The circumstances by which putrefaction is counteracted, are, 1. the
chemical change of the azotized juices; 2. the abstraction of the water;
3. the lowering of the temperature; and 4. the exclusion of oxygen.
1. _The chemical change of the azotized juices._--The substance which in
dead animal matter is first attacked with putridity, and which serves to
communicate it to the solid fibrous parts, is albumen, as it exists
combined with more or less water in all the animal fluids and soft
parts. In those vegetables also which putrefy, it is the albumen which
first suffers decomposition; and hence those plants which contain most
of that proximate principle, are most apt to become putrid, and most
resemble, in this respect, animal substances; of which fact, mushrooms,
cabbages, coleworts, &c., afford illustrations. The albumen, when
dissolved in water, very readily putrefies in a moderately warm air; but
when coagulated, it seems as little liable to putridity as fibrin
itself. By this change, it throws off the superfluous water, becomes
solid, and may then be easily dried. Hence, those means which by
coagulation make the albumen insoluble, or form with it a new compound,
which does not dissolve in water, but which resists putrefaction, are
powerful antiseptics. Whenever the albumen is coagulated, the uncombined
water may be easily evaporated away, and the residuary solid matter may
be readily dried in the air, so as to be rendered unsusceptible of
decomposition.
In this way acids operate, which combine with the albumen, and fix it in
a coagulated state, without separating it from its solution: such is the
effect of vinegar, citric acid, tartaric acid, &c.
Tannin combines with the albuminous and gelatinous parts of animals, and
forms insoluble compounds, which resist putrefaction; on which fact the
art of tanning is founded.
Alcohol, oil of turpentine, and some other volatile oils, likewise
coagulate albumen, and thereby protect it from putrescence. The most
remarkable operation of this kind is exhibited by wood vinegar, in
consequence of the _creosote_ contained in it, according to the
discovery of Reichenbach. This peculiar volatile oil has so decided a
power of coagulating albumen, that even the minute portion of it present
in pyrolignous vinegar is sufficient to preserve animal parts from
putrefaction, when they are simply soaked in it. Thus, also, flesh is
cured by wood smoke. Wood tar likewise protects animal matter from
change, by the creosote it contains. The ordinary pyrolignous acid
sometimes contains 5 per cent. of creosote.
In circumstances where a stronger impregnation with this antiseptic oil
may be necessary, common wood vinegar may be heated to 167° F., and
saturated with effloresced Glauber’s salts, by which expedient the oil
is separated and made to float upon the surface of the warm liquid;
whence it should be immediately skimmed off; because, by cooling and
crystallizing, the solution would so diminish in density as to allow the
oil to sink to the bottom; for its specific gravity is considerably
greater than that of water. This oil, which contains, besides creosote,
some other volatile constituents, may be kept dissolved ready for use in
strong vinegar or alcohol. Water takes up of pure creosote only 1-3/4
per cent.; but alcohol dissolves it in every proportion.
The earthy and metallic salts afford likewise powerful means for
separating albumen from its watery solution, their bases having the
property of forming insoluble compounds with it. The more completely
they produce this separation, the more effectually do they counteract
putrefaction. The alkaline salts also, as common salt, sal ammoniac,
saltpetre, and tartar, operate against putrescence, though in a smaller
degree, because they do not precipitate the albumen; but, by abstracting
a part of its water, they render it less liable to become putrid. Among
the earthy salts, alum is the most energetic, as it forms a subsalt
which combines with albumen; it is three times more antiseptic than
common salt, and from seven to eight times more so than saltpetre.
Muriate of soda, however, may be employed along with alum, as is done in
the tawing of sheepskins.
The metallic salts operate still more effectually as antiseptics,
because they form with albumen still more intimate combinations. Under
this head we class the green and red sulphates of iron, the chloride of
zinc, the acetate of lead, and corrosive sublimate; the latter, however,
from its poisonous qualities, can be employed only on special occasions.
Nitrate of silver, though equally noxious to life, is so antiseptic,
that a solution containing only 1/500 of the salt is capable of
preserving animal matters from corruption.
2. _Abstraction of water._--Even in those cases where no separation of
the albumen takes place in a coagulated form, or as a solid precipitate,
by the operation of a substance foreign to the animal juices,
putrefaction cannot go on, any more than other kinds of fermentation, in
bodies wholly or in a great measure deprived of their water. For the
albumen itself runs so much more slowly into putrefaction, the less
water it is dissolved in; and in the desiccated state, it is as little
susceptible of alteration as any other dry vegetable or animal matter.
Hence, the proper drying of an animal substance becomes a universal
preventive of putrescence. In this way fruits, herbs, cabbages, fish,
flesh, may be preserved from corruption. If the air be not cold and dry
enough to cause the evaporation of the fluids before putrescence may
come on, the organic substance must be dried by artificial means, as by
being exposed in thin slices in properly constructed air-stoves. At
temperatures under 140° F., the albumen dries up without coagulation,
and may then be re-dissolved in cold water, with its valuable properties
unaltered. By such artificial desiccation, if flesh is to be preserved
for cooking or boiling, it must not be exposed, however, to so high a
degree of heat, which would harden it permanently, like the baked
mummies of Egypt. Mere desiccation, indeed, can hardly ever be employed
upon flesh. Culinary salt is generally had recourse to, either alone or
with the addition of saltpetre or sugar.
These alkaline salts abstract water in their solution, and,
consequently, concentrate the aqueous solution of the albumen; whence,
by converting the simple watery fluid into salt water, which is in
general less favourable to the fermentation of animal matter than pure
water, and by expelling the air, they counteract putridity. On this
account, salted meat may be dried in the air much more speedily and
safely than fresh meat. The drying is promoted by heating the meat
merely to such a degree as to consolidate the albumen, and eliminate the
superfluous water.
Alcohol operates similarly, in abstracting the water essential to the
putrefaction of animal substances, taking it not only from the liquid
albumen, but counteracting its decomposition, when mixed among the
animal solids. Sugar acts in the same way, fixing in an unchangeable
syrup the water which would otherwise be accessory to the fermentation
of the organic bodies. The preserves of fruits and vegetable juices are
made upon this principle. When animal substances are rubbed with
charcoal powder or sand, perfectly dry, and are afterwards freely
exposed to the air, they become deprived of their moisture, and will
keep for any length of time.
3. _Defect of warmth._--As a certain degree of heat is requisite for the
vinous fermentation, so is it for the putrefactive. In a damp
atmosphere, or in one saturated with moisture, if the temperature stand
at from 70° to 80° F., the putrefaction goes on most rapidly; but it
proceeds languidly at a few degrees above freezing, and is suspended
altogether at that point. The elephants preserved in the polar ices are
proofs of the antiseptic influence of low temperature. In temperate
climates, ice-houses serve the purpose of keeping meat fresh and sweet
for any length of time.
4. _Abstraction of oxygen gas._--As the putrefactive decomposition of a
body first commences with the absorption of oxygen from the atmosphere,
so it may be retarded by the exclusion of this gas. It is not, however,
enough to remove the aerial oxygen from the surface of the body, but we
must expel all the oxygen that may be diffused among the vessels and
other solids, as this portion suffices in general to excite
putrefaction, if other circumstances be favourable. The expulsion is
most readily accomplished by a moderate degree of heat, which, by
expanding the air, evolves it in a great measure, and at the same time
favours the fixation of the oxygen in the extractive matter, so as to
make it no longer available towards the putrefaction of the other
substances. Milk, soup, solution of gelatine, &c., may be kept long in a
fresh state, if they be subjected in an air-tight vessel every other day
to a boiling heat. Oxygenation may be prevented in several ways: by
burning sulphur or phosphorus in the air of the meat receiver; by
filling this with compressed carbonic acid; or with oils, fats, syrups,
&c., and then sealing it hermetically. Charcoal powder recently calcined
is efficacious in preserving meat, as it not only excludes air from the
bodies surrounded by it, but intercepts the oxygen by condensing it.
When butcher-meat is enclosed in a vessel filled with sulphurous acid,
it absorbs the gas, and remains for a considerable time proof against
corruption. The same result is obtained if the vessel be filled with
ammoniacal gas. At the end of 76 days such meat has still a fresh look,
and may be safely dried in the atmosphere.