are described as mesophilic starters, strains ofLactococcus lactisand its
subspecies. Thermophilic starters such as Lactobacillus helveticus,
Lb. casei,Lb. lactis,Lb. delbrueckiisubsp.bulgaricusandStrep. therm-
ophilusare used in the production of cheeses like Emmental and Parme-
san where a higher incubation temperature is employed.
The role of starter organisms in cheesemaking is both crucial and
complex. Their central function is the fermentation of the milk sugar
lactose to lactic acid. This and the resulting decrease in pH contribute to
the shelf-life and safety of the cheese and gives a sharp, fresh flavour to
the curd. The stability of the colloidal suspension of casein is also
weakened and calcium is released from the casein micelles improving
the action of chymosin. After the protein has been coagulated, the acid
aids in moisture expulsion and curd shrinkage, processes which govern
the final cheese texture.
There are two different systems for uptake and metabolism of lactose
in LAB. In most lactobacilli andStrep. thermophilus, lactose is taken
up by a specific permease and is then hydrolysed intracellularly by
b-galactosidase. The glucose produced is fermented by the EMP pathway
which the galactose also enters after conversion to glucose-6-phosphate
by the Leloir pathway (Figure 9.7). Most lactococci and some lactobacilli
such as Lb. casei take up lactose by a phosphoenolpyruvate (PEP)-
dependent phosphotransferase system (PTS) which phosphorylates lac-
tose as it is transported into the cell. The lactose phosphate is then
hydrolysed by phospho-b-galactosidase to glucose, which enters the
EMP pathway, and galactose-6-phosphate which is eventually converted
to pyruvate via the tagatose-6-phosphate pathway. These pathways are
of practical import in cheesemaking; in the lactococci, lactose utilization
is an unstable, plasmid encoded characteristic and loss of these genes can
Figure 9.7 Lactose uptake systems
332 Fermented and Microbial Foods