Food Chemistry

1.4 Proteins 83

Fig. 1.47.Viscosity curves during reduction of different
wheat glutens. For sample designation see Fig. 1.44.
(according toLasztity, 1975)

modified by reduction of its disulfide bonds
to sulfhydryl groups and subsequent reoxida-
tion of these groups under various conditions
(Fig. 1.48). Reoxidation of a diluted suspen-

Fig. 1.48.Reaction of proteins withD,L-alanine car-
boxy anhydride. (according toSelaet al., 1962 andSt.
Angeloet al., 1966)

Fig. 1.49.Covalent binding of lysine to gluten (ac-
cording toLi-Chanet al., 1979) and of methionine or
tryptophan to soya protein (according toVoutsinasand
Nakai, 1979), by applying a carbodiimide procedure

sion in the presence of urea results in a weak,

soluble, adhesive product (gluten A), whereas

reoxidation of a concentrated suspension in

the presence of a higher concentration of urea

yields an insoluble, stiff, cohesive product

(gluten C). Additional viscosity data have

shown that the disulfide bridges in gluten

A are mostly intramolecular while those in

gluten C are predominantly intermolecular

(Fig. 1.49).

1.4.6.3 EnzymaticModification..................................

Of the great number of enzymatic reactions with

protein as a substrate (cf. 1.4.5), only a small

number have so far been found to be suitable for

use in food processing.

1.4.6.3.1 Dephosphorylation

Figure 1.50 usesβ-casein as an example to show

that the solubility of a phosphoprotein in the pres-

ence of calcium ions is greatly improved by par-

tial enzymatic dephosphorylation.

1.4.6.3.2 PlasteinReaction........................................

The plastein reaction enables peptide fragments

of a hydrolysate to join enzymatically through

peptide bonds, forming a larger polypeptide of

Fig. 1.50.Solubility ofβ-casein, partially dephospho-

rylated by phosphoprotein phosphatase: Precipitation:

pH 7.1: 2.5mg/ml protein: 10 mmol/LCaCl 2 :35◦C;

1 h. (according toYoshikawaet al., 1974)