2009_Book_FoodChemistry

1.4 Proteins 81
For example, succinylated wheat gluten is quite
soluble at pH 5 (cf. Fig. 1.40). This effect is
related to disaggregation of high molecular
weight gluten fractions (cf. Fig. 1.41). In the
case of succinylated casein it is obvious that the
modification shifts the isoelectric point of the
protein (and thereby the solubility minimum) to
a lower pH (cf. Fig. 1.42). Succinylation of leaf
proteins improves the solubility as well as the
flavor and emulsifying properties.
Succinylated yeast protein has not only an increased
solubility in the pH range of 4–6, but
is also more heat stable above pH 5. It has better
emulsifying properties, surpassing many other
proteins (Table 1.37), and has increased whippability.
Introduction of aminoacyl groups into protein can
be achieved by reactions involving amino acid
carboxy anhydrides (Fig. 1.44), amino acids and
carbodiimides (Fig. 1.46) or by BOC-amino acid
hydroxysuccinimides with subsequent removal
of the aminoprotecting group (BOC) (cf. 1.161):
Fig. 1.42. Solubility of succinylated wheat protein as
a function of pH (0.5% solution in water). (according
to Grant, 1973)
Fig. 1.43. Gel column chromatography of an acetic acid
(0.2mol/L) wheat protein extract. Column: Sephadex
G-100 (— before and - - - after succinylation). (according
to Grant, 1973)
(1.161)
Feeding tests with casein with attached methionine,
as produced by the above method,
have demonstrated a satisfactory availability
of methionine (Table 1.38). Such covalent
attachment of essential amino acids to a protein
may avoid the problems associated with food
supplementation with free amino acids: losses in
Fig. 1.44. Solubilities of native (—) and succinylated
casein (— 50% and −·−·−76%) as a function of pH.
(according to Schwenke et al., 1977)
processing, development of undesired aroma due
to methional, etc.
Table 1.39, using β-casein as an example, shows
to what extent the association of a protein is affected
by its acylation with fatty acids of various
chain lengths.