Flavours and Off-Flavours in Milk and Dairy Products - Advanced Dairy Chemistry - page 638

1-13 and 1-14) and C-terminal ( s1 -casein f14-17, 17-21) sequences, which
were found to be bitter in taste (Richardson and Creamer, 1973; Lee et al.,
1996). The large peptide produced by chymosin, s1 -casein f24-199, is further
hydrolyzed by chymosin and CEP (for further details see McSweeney et al.,
1994; Singh et al., 1995, 1997).

14.2.3.5.2. Hydrolysis of -Casein

Chymosin has limited action on -casein in Cheddar, although some
activity is indicated by the presence of the peptide -casein f1-192 (McSweeney
et al., 1994). Hydrolysis of the bond Leu 192 -Tyr 193 of -casein by chymosin
releases a small C-terminal fragment, -casein f193-209, which is extremely
bitter (Singh et al., 2004b). Nearly half of the -casein in Cheddar cheese is
hydrolyzed during ripening by plasmin, an indigenous milk proteinase. Plasmin
hydrolysis of -casein results in the formation of three -caseins [ 1 -( -casein
f29-209), 2 -( -casein f106-209) and 3 -( -casein f108-209)], representing
the C-terminal region, and five proteose-peptones ( -casein f1-28, - casein
1-105/107 and -casein f29-105/107), representing the corresponding
N-terminal region. The -caseins seem to accumulate in Cheddar over the
ripening period. The proteose-peptones are extensively hydrolyzed by the
starter bacterial CEP and peptidases to produce small peptides and free
amino acids (Singh et al., 1995, 1997).
Proteolysis in cheese seems to be a sequential process involving enzymes
from rennet, milk proteinases (particularly plasmin), the starter culture,
secondary microorganisms and non-starter lactic acid bacteria (NSLAB):

The hydrolysis of casein to high molecular weight peptides is thought
to be primarily the result of chymosin and plasmin.
The subsequent hydrolysis of high molecular weight peptides into
small peptides and free amino acids is primarily the result of proteo-
lytic enzymes from lactic acid bacteria (Singh et al., 2003a).

Proteolytic degradation of caseins into peptides in Cheddar (Singh
et al., 1994, 1995, 1997; Fernandez et al., 1998), Emmental (Gagnaire et al.,
2001) and Parmigiano-Reggiano (Addeo et al., 1992, 1994) cheeses has been
characterized in detail.

14.2.3.6. Catabolism of Amino Acids

In lactococci, the first step in the degradation of amino acids is transa-
mination (Figure 14.7; Gao et al., 1997), leading to the formation of -keto
acids ( -KAs). Aromatic aminotransferases have been characterized from
L. lactis ssp cremoris (Yvon et al., 1997; Rijnen et al., 1999a) and L. lactis ssp