Breakdown
of protein
The process where protein
is broken down is called proteolysis where pro-
teo stands for
protein and lysis for breakdown. The major enzymes con-
cerned
are proteases, e.g. rennin, pepsin and trypsin. These enzymes
de-
grade proteins into peptides, which are then defraded by
various peptidas-
es to smaller peptides and free amino acids.
Amino acids can be reutilised
for protein synthesis by
the cell; however, they can also be broken down
oxidatively or
fermentatively.
Proteins
and their constituent amino acids have awide combination of
chemical
elements and contain carbon, hydrogen, oxygen, sulphur, nitrogen
and
phosphorus. Breakdown of protein therefore results in a much
larger
range of acids, alcohols gases (hydrogen, carbon dioxidem,
hydrogen sul-
phide and ammonia) and other compounds. Breakdown
of portein nearly
always results in ammonia, which is alkaline
and has a strong smell.
Three
amino acids, cystine, cysteine and methionine, contain sulphur
and
result in hydrogen sulphide which also gives off a strong smell.
Breakdown
of protein in liquid milk takes place in two major stages
called
peptonisation and consists of:
- curdling (sweet
as opposed to sour) or clotting of the milk by rennin-like
enzymes. This fault in milk is called sweet curdling, a defect which is
common in pasteurised milk which is stored warm. - proteolysis of
the protein, resulting in production of ammonia, which is
alkline.
The degree of amino
free acids and ammonia in cheese gives an indication
of its age
and maturity as proteolysis progresses. Blue, or mould ripened,
cheese
has rapid proteolysis, resulting in production of laarge amounts
of
ammonia
Breakdown of fat
The process where fat
is broken down by enzymes is called lipolysis, from
the Greek
roots lipo meaning fat and lysis meaning vreakdown. THe major
enzymme
concerned is lipase. During lipolysis the fat is hydrolysed
to glycerol
and three separate fatty acids. Some of the
fatty acids are volatile and give
off strong smells. One
example is buryric acid, which gives the characteris-
tic, rancid
taste.
Pure
fat is relatively resistant to microbiological breakdown. Mikl
fat, in
the form of butter and cream, contains protein, carbohydrate,
minerals, etc.
for frowth and is therefore more susceptible.
Many
bacteria and moulds which break down proteins also break down
fat
oxidatively.
Breakdown of lecithin
Lecithin, the phospholipid
included in the membranes round the fat glob-
ules, is a chemical
combination of glycerol, two fatty acids, phosphoric acid
and
choline, an organic alkali, Strains of Bacillus cereus produce enzymes,
lecithinases,
which hydrolyse the lecithin into diflyceride and phosphoryl
choline.
The membranes of the fat globules are split, resulting in
an unstable
fat emulsion often appearing in the form of lumps
floating on the
surface of the milk or cream. This fault
in milk or cream is called "bitty
cream"or "broken
cream"
Further
breakdown of the choline into trimethyl amine will result in a fishy
smell
and taste.
Pigment and colour production
The proces of colour
production is called chromogenesis and the organ-
ism causing
the production is referred to as chromogenic after the Greek
roots
chromo meaning colur and genesis meaning birth or origin.
This
process of metabolism is a feature of certain micro-organisms. It
is
greater in certain foods than others and takes place at lower
temperatures.
There are two types of pigment:
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Dairy
Processing Handbook/chapter 2 44
[Chapter3]
[45][46][47][48][49][50][51][52][53][54]
[55][56][57][58][59][60][61][62][63][64]
[Chapter5]
