A bacteriophage is a bacterial virus which in its virulent state infects
the bacterial cell, multiplies within it, eventually causing the cell to burst
(lysis). When this occurs during a cheese fermentation, acidification
slows or even stops causing financial losses to the producer as well as
an increased risk that pathogens might grow. An important source of
phage in cheesemaking is thought to be the starter culture organisms
themselves which carry within them lysogenic phages that can be induced
into a virulent state. Problems occur particularly when starters contain a
single strain or only a few strains and the same culture is reused over an
extended period. During this time, phages specific to that organism build
up in the plant and can be isolated from the whey and from environ-
mental sources such as drains and the atmosphere, increasing the chance
of fermentation failure. In the past, control of this problem has been
based on the observation of rigorous hygiene in the dairy, the rotation of
starter cultures with differing phage susceptibilities and propagation of
starters in phage-inhibitory media which contain phosphate salts to
chelate Ca^21 and Mg^21 required for successful phage adsorption to
the bacterial cell. LAB possess their own resistance mechanisms to phage
infection which include restriction/modification of non-host DNA, inhi-
bition of phage adsorption by alteration or masking of specific receptors
on the cell surface, and reduction of burst size (the number of phages
released per infected cell). Most of these mechanisms appear to be
plasmid encoded and this has opened the way for new strategies for
phage control so that transconjugants with enhanced phage resistance
are now available.
A time course for the production of Cheddar cheese showing pH
changes and the timing of different process stages is shown in Figure 9.8.
A good starter should produce around 0.2% acidity within an hour’s
incubation. It will multiply up to around 10^8 –10^9 cfu g^1 in the curd
producing an acidity of 0.6–0.7% before its growth is stopped by salting.
Figure 9.8 pH changes during Cheddar cheese manufacture
334 Fermented and Microbial Foods