Flavours and Off-Flavours in Milk and Dairy Products - Advanced Dairy Chemistry - page 637

Starter and non-starter bacterial enzymes
a) Cell envelope-associated proteinases (lactocepins)
b) Peptidases (including endopeptidases, aminopeptidases, di-/tri-
peptidases and proline-specific peptidases)

Secondary starter enzymes
Exogenous enzymes used as ripening aids

Enzymes from the first three sources are active in most ripened cheeses.
The secondary starters (microorganisms added to cheese milk or curd for
purposes other than acidification, e.g., surface smear organisms, blue/white
moulds) exert considerable influence on the maturation of cheese varieties in
which they are used. Exogenous enzymes, when present can be very

influential.
The correct pattern of proteolysis is generally considered to be a pre-
requisite for the development of the correct flavour of cheese. Products of
proteolysis per se (i.e., peptides and free amino acids) probably are significant
in cheese taste, at least to 'background' flavour, and some off-flavours, e.g.,
bitterness, but are unlikely to contribute aroma. Compounds arising from the
catabolism of amino acids contribute directly to cheese taste and aroma.

14.2.3.5.1. Hydrolysis of s1 -Casein
In the cheese environment, with a high ionic strength and a low a w ,
rennet-induced breakdown of s1 -casein proceeds much faster than that of
-casein ( s2 - and para- -caseins are quite resistant to hydrolysis by the
enzymes of rennet) (Visser, 1993). The residual chymosin rapidly hydrolyses
s1 -casein at the Phe 23 -Phe 24 bond during the initial stages of ripening
(Creamer and Richardson, 1974). Hydrolysis of this single bond of s1 -casein
causes a rapid change in the rubbery texture of young Cheddar curd into a
smoother, more homogeneous product (Lawrence et al., 1987). However,
more recent work of O'Mahony et al. (2005) showed that the initial softening
of texture is more highly correlated with the level of insoluble calcium than
with the level of intact S1 -casein in cheeses early in ripening. The authors of
this study concluded that the hydrolysis of S1 -casein at Phe 23 -Phe 24 is not a
prerequisite for softening which is due principally to the partial solubilization
of colloidal calcium phosphate associated with the para- -casein matrix of
Cheddar cheese during the early stages of ripening. The peptide s1 -casein
f1-23, produced by chymosin action on the bond Phe 23 -Phe 24 of s1 -casein, is
further hydrolyzed in Cheddar cheese (Singh et al., 1994) by the CEP from
starter L. lactis ssp. cremoris, resulting in the production of a range of small
molecular weight peptides, representing N-terminal ( s1 -casein f1-7, 1-9,