Food Biochemistry and Food Processing

10 Chymosin in Cheese Making 247

The macropeptide, also known as caseinmacro-
peptide, contains approximately 30% amino sugar,
hence the name glycomacropeptide (Lucey 2003).
In the ultrafiltration processes of cheese making, this
macropeptide is retained with whey proteins to in-
crease cheese yields significantly. In the conventional
cheese-making process, the macropeptide is found
in whey.
The two-step coagulation does not fully explain
the presence of the smooth curd or coagulum. Beau,
many years ago, established that a strong milk gel
arose because the fibrous filaments of paracasein
cross-linked to make a lattice. Bonding at critical
points between phosphorus, calcium, free amino
groups, and free carboxyl groups strengthened the
lattice, with its entrapped lactose and soluble salts.
A parallel was drawn between the gel formed when
only lactic acid was involved as in fresh, unripened
cheese and when considerable chymosin was in-
volved as in hard, ripened cheese. Both gels were
considered originally as starting with a fibrous pro-
tein cross bonding, but the strictly lactic acid curd
was considered weaker, because not having been hy-
drolyzed by a proteolytic enzyme, it possessed less
bonding material. The essential conditions for a
smooth gel developing in either case were sufficient
casein, a quiescent environment, moderate tempera-
ture, and sufficient time for reaction.

FACTORS AFFECTING
CHYMOSIN ACTION IN MILK

Rennet coagulation in milk is influenced by many
factors that ultimately have an impact on cheese
characteristics (Kosikowski and Mistry 1997). The
cheese maker is usually able to properly control
these factors, some of which have a direct affect on
the primary phase and others on the secondary.

CHYMOSINCONCENTRATION

Chymosin concentration affects gel firmness (Lom-

holt and Qvist 1999), and enzyme kinetics have

been used to study these effects. An increase in the

chymosin concentration (larger amounts of rennet

added to milk) reduces the total time required for

rennet clotting, measured by the appearance of the

first curd particle. As a result, the secondary phase

of rennet action will also proceed much earlier, with

the net result of an increase in the rate of increase in

gel firmness. This property of chymosin is some-

times used to control the rennet coagulation in con-

centrated milks, which have a tendency to form

firmer curd because of the higher protein content.

Lowering the amount of rennet reduces the rate of

curd firming. It should also be noted that with an

alteration in the amount of rennet added to milk,

there also will be a change in the amount of residual

chymosin in the cheese.

TEMPERATURE

The optimal condition for curd formation in milk

with chymosin is 40–45°C, but this temperature is

not suitable for cheese making. Rennet coagulation

for cheese making generally occurs at 30—35°C for

proper firmness. At lower temperatures rennet clot-

ting rate is significantly reduced, and at refrigeration

temperatures virtually no curd is obtained.

PH

Chymosin action in milk is optimal at approximate-

ly pH 6. This is obtained with starter bacteria. If the

pH is lowered further rennet clotting occurs at a

faster rate, and curd firmness is reduced. Lowering

pH also changes the water-holding capacity, which

will have an impact on curd firmness.

Figure 10.1.Destabilization of the casein micelle by introduction of chymosin.