2009_Book_FoodChemistry

1.4 Proteins 83
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).
Fig. 1.47. Viscosity curves during reduction of different
wheat glutens. For sample designation see Fig. 1.44.
(according to Lasztity, 1975)
modified by reduction of its disulfide bonds
to sulfhydryl groups and subsequent reoxidation
of these groups under various conditions
(Fig. 1.48). Reoxidation of a diluted suspen-
1.4.6.3 Enzymatic Modification
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 uses β-casein as an example to show
that the solubility of a phosphoprotein in the presence
of calcium ions is greatly improved by partial
enzymatic dephosphorylation.
1.4.6.3.2 Plastein Reaction
The plastein reaction enables peptide fragments
of a hydrolysate to join enzymatically through
peptide bonds, forming a larger polypeptide of
Fig. 1.48. Reaction of proteins with D,L-alanine carboxy
anhydride. (according to Sela et al., 1962 and St.
Angelo et al., 1966)
Fig. 1.49. Covalent binding of lysine to gluten (according
to Li-Chan et al., 1979) and of methionine or
tryptophan to soya protein (according to Voutsinas and
Nakai, 1979), by applying a carbodiimide procedure
Fig. 1.50. Solubility of β-casein, partially dephosphorylated
by phosphoprotein phosphatase: Precipitation:
pH 7.1: 2.5mg/ml protein: 10 mmol/L CaCl 2 :35 ◦ C;
1 h. (according to Yoshikawa et al., 1974)