Food Chemistry

86 1 Amino Acids, Peptides, Proteins

Fig. 1.56.Effect of pH on solubility of soy protein and
modified products (1 g/100 ml water). 1 Soy protein,
24 .1% Glu; 2 Plastein 24.8% Glu; 3 Glu-plastein with
41 .9% Glu. (according toYamashitaet al., 1975)

Fig. 1.57.Solubility of soy protein and modified prod-
ucts (800 mg/10 ml water) as a function of heating
time at 100◦C. 1 Soy protein 24.1% Glu; 2 Plastein
24 .8% Glu; 3 Glu-plastein, 41.9% Glu. (according to
Yamashitaet al., 1975)

Elimination of the bitter taste from a protein
hydrolysate is also possible without incorpora-
tion of hydrophilic amino acids. Bitter-tasting
peptides, such as Leu-Phe, which are released by
partial hydrolysis of protein, react preferentially
in the subsequent plastein reaction and are in-
corporated into higher molecular weight peptides
with a neutral taste.
The versatility of the plastein reaction is also

demonstrated by examples wherein undesired
amino acids are removed from a protein.
A phenylalanine-free diet, which can be prepared
by mixing amino acids, is recommended for
certain metabolic defects. However, the use of
a phenylalanine-free higher molecular weight
peptide is more advantageous with respect to
sensory and osmotic properties. Such peptides
can be prepared from protein by the plastein
reaction. First, the protein is partially hydrolyzed
with pepsin. Treatment with pronase under

Table 1.43.Taste of glutamic acid enriched plasteins

Enzyme pH Sub-strateaHydro-lysisbTastec

bitter meat

broth

type

Pepsin 1.5G 67 1 1. 3

P73 4. 51. 0

α-Chymo- 8.0G 48 1 1. 0

trypsin P 72 4. 51. 0

Molsin 3.0G 66 1. 05. 0

P74 1. 31. 3

Pronase 8.0G 66 1. 04. 3

P82 1. 31. 2

aG: Glu-plastein, P: plastein; 1 g/100 ml.

bNsol(10% TCA)/Ntotal(%).

c1: no taste, 5: very strong taste.

suitable conditions then preferentially releases

amino acids with long hydrophobic side chains.

The remaining peptides are separated by gel

chromatography and then subjected to the plas-

tein reaction in the presence of added tyrosine

and tryptophan (Fig. 1.58). This yields a plastein

that is practically phenylalanine-free and has

a predetermined ratio of other amino acids,

including tyrosine (Table 1.44).

The plastein reaction can also be carried out as

a one-step process (Fig. 1.59), thus putting these

reactions to economic, industrial-scale use.

1.4.6.3.3 Cross-Linking

Cross-linking between protein molecules is

achieved with transglutaminase (cf. 2.7.2.4)

and with peroxidase (cf. 2.3.2.2). The cross-

linking occurs between tyrosine residues when

a protein is incubated with peroxidase/H 2 O 2

(cf. Reaction 1.163).

(1.163)