44 Handbook of Food Science and Technology 3
Several
factors influence coagulation, such as enzyme concentration,
temperature, pH, calcium content, casein composition, micelle size and pre-
treatment
of milk such as cooling, heat treatment and homogenization.
The network formed at pH 6.6 is rich in minerals, given the numerous
interactions between calcium and casein at this pH; this type of gel tends to
contract, which leads to an expulsion of whey.
Mixed coagulation
Mixed coagulation results from the combined
action of rennet and
acidification. The variety of combinations, resulting in casein micelles with
different mineral content when gelation occurs and whey is drawn off the curd,
is the source of a wide range of soft cheeses, semi-soft cheeses and medium-
hard cheeses.
1.3.4.3.
Draining
Lactic acid and rennet gels
The draining stage involves removing some of the whey trapped in the gel
network formed by acidification and/or enzymatic action.
It begins in the
coagulation tanks and continues in the moulds and finally in the cheese-
ripening rooms. It is possible to express the whey flow rate based on Darcy’s
law (see Chapter 3, Volume 2):
A
V
R
P
η
•
Δ
=
[1.4]
where
Δ
P
is the differential pressure exerted on the gel (Pa),
η
is the viscosity
of the whey (Pa s), R is the hydrodynamic resistance of the gel
(m
-1
) and A is the surface area of the gel (m
2
).
As a result, whey removal depends on:
– the type of curd, which affects permeability in particular (
1
R
term
of [1.4]); gel porosity decreases during acidification (increase in R), but this is
offset by the reduced water retention capacity of
protein close to its pI;
– extent of the mechanical and thermal treatments applied on the curd in
the tank, which involves slicing the curd (increase in A), subsequent stirring to
From Milk to Dairy Products 45
avoid sticking (maintaining A) and heating (decrease in
η
; energy supply
strengthens the protein network and contractability of the gel resulting in an
increase in
Δ
P
);
– the pressing stage after molding (increase in
Δ
P
), which removes the
remaining whey and strengthens the cohesion of the curd in the case of hard
cheeses.
The kinetics of the removal of whey from the mould can be described by
equation [6.9] (see Chapter 6, Volume 2). In this case,
flow resistance
increases due to the obstruction of the mould perforations by the curd grains,
making it necessary to turn the cheese regularly so as to promote draining.
The natural drainage of a lactic gel is slow and limited. It results in a
heterogeneous curd with a low dry matter and mineral content: the weakly
cross-linked network contracts only slightly. Processes such as centrifugation
or ultrafiltration of the curd can significantly increase drainage compared with
traditional methods (strainer, bag or filter drainage). Subjecting milk and/or
acid gel to intense heat treatment (80–95°C for several minutes) can increase
cheese yield by denaturation and retention of whey proteins. However, heat
treatment, as well as homogenization, limits the rate and intensity of drainage.
Rennet gel has strong cohesion,
elasticity and porosity, but low
permeability leading to limited natural drainage. As a result, it is necessary to
carry out different operations in the tank (slicing, mixing, slow and steady
heating up to 56°C for hard cheeses) to allow drainage of the gel. The higher
the dry matter content required, the more intense these processing steps
become; however, they also reduce cheese yield and the recovery coefficients
of the cheese components.
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