clearly have serious consequences for milk fermentation. Using trans-
duction techniques, molecular biologists have produced strains ofLacto-
coccus lactisin which this property has been stabilized by integration of
the lactose utilization genes in the chromosome.
The thermophilic lactobacilli, which employ a lactose permease and
b-galactosidase, metabolize the glucose produced preferentially, turning to
galactose only when lactose becomes limiting. This can be a problem in
some products. The accumulation ofgalactose can give rise to a brown
discolouration during theheat processing of Mozzarella cheese. In Swiss
cheeses such as Emmental, residual galactose can affect product flavour
since propionic acid bacteria fermentit in preference to lactate. In doing so
they produce a preponderance of acetic (ethanoic) acid which does not
confer the usual nutty flavour associated with the equimolar concentrations
of acetate and propionate produced by thePropionibacteriumfrom lactate.
Lactic acid bacteria are nutritionally fastidious and require preformed
nucleotides, vitamins, amino acids and peptides to support their growth.
To grow to high cell densities and produce acid rapidly in milk, dairy
starters must have proteolytic activity to overcome the limitation im-
posed by the low non-protein nitrogen pool in native milk. These systems
are comprised of proteinases, associated with the surface of the bacterial
cell wall, which can hydrolyse casein proteins. Peptidases in the cell wall
degrade the oligopeptides produced down to a size that can be trans-
ported into the cell (4–5 amino acid residues) where they are further
degraded and utilized. While this ability is essential to starter function, it
also plays an important role in the development of cheese flavour during
ripening or maturation (see below).
Citrate fermentation to diacetyl is required in some cheese varieties
and starter cultures for these include species such asLactococcus lactis
subsp.lactisorLeuconostoc cremoris. Carbon dioxide is another product
of this pathway and is important in producing the small eyes in Dutch
cheese like Gouda or giving an open texture that will facilitate mould
growth in blue-veined cheeses. In other cheese, such as Cheddar, this
would be regarded as a textural defect.
To produce Cheddar cheese, starter culture is added at a level to give
106 –10^7 cfu ml^1. In the past these cultures were grown-up in the dairy
from stock cultures or from freeze-dried preparations bought in from
commercial suppliers. Nowadays frozen, concentrated cultures that are
added directly to the cheese vat are increasingly used because of their
ease of handling and the greater security they offer the cheesemaker. This
applies particularly to the risk of bacteriophage inhibition of the fer-
mentation which has been a major preoccupation of the cheesemaker
since it was first identified in New Zealand in the 1930s. Problems of
phage infection are not confined to cheesemaking but have also been
encountered in the production of yoghurt and fermented meats.
Chapter 9 333