incidence ofC. botulinumspores in milk and the fact that manufacturers
employ heat processes well in excess of the minimum legal requirement.
Sterilized milk described in Table 5.4 is a rather specialized product of
diminishing importance. In the UK it is defined as a product which is
heat processed in-bottle at temperatures above 100 1 C for sufficient time
for it to pass the turbidity test. This test is based on the principal that the
heat process is sufficient to denature whey proteins. In it, casein is
precipitated with ammonium sulfate and filtered off. The filtrate is then
heated; if it remains clear the milk is acceptable because the whey
proteins have already been denatured and removed with the precipitated
casein. Turbidity indicates an inadequate heat process which has left
some whey protein undenatured.
Gram-negative psychrotrophs will not survive pasteurization,
although some pseudomonads produce extracellular lipases and proteas-
es which are heat resistant. If enough of these bacteria are present
( 4105 ml^1 ), sufficient enzyme can be produced pre-pasteurization to
cause rancidity and casein degradation in the processed milk.
Raw milk may also contain a number of organisms known as ther-
modurics that can survive mild pasteurization treatments. These are
generally Gram-positives such as the sporeforming bacteria and mem-
bers of the genera Microbacterium, Micrococcus, Enterococcus and
Lactobacillus, but 1–10% of strains of the Gram-negativeAlcaligenes
toleransmay also survive.
In the main, spoilage of pasteurized milk is due to the growth of
psychrotrophic Gram-negative rods such asPseudomonas, Alcaligenes,
and Acinetobacter, introduced as post-pasteurization contaminants.
Product shelf-life will depend on the number of contaminants introduced
and the efficiency of the cold chain; a 2 1 C decrease in storage temper-
ature will approximately double the shelf life, but pasteurized milk
produced under conditions of good manufacturing practice should keep
for more than 10 days under refrigeration. Spoilage usually manifests
itself as off odours and flavours described as fruity or putrid but visual
defects such as clotting due to proteolytic activity can also arise. The
souring traditionally associated with milk spoilage and due to the growth
of lactic acid bacteria is now rare.
In milk which is subject to very low levels of post-pasteurization
contamination, spoilage can result from the growth of thermoduric
Bacillusspp. This may be associated with flavour defects but the most
studied example is the bitty cream phenomenon produced by the leci-
thinase activity ofBacillus cereus. This enzyme hydrolyses the phospho-
lipids associated with the milk fat globule membrane to produce small
proteinaceous fat particles which float on the surface of hot drinks and
adhere to surfaces of crockery and glasses. Bitty cream is associated
mostly with milk that has been subject to temperature abuse, although
Chapter 5 129