Agriculture Reference
In-Depth Information
Loss of bloom
Bloom
is defined as the colour and general appearance of
a carcase surface when viewed through the semi-
transparent layers of connective tissue, muscle and fat
which form the carcase surface. If these tissues become
moist, the collagen fibres in the connective tissue swell
and become opaque and the meat surface assumes a dull,
lifeless appearance. Loss of surface bloom in beef carcases
may also be caused by dehydration or undue oxidation,
but it may be prevented by avoiding temperature fluctua-
tions that permit alternate drying and dampening of the
carcase surface. It is also important to keep the relative
humidity of cooling chambers high and ensure free
circulation of air. Muscular tissue also tends to become
brownish on exposure to air as myohaemoglobin changes
to the brown pigment methaemoglobin, but the actual
amount of exposed muscle in a side of beef is so small
that this is of little or no consequence. Refrigeration has
little effect on the carcase fat except in the case of frozen
meat which has undergone a prolonged period of storage,
in which case rancidity may develop.
solutes are introduced, these orient the water molecules
around them and make them much less available for use by
micro-organisms. With the exception of
Staphylococcus
aureus
and most moulds, micro-organisms are poor at
competing with solutes for the water molecules.
The
a
w
varies little with temperature over the normal
growth range of micro-organisms. Pure water has the
highest
a
w
possible (1.0), and the
a
w
decreases with the
addition of solute (always <1).
Various NaCl solutions will give the following
a
w
.
%NaCl (w/v)
a
w
0.9
0.995
3.5
0.98
7.0
0.96
16.0
0.90
22.0
0.86
The
a
w
may greatly affect the ability of an organism to
survive heat. The thermal death time (
D
60
) for
Salmonella
typhimurium
at 60°C is 0.18 minutes at an
a
w
of 0.94.
The
a
w
stated for an organism is usually the minimum
at which growth will take place, but growth will increase
with increasing
a
w
. Lower than minimal
a
w
will not
necessarily kill the organisms, and they will remain
infectious. It is difficult to standardise an
a
w
for a specific
food as this may vary depending on the source, the age of
the food and even different parts of the food.
Chemical changes in stored meat
The chemical changes that take place after slaughter are
indicative of a slight degree of breakdown in protein, due
either to endogenous enzymes or to those of micro-
organisms. The odour of the meat becomes progressively
more marked but never undesirable; the flavour may be
described as stale, rendering the meat unpalatable but
not repulsive.
The storage life of meat is more dependent on the
chemical changes that take place in fat rather than in
muscle, for fat rancidity, even if only slight, is objection-
able. The condition of the fat therefore determines the
length of storage, for while the lean muscle of a carcase
may be still improving in flavour, the changes in fat may
render the meat repugnant and unmarketable.
Meat curing
While curing may be applied to all kinds of meat, it is
best adapted to those with a high fat content, for example,
pork or fine-fibred beef intermixed with fat, and it is for
this reason that brisket and flank of beef make high-
quality pickled meat. On the other hand, lean beef, veal
or mutton becomes dry and unpalatable on pickling.
Drying
Although drying as such plays only a minor role in pres-
ervation today, the whole vast process of refrigeration is
largely based on the principle of drying, that is, the
removal of water available for microbial growth. Again,
salting largely owes its preservation action to the extrac-
tion of water by osmosis.
It is essential that as little water as possible is put on
sheep and cattle carcases during dressing.
Salt
Salt is the principal preserving material used in curing
on a commercial scale, though it appears to have little
directly harmful effect on bacteria; large quantities of
salt or sugar produce the same result as would be
obtained by the extraction of water. Indeed, the osmotic
pressure of the strong salt or sugar solution removes the
water necessary for bacterial growth from the meat.
Halophilic (salt-loving) bacteria require salt for optimum
growth and are not affected; they are, however, slower
growing than non-halophilic bacteria.
Distinction must be made between salted meats (beef,
pork) and cured meats (bacon, ham, corned beef ). In
salted meats, the dry salt first dissolves in the surface
Water activity or water availability (
a
w
)
Water activity or water availability,
a
w
, is a measure of the
partial vapour pressure of the foodstuffs compared to that
of pure water at its surface. Water molecules are loosely ori-
entated in pure liquid water and can easily rearrange. When
