Food Chemistry

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 pro-
duced 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 proper-
ties. Some brands are used as food gelatins, oth-
ers play an important role in industry (film emul-
sions, glue manufacturing).

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 (Mr 70 ,000) which forms
elastic fibers. The protein has rubber-like prop-
erties. It can stretch and then return to its origi-
nal length or shape. Large amounts of elastin are
present in ligaments and the walls of blood ves-
sels. The ligament located in the neck of graz-
ing 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 con-
tent of amino acids with nonpolar side chains
(Ala, Val) is greatly increased. This difference ex-
plains that, unlike collagen, elastin lacks the capa-
bility 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 nonaro-
matic, nonpolar side chain.

12.3.3 Free Amino Acids

Fresh beef muscle contains 0.1–0.3% free amino
acids (fresh weight basis). All amino acids are de-

tectable 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 re-

garded 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):

(12.22)

The biochemical role of taurine includes derivati-

zation of bile acids (taurocholic and taurodeoxy-

cholic 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-, diethyl-

and isopropylamine) are detected only in trace

amounts.

Of the biogenic amines produced on the de-

carboxylation 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