Casein, Caseinates, and Milk Protein Concentrates 165eventually forming a viscoelastic, three -
dimensional gel (Ennis and Mulvihill 1999 ).
The rennet coagulum consists of casein,
whey protein, fat, lactose, and the minerals
of the milk, and has a fl uffi er and spongier
texture than acid precipitated casein.Wet Processing of Casein Curd
Once the casein has been precipitated/
coagulated it is broken up and then heated
(60 ° C to 75 ° C; 140 ° F to 167 ° F for up to 30
minutes) (Ennis and Mulvihill 1999 ). Heating
the precipitated casein expels excess mois-
ture and causes the particles to shrink and
agglomerate together to form clumps of curd.
The curd is separated from the whey and
washed several times in water (30 ° C to 40 ° C;
86 ° F to 104 ° F) (Ennis and Mulvihill 1999 ).
The curd is then mechanically pressed or
centrifuged to approximately 55% moisture
content (Ennis and Mulvihill 1999 ).Drying and Dry - processing Operations
The pressed casein curd is dried by hot air
(approximately 130 ° C; 266 ° F) (e.g., using
fl uidized beds) to approximately 10% to
12% moisture content (Ennis and Mulvihill
1999 ). The warm, “ dry ” casein is then sub-
jected to a series of dry - processing steps
that involve cooling, tempering (to ensure
the homogenous distribution of moisture),
milling, sifting (to produce coarse, medium,
and fi ne casein particles), and blending (to
ensure homogeneity) (Ennis and Mulvihill
1999 , Southward 1998 ). The casein is then
bagged and either placed in storage or shipped
immediately.
The yield of commercial casein using
these processes is typically approximately
3 kg/100 kg skim milk (Southward 1998 ).
Most of the acid casein is converted to a
water - soluble salt form (i.e., caseinate). The
rennet casein that is produced is used on an
as - is basis.chloric) at 20 ° C (68 ° F), to pH 4.6 (Southward
1998 ). The casein precipitates as fi ne, dis-
crete particles in a whey medium. An alterna-
tive acid for precipitation is lactic acid; the
pasteurized skim milk is cooled to approxi-
mately 22 ° C to 26 ° C (71.6 ° F to 78.8 ° F) and
then inoculated with lactic acid starter cul-
tures (e.g., Lactococcus lactis cremoris ) at
0.1% to 0.2% milk volume and incubated
without agitation for 14 to 16 hours
(Southward 1998 ). During incubation, the
starter cultures ferment some of the lactose
in milk to lactic acid and the pH decrease (pH
approximately 4.6) causes coagulation of the
milk. A soft casein gel (i.e., coagulum) is
produced.
Rennet - induced coagulation: In the
manufacture of rennet casein, the caseins are
coagulated through the action of rennet. In
contrast to the process used to prepare acid
casein there is no change in the pH of the
milk prepared by enzyme coagulation.
Chymosin, the principal protease in bovine
rennet, or various other types of proteases
(e.g., microbial rennets, fermentation -
produced chymosin) coagulate milk in a two -
stage process. The primary stage involves the
rennet - specifi c hydrolysis of the Phe 105 -
Met 106 peptide bond of κ - casein to release the
casein macropeptide (CMP) into the sur-
rounding whey (Southward 1998 ). This
results in a decrease in zeta - potential and loss
of casein micelle steric stabiliza tion. The
remaining (N - terminal) κ - casein, referred to
as para - κ - casein, is unable to protect the
casein - sensitive caseins, and the caseins
coagulate (i.e., clot) in the presence of
calcium ions (Southward 1998 ).
When approximately 80% of the κ - casein
has been hydrolyzed, the second stage of
coagulation occurs (Fox and Kelly 2004 ).
During this stage, under quiescent condi-
tions, and providing a critical concentration
of calcium is present and the temperature is
above 18 ° C (64.4 ° F), the CMP - depleted
micelles aggregate into chains or clumps,