2009_Book_FoodChemistry

584 12 Meat
The transition of collagen to gelatin outlined
above occurs during the cooking and roasting of
meat. The extent of gelatinization is affected by
the collagen cross-linking as determined by the
age of the animal and the amount of heat applied
(temperature, time, pressure).
Gelatin plays a role as a gelling agent. It is produced
on a large scale from animal bones or
skin by treatment with alkali or acid, followed by
a water extraction step. Depending on the process,
products are obtained which differ in molecular
weight and, consequently, in their gelling properties.
Some brands are used as food gelatins, others
play an important role in industry (film emulsions,
glue manufacturing).
Fresh beef muscle contains 0.1–0.3% free amino
acids (fresh weight basis). All amino acids are detectable
in low amounts (<0.005), with alanine
(0.01–0.05%) and glutamic acid (0.01–0.05%)
being most predominant.
The free amino acid fraction also contains 0.02–
0.1% taurine (I). As such, taurine should be regarded
as a major constituent of this fraction. It is
obtained biosynthetically from cysteine through
cysteic acid and/or from a side pathway involving
cysteamine and hypotaurine (II):
12.3.2.3.2 Elastin
Elastin is found in lower amounts in connective
tissue along with collagen. It is a nonswelling,
highly stable protein (M r 70,000) which forms
elastic fibers. The protein has rubber-like properties.
It can stretch and then return to its original
length or shape. Large amounts of elastin are
present in ligaments and the walls of blood vessels.
The ligament located in the neck of grazing
animals is an exceptionally rich source of this
protein. Table 12.6 shows that the amino acid
composition is different from that of collagen.
The amount of basic and acidic amino acids is
low, e. g., hydroxylysine is absent, while the content
of amino acids with nonpolar side chains
(Ala, Val) is greatly increased. This difference explains
that, unlike collagen, elastin lacks the capability
of swelling on heating in water. The elastic
properties are based on very strong cross links,
which involve the desmosines described in the
section on collagen (cf. Formula 12.15).
Elastin is hydrolyzed by the serine proteinase
elastase, which is excreted by the pancreas. This
enzyme preferentially cleaves peptide bonds at
sites where the carbonyl residue has a nonaromatic,
nonpolar side chain.
12.3.3 Free Amino Acids
(12.22)
The biochemical role of taurine includes derivatization
of bile acids (taurocholic and taurodeoxycholic
acids). A neurotransmitting function has
also been ascribed to this compound.
12.3.4 Peptides
The characteristic β-alanyl histidine peptides,
carnosine, anserine and balenine, of muscle are
described in section 1.3.4.2. Their contribution to
taste is discussed in 12.9.1.
12.3.5 Amines
Methylamine in fresh beef muscle is present
at 2 mg/kg, while the other volatile aliphatic
amines (dimethyl-, trimethyl-, ethyl-, diethyland
isopropylamine) are detected only in trace
amounts.
Of the biogenic amines produced on the decarboxylation
of amino acids (cf. 10.2.8.3),
histamine, tyramine, putrescine and cadaverine
have been identified in beef and pork. Since these
substances are microbial metabolic products, they
were the proposed indicators of microbial quality
and were listed in the biogenic amine index (BAI
= concentration of the four amines in mg/kg).
ABAIvalueof<5 indicates clean meat, 5–20