2009_Book_FoodChemistry

4.4 Polysaccharides 333
4.4.4.23 Polyvinyl Pyrrolidone (PVP)
4.4.4.23.1 Structure, Properties
This compound is used as if it were a polysaccharide-type
additive. Therefore, it is described
here. The molecular weight of PVP can
range from 10–360 kdal.
(4.165)
It is quite soluble in water and organic solvents.
The viscosity of the solution is related to the
molecular weight.
4.4.4.23.2 Utilization
PVP forms insoluble complexes with phenolic
compounds and, therefore, is applied as a clarifying
agent in the beverage industry (beer, wine,
fruit juice). Furthermore, it serves as a binding
and thickening agent, and as a stabilizer, e. g., of
vitamin preparations. Its tendency to form films is
utilized in protective food films (particle solubility
enhancement and aroma fixation in instant tea
and coffee production).
4.4.5 Enzymatic Degradation
of Polysaccharides
Enzymes that cleave polysaccharides are of
interest for plant foods. Examples are processes
that occur in the ripening of fruit (cf. 18.1.3.3.2),
in the processing of flour to cakes and pastries
(cf. 15.2.2.1), and in the degradation of cereals
in preparation for alcoholic fermentation
(cf. 20.1.4). In addition, enzymes of this type are
used in food technology (cf. 2.7.2.2) and in carbohydrate
analysis (cf. Table 2.16 and 4.4.6). The
following hydrolases are of special importance.
4.4.5.1 Amylases
Amylases hydrolyze the polysaccharides of
starch.
4.4.5.1.1 α-Amylase
α-Amylase hydrolyzes starch, glycogen, and
other 1,4-α-glucans. The attack occurs inside
the molecule, i. e., this enzyme is comparable
to endopeptidases. Oligosaccharides of 6–7
glucose units are released from amylose. The
enzyme apparently attacks the molecule at the
amylose helix (cf. 4.4.4.14.3) and hydrolyzes
“neighboring” glycoside bonds that are one turn
removed. Amylopectin is cleaved at random; the
branch points (cf. 4.4.4.14.4) are overjumped.
α-Amylase is activated by Ca 2+ ions (cf. 2.3.3.1
and 2.7.2.2.2).
The viscosity of a starch solution rapidly decreases
on hydrolysis by α-amylase (starch
liquefaction) and the iodine color disappears. The
dextrins formed at first are further degraded on
longer incubation, reducing sugars appear and,
finally, α- maltose is formed. The activity of the
enzyme decreases rapidly with decreasing degree
of polymerization of the substrate.
Catalysis is accelerated by the gelatinization of
starch (cf. 4.4.4.14.2). For example, the swollen
substrate is degraded 300 times faster by a bacterial
amylase and 10 5 times faster by a fungal
amylase than is native starch.
4.4.5.1.2 β-Amylase
This enzyme catalyzes the hydrolysis of 1,4-α-Dglucosidic
bonds in polysaccharides (mechanism,
2.4.2.5), effecting successive removals of maltose
units from the nonreducing end. Hydrolysis is
linked to a Walden inversion at C-1, giving rise to
β-maltose. This inversion, which can be detected
polarimetrically, represents a definite characteristic
of an exoglycanase.
In contrast to amylose, amylopectin is not completely
hydrolyzed. All reaction stops even before
branch points are reached.
4.4.5.1.3 Glucan-1,4-α-D-glucosidase
(Glucoamylase)
This glucoamylase starts at the nonreducing end
of 1,4-α-D-glucans and successively liberates β-
D-glucose units. In amylopectin, α-1,6-branches
are cleaved ca. 30 times slower than α-1,4-bonds.