Food Chemistry

15.2 Individual Constituents 695

Fig. 15.14.Tensile tests of glutens with varying con-
tent of gliadin (gluten K from retail wheat flour was
extracted with 70% aqueous ethanol. The extracted
gliadin and the remaining glutenin were freeze dried,
remixed in different proportions, and then hydrated.
Gliadin content of the glutens: K) 33.9%, 1) 55.9%,
14) 22.6%; the gliadin contents of the other samples
are in between, according toKimet al., 1988)

amount of polymerizable gluten proteins (HMW
subunits of the x-type, LMW subunits) is required
with the lowest possible amounts of terminators
(low molecular thiol compounds, gliadins with
an odd number of cysteine residues, possibly also
HMW subunits of the y-type).

15.2.1.5 Puroindolins

The wheat endosperm contains two basic,
cysteine-rich proteins, puroindolin a and b (PIN-
a and -b). The name is derived from the presence
of tryprophan-rich segments in the amino aicd
sequences: Trp-Arg-Trp-Trp-Lys-Trp-Trp-Lys in
PIN-a and Trp-Pro-Thr-Trp-Trp-Lys in PIN-b.
PIN-a consists of a peptide chain with
115 residues (Mr 12 ,479) and five disulfide
bridges. The peptide chains of PIN-a and PIN-b
are homologous to an extent of 60%. It has been
shown that the puroindolins are identical to the
basic friabilins which have been discovered on

the surface of starch granules. PIN-a and -b are

positively charged and bind negatively charged

phospholipids with a high affinity. PIN-a also

forms stable complexes with glycolipids while

PIN-b is less suitable. It is assumed that the

indolyl residues of the tryptophan-rich segments

are involved in the stabilization of the complexes,

hydrogen bridges being formed between the

indole-NH and the OH-groups of the glycolipids.

Thus, the higher stability of PIN-a compared

with PIN-b complexes is based on the longer

tryprophan-rich segment.

Foams of PIN-a and, to a smaller extent, of PIN-b

are stabilized especially in the presence of polar

wheat lipids. In this respect, PIN-a is clearly su-

perior to egg white proteins as shown by the fol-

lowing comparison. After a drip off time of 5 min,

a foam density of 0.028 was obtained with 0.3mg

of PIN-a/ml, while 1.25 mg/ml of egg white pro-

teins were required for this purpose.

For the baking process, it is expected that the

puroindolins protect the foam-like texture of the

dough from destabilization by lipids.

15.2.2 Enzymes

Of the enzymes present in cereal kernels, those

which play a role in processing or are involved in

the reactions which are decisive for the quality of

a cereal product will be described.

15.2.2.1 Amylases

α-andβ-amylases (for their reactions, see

4.4.5.1) are present in all cereals. Wheat and rye

amylases are of particular interest; their optimum

activities are desirable in dough making in the

presence of yeast (cf. 15.4.1.4.8). In mature

kernels,α-amylase activity is minimal, while it

increases abruptly during sprouting or germina-

tion. Unlike the situation with wheat, dormancy

in rye is not very pronounced. Unfavorable har-

vest conditions (high moisture and temperature)

favor premature germination (“sprouting”), not

visible externally. During this time,α-amylase

activity rises, resulting in extensive starch degra-

dation during the baking process. Bread faults

appear, as mentioned under 15.4.1.2.