Dairy Chemistry And Biochemistry

CHEMISTRY AND BIOCHEMISTRY OF CHEESE AND FERMENTED MILKS 405

lactose which is normally metabolized to L-lactic acid by the Lactococcus

starter within a day for most varieties or a few weeks for Cheddar. In most

varieties, the L-lactate is racemized to DL-lactate by NSLAB within about 3

months and a small amount is oxidized to acetic acid at a rate dependent

on the oxygen content of the cheese and hence on the permeability of the

packaging material.

In cheese varieties made using Streptococcus salvarius ssp. thermophilus

and Lactobacillus spp. as starter, e.g. Swiss types and Mozzarella, the

metabolism of lactose is more complex than in cheese in which a

Lactococcus starter is used. In these cheeses, the curd is cooked to 52-55"C,

which is above the growth temperature for both components of the starter;

as the curd cools, the Streptococcus, which is the more heat-tolerant of the

two starters, begins to grow, utilizing the glucose moiety of lactose, with the

production of L-lactic acid, but not galactose, which accumulates in the

curd. When the curd has cooled sufficiently, the Lactobacillus spp. grow,

and, if a galactose-positive species/strain is used, it metabolizes galactose,

producing DL-lactate (Figure 10.15). If a galactose-negative strain of

Lactobaciilus is used, galactose accumulates in the curd and can participate

in Maillard browning, especially during heating, which is undesirable,

especially in Pizza cheese.

Swiss-type cheeses are ripened at about 22°C for a period to encourage

the growth of Propionibacterium spp. which use lactic acid as an energy

2

h 0

Y

c

M

0

M

(^0).
v
.- C
3
-I 2
c
a,
S
0
0 10 20 I0 20 30 411
Time (h) Time (days)
Figure 10.15 Metabolism of lactose, glucose, galactose, D- and L-lactic acid in Emmental
cheese. Cheese transferred to hot room (22-24°C) at 14 days. 0, D-lactate; 0, acetate;
H, galactose; 0, L-lactate; +, glucose; 0, lactose; A, propionate.