exposure to these conditions is minimized. Food freezing processes are not
designed however to maximize microbial lethality but to minimize loss of
product quality. Formation of large ice crystals and prolonged exposure
to high osmotic pressure solutions during slow cooling also damage cells
of the food material itself causing greater drip loss and textural deterio-
ration on thawing, so fast freezing in which the product is at storage
temperature within half an hour is the method of choice commercially.
The rate of freezing in domestic freezers is much slower so, although
microbial lethality may be greater, so too is product quality loss.
Thawing of frozen foods is a slower process than freezing. Even with
moderate size material the outside of the product will be at the thawing
temperature some time before the interior. So with high thawing tem-
perature, mesophiles may be growing on the surface of a product while
the interior is still frozen. Slow thawing at lower temperature is generally
preferred. It does have some lethal effect as microbial cells experience
adverse conditions in the 0 to 101 C range for longer, but it will also
allow psychrotrophs to grow. Provided the product is not subject to
contamination after thawing, the microflora that develops will differ from
that on the fresh material due to the selective lethal effect of freezing.
Lactic acid bacteria are often responsible for the spoilage of defrosted
vegetables whereas they generally comprise only about 1% of the micro-
flora on fresh chilled produce which is predominantly Gram-negative.
Freezing and defrosting may make some foods more susceptible to
microbiological attack due to destruction of antimicrobial barriers in the
product and condensation, but defrosted foods do not spoil more rapidly
than those that have not been frozen. Injunctions against refreezing
defrosted products are motivated by the loss of textural and other
qualities rather than any microbiological risk that is posed.
4.5 Chemical Preservatives
The addition of chemicals to food is not a recent innovation but has been
practised throughout recorded history. Doubtless too, there has also
always been a certain level of misuse but this must have gone largely
undetected until modern analytical techniques became available. When
chemical analysis and microscopy were first applied to foods in the early
19th century, they revealed the appalling extent of food adulteration then
current. Pioneering work had been done by the 18th century chemist
Jackson, but publication of the book ‘A Treatise on Adulterations of
Food, and Culinary Poisons’ by Frederick Accum in 1820 marks a
watershed. Accum exposed a horrifying range of abuses such as the sale
of sulfuric acid as vinegar, the use of copper salts to colour pickles, the
use of alum to whiten bread, addition of acorns to coffee, blackthorn
leaves to tea, cyanide to give wines a nutty flavour and red lead to colour
98 The Microbiology of Food Preservation