Food Biochemistry and Food Processing

12 Part I: Principles

pancreatic elastase and collagenase are responsible
for the breakdown of animal connective tissues.

TRANSGLUTAMINASEACTIVITY INSEAFOOD
PROCESSING

Transglutaminase (TGase, EC 2.3.2.13) has the sys-
tematic name of protein-glutamine-glutamyltrans-
ferase. It catalyzes the acyl transfer reaction between
-carboxyamide groups of glutamine residues in pro-
teins, peptides, and various primary amines. When
the-amino group of lysine acts as acyl acceptor, it
results in polymerization and inter- or intramolecular
cross-linking of proteins via formation of-(-glu-
tamyl) lysine linkages. This occurs through exchange
of the-amino group of the lysine residue for ammo-
nia at the carboxyamide group of a glutamine residue
in the protein molecule(s). Formation of covalent
cross-links between proteins is the basis for TGase to
modify the physical properties of protein foods. The
addition of microbial TGase to surimi significantly
increases its gel strength, particularly when the suri-
mi has lower natural setting abilities (presumably
due to lower endogenous TGase activity). Thus far,
the primary applications of TGase in seafood pro-
cessing have been for cold restructuring, cold gela-
tion of pastes, or gel-strength enhancement through
myosin cross-linking. In the absence of primary
amines, water may act as the acyl acceptor, resulting
in deamination of-carboxyamide groups of gluta-
mine to form glutamic acid (Ashie and Lanier 2000).

PROTEOLYSIS DURINGCHEESEFERMENTATION

Chymosin (rennin) is an enzyme present in the calf
stomach. In cheese making, lactic acid bacteria
(starter) gradually lower the milk pH to the 4.7 that
is optimal for coagulation by chymosin. Most lactic
acid starters have limited proteolytic activities. How-
ever, other added lactic acid bacteria have much
stronger proteolytic activities. These proteases and
peptidases break down the milk caseins to smaller
protein molecules and, together with the milk fat,
provide the structure of various cheeses. Other en-
zymes such as decarboxylases, deaminases, and
transaminase are responsible for the degradations of
amino acids into secondary amines, indole,
-keto
acids, and other compounds that give the typical fla-
vor of cheeses. Table 1.6 lists some of these en-
zymes and their reactions.

PROTEOLYSIS INGERMINATINGSEEDS

Proteolytic activities are much lower in germinating

seeds. Only aminopeptidase and carboxypeptidase

A are better known enzymes (Table 1.7). They pro-

duce peptides and amino acids that are needed in the

growth of the plant.

PROTEASES FORCHILL-HAZEREDUCTION IN

BEERPRODUCTION

In beer production, a small amount of protein is dis-

solved from the wheat and malt into the wort.

During extraction of green beer from the wort, this

protein fraction is also carried over to the beer.

Because of its limited solubility in beer at lower

temperatures, it precipitates out and causes hazing in

the final product. Proteases of plant origin such as

papain, ficin, and bromelain, and possibly other

microbial proteases, can break down these proteins.

Addition of one or more of these enzymes is com-

monly practiced in the brewing industry to reduce

this chill-haze problem.

BIOCHEMICAL CHANGES OF

LIPIDS IN FOODS

CHANGES INLIPIDS INFOODSYSTEMS

Research reports on enzyme-induced changes in

lipids in foods are abundant. In general, they are

concentrated on changes in the unsaturated fatty

acids or the unsaturated fatty moieties in acylglyc-

erols (triglycerides). The most studied are linoleate

(linoleic acid) and arachidonate (arachidonic acid)

as they are quite common in many food systems

(Table 1.8). Because of the number of double bonds

in arachidonic acid, enzymatic oxidation can occur

at various sites, and the responsible lipoxygenases

are labeled according to these sites (Table 1.8).

CHANGES INLIPIDS DURINGCHEESE

FERMENTATION

Milk contains a considerable amount of lipids and

these milk lipids are subjected to enzymatic oxida-

tion during cheese ripening. Under proper cheese

maturation conditions, these enzymatic reactions

starting from milk lipids create the desirable flavor

compounds for these cheeses. These reactions are

numerous and not completely understood, so only