242 Chapter 10
Pasteurization of rennet destroys its
activity.- The amount and structure of calcium in
milk alter rennet activity. In general,
Ca^2 +^ ions accelerate clotting time and
lead to the formation of fi rmer coagu-
lum. It is customary to add 0.02% CaCl 2
to milk to assist in rapid coagulation. Too
much calcium may cause corky body
and other defects in cheese. Mastitis
milk gives poor coagulation and should
be avoided. - Dilution of milk or high - fat content leads
to soft and fragile coagulum. - The pH of milk infl uences clotting time.
Milk with lower pH (higher titratable
acidity) clots faster. Rennet is more
soluble at the lower pH developed during
cooking of cheddar cheese curd (ched-
daring process). Rennet retention is
enhanced in curd under these conditions.
Microbial rennet ’ s solubility or retention
is not infl uenced by low - draining pH. - Rennet is inactivated rapidly by alkaline
pH values greater than 7. Because cheese
color is normally alkaline, mixing rennet
and color extract causes coagulation
problems. Similarly, chlorinated water
is inhibitory to rennet. As much as
40% of rennet activity is destroyed by
three - minute exposure to 2 ppm of chlo-
rine in water. Rennet should be diluted
with 20 to 40 volumes of non - alkaline
water immediately prior to addition
to the vat. Agitating the vat for two
to three minutes should disperse the
enzyme thoroughly. - Agitation and vibration of milk during
rennet setting/clotting leads to an unde-
sirable weak curd. - Rennet should be stored in cold and dark
place to preserve its activity.
Acid Coagulation
Acid coagulation, also known as isoelectric
precipitation of casein, is used in ricotta,to survive high cooking conditions. Also,
they end up in whey and may interfere with
the functional properties of whey products;
consequently, appropriate heat destruction
procedures are necessary to ensure that the
whey products are rennet free.
Handling and Use of Rennet in the
Cheese Plant
It is necessary to understand the characteris-
tics of rennet and its behavior under various
conditions to achieve consistent coagulant
activity, curd character, and cheese quality:
- Heat - treatment of milk more severe than
pasteurization causes heat denaturation
and interaction of whey proteins with
caseins. The heat - modifi ed proteins do
not form a strong gel. Therefore, high -
heat treatment is detrimental for optimum
coagulation and curd strength. - Homogenization of milk causes consid-
erable changes in the reaction of milk to
rennet as well as acid. The curd becomes
whiter in color and more moisture is
retained. Whey separation from the curd
is reduced, and fi ner (softer) gel is pro-
duced due to the fi ner fat globule size. In
soft cheeses such as cream cheese,
homogenization is desirable because it
enhances fat recovery in the cheese curd.
However, in hard cheeses, it produces
curd with a rubbery texture. - The optimum coagulation temperature
for most cheeses is 30 ° C to 32 ° C (86 to
89.6 ° F). For Swiss varieties, it is 37 ° C
(99 ° F). The curd is relatively fragile at
temperatures below 30 ° C (86 ° F), thus
making the curd - cutting step ineffi cient.
Curd losses as fi nes are observed and the
cheese yield is lowered. At temperatures
below 20 ° C (68 ° F), coagulation is pro-
gressively weak but it is regained upon
warming to 30 ° C (86 ° F). No discernible
enzymatic or even acid coagulation of
milk is observed at 4 ° C (39 ° F). Again,
milk coagulation sets in on warming.