Reading references and experiments illustrating the principles upon
which fireless cookery is based.
_1. A test of the insulating powers of different materials._
_Apparatus:_
One or more boxes and fittings, described on pages 9 to 11.
One or more pails of the same size, shape and material, preferably of
from two to four quarts’ capacity, with close fitting covers.
Cooking thermometer
Wool
Mineral wool
Cotton batting or waste
Excelsior
Hay
Sawdust
Newspapers
Ground cork
Southern moss
Pencil
Notebook
Pack the box successively with as many of the different packing
materials given above as are to be tested, following the directions
given on page 15; or have several exactly similar boxes packed at the
same time. For all tests fill the cooker-pail with water, bring it to
the boiling point, let it boil one minute, to permit all parts of the
utensil and its contents to reach the same temperature; then put it at
once into the cooker-box and leave it for an equal length of time, not
less than one hour. Record the temperature of the contents of the pail
at the expiration of this period. In order to get a full record and a
fair comparison it would be well to repeat this experiment with varying
periods of time, taking the temperature, for instance, at the end of
one, three, six, nine, and twelve hours. In taking temperatures do not
wholly remove the cushion and cover of the pail, but slip them to one
side, enough to insert the thermometer. This is, of course, a crude
method of taking temperatures, but answers for purposes of comparison.
If it is desired to make more accurate records this can be done by
boring the cover of the box, the cushion and the pail cover, and
inserting a thermometer through corks which are used to close the bored
holes. The temperature can then be read while the apparatus is closed.
However, the first method, if carefully done, will give probably within
one degree of the correct temperature. Record the results in tabular
form.
Which material do you find gives the best insulation?
Winkelmann,[4] Duff,[5] and other writers on physics give tables of the
conductivity of felt, asbestos paper, paper, cotton, flannel, and other
materials; but as different figures are shown, from different sources,
for the same material, it is likely that the insulating power of any
material used for packing a cooker will depend as much or more upon the
way it is packed as upon the material used.
[4] “Handbuch der Physik.”
[5] “Textbook of Physics.”
_Experiment: Conductivity of different materials._
Take a piece of copper wire about six inches long in one hand, and a
piece of steel wire of the same length and thickness in the other. Put
one end of each piece in a flame, holding the wire by the extreme end.
Notice which first becomes too hot to hold at the end farthest from the
flame. This illustrates the different conductivity of the two materials,
steel and copper. There is not a great deal of difference in the
conductivity of different materials, but metals are relatively good
conductors, and air is a very poor conductor.
2. _Heat is carried from the pail partly by convection_, except where
solid insulating material, such as wood or indurated fibre, is used;
and that manner of packing which best entangles the air and prevents
air currents will, therefore, most increase the effectiveness of the
insulation.
_Experiment: Convection._
Into a glass flask of cold water drop a few crystals of potassium
permanganate, being careful not to agitate the flask. Apply a flame to
the bottom of the flask. As the water becomes heated its density is
reduced and it rises, forming convection currents which are coloured by
the permanganate and may be distinctly seen.
Convection currents may be formed in any liquid or gas; for instance,
air. By means of them heat will be carried from one part of the liquid
or gas to another. Thus air heated by contact with a kettle of food
will, if allowed to flow freely, carry the heat away from the food.
3. _Heat is also lost by radiation._ This takes place less rapidly from
a bright, highly polished surface, and for this reason “Thermos” and
similar bottles are encased in polished nickle. A cooker-pail with
polished outside surface retains heat better than one with a dull
finish. In those cookers made with a metal outside retainer, the
surface should not be painted or roughened or dulled by any means.
_Experiment: Radiation._
Take two empty tin cans of the same size and shape. Wash off the paper
labels. Keep one of them bright and shining, but move the other through
a candle flame until the entire outer surface is smoked. Into each pour
exactly the same quantity of water at the same temperature. Note
carefully the temperature and the time. At the end of any given period,
say one hour, again take the temperature of each. Which has lost the
most heat, that in the bright can or that in the dull can?
_4. The effect of different degrees or thicknesses of insulation._
_Materials:_
The same as those used in the experiment, section 1, with the addition
of boxes of various sizes, some smaller, some larger, than the one used
in the first experiment.
Pack the boxes with one or more of the various insulating materials used
in the first experiment, so as to allow varying thicknesses of
insulation around the cooker-pail. This should be the same or an exactly
similar pail in each case. Fill the pail for all tests with an equal
quantity of water, boil it for one minute, and leave it in the boxes for
an equal length of time. Record the temperature maintained in each test.
Keep the record in tabular form.
What thickness of insulation do you find gives the best result with the
materials used in your experiment? Is it necessary to assume that the
same thickness will be required with all insulating materials?
_5. The effect of the density of foods upon the temperature maintained._
_Materials:_
One cooker or hay-box
Starch
Water
Salt
Cooking thermometer
Scales
Litre or quart measure
Notebook and pencil
Bring one or more litres or quarts of water to a boil, boil it for one
minute, and put it into the cooker for one hour or more. Repeat the
test, using, successively, five grams of salt to each litre, or one
teaspoonful to each quart, and 5, 10, and 20 per cent. mixtures of
starch with water. Record the temperatures in tabular form, and compare
the results. What would you gather to be the effect of density upon the
temperatures maintained?
_6. The effect on temperature of filling the cooker-pails one-fourth,
one-half, three-quarters, and entirely full._
_Materials:_
Cooker or hay-box pail of eight quarts’ capacity
Pail of two quarts’ capacity
“Space adjuster”
Water
Thermometer
Notebook and pencil
Fill the large cooker-pail one-fourth full of water. Bring it to a boil
and put it into the cooker for a definite period of time, not less than
one hour. Record the resulting temperature. If desired to make the test
more comprehensive, leave the water in the cooker for six, nine, or
twelve hours, being careful to allow the cooker to become cold between
each test. Perform the same experiment with the same pail one-half full,
again when it is three-fourths full, and again when entirely full.
Record the results in tabular form and compare them. Repeat these tests
with a pail of two quarts’ capacity. What is the influence on
temperature of having pails partially, or completely, filled?
The explanation is that evaporation takes place in partially filled
pails.
_7. Chemistry of the action of food materials (salt, soda, acids, water,
etc.) upon cooking utensils made of tin, or aluminum, when used in a
cooker or hay-box._
The amount of tin dissolved by foods is indicated by the corrosion of
the utensil, which can often be seen by the naked eye to be altered in
appearance. The exact quantity of tin salts or other tin compounds which
may be formed can only be determined by careful chemical analysis. It
has been found that many canned goods supposed to be inert, such as
squash and pumpkin, have a marked effect upon tin. Crude tests with a
number of different foods can be made with tin, iron, aluminum, and
copper utensils, as in many cases there is evidence to the eye of action
upon the metals. It must be borne in mind, however, that such tests are
crude and not decisive of the fact of there being no action in case no
action is plainly visible. Only chemical analysis can prove this.
The action of foods upon tin cans bears a close relation to their action
upon the utensils when used in fireless cookery, since there is time
with the long cooking involved for similar reactions to take place in
the cooker.[6]
[6] See “Food Inspection and Analysis,” by Leach, published by John
Wiley Sons, New York, 1904, page 694.
Tin utensils rust badly after short use in a cooker, and thus affect the
flavour of food cooked in them. This is due to the action of acids and
water on the iron which forms the basis of sheet tin. When the thin
plating of tin is worn off, the iron is left exposed to the action of
water, etc.
Soda dissolves aluminum, and leaves a black surface on aluminum
utensils. This black substance is iron, which is present with the
aluminum in the utensils. To remove the black appearance, clean the
utensil with acid. Do not try to remove it by scouring, as this will not
do the work well, and is laborious and injurious to the pail.
=Detection of poisonous metals that may be dissolved from the cooker
utensils.=
_Experiment A. Tin._ In a tin cooker-pail boil such foods as apple
sauce, tomatoes, squash, or others that act on tin, and put them into a
cooker for twelve hours. Transfer them to an agate ware or porcelain
utensil, evaporate them over steam until they may be burned in a
porcelain dish until charred and brittle. Pulverize this charred mass,
and extract it with hydrochloric acid. Filter and wash it. Saturate the
filtrate with hydrogen sulphide gas; add a saturated solution of
potassium acetate to neutralize the hydrochloric acid present and assist
in the coagulation of sulphide of tin. Warm it slightly, filter and wash
out the stannic sulphide, dry it and weight it as stannic oxide, from
which the tin dissolved may be calculated.
_Experiment B. Aluminum._ To simplify the experiment a weak solution of
malic acid may be used (seven grams per litre being about the average
amount found in apples). Bring this to a boil in an aluminum cooker-pail
and put it into a cooker for twelve hours. Transfer it to a porcelain
vessel and add ammonia to precipitate the alumina. Filter and wash this,
dry and weigh the aluminum oxide. It is probable that a smaller quantity
of aluminum would be dissolved by foods of a mushy consistency than
would be found in this clear solution.
_8. The efficiency of home-made refrigerating boxes compared with other
means of keeping foods cold._
_Materials:_
One box fitted as for fireless cooking, with two or three covered
crocks of at least one-half gallon capacity, packed as directed on
page 37, with either sawdust, hay, straw, excelsior or paper. Sawdust
is specially recommended.
Thermometer
Ice
Notebook and pencil
Fill the central crock with a weighed quantity of ice. Fill one or both
of the other crocks with water at room temperature. Cover the crocks
and close the box. Record the temperature of the water at the end of
six, twelve, twenty-four, and forty-eight hours.
Make repeated observations of the temperatures found in ordinary
household refrigerators, cellars, cold storage rooms, and any other
places used for keeping foods cold. Compare these with the temperatures
obtained with a home-made refrigerating box. Is there any economy in
using these boxes?
=Bacteriology of Insulating Boxes=
_9. Temperatures which kill disease and putrefactive germs, or check
their growth._
It is taken for granted that the student of this subject will be more or
less familiar with the nature of bacteria and the elements of
bacteriology. It will be recalled that bacteria are a vegetable form of
life; that, like all plants, they have, under certain conditions, the
power of growth which is shown, largely, by their reproduction; and that
under other conditions they are killed. When their growth is merely
checked, they are in a dormant state, or perhaps form spores, in either
of which cases they are ready to develop as soon as their environment
permits. Temperature has much to do with the state of bacteria. If the
temperature and other conditions are such that they are in an active or
growing state, they will multiply with enormous rapidity. When in food
stuffs they effect certain changes by reason of the products which they
form as a result of their life processes, or of the alteration in the
food materials, owing to their abstraction of some chemical elements or
compounds used for their nutrition. When bacteria form unpleasant
smelling or tasting substances we speak of them as “putrefactive
bacteria.” Those which, if introduced into the bodies of humans or
animals, will cause diseases, are called “disease bacteria.” Foods are
liable to contain both kinds; and, therefore, it is, obviously, wise to
do all that is possible to kill them or prevent their growth.
Most forms occurring in foods grow best at from 80 degrees to 98 degrees
Fahrenheit. Few bacteria grow at above 100 degrees, and, if kept at 125
degrees, the weaker ones soon die. After subjection to a temperature of
150 degrees to 160 degrees Fahrenheit, for ten minutes, if water is
present, almost all kinds are killed unless they are in the spore state.
Prolonged boiling will often be resisted by spores. Dry heat is not as
effective in killing bacteria as moist, and a higher temperature must,
therefore, be reached to effect this end. Below 70 degrees Fahrenheit
the growth of bacteria is more and more retarded, but not entirely
checked until freezing point is reached. The popular idea that freezing
may be relied upon to destroy bacteria is not true.
The bearing of these facts upon the subject of bacteria in foods cooked
in insulating boxes is evident. Whether foods are cooked or kept cold,
care must be taken that such a temperature is reached that bacteria may
not grow.
In application of these principles we see that foods must be heated
sufficiently to kill bacteria before it will be safe to subject them to
the comparatively low temperature of the cooker for the long period
necessary. This is one reason why foods in large pieces, such as roasts
of meat, whole vegetables, and moulds containing a mass of food, must be
boiled for a considerable time before being put into the cooker. Heat
will not penetrate at once to the centre of such foods, and they would
be likely to ferment or putrefy unless boiled long enough to heat the
centre beyond the point where bacteria thrive. The fact that meats,
cereals, and other foods have been known to sour or ferment, even after
such boiling, if left in the cooker for a very long time, may be
explained by the fact that, though all growing bacteria were killed,
spores, which resisted the boiling, might have been present in the food,
and when it cooled to a point conducive to the germination of these
spores, and remained at this temperature for long, they might have
developed, become active, and produced the objectionable changes
characteristic of their kind.
In the case of foods to be kept in refrigerating boxes, a temperature
considerably below 70 degrees Fahrenheit must be maintained. 50 degrees
Fahrenheit, or lower, will be found an excellent preventive of germ
growth.
Mr. L. A. Rogers has written a clear and concise description of the
nature, growth, and conditions necessary to combat bacteria such as are
found in food, in his paper entitled “Bacteria in Milk,” published in
the Yearbook of the Department of Agriculture, 1907, pages 180 to 196.
Other books which give information on this subject are “Bacteria,
Yeasts, and Molds in the Home,” by Conn, and “Household Bacteriology,”
by S. Maria Elliott.
Yeasts and moulds also may take part in the changes which spoil foods;
but the temperature conditions which control bacteria would be
practically the same for them.
_10. Cooking temperatures of different starches._
_Experiment: Cooking starch._
Pare and grate one or more potatoes. Wash the gratings by placing them
in a cheesecloth bag and immersing them in cold water. Squeeze and
press the contents of the bag until no more starch seems to pass through
the cloth. Let it settle, pour off the water; add clear water and let
the starch settle again. Pour off the second water. Take one
tablespoonful of the starch, mix it with one cupful of cold water. Heat
it slowly over a moderate fire, stirring it constantly, and recording
the temperature at which the mixture becomes noticeably clearer and
thickens.
Repeat this experiment with corn-starch; wheat starch, washed from wheat
flour, as is done with the grated potato; with starch washed from rye
flour; and, if desired, with rice, bean, pea, oat and tapioca starches,
also.
“Food and the Principles of Dietetics,” by Hutchison, gives, on page
378, a list of different starches and the temperatures at which they
gelatinize.
In a bulletin entitled “Digestibility of Starch of Different Sorts as
Affected by Cooking,” by Edna D. Day, Ph.D. (U. S. Dept, of Agriculture,
Office of Experiment Stations, Bulletin No. 202, page 40), we read that
starch takes up water at 60 degrees to 80 degrees Centigrade (140
degrees to 176 degrees Fahrenheit) and forms a sticky, colloidal
substance known as starch paste, in which form it is very easily
digested. Long boiling, at least to the extent of three hours, does not
make it more quickly digestible.
There is something to be considered besides the mere starch in cooking
starchy foods, and the fact that potato starch will form paste at 149
degrees while rice starch requires 176 degrees does not mean that less
cooking will be needed for potatoes than for rice. The woody fibre or
other constituents of foods, as well as their density and difference in
size, must be taken into account.
_11. Cooking temperatures of proteids._
=Egg Albumen=
In the bulletin entitled “Eggs and Their Uses as Food,” by C. F.
Langworthy, Ph.D., published as Farmers’ Bulletin, No. 128, by the U. S.
Department of Agriculture, the statement is made that “egg white begins
to coagulate at 134 degrees Fahrenheit. White fibres appear which become
more numerous until at about 160 degrees Fahrenheit the whole mass is
coagulated, the white almost opaque, yet it is tender and jelly-like. If
the temperature is raised to 212 degrees Fahrenheit, and continued, the
coagulated albumen becomes much harder and eventually more or less tough
and horn-like; it also undergoes shrinkage. It has been found by
experiment that the yolk of egg coagulates firmly at a lower temperature
than the white.”
It also says that these changes in the albumen suggest the idea that it
is not advisable to cook eggs in boiling water in order to secure the
most desirable result.
_Experiment A:_ To show the changes that take place in egg white at
various temperatures.
_Materials:_
Test-tube and holder
Beaker or saucepan of water
Thermometer
Egg white
Put the white of egg into the test-tube. Insert the thermometer. Hold
the test-tube in the pan of cold water to the depth of the egg white.
Gradually heat the water and observe the temperature at which the first
change in the egg albumen takes place. Notice also the temperature of
the water at this point. Continue the experiment until the water in the
outer vessel has boiled ten or twenty minutes, noting the temperatures
at which the various changes occur.
_Experiment B:_ To show the temperatures obtained in the proper cooking
of eggs.
_Materials:_
Fireless cooker
Eggs
Water
Thermometer
Cook eggs as directed for soft-cooked eggs on page 190, observing the
temperature of the water after the eggs are added to it, and when they
are removed from the cooker; also the condition, flavour, etc., of the
eggs.
=Cereal Proteids=
Professor Harcourt, in his bulletin, “Breakfast Foods,” published by the
Ontario Department of Agriculture, pp. 20 and 29, says that long cooking
of cereals renders the protein more digestible. The cooking which he
describes was carried on in a double boiler, and, therefore, below
boiling temperature, and in this respect is similar to fireless cookery.
He says that while short cooking, which was done at boiling temperature,
seemed to make cereal proteids less digestible, the long cooking at
below boiling temperature, which followed, somewhat changed them and
made them more digestible.
While little study appears to have been made of the digestibility of
cereal proteids when cooked for a long time at a low temperature, it is
probably fair, in the absence of further definite information, to assume
that, like animal proteids, it is better to cook them at a low
temperature such as that of the fireless cooker, than at the temperature
of boiling water or higher.
=Meat Proteids=
In the bulletin entitled “A Precise Method of Roasting Meat,” by
Elizabeth A. Sprague and H. S. Grindley, published by the University of
Illinois, a study is made of the temperatures at which the changes take
place from raw meat to “rare”; from “rare” to “medium rare,” and from
this to “well done” meat. The authors found that if the centre of the
meat is between 130 degrees and 148 degrees Fahrenheit (55 degrees and
65 degrees Centigrade), it is rare; if it is between 148 degrees and 158
degrees Fahrenheit (65 degrees and 70 degrees Centigrade), it is medium
rare; and if it is between 158 degrees and 176 degrees Fahrenheit (70
degrees and 80 degrees Centigrade), it is well done. They found no
advantage in cooking meat in a very hot oven (385 degrees Fahrenheit, or
195 degrees Centigrade), but rather a difficulty to keep it from
burning; that in an oven which was about 350 degrees Fahrenheit (175
degrees Centigrade), the meat cooked better; and that in an Aladdin oven
which kept the meat at about 212 degrees Fahrenheit (100 degrees
Centigrade), it cooked best of all; that is, it was of more uniform
character all through, more juicy, and more high flavoured. This seems
to point to an advantage in fireless cookery for meats, and practical
experience bears it out.
The initial heat of the insulated oven serves to sear and brown the
meat, and when this heat is reduced by the cooling of the stones, the
low temperature found to be best for completing the roasting is
obtained. With regard to meats cooked in water in the cooker, experience
has shown that they become well done and are more tender than when
boiled, showing that the temperatures necessary to reach that degree of
cooking are obtained even in the centre of a large piece of meat,
without toughening or hardening the outside of the meat, as is done when
more intense heat is applied.
The hardening effect of long cooking at a high temperature on meat
proteids can be demonstrated by broiling a tender piece of steak until
it is rare, cutting off a small piece, continuing the broiling for a few
minutes, cutting off another piece and comparing these pieces with the
remainder, which should be broiled until very well done.