1.4 Proteins 73Fig. 1.37.Amino acid contents of a sunflower seed
protein isolate heated in sodium hydroxide solutions at
80 ◦C for 16 h. (according toMauron, 1975)
are markedly decreased with increasing concen-
trations 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-
Table 1.30.Formation ofD-amino acids by alkali treat-
ment of proteinsa(1% solution in 0.1 N NaOH, pH∼
12 .5, temperature 65◦C)Protein HeatingD- D- D- D- D- D- D-
time Asp Ala Val Leu Pro Glu Phe
(h) (%)Casein 0 2. 22. 32. 12. 33. 21. 82. 8
121. 84. 22. 75. 03. 010. 016. 0
330. 213. 36. 17. 05. 317. 422. 2
832. 819. 47. 313. 63. 925. 930. 5
Wheat 0 3. 32. 02. 11. 83. 22. 12. 3
gluten 3 29. 013. 53. 95. 63. 225. 923. 3
Promine D
(soya 0 2. 32. 32. 63. 33. 21. 82. 3
protein) 3 30. 115. 86. 68. 05. 818. 824. 9
Lactal- 0 3. 12. 22. 92. 73. 12. 92. 3
bumin 3 22. 79. 24. 85. 83. 612. 216. 5
aResults in % correspond toD-+L-amino acids =
100%.Fig. 1.38.Formation of lysinoalanine (LAL) by heating
casein (5% solution at 100◦C) (according toSternberg
andKim, 1977) 1 pH 5.0, 2 pH 7.0, 3 pH 8. 0lated 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-