Food Chemistry

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 poly-
saccharide-type additive. Therefore, it is de-
scribed 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 clari-
fying 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 solubil-
ity 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 cere-
als 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 car-
bohydrate 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 de-

creases 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 bac-

terial 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-D-

glucosidic 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 characteris-

tic of an exoglycanase.

In contrast to amylose, amylopectin is not com-

pletely 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 liberatesβ-

D-glucose units. In amylopectin,α-1,6-branches

are cleaved ca. 30 times slower thanα-1,4-bonds.