2009_Book_FoodChemistry

150 2 Enzymes
Table 2.21. Michaelis constants for aldehyde dehydrogenase
(ALD) from various sources
Substrate
K m (µmol/l)
ALD (bovine liver)
ALD
Mitochon- Cytosol Micro- Yeast
dria
somes
Ethanal 0.05 440 1500 30
n-Propanal – 110 1400 –
n-Butanal 0.1 < 1 – –
n-Hexanal 0.075 < 1 < 1 6
n-Octanal 0.06 < 1 < 1 –
n-Decanal 0.05 – – –
Such a process is improved by the utilization of
yeast cells which, in addition to the enzyme and
NADH, contain a system able to regenerate the
cosubstrate. In order to prevent contamination of
beer with undesirable cell constituents, the yeast
cells are encapsulated with gelatin.
2.7.2.2 Hydrolases
Most of the enzymes used in the food industry
belong to the class of hydrolase enzymes (cf. Table
2.20).
2.7.2.2.1 Peptidases
The mixture of proteolytic enzymes used in
the food industry contains primarily endopeptidases
(specificity and classification under
section 1.4.5.2). These enzymes are isolated from
animal organs, higher plants or microorganisms,
i. e. from their fermentation media (Table 2.22).
Examples of their utilization are as follows. Proteinases
are added to wheat flour in the production
of some bakery products to modify rheological
properties of dough and, thus, the firmness of the
endproduct. During such dough treatment, the
firm or hard wheat gluten is partially hydrolyzed
to a soft-type gluten (cf. 15.4.1.4.5).
In the dairy industry the formation of casein
curd is achieved with chymosin or rennin (cf.
Table 2.20) by a reaction mechanism described
under section 10.1.2.1.1. Casein is also precipitated
through the action of other proteinases by
a mechanism which involves secondary proteolytic
activity resulting in diminished curd yields
and lower curd strength. Rennin is essentially
free of other undesirable proteinases and is,
therefore, especially suitable for cheesemaking.
However, there is a shortage of rennin since it
has to be isolated from the stomach of a suckling
calf. However, it is now possible to produce
this enzyme using genetically engineered microorganism.
Proteinases from Mucor miehei, M.
pusillus and Endothia parasitica areasuitable
replacement for rennin.
Plant proteinases (cf. Table 2.22) and also those
of microorganisms are utilized for ripening and
tenderizing meat. The practical problem to be
solved is how to achieve uniform distribution of
the enzymes in muscle tissue. An optional method
appears to be injection of the proteinase into the
blood stream immediately before slaughter, or rehydration
of the freeze-dried meat in enzyme solutions.
Cold turbidity in beer is associated with protein
sedimentation. This can be eliminated by hydrolysis
of protein using plant proteinases (cf. Table
2.22). Utilization of papain was suggested by
Wallerstein in 1911. Production of complete or
partial protein hydrolysates by enzymatic methods
is another example of an industrial use of proteinases.
This is used in the liquefaction of fish
proteins to make products with good flavors.
One of the concerns in the enzymatic hydrolysis
of proteins is to avoid the release of bitter-tasting
peptides and/or amino acids (cf. 1.2.6 and 1.3.3).
Their occurrence in the majority of proteins
treated (an exception is collagen) must be expected
when the molecular weight of the peptide
fragments falls below 6000. Bitter-tasting peptides,
e. g., those which are formed in the ripening
of cheese, can be converted to a hydrolyzate
which is no longer bitter by adding a mixture of
endo- and exopeptidases from Latobacilli.
2.7.2.2.2 α- andβ-Amylases
Amylases are either produced by bacteria
or yeasts (Table 2.20) or they belong to the
components of malt preparations. The high
temperature-resistant bacterial amylases, particularly
those of Bac. licheniformis (Fig. 2.49)
are of interest for the hydrolysis of corn starch
(gelatinization at 105–110 ◦ C). The hydrolysis
rate of these enzymes can be enhanced further