BLBS102-c11 BLBS102-Simpson March 21, 2012 13:9 Trim: 276mm X 219mm Printer Name: Yet to Come
11 Chymosin in Cheese Making 229The amount of rennet required to coagulate milk within
30 minutes can be considerably below that required to prop-
erly break down the paracaseins during cheese ripening. Conse-
quently, using too little rennet in cheese making retards ripening,
as discerned by the appearance of the cheese and its end products.
A lively, almost translucent looking cheese, after ripening sev-
eral months, indicates adequate rennet; an opaque, dull-looking
cheese indicates inadequate amounts. Apparently, conversion to
peptides and later hydrolysis to smaller molecules by bacterial
enzymes give the translucent quality. Residual rennet in cheese
hydrolyzes alpha-s-1-casein to alpha-s-1-I-casein, which leads
to a desirable soft texture in the aged cheese (Lawrence et al.
1987).
Electrophoretic techniques (Ledford et al. 1966) have demon-
strated that in most ripening Cheddar cheese from milk coag-
ulated by rennet,para-beta-casein largely remains intact while
para-alpha-casein is highly degraded. According to Fox et al.
(1993), if there is too much moisture in cheese or too little
salt, the residual chymosin will produce bitter peptides due
to excessive proteolysis. In cheeses with high levels of beta-
lactoglobulin, as in cheeses made by ultrafiltration, proteolysis
by residual chymosin is retarded because of partial inhibition of
chymosin by the whey protein (Kosikowski and Mistry 1997).
Edwards (1969) found that milk-coagulating enzymes from
the mucors hydrolyze the paracaseins somewhat similarly to
rennet, while enzymes fromCr. parasiticaand the papaya plant
completely hydrolyze para-alpha- and para-beta-caseins during
ripening.
Proteolytic activity of rennet substitutes, especially microbial
and fungal substitutes, is greater than that of chymosin. Cheese
yield losses, though small, do occur with these rennets. Barbano
and Rasmussen (1992) reported that fat and protein losses to
whey were higher withRh. mieheiandRh. pusilluscompared
with those for fermentation-derived chymosin. As a result, the
cheese yield efficiency was higher with the latter than with the
former two microbial rennets.EFFECT OF CHYMOSIN ON CHEESE
TEXTUREThe formation of a rennet curd is the beginning of the formation
of a cheese mass. Thus, a ripened cheese assumes its initial
biological identity in the vat or press. This is where practically all
of the critical components are assembled and the young cheese
becomes ready for further development.
Undisturbed fresh rennet curd is not cheese because it still
holds considerable amounts of water and soluble constituents
that must be removed before any resemblance to a cheese oc-
curs. This block of curd is cut into thousands of small cubes.
Traditionally, this is done with wire knives (Fig. 11.2) but in
large automated vats, built-in blades rotate in one direction at a
selected speed to systematically cut the curd. Then, the whey,
carrying with it lactose, whey proteins, and soluble salts, streams
from the cut pores, accelerated by gentle agitation, slowly in-
creasing temperature, and a rising rate of lactic acid production.
These factors are manipulated to the degree desired by the cheese
maker. Eventually, the free whey is separated from curd.Figure 11.2.Cutting of rennet curd.Optimum acid development is essential for forming rennet
curd and creating the desired cheese mass. This requires viable,
active microbial starters to be added to milk before the addi-
tion of rennet. These bacteria should survive the cheese making
process.
Beta-D-galactosidase (lactase) and phosphorylating enzymes
of the starter bacteria hydrolyze lactose and initiate the glucose-
phosphate energy cycles for the ultimate production of lactic
acid through various intermediary compounds. Lactose is hy-
drolyzed to glucose and galactose. Simultaneously, the galac-
tose is converted in the milk to glucose, from which point lactic
acid is produced by a rather involved glycolytic pathway. In
most cheese milks, the conversion of glucose to lactic acid is
conducted homofermentatively by lactic acid bacteria, and the
conversions provide energy for the bacteria. For each major
cheese type, lactic acid must develop at the correct time, usually
not too rapidly, nor too slowly, and in a specific concentration.
The cheese mass, which evolves as the whey drains and
the curds coalesce, contains the insoluble salts, CaHPO 4 and
MgHPO 4 , which serve as buffers in the pressed cheese mass,
and should the acid increase too rapidly during cooking, they
will dissolve in the whey. In the drained curds or pressed cheese
mass under these circumstances, since no significant pool of in-
soluble divalent salt remains for conversion into buffering salts,
the pH will lower to 4.7–4.8 from an optimum 5.2, leading to a
sour acid-ripened cheese. Thus, conservation of a reserve pool
of insoluble salts is necessary until the cheese mass is pressed.
Thereafter, the insoluble salts are changed into captive soluble
salts by the lactic acid and provide the necessary buffering power
to maintain optimum pH at a level that keeps the cheese sweet.
Such retention also is aided or controlled by the skills of the
cheese maker. For Cheddar, excess wetness of the curd before
pressing or too rapid an acid development in the whey causes
sour cheese. For a Swiss or Emmental, the time period for acid