2009_Book_FoodChemistry

86 1 Amino Acids, Peptides, Proteins
Table 1.43. Taste of glutamic acid enriched plasteins
Enzyme
pH Sub-strate a Hydro-lysis b Taste c
bitter meat
broth
type
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 to Yamashita et al., 1975)
Pepsin 1.5 G 67 1 1.3
P 73 4.5 1.0
α-Chymo- 8.0 G 48 1 1.0
trypsin P 72 4.5 1.0
Molsin 3.0 G 66 1.0 5.0
P 74 1.3 1.3
Pronase 8.0 G 66 1.0 4.3
P 82 1.3 1.2
a G: Glu-plastein, P: plastein; 1 g/100 ml.
b N sol (10% TCA)/N total (%).
c 1: no taste, 5: very strong taste.
Fig. 1.57. Solubility of soy protein and modified products
(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
Yamashita et al., 1975)
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 plastein
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.
Elimination of the bitter taste from a protein
hydrolysate is also possible without incorporation
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 incorporated
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
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 crosslinking
occurs between tyrosine residues when
a protein is incubated with peroxidase/H 2 O 2
(cf. Reaction 1.163).
(1.163)