Dairy Chemistry And Biochemistry

412 DAIRY CHEMISTRY AND BIOCHEMISTRY

CASEINS, CASEIN-DERIVED PEPTIDES

I

PEPTIDES

Pepo

pyro-GI u-Lys-Ala-Glx-Gly-Pro-Leu-Leu-Leu-Pro-His-Phe J.

PIP

JL 4.

PCP

pyro-Glu.Lyr-Ala-Glx-Gly-Pro-Leu~Leu~Leu i Pro-His-Phe

Lys- pTN Ala-Glx-Gly-Pro-Leu-Leu-Leu

Glx-GI 7" y-Pro-Leu-Leu-Leu

PepN

*lfGix-Oly-PR1-Leu-Leu.Leu

PepX

Gly-Pro-Leu-Leu-Leu i

Gly-Pro pr J :L

DIP

Leu-Leu^4

Figure 10.22 Schematic representation of the hydrolysis of casein (a) by lactococcal cell
envelope proteinase (CEP), and (b) degradation of an hypothetical dodecapeptide by the
combined action of lactococcal peptidases: oligopeptidase (PepO), various aminopeptidases
(PCP, PepN, PepA, PepX), tripeptidase (TRP), prolidase (PRD) and dipeptidase (DIP).

are mainly responsible for primary proteolysis, i.e. the formation of water
(or pH 4.6)-soluble N, as summarized in Figure 10.20.
Although in vim, the cell wall-associated proteinase of the Lactococcus
starters is quite active on /?-casein (and that from some strains on a,,-casein
also), in cheese, they appear to act mainly on casein-derived peptides,
produced by chymosin from a,,-casein or by plasmin from /?-casein.
The starter cells begin to die off at the end of curd manufacture (Figure
10.21); the dead cells may lyse and release their intracellular endopeptidases
(Pep 0, Pep F), arninopeptidases (including Pep N, Pep A, Pep C, Pep X),
tripeptidases and dipeptidases (including proline-specific peptidases) which
produce a range of free amino acids (Figure 10.22). About 150 peptides have