Chemistry Reference
In-Depth Information
9.7.1.
Rennet-Induced Gels
Generally, it is thought that Ca does not directly affect the enzymatic
phase, although addition of CaCl
2
does reduce milk pH, which accelerates the
hydrolysis reaction (Lucey and Fox, 1993). Rennet-altered micelles will
aggregate only in the presence of Ca
2+
, and gelation occurs only if there is
sufficient CCP present (i.e. needs some type of casein micellar structure, as
sodium caseinate does not form a rennet-induced gel, even though there is
release of the macropeptide). Addition of (
<
50 mM) Ca reduces the rennet
coagulation time, even at a constant milk pH, and flocculation occurs at a
lower degree of
-casein hydrolysis. Addition of Ca increases the rate of
firming of renneted milk gels, mainly by neutralization of the negatively
charged groups on the micelle surface and possibly by the formation of Ca
bridges. Addition of high concentrations of Ca (e.g.
>
0.1 M) reduces the rate
of gel firming, probably by increasing the effective (positive) surface charge
on the micelles. Addition of up to 10 mM Ca increases the strength of rennet-
induced gels (Lucey and Fox, 1993). Low levels (
0.02%) of CaCl
2
are often
added by cheesemakers to help standardize the coagulation process (e.g.
cutting time).
Reduction of the CCP content of casein micelles by
30% prevents
coagulation unless [Ca
2+
] is increased (Shalabi and Fox, 1982). Udabage et al.
(2001) investigated the effects of mineral salts and Ca sequestrants or chelat-
ing agents on the gelation of renneted skim milk. They found that depending
on the level of chelating agent, addition of citrate or ethylenediaminetetraa-
cetic acid (EDTA) reduced the storage modulus (G
0
) of rennet-induced gels
and above a certain concentration rennet gelation was inhibited completely
(10 mmol/kg milk).
Choi et al. (2007) demonstrated that the concentration of insoluble Ca
phosphate (CCP) associated with the casein micelles had an important influence
on the properties of rennet-induced gels. Removal of some CCP from milk prior
to gelation using a Ca-chelator lowered the storage modulus of rennet-induced
gels due to the reduction in the amount of CCP crosslinking in casein micelles.
Reduction in the CCP content prior to rennet-induced gelation resulted in gels
with higher loss tangent values, indicating greater bond mobility.
The swelling, hydration and solubility of casein micelles in renneted
milk are greatly increased in the presence of NaCl but markedly reduced if the
brine solution contains Ca (Lucey and Fox, 1993). The addition of high
concentrations of NaCl causes a reduction in rennet coagulation time. In
some cheese varieties, salt is added to the cheesemilk (e.g. Domiati) resulting
in a slower set and weaker curd (Fahmi and Shahara, 1950). These changes
are largely reversible on removal of the excess NaCl by exhaustive dialysis
against bulk milk (Huppertz, 2007).
