Agriculture Reference
In-Depth Information
the storage of practically every foodstuff. An unpleasant
odour or flavour in fat may be due to absorption of
foreign odours, as in the tainting of meat or butter stored
in a chamber previously used for fruit, atmospheric oxi-
dation or the action of micro-organisms, which may give
rise to extensive hydrolysis of fat. A small amount of free
fatty acids, however, has little effect on flavour; it is more
likely that the tainted flavour normally accompanying
bacterial growth is mainly due to nitrogenous break-
down products of connective tissue.
Rancidity from atmospheric oxidation does not
require the presence of micro-organisms and is the
most common type of deleterious fatty change. The
important oxidative lipids in food are the unsaturated
fatty acids, particularly oleic, linoleate and linolenate,
with the susceptibility and rate of oxidation increasing
with their degree of unsaturation. Oxidation continues
at low temperature since little energy is required for
the biochemical reaction. Exposure to light is another
factor which can predispose fatty tissues to oxidation.
The natural resistance of some fats, for example,
chicken fat, to the development of rancidity is due to
the presence of antioxidants such as vitamin E. Beef
and mutton fats are relatively resistant and cause little
trouble except when frozen and stored at −10°C for
periods longer than 18 months.
Bacon fat, particularly when exposed to light, is
much more susceptible to oxidation. The type of feed-
ing of the bacon pig also affects the rapidity of oxida-
tion, for example, extensive swill feeding produces a
soft fat with a high proportion of unsaturated fatty
acids which tend to be converted to aldehydes and
ketones, imparting the acid flavour associated with
rancidity. The rapid onset of rancidity, together with a
yellow colouration of the fat, has frequently been
observed in carcases of pigs fed on cod liver oil or fish
meal. A further deleterious factor in bacon manufac-
ture is that during curing much of the natural resist-
ance of the pig tissues to oxidation is broken down by
the specific action of the pickling salts (sodium chlo-
ride is known to have a catalytic effect in fat autoxida-
tion), and for this reason, bacon fat is more liable to
develop rancidity than pork fat. Even at −10°C oxida-
tion of bacon fat is appreciable within a few weeks,
though if bacon is smoked subsequently to curing the
absorption of phenolic substances confers a certain
amount of protection against oxidation during storage.
The time factor in the curing of bacon and ham is
therefore of particular importance with bacon which
undergoes a long period of manufacture more liable to
become rancid.
Marked rancidity in pig fat is usually associated with a
change in colour of the fat from white to yellow, and the
abdominal wall, as a current of air cools the abdominal
viscera and delays migration of putrefactive bacteria
from the intestinal tract. Venison, which is particularly
rich in connective tissue and therefore exceedingly
tough after slaughter, requires conditioning by hang-
ing before it is rendered palatable. It can hang for long
periods without decomposition, and it is stated that
the muscular tissue of deer possesses antibacterial
substances which have an inhibitory action on a great
number of putrefactive bacteria as well as on the
bacteria responsible for food poisoning; in this way,
conditioning or ripening can take place in venison
un-associated with decomposition.
The smaller animals such as
game
or
hares
lose heat
rapidly after death and at an atmospheric temperature of
16-18.5°C. Small carcases such as rabbits cool to air tem-
perature in about 12 hours, whereas larger carcases such
as sheep require about 24 hours. As this rapid heat dis-
sipation inhibits the growth of putrefactive bacteria, it is
practicable to consign feathered game and hares to mar-
ket without removal of the abdominal viscera and packed
in crates or baskets.
In British fresh
sausage
, the early stages of decomposi-
tion usually take place simultaneously throughout the
meat substance, but all the accepted changes associated
with decomposition may not be present. Valuable indi-
cations of unsoundness in fresh sausage, as distinct from
smoked, are stickiness on the surface of the casing; in the
early stages, a sour rather than a foetid odour as a result
of the activity of lactic acid bacteria and
Brochothrix
thermosphactum
; easy separation of the sausage meat
from the casing; and a grey colour on section of the sau-
sage. The odour of early decomposition may be detected
by a boiling test, especially if a little lime water is added
to the water before boiling.
It is important to remember that while the spoilage
organisms indicate their presence by off colours, odours
and tastes as well as changes in consistency, most food-
poisoning organisms give no indication of their presence
in food. Some, such as
B. cereus
,
E. coli
and
Pseudomonas
,
as well as causing spoilage, can also on occasions cause
food poisoning.
E. coli
is commonly found in foods of ani-
mal origin and is an indication of sewage pollution of water
and unhygienic methods of preparation. The factors
responsible for the onset of food poisoning (mainly lack of
hygiene and careless storage of cooked and uncooked
foods at temperatures suitable for bacterial growth) are vir-
tually the same as those that lead to the spoilage of food.
Decomposition of fat
Rancidification is the chemical decomposition of fats,
oils and lipids and follows oxidative, hydrolytic or micro-
bial pathways. The problem of
fat rancidity
crops up in
