variety of new products including complete meals, prepared and delica-
tessen salads, dairy desserts and many others. Three main factors have
contributed to this development:
(1) the food manufacturers’ objective of increasing added value to
their products;
(2) consumer demand for fresh foods and ease of preparation while
at the same time requiring the convenience of only occasional
shopping excursions; and
(3) the availability of an efficient cold chain – the organization and
infrastructure which allows low temperatures to be maintained
throughout the food chain from manufacture/harvest to con-
sumption.Chill storage can change both the nature of spoilage and the rate at
which it occurs. There may be qualitative changes in spoilage character-
istics, as low temperatures exert a selective effect preventing the growth
of mesophiles and leading to a microflora dominated by psychrotrophs.
This can be seen in the case of raw milk which in the days of milk churns
and roadside collection had a spoilage microflora comprised largely of
mesophilic lactococci which would sour the milk. Nowadays in the UK,
milk is chilled almost immediately it leaves the cow so that psychrotro-
phic Gram-negative rods predominate and produce an entirely different
type of spoilage. Low temperatures can also cause physiological changes
in micro-organisms that modify or exacerbate spoilage characteristics.
Two such examples are the increased production of phenazine and
carotenoid pigments in some organisms at low temperatures and the
stimulation of extracellular polysaccharide production in Leuconostoc
spp. and some other lactic acid bacteria. In most cases, such changes
probably represent a disturbance of metabolism due to the differing
thermal coefficients and activation energies of the numerous chemical
reactions that comprise microbial metabolism.
Though psychrotrophs can grow in chilled foods they do so only
relatively slowly so that the onset of spoilage is delayed. In this respect
temperature changes within the chill temperature range can have pro-
nounced effects. For example, the generation time for one pseudomonad
isolated from fish was 6.7 hours at 5 1 C compared with 26.6 hours at
01 C. Where this organism is an important contributor to spoilage, small
changes of temperature will have major implications for shelf-life. The
keeping time of haddock and cod fillets has been found to double if the
storage temperature is decreased from 2.8 1 Cto0.3 1 C. Mathematical
modelling techniques of the sort described in Section 3.4 can be useful in
predicting the effect of temperature fluctuations on shelf-life, but, as a
94 The Microbiology of Food Preservation