Physical Chemistry of Foods

It may be added that not all variables have been discussed, such as pH,
ionic strength, and solvent quality, or the time scale of deformation. See
Figure 17.14c for the effects of formation temperature and measurement
temperature, which are opposite in a sense.
A generalconclusionmay be that fractal particle gels follow some
simple scaling laws that provide much insight. However, it needs much more
than a knowledge of the values ofj,D, andaeffto predict quantitatively the
mechanical properties. For every gel type, careful study at a range of
conditions is needed to obtain a full picture.

Question

Consider the making of curd (green cheese) from milk. The milk is made to gel by the
addition of rennet. Then whey has to be removed by syneresis. What measures can
you think of to enhance the rate of syneresis?

Answer

As is mentioned above, the syneresis rate increases with increasing temperature and
decreasing pH. A look at Eq. (17.17) shows that the total outflow of whey is
proportional to the surface area of the gel and inversely proportional to the distance
over which the whey has to flow. Cutting the gel into small pieces thus very much
enhances the syneresis rate (in fact, syneresis is almost imperceptible before the gel is
cut). The equation contains the endogenous syneresis pressurepsyn. However, for a
system that can show structural rearrangement leading to syneresis, any externally

TABLE17.4 Some Properties of Various Casein Gels Made of Skim Milk

Property Acid gel, type 1a Acid gel, type 2b Rennet gelc

Strands are Hinged Stretched Stretched

PermeabilityB/mm^2 0.15 0.15 0.25

Fractal dimensionalityD 2.35 2.4 2.25

Syneresis no no yes

ModulusG^0 /Pa 20 180 30

Fracture stresssfr/Pa 100 100 10

Fracture strainefr 1.1 0.5 2–3d

aMade by slow acidification at 30 8 C.

bMade by acidification at 2 8 C and slowly heating to 30 8 C.

cMade by renneting at 30 8 C.

dDepends substantially on strain rate.

Approximate results for well-developed gels tested at 30 8 C.