOTAL.

-------+-----------+-----------+--------+-----------+-----------+--------

228 | 12 | 5.2 | 67 | 9 | 13.4 | 295

-------+-----------+-----------+--------+-----------+-----------+--------

Such results are very significant, and indicate the importance of all

large corporations obtaining the service of systematic and periodic

bacteriological examination of the milk supply. Nor are the results

surprising, for when we remember the habits of the tubercle bacillus

we cannot conceive a more favourable nurture ground than the typical

byre. "Nothing worse than the insanitary conditions of the life of

the average dairy cow," says Sir George Brown, late of the Board of

Agriculture, "can be imagined." It will be obvious that the above facts

make it incumbent upon responsible authorities to see that not a stone

is left unturned to enforce cleanliness in all dairy work, isolation of

diseased cows, and strict treatment of all infected milk.

_Typhoid Fever._ Jaccoud in France and Hart in England have shown that

enteric fever (typhoid) is not infrequently spread by milk. An epidemic

affecting 386 persons in Stamford, Conn., U.S.A., was traced to milk,

97 per cent. of the cases coming from one single milk supply. Dr.

McNail recently recorded an outbreak of twenty-two cases of enteric,

due to a polluted milk supply.

Within the last twelve months much attention has been drawn to a milk

source of typhoid infection by the epidemic of typhoid at Bristol.

Dr. D. S. Davies has pointed out that a brook received the sewage of

thirty-seven houses, the overflow of a cesspool serving twenty-two

more, the washings from fields over which the drainage of several

others was distributed, and the direct sewage from at least one other,

and then flowed directly through a certain farm. The water of this

stream supplied the farm pump, and the water itself, it is scarcely

necessary to add, was highly charged with putrescent organic matter and

micro-organisms. This water was used for washing the milk-cans from

this particular farm, otherwise the dairy arrangements were efficient.

Part of the milk was distributed to fifty-seven houses in Clifton;

in forty-one of them cases of typhoid occurred. Another part of the

milk was sold over the counter; twenty households so obtaining it were

attacked with typhoid fever, and a number of further infections and

complications arose. This evidence would appear to support the fact

that milk may act in the same way, though not in such a high degree, as

water in the conveyance of typhoid fever.

It may be pointed out that specific typhoid is not a disease of

animals; consequently no danger need be apprehended from milk if it

is properly cared for _after_ it comes from the cow. Typhoid milk is

almost invariably due to the addition of typhoid-infected water,

either by way of adulteration or in the process of washing out the

milk-cans. Cases have, however, been recorded in which there has been

direct transmission to the milk from a person convalescing from the

disease, and also indirect transmission by a milker serving also in the

capacity of nurse to a patient in his own family.

Though the typhoid bacillus appears not to have the power of

multiplying in milk, it has the faculty of existing and thriving in

milk, even when it has curdled or soured, for a considerable time, and

may thus infect milk products like butter and cheese. But infection by

milk products may be eliminated as of too rare occurrence to deserve

attention. The bacillus does not coagulate the milk like its ally

the _Bacillus coli communis_, which is a much more frequent and less

injurious inhabitant of milk.

_Cholera._ The cholera bacillus, as we have already pointed out, is

unable to live in an acid medium. Hence its life in milk is a limited

one, and generally depends on some alkaline change in the milk. Heim

found that cholera bacilli would live in raw milk from one to four

days, depending upon the temperature. D. D. Cunningham, from the

results of a large number of investigations in India, concludes that

the rapidly developing acid fermentations normally or usually setting

in, connected with the rapid multiplication of other common bacteria

and moulds, tend to arrest the multiplication of cholera bacilli, and

eventually to destroy their vitality. Boiling milk appears, on the

contrary, to increase the suitability of milk as a nidus for cholera

bacilli, partly by its germicidal effect upon the acid-producing

microbes, and partly because it removes from the milk the enormous

numbers of common bacteria, which in raw milk cause such keen

competition that the cholera bacillus finds existence impossible.

Professor W. J. Simpson, lately the Medical Officer of Health for

Calcutta, has placed on record an interesting series of cholera cases

on board the _Ardenclutha_, in the port of Calcutta, which arose from

drinking milk which had been polluted with one quarter of its volume

of cholera-infected water. This water came from a tank into which some

cholera dejecta had passed. Of the ten men who drank the milk four

died, five were severely ill, and one, who drank but very little of

the milk, was only slightly ill. There was no illness whatever amongst

those who did not drink the milk.

_Diphtheria._ Recent observations on the infectivity of diphtheria

in milk by Schottelius have established the fact that milk is a good

medium for the bacillus of diphtheria, but that it rarely acts as

a vehicle for transmitting the disease. Klein has emphasised the

possibility of this means of infection. In the first place, it is

obvious that the milk may become infected from a human source--from

pollution with diphtheritic discharges or dried "fomites." Secondly,

from a variety of different quarters evidence has been forthcoming

to throw some suspicion upon the cow itself as the agent. Klein

states that "a new eruptive disease on the teats and udder of the

cow," consisting of papules, vesicles, and induration, may be set up

by the subcutaneous inoculation of a pure culture of the _Bacillus

diphtheriæ_. In these eruptions a bacillus similar to the _B.

diphtheriæ_ was demonstrated. On _a priori_ grounds this evidence

substantiates a belief that diphtheria, in some form or other, may be

a disease of cows. Other observers have not been able to confirm these

observations, and the whole matter of cow diphtheria must remain for

the present _sub judice_.

As long ago as 1879 W. H. Power traced an epidemic of diphtheria in

North London to the milk supply. In 1887 the same authority studied

another outbreak, and other observers have produced further evidence

in favour of the conveyance of this disease by milk. Air infection of

milk by the _Bacillus diphtheriæ_ probably occurs only very rarely, on

account of the fact that the organism is readily killed by desiccation,

and yet such is necessary before it can be airborne. The most frequent

mode of infection of milk with this disease is from the throats, hands,

bodies, or clothing of dairy workers suffering from a mild or acute

form of the disease.

The specific and proved cases in which milk has acted as the vehicle of

diphtheria are, it is true, comparatively few. Yet, nevertheless, the

possibility of milk infection in this disease is not one which we can

afford to neglect.

_Scarlet Fever._ Here again the evidence is not complete, chiefly

owing to the fact that no specific organism of scarlet fever has yet

been discovered. Many cases have, however, illustrated the undeniable

conveyance of the disease by milk. Even before 1881 a number of milk

epidemics of scarlet fever had been traced out. In 1882 these were

further added to by Mr. W. H. Power's report concerning a series

of cases in Central London. That report was remarkable for the

introduction of a new feature, viz., the evidence produced in favour

of the infection of milk from some disease of the cow. The Medical

Department of the Local Government Board from that time took up a

position of suspended judgment concerning the belief hitherto credited

that milk could only be infected by _human_ scarlet fever. In 1886

there was a remarkable epidemic in Marylebone, and the theory was

suggested by Dr. Klein and Mr. Power that the cow from which the milk

was derived suffered from scarlet fever.

Into the extensive controversy which raged round "the Hendon disease,"

as it was called, affecting the cows supplying the Marylebone milk,

we cannot here enter. It will be sufficient to say that a long

discussion took place as to whether or not this Hendon disease was or

was not scarlet fever. The difficulty of course largely arose from the

fact before mentioned that we do not at present know the specific

micro-organism of scarlet fever. The Agricultural Department supported

the view of Professor Crookshank that the cow disease at Hendon was

_cow-pox_, and Professor Axe further pointed out that there was evidence

of the Hendon milk having been contaminated with human scarlet fever.

Whichever conclusion was adopted, all were agreed upon one point, viz.,

that the disease had been conveyed from Hendon to persons in Marylebone

by means of the milk.

Mr. Ernest Hart in 1897 published a very large number of records of

scarlatinal milk infection from all parts of the country, and though

the cause of the disease is obscure, there is now no doubt that it may

be and is conveyed by means of milk.

_Other Diseases Conveyed by Milk._ In addition to the above, there are

other diseases spread by means of polluted milk. From time to time

exceptional cases have occurred in which a disease like anthrax has

been spread by this means. But it is not to such rare cases that we

refer. There are two very common diseases in which milk has been proved

to play a not inconsiderable part, viz., _thrush_ and _diarrhœa_.

The mould which gives rise to the curd-like patches in the throats of

children, and which is known as _Oidium albicans_, frequently occurs

in milk. Soft white specks are seen on the tongue and mucous membrane

of the cheeks and lips, looking not unlike particles of milk curd.

If a scraping be placed upon a glass slide with a drop of glycerine

and examined by means of the microscope, the spores and mycelial

threads of this mould will be seen. The spores are oval, and possess

a definite capsule. The threads are branched and jointed at somewhat

long intervals. Milk affords an excellent medium for the growth of this

parasite. Thus undoubtedly we must hold milk partly responsible for

spreading this complaint. _Penicillium_, _Aspergillus_, and _Mucor_ are

also frequent moulds in milk.

Professor MacFadyen[58] has given a full account of the ways in which

milk becomes pathogenic, and his views have received further support

from Professor Sheridan Delépine, who has examined more than one

hundred samples of milk from Liverpool and Manchester. The result of

this investigation has been that milk must be held to be one of the

most potent causes of the summer diarrhœa of children. Indeed, a

bacillus has been isolated identical with one which was apparently

the cause of this complaint, which carries off such a large number

of infants every summer. It resembles closely the _Bacillus coli

communis_, which is an almost constant inhabitant of the alimentary

canal, and is held by many bacteriologists to play, especially in

conjunction with yeasts and other saprophytic organisms, an active rôle

in the intestine of man.

In a recent official report[59] Dr. Hope, of Liverpool, states that

"the method of feeding plays a most important part in the causation

of diarrhœa; when artificial feeding becomes necessary, the most

scrupulous attention should be paid to feeding-bottles." Careless

feeding, in conjunction with a warm, dry summer, invariably results

in a high death-rate from this cause. These two causes interact upon

each other. A warm temperature is a favourable temperature for the

growth of the poisonous micro-organism; a dry season affords ample

opportunity for its conveyance through the air. Unclean feeding-bottles

are obviously an admirable nidus for these injurious bacteria, for in

such a resting-place the three main conditions necessary for bacterial

life are well fulfilled, viz., heat, moisture, and pabulum. The heat is

supplied by the warm temperature, the moisture and food by the dregs of

milk left in the bottle; and the dry air assists in transit.

Before passing on to other matters, reference must be made to

poisonous products other than bacteria which occur in milk and set up

ill-health. Vaughan, of Michigan, pointed out at the London Congress

of Hygiene in 1891 that he had separated a poisonous alkaloid, which

he called _tyrotoxicon_. This, as its name denotes, was a toxic or

poisonous substance, probably produced by some form of microbe. It

may be taken as a type of the organic chemical substances frequently

occurring in milk.

METHODS OF PRESERVING MILK

From the somewhat extensive category of diseases which may be

milk-borne, it will be suitable now to speak of some of the means at

our disposal for obtaining and preserving good, pure milk.

We considered at the commencement of this chapter the most frequent

channels of contamination. If these be avoided or prevented, and if

the milk be derived from cows in good health and well kept, the risk

of infection is reduced to a minimum. But we have seen that much, if

not most, of the pollution of milk arises after the milking process

and during transit and storage preparatory to use. Bacteria are so

ubiquitous that to prevent the entrance of any at all is almost beyond

hope. Can anything be done to prevent their multiplication or to kill

them in the milk? Fortunately the answer is in the affirmative.

There are two means at hand to secure these results. First, we may

add to the milk various chemical or physical preservatives. Borax or

boric acid, formaldehyde, salicylic acid, and other chemical bodies

are used for this purpose. The commonest of these is that named first.

The Food and Drugs Act (Section VI., 1875) permits the addition of

an ingredient not injurious to health if the same is required for

protection or preparation of the article in question. It is, however,

a difficult matter to determine what amount of boric acid is injurious

to health, for this differs widely in different persons. It has been

laid down by one authority that even so small an amount as one-tenth

per cent. might have inconvenient results, owing to its cumulative

effect. _Formaldehyde_ is without doubt an excellent antiseptic, and

the more its efficacy becomes known so much the more probably will

it be used. The _salicylates_, which are mild antiseptics, have long

been used as preservatives. These substances, then, can be added to

milk in quantities not recognisable to the taste (salicylic acid

about .75 grain, and boracic acid .4 grain, to the litre of milk).

They will materially increase the time that milk will remain sweet,

they will prevent a number of micro-organisms living in the milk, and

will inhibit multiplication of others.[60] Secondly, it is possible

very perceptibly to remove the infectivity of milk by filtration and

temperature variations.

_Filtration_ has been practised for some time by the Copenhagen Dairy

Company and by Bolle, of Berlin. The filters used consist of large

cylindrical vessels divided by horizontal perforated diaphragms into

five superposed compartments, of which the middle three are filled

with fine sand of three sizes. At the bottom is the coarsest sand,

and at the top the finest. The milk enters the lowest compartment by

a pipe under gravitation pressure, and is forced upwards, and finally

is run off into an iced cooler, and from that into the distribution

cans. By this means the number of bacteria is reduced to one-third.

The difficulty of drying and sterilising enough sand to admit a

large turnover of milk is a serious one. This, in conjunction with

the belief that filtration removes some of the essential nutritive

elements of milk, has caused the process to be but little adopted.

Dr. Seibert states that if milk be filtered through half an inch of

compressed absorbent cotton, seven-eighths of the contained bacteria

will be removed, and a second filtration will further reduce the number

to one-twentieth. One quart of milk may thus be filtered in fifteen

minutes.

The common methods now in vogue for the protection of milk are based

upon _germicidal temperatures_. Low temperatures, it is true, do

not easily destroy life, but they have a most beneficial effect

upon the keeping quality of milk. At the outset of the process of

cooling, strong currents of air are started in the milk-can, which

act mechanically as deodorisers. But if the temperature be lowered

sufficiently, the contained bacteria become inactive and torpid, and

eventually are unable to multiply or produce their characteristic

fermentations. At about 50° F. (10° C.) the activity ceases, and

at temperatures of 45° F. (7° C.) and 39° F. (4° C.) organisms are

deprived of their injurious powers. If it happens that the milk is to

be conveyed long distances, then even a lower temperature is desirable.

The most important point with regard to the cooling of milk is that it

should take place quickly. Various kinds of apparatus are effective in

accomplishing this. Perhaps those best known are Lawrence's cooler and

Pfeiffer's cooler, the advantage of the latter being that during the

process the milk is not exposed to the air. It must not be forgotten

that cooling processes are not sterilising processes. They do not

necessarily kill bacteria; they only inhibit activity, and under

favourable circumstances the torpid bacteria may again acquire their

injurious faculties. Hence during the cooling of milk greater care must

be taken to prevent aërial contamination than is necessary during the

process of sterilising milk. No cooling whatever should be attempted in

the stable; but, on the other hand, there should be no delay. Climate

makes little or no difference to the practical desirability of cooling

milk, yet it is obvious that less cooling will be required in the cold

season.

We now come to the protective processes known as _sterilisation_ and

_pasteurisation_. As we have already seen, sterilisation indicates

a complete and final destruction of bacteria and their spores. As

applied to methods of preserving milk, sterilisation means the use of

heat at, or above, boiling-point, or boiling under pressure. This may

be applied in one application of one to two hours at 250° F., or it

may be applied at stated intervals at a lower temperature. The milk

is sterilised--that is to say, contains no living germs--is altered

in chemical composition, and is also boiled or "cooked," and hence

possesses a flavour which to many people is unpalatable.

Now, such a radical alteration is not necessary in order to secure

non-infectious milk. The bacteria causing the diseases conveyable

by milk succumb at much lower temperatures than the boiling-point.

Advantage is taken of this in the process known as "pasteurisation."

By this method the milk is heated to 167-185° F. (75-85° C.). Such a

temperature kills harmful microbes, because 75° C. is decidedly above

their average thermal death-point, and yet the physical changes in the

milk are practically _nil_, because 85° C. does not relatively approach

the boiling-point. There is no fixed standard for pasteurisation,

except that it must be above the thermal death-point of pathogenic

bacteria, and yet below the boiling-point. As a matter of fact, 158° F.

(70° C.) will kill all souring bacteria as well as disease-producing

organisms found in milk. If the milk is kept at that temperature for

ten or fifteen minutes, we say it has been "pasteurised." If it has

been boiled, with or without pressure, for half an hour, we say it

has been "sterilised." The only practical difference in the result is

that sterilised milks have a better keeping quality than pasteurised,

for the simple reason that in the latter some living germs have been

unaffected.

Sterilisation may of course be carried out in a variety of

modifications of the two chief ways above named. When the process

is to be completed in one event an _autoclave_ is used, in order to

obtain increased pressure and a higher temperature. Milk so treated

is physically changed in greater degree than in the slower process.

The slow or _intermittent_ method is, of course, based on Tyndall's

discovery that actively growing bacteria are more easily killed than

their spores. The first sterilisation kills the bacteria, but leaves

their spores. By the time of the second application the spores have

developed into bacteria, which in turn are killed before they can

sporulate.

The methods of pasteurisation are continually being modified and

improved, especially in Germany and America. Most of the variations in

apparatus may be classed under two headings. There are, first, those

in which a sheet of milk is allowed to flow over a surface heated

by steam or hot water. This may be a flat, corrugated surface or a

revolving cylinder. The milk is then passed into coolers. Secondly,

milk is pasteurised by being placed in reservoirs surrounded by an

external shell containing hot water or steam. Dr. A. L. Russell[61] has

described one apparatus consisting of a pasteuriser, a water-cooler,

and an ice-cooler. The pasteuriser is heated by hot water in the

outside casement. To equalise rapidly the temperature of the water

and milk a series of agitators must be used. These are suspended on

movable rods, and hang vertically in the milk and water chambers. By

this ingenious arrangement the heat is diffused rapidly throughout the

whole mass, and as the temperature of the milk reaches the proper point

the steam is shut off, and the heat of the whole body of water and milk

will remain constant for the proper length of time.

The somewhat difficult problem of drawing off the pasteurised milk from

the vat without reinfecting it by contact with the air is solved by

placing a valve inside the chamber, and by means of a pipe leading the

pasteurised milk directly and rapidly into the coolers. These are of

two kinds, which may be used separately or conjointly. In one set of

cylinders there is cold circulating water, in the other finely crushed

ice.

Domestic pasteurisation can be accomplished readily by heating the milk

in vessels in a water-bath raised to the required temperature for half

an hour.

Without entering into a long discussion upon the various methods

adopted, we may summarise some of the chief essential conditions. It

need scarcely be said that the operation must be efficiently conducted,

and in such a way as to maintain absolute control over the time and

temperature. The apparatus should be simple enough to be easily

cleansed, sterilised, and economical in use. Arrangements must always

be made to protect the milk from reinfection during and after the

process. The entire preparation of the milk for market may be summed up

in four items:

1. Pasteurisation in heat reservoir.

2. Rapid cooling in water-or ice-coolers.

3. All cans, pails, bottles, and other utensils to be

thoroughly sterilised in steam.

4. The prepared milk must be placed in sterilised bottles and

sealed up.

The quality of the milk to be pasteurised is an important point. All

milks are not equally suited for this purpose, and those containing a

large quantity of contamination, especially of spores, are distinctly

unsuitable. Such milks, to be purified, _must_ be sterilised. Dr.

Russell has laid down a standard test for the degree of contamination

which may be corrected by pasteurisation by estimating the degree of

acidity, a low acidity (_e. g._, 0.2 per cent.) usually indicating a

smaller number of spore-bearing germs than that which contains a high

percentage of acid.

Lastly, while the heating process is of course the essential feature

of efficient pasteurisation, it must not be forgotten that rapid

and thorough cooling is almost equally important. As we have seen,

pasteurisation differs from complete sterilisation in that it leaves

behind a certain number of microbes or their spores. Cooling inhibits

the germination and growth of this organismal residue. If after the

heating process the milk is cooled and kept in a refrigerator, it will

probably keep sweet from three to six days, and may do so for three

weeks.

Before leaving this subject we may glance for a moment at the bacterial

results of pasteurisation and sterilisation. The chief two of these

are the enhanced keeping quality and the removal of disease-producing

germs. The former is due in part to the latter, and also to the

removal of the lactic acid and other fermentative bacteria. As a

general rule these bacteria do not produce spores, and hence they are

easily annihilated by pasteurisation. True, a number of indifferent

bacteria are untouched, and also some of the peptonising species. The

cooling itself contributes to the increased keeping power of the milk,

especially in transit to the consumer.

Pasteurised milks have the following three economical and commercial

advantages over sterilised milks, namely, they are more digestible, the

flavour is not altered, and the fat and lact-albumen are unchanged.

Professor Hunter Stewart, of Edinburgh, about two years ago,

compiled from a number of experiments the following instructive and

comprehensive table (page 212).

It will be admitted that this table exhibits much in favour of

pasteurisation; yet the crucial test must ever be the effect upon

pathogenic bacteria. Flügge has conducted a series of experiments upon

the destruction of bacteria in milk, and he states that a temperature

of 158° F. (70° C.) maintained for thirty minutes will kill the

specific organisms of tubercle, diphtheria, typhoid, and cholera.

MacFadyen and Hewlett have demonstrated,[62] by sudden alternate

heating and cooling, that 70° C. maintained for half a minute is

generally sufficient to kill suppurative organisms and such virulent

types of pathogenic bacteria as _Bacillus diphtheriæ_, _B. typhosus_,

and _B. tuberculosis_.

------------+-----------+--------------+----------------+---------+-----------+-----------

No. of |per cc. in | of | per cc. in | in | in | of

------------+-----------+--------------+----------------+---------+-----------+-----------

5 | 136,262 | 10' 60° C. | 1722 average | 0.423 | 0.418 |Unaffected

4 | 53,656 | 30' 60° C. | 1 sterile | 0.435 | 0.427 | "

12 | 78,562 | 10' 65° C. | 6 sterile | 0.395 | 0.362 | Not

12 | 132,833 | 30' 65° C. | 9 sterile | 0.395 | 0.333 | "

13 | 49,867 | 10' 70° C. | sterile | 0.422 | 0.269 | Slightly

9 | 38,320 | 30' 70° C. | " | 0.421 | 0.253 | "

2 | 77,062 | 10' 75° C. | " | 0.38 | 0.07 | Boiled

3 | 48,250 | 30' 75° C. | " | 0.38 | 0.05 | "

1 | 1,107,000 | 10' 80° C. | " | 0.375 | 0.00 | "

1 | 1,107,000 | 30' 80° C. | " | 0.375 | 0.00 | "

------------+-----------+--------------+----------------+---------+-----------+-----------

Respecting the numerical diminution of microbes brought about by

pasteurisation and sterilisation, respectively, we may take the

following two sets of experiments. Dr. N. L. Russell[63] tabulates the

immediate results of pasteurisation as follows:

----------------------------------------------+---------------------------

UNPASTEURISED. | PASTEURISED.

-----------+---------+------------+-----------+--------+---------+--------

milk. | 25,300 | 18,827,000 | 3,674,000 | 0 | 37,500 | 6,140

Cream, 25%.| 425,000 | 32,800,000 | 8,700,000 | 0 | 57,000 | 24,250

-----------+---------+------------+-----------+--------+---------+--------

As regards the later effect of the process, he states that in fifteen

samples of pasteurised milk examined from November to December nine

of them revealed no organisms, or so few that they might almost be

regarded as sterile; in those samples examined after January the lowest

number was 100 germs per cc., while the average was nearly 5,000. With

the pasteurised cream a similar condition was to be observed.

Dr. Hewlett[64] defines pasteurisation briefly as heating the milk to

68° C. for twenty or thirty minutes, and this treatment he quotes as

destroying 99.75 per cent. of the total number of organisms. Bitter's

table of results at 158° F. bears out the same:

BEFORE PASTEURISATION. AFTER PASTEURISATION.

No. of Bacteria in 10 Drops. No. of Bacteria in 10 Drops.

1. 102,600 2-3

2. 251,600 30-40

3. 25,000 3-5

4. 37,500 2-5

5. 94,000 2

BACTERIA IN MILK PRODUCTS

Cream is generally richer in bacteria than milk. _Set_ cream contains

more bacteria than _separated_ cream, but germs are abundant in both.

Yet whilst it is true that cream contains a large number of bacteria,

it must be pointed out that the butter fat in cream is a less suitable

food for organisms than is the case with milk. Hence the fermentative

changes set up in cream are of less degree than in milk, particularly

so if separated from the milk. Butter-milk and whey vary much in their

bacterial content. Butter necessarily follows the standard of the

cream. But as the butter fat is not well adapted for bacterial food,

the number of bacteria in butter is usually less than in cream.[65]

Moreover, they are soon reduced both in quality and quantity. Butter

examined after it is several months old is often found to be almost

free from germs; yet in the intervening period a variety of conditions

are set up directly or indirectly through bacterial action.

_Rancid_ butter is partly due to organisms. _Putrid_ butter is caused,

according to Jensen, by various putrefactive bacteria, one form of

which is named _Bacillus fœtidus lactis_. This organism is killed at

a comparatively low temperature, and is therefore completely removed

by pasteurisation. _Ill-flavoured_ butter may be due to germs or an

unsuitable diet of the cow and a retention of the bad quality of the

resulting milk. _Lardy_ and _oily_ butters have been investigated by

Storch and Jensen and traced to bacteria. Lastly, _bitter_ butter

occasionally occurs, and is due to fermentative changes in the milk.

Butter may also contain pathogenic bacteria, like tubercle. The _B.

coli_ can live for one month in butter.

Cheese suffers from very much the same kind of "diseases" as butter,

except that chromogenic conditions occur more frequently. The latter

are, under certain circumstances, more the result of chemical than

bacterial action. Most of the troubles in cheese originate in the milk.

_Method of Examination of Butter._ Several grams of the butter should

be placed in a large test-tube, which is then two-thirds filled with

sterilised water and placed in a water-bath at about 45° C. until the

butter is completely melted. A small quantity may then be added to

gelatine or agar and plated out on Petri dishes or in flat-bottomed

flasks in the usual way. After which the tube may be well shaken

and returned to the bath inverted. In the space of twenty or thirty

minutes the butter has separated from the water with which it has been

emulsified. It is then placed in the cold to set. The water may be

now either centrifugalised or placed in sedimentation flasks, and the

deposit examined for bacteria.

_The Uses of Bacteria in Dairy Produce._ In considering the relation

of bacteria to milk we found that many of the species present were

injurious rather than otherwise, and when we come to consider bacteria

in dairy products, like butter and cheese, we find that the dairyman

possesses in them very powerful _allies_. Within recent years almost

a new industry has arisen owing to the scientific application of

bacteriology to dairy work.

As a preliminary to butter-making the general custom in most countries

is to subject the cream to a process of "ripening." As we have seen,

cream in ordinary dairies and creameries invariably contains some

bacteria, a large number of which are in no sense injurious. Indeed,

it is to these bacteria that the ripening and flavouring processes are

due. They are perfectly consistent with the production of the best

quality of butter. The aroma of butter, as we know, controls in a large

measure its price in the market. This aroma is due to the decomposing

effect upon the constituents of the butter of the bacteria contained

in the cream. In the months of May and June the variety and number

of these types of bacteria are decidedly greater than in the winter

months, and this explains in part the better quality of the butter at

these seasons. As a result of these ripening bacteria the milk becomes

changed and soured, and slightly curdled. Thus it is rendered more

fit for butter-making, and acquires its pleasant taste and aroma. It

is then churned, after which bacterial action is reduced to a minimum

or is absent altogether. Sweet-cream butter lacks the flavour of

ripened or sour-cream butter. The process is really a fermentation,

the ripening bacteria acting on each and all of the constituents of

the milk, resulting in the production of various bye-products. This

fermentation is a decomposition, and just as we found when discussing

fermentation, so here also the action is beneficial only if it is

stopped at the right moment. If, for example, instead of being stopped

on the second day, it is allowed to continue for a week, the cream will

degenerate and become offensive, and the pleasant ripening aroma will

be changed to the contrary.

Bacteriologists have demonstrated that butters possessing different

flavours have been ripened by different species of bacteria.

Occasionally one comes across a dairy which seems to be impregnated

with bacteria that improve cream and flavour well. In other cases

the contrary happens, and a dairy becomes impregnated with a species

having deleterious effects upon its butter. This species may arise

from unclean utensils and dairying, from disease of the cow, or from

a change in the cow's diet. Thus it comes about that the butter-maker

is not always able to depend upon good ripening for his cream. At

other times he gets ripening to occur, but the flavour is an evil one,

and the results correspond. It may be bitter or tainted, and just as

certainly as these flavours develop in the cream, so is it certain

that the butter will suffer. Fortunately the bacterial content of the

cream is generally either favourable or indifferent in its action. Thus

it comes about that the custom is to allow the cream simply to ripen,

so to speak, of its own accord, in a vat exposed to the influence of

any bacteria which may happen to be around. This generally proves

satisfactory, but it has the great disadvantage of being indefinite and

uncertain. Occasionally it turns out wholly unsatisfactory, and results

in financial loss.

There are various means at our command for improving the ripening

process. Perfect cleanliness in the entire manipulation necessary

in milking and dairying, combined with freedom from disease in the

milch cows, will carry us a long way on the road towards a good

cream-ripening. Recently, however, a new method has been introduced,

largely through the work and influence of Professor Storch in Denmark,

which is based upon our new knowledge respecting bacterial action in

cream-ripening. We refer to the artificial processes of ripening set

up by the addition of _pure cultures of favourable germs_.[66] If a

culture of organisms possessing the faculty of producing in cream a

good flavour be added to the sweet cream, it is clear that advantage

will accrue. This simple plan of _starting_ any special or desired

flavour by introducing the specific micro-organism of that flavour

may be adopted in two or three different ways. If cream be inoculated

with a large, pure culture of some particular kind of bacteria, this

species will frequently grow so well and so rapidly that it will check

the growth of the other bacteria which were present in the cream at

the commencement and before the starter was added. That is, perhaps,

the simplest method of adding an artificial culture. But secondly, it

will be apparent to those who have followed us thus far, that if the

cream is previously _pasteurised_ at 70° C. these competing bacteria

will have been mostly or entirely destroyed, and the pure culture, or

starter, will have the field to itself. There is a third modification,

which is sometimes termed ripening by _natural starters_. A natural

starter is a certain small quantity of cream taken from a favourable

ripening--from a clean dairy or a good herd--and placed aside to

sour for two days until it is heavily impregnated with the specific

organism which was present in the whole favourable stock of which the

natural starter is but a part. It is then added to the new cream the

favourable ripening of which is desired. Of the species which produce

good flavours in butter the majority are found to be members of

the acid-producing class; but probably the flavour is not dependent

upon the acid. Moreover, the aroma of good ripening is also probably

independent of the acid production.

Of all the methods of ripening--natural ripening, the addition of

natural starters, the addition of pure cultures with or without

pasteurisation--there can be no doubt that pure culture after

pasteurisation is the most accurate and dependable. The use of natural

starters is a method in the right direction; yet it is, after all,

a _mixed_ culture, and therefore not uniform in action. In order to

obtain the best results with the addition of pure cultures, Professor

Russell has made the following recommendations:

1. The dry powder of the pure culture must be added to a small amount

of milk that has been first pasteurised, in order to develop an active

growth from the dried material.

2. The cream to be ripened must first be pasteurised, in order to

destroy the developing organisms already in it, and thus be prepared

for the addition of the pure culture.

3. The addition of the developing starter to the pasteurised cream and

the holding of the cream at such a temperature as will readily induce

the best development of flavour.

4. The propagation of the starter from day to day. A fresh lot of

pasteurised milk should be inoculated daily with some of the pure

culture of the previous day, not the ripening cream containing the

culture. In this way the purity of the starter is maintained for a

considerable length of time. Those starters are best which grow rapidly

at a comparatively low temperature (60-75° F.), which produce a good

flavour, and which increase the keeping qualities of the butter. Now,

whilst it is true that the practice of using pure cultures in this way

is becoming more general, very few species have been isolated which

fulfil all the desirable qualities above mentioned. In America starters

are preferred which yield a "high" flavour, whereas in Danish butter

a mild aroma is commoner. In England as yet very little has been done,

and that on an experimental scale rather than a commercial one.[67]

In 1891 it appears that only 4 per cent. of the butter exhibited at

the Danish butter exhibitions was made from pasteurised cream plus a

culture starter; but in 1895, 86 per cent. of the butter was so made.

Moreover, such butter obtained the prizes awarded for first-class

butter with preferable flavour. Different cultures will, of course,

yield different flavoured butter. If we desire, say, a Danish butter,

then some species like "Hansen's Danish Starter" would be added; if we

desire an American butter, we should use a species like that known as

"Conn's Bacillus, No. 41." But whilst these are two common types, they

are not the only suitable and effective starters. On certain farms in

England there are equally good cultures, which, placed under favourable

temperatures in new cream, would immediately commence active ripening.

Professor H. W. Conn, who, with Professor Russell, has done so much

in America for the advancement of dairy bacteriology, reports[68] a

year's experience with the bacillus to which reference has been made,

and which is termed No. 41. It was originally obtained from a specimen

of milk from Uruguay, South America, which was exhibited at the World's

Fair in Chicago, and proved the most successful flavouring and ripening

agent among a number of cultures that were tried. The conclusions

arrived at after a considerable period of testing and experimentation

appear to be on the whole satisfactory. A frequent method of testing

has been to divide a certain quantity of cream into two parts, one

part inoculated with the culture and the other part left uninoculated.

Both have then been ripened under similar conditions, and churned in

the same way; the differences have then been noted. It is interesting

to know that, as a result of the year's experience, creameries have

been able to command a price varying from half a cent to two cents

a pound _more_ for the "culture" butters than for the uninoculated

butters. The method advised in using this pure culture is to pasteurise

(by heating at 155° F.) six quarts of cream, and after cooling to

dissolve in this cream the pellet containing bacillus No. 41. The cream

is then set in a warm place (70° F.), and the bacillus is allowed to

grow for two days, and is then inoculated into twenty-five gallons of

ordinary cream. This is allowed to ripen as usual, and is then used

as an infecting culture, or "starter," in the large cream vats in the

proportion of one gallon of infecting culture to twenty-five gallons of

cream, and the whole is ripened at a temperature of about 68° F. for

one day. The cream ripened by this organism needs to be churned at a

little lower temperature (say 52°-54° F.) but to be ripened at a little

higher temperature than ordinary cream to produce the best results.

Cream ripened with No. 41 has its keeping power much increased, and

the body or grain of the butter is not affected. More than two hundred

creameries in America used this culture during 1895, and Professor Conn

reports that this has proved that its use for the production of flavour

in butter is feasible in ordinary creameries and in the hands of

ordinary butter-makers provided they will use proper methods and proper

discretion.

_Bacteria in Cheese-making._ The cases where it has been possible to

trace bacterial disease to the consumption of butter and cheese have

been rare. Notwithstanding this fact, it must not be supposed that

therefore cheese contains few or no bacteria. On the contrary, for

the making of cheese bacteria are not only favourable, but actually

essential, for in its manufacture the casein of the milk has to be

separated from the other products by the use of _rennet_, and is then

collected in large masses and pressed, forming the fresh cheese. In the

course of time this undergoes ripening, which develops the peculiar

flavours characteristic of cheese, and upon which its whole value

depends.

We have said that the casein is separated by the addition of rennet,

which has the power of coagulating the casein. But this precipitation

may also be accomplished by allowing acid to develop in the milk

until the casein is precipitated, as in some sour-milk or cottage

cheeses. The former method is of course the usual one in practice. It

has been suggested that the bacteria contained in the rennet exert

a considerable influence on the cheese, but this, although rennet

contains bacteria, is hardly established. It is not here, however, that

bacteria really play their rôle. After this physical separation, when

the cheese is pressed and set aside, is the period for the commencement

of the ripening process.

That bacteria perform the major part of this ripening process, and

are essential to it, is proved by the fact that when they are either

removed or opposed the curing changes immediately cease. If the milk be

first sterilised, or if antiseptics, like thymol, be added, the results

are negative. It is not yet known whether this peptonising process is

due to the influence of a single organism or not. The probability,

however, is that it is to be ascribed to the action of that group of

bacteria known as the _lactic-acid_ organisms. Nor is it yet known

whether the peptonisation of the casein and the production of the

flavour are the results of one or more species. Freudenreich believes

them to be due to two different forms.

However that may be, we meet with at least four common groups of

bacteria more or less constantly present in cheese-ripening, either

in the early or late stages. First, there are the _lactic-acid

bacteria_, by far the largest group, and the one common feature of

which is the production by fermentation of lactic acid; secondly,

there are the _casein-digesting bacteria_, present in relatively small

numbers; thirdly, the _gas-producing bacteria_, which give to cheese

its honeycombed appearance; lastly, an indifferent or _miscellaneous_

group of extraneous bacteria, which were in the milk at the outset of

cheese-making, or are intruders from the air or rennet. All these four

groups may bring about a variety of changes, beneficial and otherwise,

in the cheese-making.

In order that the relation of bacteria to cheese may be more fully

understood, we may draw attention to some experiments conducted by

Professor H. L. Russell as to the numbers of bacteria present during

different stages of the ripening, excluding those already referred to

as present in the rennet. It appears that there is always at first

a marked increase in the number of micro-organisms, which is soon

followed by a more gradual decline. While the casein-digesting and

gas-producing classes suffer a general and more or less rapid decline,

the lactic-acid bacteria develop to an enormous extent, from which fact

it would appear that cheese offers ideal conditions for the development

of the latter. In some most interesting records Professor Russell has

divided the ripening process into three divisions:

1. _Period of Initial Bacterial Decline in Cheese._ Where the green

cheeses were examined immediately after removing from the press, it was

usually found that a diminution in numbers of bacteria had taken place.

This period of decline lasts but a short time, not beyond the second

day. Lower temperature and expulsion of the whey would account for this

general decline in all species of bacteria.

2. _Period of Bacterial Increase._ Soon after the cheese is removed

from the press a most noteworthy change takes place in green cheese.

A very rapid increase of bacteria occurs, confined almost exclusively

to the lactic-acid group. This commences in green cheese about the

eighth day, and continues more or less for twenty days. In Cheddar

cheese it commences about the fifth day, reaches its maximum about the

twentieth day, declines rapidly to the thirtieth day, and gradually for

a hundred following days. During the first forty days of this period

the casein-digesting and gas-producing organisms are present, and at

first increasing, but relatively to only a very slight degree. With

this rapid increase in organisms the curd begins to lose its elastic

texture, and before the maximum number of bacteria is reached the

curing is far advanced. Freudenreich has shown that acid inhibits the

growth of the casein-digesting microbes and _vice versâ_.

3. _Period of Final Bacterial Decline._ The cause of this decline

can only be conjectured, but it is highly probable that it is due

to a general principle to which reference has frequently been made,

viz., that after a certain time the further growth of any species of

bacteria is prevented by its own products. We may observe that the

gas-producing bacteria in Cheddar cheese last much longer than the

peptonising organisms, for they are still present up to eighty days.

Professor Russell aptly compares the bacterial vegetation of cheese

with its analogue in a freshly seeded field. "At first multitudes of

weeds appear with the grass. These are the casein-digesting organisms,

while the grass is comparable to the more native lactic-acid flora. In

course of time, however, grass, which is the natural covering of soil,

'drives out' the weeds, and in cheese a similar condition occurs." In

milk the lactic-acid bacteria and peptonising organisms grow together;

in ripening cheese the former eliminate the latter.

We have seen that the conclusion generally held respecting these

lactic-acid bacteria is that they are the main agents in curing the

cheese. Upon this basis a system of pure starters has been adopted,

the characteristics of which must be as follows: (_a_) The organism

shall be a pure lactic-acid-producing germ, incapable of producing

gaseous products; (_b_) it should be free from any undesirable aroma;

(_c_) it should be especially adapted for vigorous development in

milk. The starter may be propagated in pasteurised or sterilised milk

from a pure culture from the laboratory. The advantages accruing from

the uses of this lactic-acid culture, as compared with cheese made

without a culture, are that with sweet milk it saves time in the

process of manufacture; that with tainted milk, in which acid develops

imperfectly, it is an aid to the development of a proper amount of

acid for a typical Cheddar cheese; and that the flavour and quality of

such cheese is preferable to cheese which has not been thus produced.

Professor Russell is of opinion that the lactic-acid organisms are to

be credited with greater ripening powers than the casein-digesting

organisms, but it must not be forgotten that these two great families

of bacteria are still more or less on trial, and it is not yet possible

finally to dispose of either of them. Mr. F. J. Lloyd holds that though

"the greater the number of lactic-acid bacilli in the milk the greater

the chance of a good curd," still "this organism alone will not produce

that nutty flavour which is so sought after as being the essential

characteristic of an excellent Cheddar cheese."[69]

There are several difficulties to be encountered by dairymen starting a

ripening by the addition of a pure culture. To begin with, there is the

initial difficulty of not being able to pasteurise milk intended for

cheese, as rennet will not coagulate pasteurised milk (Lloyd). Hence

it is impossible to avoid some contamination of the milk previous to

the addition of the culture. The continual uncontaminated supply of

pure culture is by no means an easy matter. The maintenance of a low

temperature to prevent the rapid multiplication of extraneous bacteria

will, in some localities, be a serious difficulty. These difficulties

have, however, not proved insurmountable, and by various workers in

various localities and countries culture-ripening is being carried on.

_Abnormal Ripening._ Unfortunately, from one cause or another, faulty

fermentations and changes are not infrequently set up. Many of these

may be prevented, being due to lack of cleanliness in the process or

in the milking; others are due to the gas-producing bacteria being

present in abnormally large numbers. When this occurs we obtain what

is known as "gassy" cheese, on account of its substance being split

up by innumerable cavities and holes containing carbonic acid gas,

or sometimes ammonia or free nitrogen. Some twenty-five species of

micro-organisms have been shown by Adamety to cause this abnormal

swelling. In severe cases of this gaseous fermentation the product is

rendered worthless, and even when less marked the flavour and value are

much impaired. Winter cheese contains more of this species of bacteria

than summer. Acid and salt are both used to inhibit the action of these

gas-producing bacteria and yeasts, and with excellent results.

We may remark that the character of the gas holes in cheese is not of

import in the differentiation of species. If a few gas bacteria are

present, the holes will be large and less frequent; if many, the holes

will be small, but numerous. (Swiss cheese having this characteristic

is known as Nissler cheese.)

Many of these gas germs belong to the lactic-acid group, and are

susceptible to heat. A temperature of 140° F. maintained for fifteen

minutes is fatal to most of them, largely because they do not form

spores. The sources of the extensive list of bacteria found in cheese

are of course varied, more varied indeed than is the case with milk.

For there are, in addition to the organisms contained in the milk

brought to the cheese factory, the following prolific sources, viz.,

the vats and additional apparatus; the rennet (which itself contains a

great number); the water that is used in the manufacture.

In addition to the abnormalities due to gas, there are also other

faulty types. The following chromogenic conditions occur: _red cheese_,

due to a micrococcus; _blue cheese_, produced, according to Vries,

by a bacillus; and _black cheese_, caused by a copious growth of

low fungi. _Bitter cheese_ is the result of the _Micrococcus casei

amari_ of Freudenreich, a closely allied form of Conn's micrococcus of

bitter milk. Sometimes cheese undergoes a putrefactive decomposition,

and becomes more or less putrid. These latter conditions, like the

gassy cheeses, are due to the intrusion of bacteria from without,

or from udder disease of the cow. Healthy cows, clean milking, and

the introduction of pure cultures are the methods to be adopted for

avoiding "diseases" of cheese and obtaining a well-flavoured article

which will keep.

Finally, we may quote five conclusions from the prolonged researches

of Mr. Lloyd[70] which cannot but prove helpful to the Cheddar cheese

industry in England:

1. To make Cheddar cheese of excellent quality, the _Bacillus acidi

lactici_ alone is necessary; other germs will tend to make the work

more rather than less difficult. Hence scrupulous cleanliness should be

a primary consideration of the cheese-maker.

2. No matter what system of manufacture be adopted, two things are

necessary. One is that the whey be separated from the curd, so that

when the curd is ground it shall contain not less than 40 per cent. of

water, and not more than 43 per cent.; the other point is that the

whey left in the curd shall contain, developed in it before the curd is

put in the press, at least 1 per cent. of lactic acid if the cheese is

required for sale within four months, and not less than 8 per cent. of

lactic acid if the cheese is to be kept ripening for a longer period.

3. The quality of the cheeses will vary with the quality of the milk

from which they have been made, and proportionately to the amount of

fat present in that milk.

4. "Spongy curd" is produced by at least five organisms, and one of

these is responsible for a disagreeable taint found in curd. They

occur in water. Hence the desirability of securing clean water for all

manipulative purposes, and also for the drinking purposes of the milch

cow.

5. The fact that certain bacteria are found in certain localities and

dairies is due more to local conditions than to climatic causes.

It is needless to remark that these conclusions once more emphasise the

fact that strict and continual cleanliness is the one desideratum for

bacteriologically good dairying. That being secured in the cow at the

milking, in the transit, and at the dairy, it is a comparatively simple

step, by means of pasteurisation and the use of good pure cultures of

flavouring bacteria, to the successful application of bacteriology to

dairy produce.

_Methods of Examination of Milk_:

1. _Preparation of Microscopic Slides._ This course might at once occur

to the mind as the first to adopt in searching for bacteria in milk.

Devices have accordingly been proposed for saponification previous to

staining. Some recommend the addition of a few drops of a solution

of sodium carbonate; others use methylene blue and chloroform. But,

whatever plan of staining is adopted, this method of examination in its

simplest form is in no degree a criterion of the bacterial content of a

large quantity of milk.

Hence it has come to be recognised that one of two manipulations must

precede such microscopic examination. These simple processes are known

by the terms of _sedimentation_ and _centrifugalisation_. Sedimentation

means merely placing the milk in conical glasses in a cool place for

twenty-four hours. The introduction of improved forms of the centrifuge

has brought the second method of securing a sediment into preference.

Five cubic centimetres of the milk are introduced into the graduated

bottle, which is then placed in the centrifuge, and whirled for one

or two minutes. Thus a deposit of particulate matter is ensured.

Cover-glass specimens of the sediment or deposit are then prepared and

stained in the ordinary way.

[Illustration: A CENTRIFUGE

Used in the Examination of Milk]

In testing for tubercle something more is generally necessary. To the

50 cc. of the milk set aside for sedimentation 10 cc. of liquefied,

colourless carbolic acid are added. The mixture is shaken and poured

into the conical glass. After standing for twenty-four hours a little

of the sediment is taken by means of a pipette and examined by ordinary

methods, though after "fixing" the films with heat they are some times

passed through equal parts of alcohol and ether. The stain is of course

that usually adopted in tubercle, namely, the Ziehl-Neelsen. Scheurlen

suggested a method for demonstrating the tubercle bacillus in milk by

steeping the cover glasses first in alcohol and then ether, after which

they were stained with Ziehl-Neelsen.

2. _Plate Culture._ The milk is to be diluted a thousand or more times

with sterile water, and ordinary plate cultures made in Petri dishes or

flat-bottomed conical flasks. The colonies should be counted as late as

possible; but even then the isolation of pathogenic germs is uncertain.

As regards further procedure, the ordinary methods of sub-culturing

adopted in water examination must be strictly followed, and the special

tests for _Bacillus typhosus_ and _B. coli_ applied. As we have already

seen, the quantitative estimation of organisms in milk is not of the

same value as in water.

3. _Inoculation._ To test the capacity of the milk for causing disease,

before or after centrifugalisation, preferably the latter, a certain

quantity of the sediment may be inoculated into guinea-pigs. In

suspected tubercle 2 cc. may be taken; in diphtheria a little less

will suffice. The inoculation should be either intraperitoneal or

subcutaneous. Many authorities hold that this test is the only safe one

to protect the public from milk containing germs of disease.

BACTERIA IN OTHER FOODS

_Shell-fish_ have recently claimed the attention of bacteriologists,

owing to the outbreak of typhoid and other epidemics apparently

traceable to _oysters_.

It is four or five years since Professor Conn startled the medical

world by tracing an epidemic of typhoid fever to the consumption

of some uncooked oysters.[71] Almost at the same time Sir William

Broadbent published in the _British Medical Journal_ a series of

cases occurring in his practice which illustrated the same channel of

infection. Since then a number of similar items of evidence to the same

effect have cropped up. Hence there is little wonder that a number of

investigators concentrated their attention upon this matter. Professors

Herdman and Boyce, of Liverpool, Dr. Cartwright Wood, Dr. Klein, and

Dr. Timbrell Bulstrode are some of the chief contributors to the

elucidation of this problem.

The mode of infection of oysters by pathogenic bacteria is briefly

as follows: The sewage of certain coast towns is passed untreated

out to sea. At or near the outfall, oyster-beds are laid down for

the purpose of fattening oysters. Thus they become contaminated with

saprophytic and pathogenic germs contained in the sewage. It will be

at once apparent that several preliminary questions require attention

before any deductions can be drawn as to whether or not oysters convey

virulent disease to consumers. To the solution of these Dr. Cartwright

Wood was one of the first to address himself.

The precise conditions which render one locality more favourable than

another in respect to oyster culture are not fully known. But it has

been observed that they do not flourish in water containing less than

three per cent. of salt. Hence they are absent from the Baltic Sea,

which, owing to the fresh water flowing into it in rivers, contains a

smaller percentage of salt than three. Oysters appear in addition, to

favour a locality where they find their chosen food of small animalculæ

and particles of organic matter. Such a favourable locality is the

mouth of a river, where tides and currents also assist in bringing

food to the oyster. Unfortunately, however, in a crowded country like

England such localities round her coasts are frequently contaminated

by sewage from outfalls. Thus the oysters and the sewage come into

intimate relation with each other.

Professor Giaxa carried out some experiments in 1889 at Naples which

appeared to show that the bacilli of cholera and typhoid rapidly

disappeared in ordinary sea-water. Other observers at about the same

time, notably Foster and Freitag, arrived at an opposite conclusion.

In 1894 Professor Percy Frankland, in a report to the Royal Society,

declared "that common salt, whilst enormously stimulating the

multiplication of many forms of water bacteria, exerts a directly

and highly prejudicial effect on the typhoid bacilli, causing their

rapid disappearance from the water, whether water bacteria are

present or not." It was at this time, when the matter was admittedly

in an unsatisfactory stage, that Dr. Cartwright Wood made his

experiments.[72] We have not space here to enter into this work. But

his conclusions seem to have been amply established, and were to the

effect that typhoid and cholera bacilli could, as a matter of fact,

exist over very lengthened periods in ordinary sea-water. The next step

was to demonstrate the length of time the bacilli of cholera remained

alive in the pallial cavity and body of the oyster. Dr. Wood found they

did so for eighteen days after infection, though in greatly diminished

numbers. This diminution was due to one or all of three reasons: (_a_)

the effect of the sea-water already referred to as finally prejudicial

to bacilli of typhoid; (_b_) the vital action of the body-cells of the

oyster; (_c_) the washing away of bacilli by the water circulating

through the pallial cavity.

It will have been noticed that up to the present we have learned that

typhoid bacilli can and do live in sea-water, and also inside oysters

up to eighteen days, but in ever-diminishing quantities. The question

now arises: What is the influence of the oyster upon the contained

bacilli? Under certain conditions of temperature organisms may multiply

with great rapidity inside the shell of the oyster. Yet, on the other

hand, the amœboid cells of the oyster, the acid secretion of its

digestive glands, or the water circulating through its pallial cavity,

may act inimically on the germs. Proof can be produced in favour of

the third and last-named mode by which an oyster can cleanse itself of

germs. So far, then, we have met with no facts which make it impossible

for oysters to contain for a lengthened period the specific bacteria

of disease. Let us now turn to their opportunity for acquiring such

disease germs. It is afforded them during the process of what is

termed "fattening." By this process the body of the oyster acquires a

plumpness and weight which enhances its commercial value. This desired

condition is obtained by growing the oyster in "brackish" water, for

thus it becomes filled out and mechanically distended with water.

But if this water contains germs of disease, what better opportunity

could such germs have for multiplication than within the body-cavity

of an oyster? "The contamination of sea-water, therefore, in the

neighbourhood of oyster-beds may undoubtedly lead to the molluscs

becoming infected with pathogenic organisms" (Wood). Yet we have

seen that, apart altogether from the individual susceptibilities or

otherwise of the consumer, there are in the series of events necessary

to infection many occasions when circumstances would practically free

the oysters from infection.

The sources of pollution of oysters are not the fattening beds alone.

The native beds also may afford opportunity for contamination.

Thirdly, in packing and transit, and fourthly, in storage in shops and

warehouses, there is frequently abundant facility for putrefactive

bacteria to gain entrance to the shells of oysters.

Dr. Klein's researches[73] into this question have been wholly

confirmatory of the facts elicited by Dr. Cartwright Wood. Despite

the tendency of the bacilli of cholera and typhoid to die out quickly

in crude sewage, the sewage is sufficiently altered or diluted at the

outfall for these organisms to exist there in a virulent state. We may

give Dr. Klein's conclusions:

1. That the cholera and typhoid bacilli are difficult of demonstration

in sewage known to have received them.

2. Both organisms may persist in sea-water tanks for two or three

weeks, the typhoid bacillus retaining its characteristics unimpaired,

the cholera bacillus tending to lose them.

3. Oysters from sources free of sewage contained no bacteria of sewage.

4. Oysters from sources exposed to risk of sewage contamination did

contain colon bacilli and other sewage bacteria.

5. In one case Eberth's typhoid bacillus was found in the mingled body

and liquor of the oyster.

Nor do typhoid bacilli lose activity or virulence by passing through an

oyster.

These researches once and for all established the fact that oysters

ordinarily grown on oyster-beds contaminated with bacteria may, and

do on occasion, contain the virulent specific bacillus of typhoid,

which can live both in sea-water and within the shell of the oyster.

This being so, it will probably appear to the reader that the risk of

infection of typhoid by oysters is very serious indeed. Yet in actual

practice many conditions have to be fulfilled. For, in addition to the

fact that the oysters must be consumed, as is usual, uncooked, the

following conditions must also be present.

(_a_) Each infective oyster must contain infected sewage, which

presupposes that typhoid excreta from patients suffering from the

disease have passed into that particular sewage untreated and not

disinfected.

(_b_) The infective oyster must be fed upon infected sewage, and still

contain the virus in its substance.

(_c_) It has to be eaten by a susceptible person.

(_d_) There must have been no period of natural cleansing after

"fattening."

Even to this formidable list of conditions we must add the further

remark that, owing to the vitality of the body-cells of the oyster, or

to the lessened vitality of the bacilli of cholera and typhoid, it is

generally the case that the tendency of these organisms is rather to

decrease and die out than live and multiply.

We shall probably maintain a satisfactory balance of truth if we place

alongside these facts the summary of the Local Government Board Report.

"There can be no doubt," wrote Sir Richard Thorne, "that

oysters which have been brought into sustained relation with

the typhoid bacillus are liable to exhibit that microbe

within the shell contents and to retain it for a while under

circumstances not only permitting its rapid multiplication

when transferred again to appropriate media, but conserving at

the same time its ability to manifest its hurtful properties."

From what has been said the preventive treatment is obvious.

All oyster-layings and shell-fish beds round the coast should

be superintended and inspected by the sanitary authority of the

Government. The importation of foreign oysters, grown on uncontrolled

beds, should, if possible, be _restricted_ or _supervised_. Further,

as a protective measure of the first importance, oysters should be

cleansed, after fattening on a contaminated bed, by being deposited

for several weeks at some point along the coast which is washed by

pure sea-water. Retention in dirty water-tanks, in uncleanly shops and

warehouses, is also to be greatly deprecated.

In order to examine oysters bacteriologically, it is necessary to pay

particular attention to the water in the pallial cavity, the contents

of the alimentary canal, and the washings of the shell itself. Ordinary

media may be used for obtaining a growth of the contained organisms.

_Other shell-fish_ than oysters do, from time to time, cause epidemics

or individual cases of gastro-intestinal irritation, and probably

contain various germs. These they acquire in all probability from their

food, which by their own choice is frequently of a doubtful character.

_Meat._ Parasites are occasionally found in meat, but bacteria are

comparatively rare. Not that they do not occur in the bodies of

animals used for human consumption, for in the glands, mesenteries,

and other organs they are common. But in those portions of the carcass

which are used by man, namely the muscles, bacteria are rare. The

reasons alleged for this are the acid reaction (sarcolactic acid) and

the more or less constant movement during life. A bacterial disease

which, perhaps more than any other, might be expected to be conveyed by

meat is tubercle. Yet the recent Royal Commission on Tuberculosis has

again emphasised the absence of bacilli in the meat substance:

"In tissues which go to form the butcher's joint, the material

of tubercle is not often found even where the organs (lungs,

liver, spleen, membranes, etc.) exhibit very advanced or

generalised tuberculosis; indeed, in muscle and muscle juice

it is very seldom that tubercle bacilli are to be met with;

perhaps they are somewhat more often to be discovered in bone,

or in some small lymphatic gland embedded in intermuscular

fat."[74]

The only way in which such meat substance becomes infected with

tubercle appears to be through carelessness in the butcher, who

perchance smears the meat substance with a knife that has been used

in cutting the organs, and so has become contaminated with infected

material. Very instructive also are the results at which Dr. Sims

Woodhead arrived in compiling evidence for the same Commission on the

effect of cooking upon tuberculous meat:

"Ordinary cooking, such as boiling and more especially

roasting, though quite sufficient to sterilise the surface,

and even the substance for a short distance from the surface

of a joint, cannot be relied upon to sterilise tubercular

material included in the centre of rolls of meat, especially

when these are more than three pounds or four pounds weight.

The least reliable method of cooking for this purpose is

roasting before a fire; next comes roasting in an oven, and

then boiling."[75]

From this statement it will be understood that rolled meat may be a

source of infection to a greater degree than the ordinary joint.

Notwithstanding this negative evidence, more than twenty species

of bacteria have been isolated from canned meats and hams, and a

considerable number of poisoning cases have occurred from meat

contaminated with bacteria or their products. The general symptoms

of such meat poisoning are vomiting, diarrhœa, fever, and more or

less prostration. Ballard and Klein isolated a specific microbe from

samples of bacon which appear to have caused an epidemic of infectious

pneumonia at Middlesborough. In 1880 occurred the well-known "Welbeck

disease" epidemic. A public luncheon was followed by severe and even

fatal illness. Seventy-two persons were affected, and four died. A

specific bacillus was isolated by Klein. In 1881 much the same thing

happened at Nottingham, in which fifteen persons were attacked, and one

died. The same bacillus was isolated from the pernicious pork. Again in

1889 an outbreak of diarrhœa at Carlisle was traced to bacterially

diseased pork. But taking these and similar cases at their worst, there

can be no doubt that under no circumstances is meat as infective as

milk.

_Ice-cream._ In 1894 Dr. Klein had occasion to bacteriologically

examine ice-creams sold in the streets of London. In all six samples

were analysed, and in each sample the conclusions resulting were of

a nature sufficiently serious to support the view that the bacterial

flora was not inferior to ordinary sewage. The water in which the

ice-cream glasses were washed was also examined, and found to contain

large numbers of bacteria.

Since that date many investigations have been made into ice-creams. It

appears that they are often made under extremely foul circumstances,

and with anything but sterilised appliances. Little wonder, then,

that the numbers of bacteria present run into millions. In nearly

all recorded cases the quality of the germs as well as the quantity

has been of a nature to cause some concern. _Bacillus coli communis_,

which, though not now considered absolutely indicative of alimentary

pollution, is looked upon as a highly unsatisfactory inhabitant of

water, has been found in considerable abundance. The _Proteus_ family,

which also possesses a putrefactive function, is common in ice-creams.

The _common water bacteria_ are nearly always present.

_Bacillus typhosus_ itself, it is said, has been isolated from some

ice-cream which was held responsible for an outbreak of enteric fever.

The material had become infected during process of manufacture in the

house of a person suffering from unnotified typhoid fever.

Now, whilst reports of the above nature appear very alarming, the

fact is that hundreds of weakly children devour ice-cream with

apparent impunity, and when evil follows it is not infrequently due

to other than bacterial conditions. The cold mass itself may inhibit

the resistance of the gastric tissues. _Tyrotoxicon_, the alkaloid

separated from cheese and cream by Vaughan, may be responsible for

some alimentary irritation. On the whole, the practical effect upon

the community is not in proportion to the bacterial content of the

ice-cream. Yet, nevertheless, we ought to be much more watchful than in

the past to preserve ice-cream from pollution with harmful bacteria.

The two chief constituents which contribute their quota of germ life

to ice-cream are _ice_ and _cream_. In addition, the uncleanly methods

of manufacture render the material likely to contain the six or

seven millions of micro-organisms per cc. which have been on several

occasions estimated. To cleanly methods of dairying we have already

fully referred; to the bacterial content of milk and cream we have

also paid some attention; but we have not had an opportunity of saying

anything of germs in ice.

_Ice_ contains bacteria in varying quantities from 20 per cc. to 10,000

or more. Nor is variation in number affected alone by the condition of

the water, for samples collected from one and the same place differ

widely. The quality follows in large measure the standard of the water.

Water bacteria, _Bacillus coli_, putrefactive bacteria, and even

pathogenic have been found in ice. Many of the latter can live without

much difficulty and are most numerous in ice containing air-bubbles.

Dr. Prudden, of New York, performed a series of experiments in 1887

to show the relative behaviour of bacteria in ice. Taking half a

dozen species, he inoculated sterilised water and reduced it to a

very low temperature for a hundred and three days, with the following

results:--_Bacillus prodigiosus_ diminished from 6,300 per cc. to

3,000 within the first four days, to 22 in thirty-seven days, and

vanished altogether in fifty-one days; a liquefying water bacillus,

numbering 800,000 per cc. at the commencement, had disappeared in four

days; _Staphylococcus pyogenes aureus_ and _B. fluorescens_ showed

large numbers present at the end of sixty-six and seventy-seven days

respectively; _B. typhosus_, which was present 1,000,000 per cc. after

eleven days, fell to 72,000 after 77 days, and 7,000 at the end of 103

days. Anthrax bacilli are susceptible to freezing, but their spores are

practically unaffected (Frankland).

From these facts it will be seen that bacteria live, but do not

multiply, in ice.

In making a bacterial investigation into the flora of ice-cream, it is

necessary to remember that considerable dilution with sterilised water

is required. The usual methods of examining water and milk are adopted.

_Bread_ forms an excellent medium for moulds, but unless specially

exposed the bacteria in it are few. Waldo and Walsh have, however,

demonstrated that baking does not sterilise the interior of bread.

These observers cultivated numerous bacteria from the centre of newly

baked London loaves.[76] The writer has recently made a series of

examinations of the air of several underground bakehouses in Central

London; but, though the air was highly impregnated with flour-dust, few

bacteria were present.

Other foods and beverages may be, and are, from time to time

contaminated in some small degree with bacteria or their spores.

Such contaminations are generally due to uncleanly manufacture or

unprotected storage. The principles of examination or of the prevention

of pollution are similar to those already described.

Bacteria Especially as they are related to the economy of nature, to industrial processes, and to the public health · 第 6 / 9 章