Dairy Chemistry And Biochemistry

392 DAIRY CHEMISTRY AND BIOCHEMISTRY

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Time (min)

Figure 10.8 Schematic representation of hydrolysis and gel formation in renneted milk;

H = hydrolysis of K-casein; V = changes in the viscosity of renneted milk (second stage of

coagulation), G = changes in the viscoelastic modulus (gel formation).

which is slow and cumbersome, or by ion-exchange high performance liquid
chromatography (HPLC). The GMP is soluble in TCA (2-12% depending
on its carbohydrate content) and can be quantified by the Kjeldahl method
or more specifically by determining the concentration of N-acetylneuraminic
acid or by reversed phase HPLC (RP-HPLC).
The activity of rennets can be easily determined using chromogenic
peptide substrates, a number of which are available.

Gel strength (curd tension). The gel network continues to develop for a
considerable period after visible coagulation (Figure 10.8). The strength of
the gel formed, which is very important from the viewpoints of syneresis
(and hence moisture control) and cheese yield, is affected by several factors

• the principal ones are summarized in Figure 10.9.
  The strength of a renneted milk gel can be measured by several types of
  viscometers and penetrometers. As discussed on p. 389, the Formograph
  gives a measure of the gel strength but the data can not be readily converted
  to rheological terms. Penetrometers give valuable information but are
  single-point determinations. Dynamic rheometers are particularly useful,
  allowing the buildup of the gel network to be studied.

Syneresis. Renneted milk gels are quite stable if undisturbed but synerese
(contract), following first-order kinetics, when cut or broken. By controlling
the extent of syneresis, the cheesemaker can control the moisture content of
cheese curd and hence the rate and extent of ripening and the stability of
the cheese - the higher the moisture content, the faster the cheese will ripen