2009_Book_FoodChemistry

1.4 Proteins 73
Table 1.30. Formation of D-amino acids by alkali treatment
of proteins a (1% solution in 0.1 N NaOH, pH ∼
12.5, temperature 65 ◦ C)
Protein Heating D- D- D- D- D- D- D-
time Asp Ala Val Leu Pro Glu Phe
(h) (%)
Casein 0 2.2 2.3 2.1 2.3 3.2 1.8 2.8
1 21.8 4.2 2.7 5.0 3.010.016.0
3 30.213.3 6.1 7.0 5.317.422.2
8 32.819.4 7.313.6 3.925.930.5
Wheat 0 3.3 2.0 2.1 1.8 3.2 2.1 2.3
gluten 3 29.013.5 3.9 5.6 3.225.923.3
Promine D
(soya 0 2.3 2.3 2.6 3.3 3.2 1.8 2.3
protein) 3 30.115.8 6.6 8.0 5.818.824.9
Lactal- 0 3.1 2.2 2.9 2.7 3.1 2.9 2.3
bumin 3 22.7 9.2 4.8 5.8 3.612.216.5
a Results in % correspond to D- + L-amino acids =
100%.
Fig. 1.37. Amino acid contents of a sunflower seed
protein isolate heated in sodium hydroxide solutions at
80 ◦ C for 16 h. (according to Mauron, 1975)
are markedly decreased with increasing concentrations
of NaOH. New amino acids (ornithine
and alloisoleucine) are formed. Initially, lysine
concentration decreases, but increases at higher
concentrations of alkali. Lysinoalanine behaves in
the opposite manner. The extent of formation of
D-amino acids as a result of alkaline treatment of
proteins is shown in Table 1.30.
Data presented in Figs. 1.38 and 1.39 clearly
show that the formation of lysinoalanine is
influenced not only by pH but also by the protein
source. An extensive reaction occurs in casein
even at pH 5.0 due to the presence of phosphory-
Fig. 1.38. Formation of lysinoalanine (LAL) by heating
casein (5% solution at 100 ◦ C) (according to Sternberg
and Kim, 1977) 1 pH 5.0, 2 pH 7.0, 3 pH 8.0
lated serine residues, while noticeable reactions
occur in gluten from wheat or in zein from
corn only in the pH range of 8–11. Figure 1.40
illustrates the dependence of the reaction on
protein concentration.
Table 1.31 lists the contents of lysinoalanine in
food products processed industrially or prepared
under the “usual household conditions”.
The contents are obviously affected by the food
type and by the processing conditions.
In the radiation of food, o-hydroxyphenylalanine
called o-tyrosine is formed through the re-