Dairy Chemistry And Biochemistry

CHEMISTRY AND BIOCHEMISTRY OF CHEESE AND FERMENTED MILKS^397

Table 10.4 Salient features of lactose metabolism in starter culture organisms (from Cogan and

Hill, 1993)

~

Organism

Cleavageb Products

Transport” enzyme Pathway‘ (mol mol- lactose)

Lactococcus spp. PTS ppgal GLY^4 L-Lactate

Leuconostoc spp.? Bgal PK^2 D-Lactate +^2 ethanol + 2C0,

Str. salicarius PMF GLY 2 L-Lactated

Lb. delbrueckii PMF? /?gal GLY 2 D-Lactated

Lb. delbrueckii PMF? jgal GLY 2 D-Lactated

Lb. helveticus PMF? Pgal GLY^4 L- (mainly) + D-lactate

subsp. thermophilus

subsp. lactis

subsp. bulgarrcus

OPTS, phosphotransferase system; PMF, proton motive force.

*ppgal, phospho-8-galactosidase; pgal, 8-galactosidase.

‘GLY, glycolysis; PK. phosphoketolase.

dThese species metabolize only the glucose moiety of lactose.

The primary starter performs several functions in addition to acid
production, especially reduction of the redox potential (Eh, from about
+250mV in milk to - 150mV in cheese), and, most importantly, plays a
major, probably essential, role in the biochemistry of cheese ripening. Many
strains produce bacteriocins which control the growth of contaminating
micro-organisms.
The ripening of many varieties is characterized by the action, not of the
primary starter, but of other micro-organisms, which we will refer to as a
secondary culture. Examples are Propionibacterium in Swiss-type cheeses,
Penicillium rogueforti in Blue cheeses, Penicillium camemberti in surface
mould-ripened cheeses, e.g. Camembert and Brie, Breuibacterium linens and
yeasts in surface smear-ripened cheese, Lactococcus lactis ssp. lactis biovar
diacetylactis and Leuconostoc spp. in Dutch-type cheeses. The specific
function of these micro-organsims will be discussed in section 10.2.7 on
ripening. Traditionally, a secondary culture was not used in Cheddar-type
cheeses but there is much current interest in the use of cultures of selected
bacteria, usually mesophilic Lactobacillus spp. or lactose-negative Lactococ-
cus spp., for Cheddar cheese with the objective of intensifying or modifying
flavour or accelerating ripening; such cultures are frequently referred to as
‘adjunct cultures’.

10.2.4 Moulding and shaping

When the desired pH and moisture content have been achieved, the curds
are separated from the whey and placed in moulds of traditional shape and
size to drain and form a continuous mass; high-moisture curds form a