Handbook of Food Science and Technology 3

Figure 1.20.
 Types of coagulation and categories of cheese. For a color
version of this figure, see www.iste.co.uk/jeantet/foodscience.zip 
Enzymatic coagulation 
Enzymatic coagulation is used to transform milk from a liquid to a gel state 
through the action of proteolytic enzymes, mostly of animal origin. 
There are three phases: 
– primary or enzymatic phase, corresponding to the hydrolysis of 
κ
-casein 
at the bond between phenylalanine (105) and methionine (106); 
– secondary phase or aggregation of hydrolyzed micelles, which, at pH 6.6, 
begins when 80 – 90% of the 
κ
-casein is hydrolyzed; 
– tertiary or cross-linking phase leading to gel formation. 

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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

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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.