42 Handbook of Food Science and Technology 3
To correct for variations in the calcium content of milk during the lactation
stage or changes in the calcium balance between
the soluble and colloidal
phase due to the effects of refrigeration or heat treatment, manufacturers add
CaCl
2
at a dose ranging usually from 80 to 200 mg L
-1
of milk, which
improves the coagulation properties of the milk.
To meet the processing time requirements (rennet
clotting time, rate of curd
formation, hardening time) and the desired mineral content in the curd, which
depend on the type of cheese desired, manufacturers
adjust the pH rennet
added into milk by fermentation (lactic starters), the addition of glucono-
δ
-
lactone,
the injection of CO
2
, or the addition of acid whey proteins.
For certain types of cheeses, the lactose content
of milk is lowered by
washing the curd (in medium-hard cheeses) or by ultrafiltration of the milk
followed by diafiltration before coagulation (in “stabilized” soft cheeses). The
partial removal of lactose slows down the activity of lactic bacteria and is a
mean of controlling the pH of the curd at the end of the acidification.
Biological standardization of cheese milk consists of the elimination of the
endogenous microorganisms in the milk
that may be undesirable
(psychrotrophs, pathogens) by heat treatment, bactofugation or microfiltration,
followed by the addition of a controlled starter culture; prematuration at a low
temperature (10–12
o
C, 12–18 h) by promoting the production of growth
factors, improves the lactic fermentation process.
1.3.4.2.
Coagulation
There are three types of coagulation (Figure 1.20).
Acid coagulation
Acid coagulation involves the precipitation of casein at its isoelectric point
(pI = 4.6) by biological acidification using lactic acid bacteria to transform
lactose to lactic acid, or by chemical acidification (injection of CO
2
, addition
of glucono-
δ
-lactone or addition of acid whey proteins). The chemical method
(organic acid) is mainly used to standardize the pH of milk before renneting,
while the addition of mineral acid is usually not permitted.
Gel formed by the solubilization of colloidal
calcium phosphate during
acidification has good permeability but high friability; the lack of structure in
From Milk to Dairy Products 43
the network (low energy hydrophobic interactions) results in almost zero
elasticity and plasticity as well as low resistance to mechanical treatment.
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