68 Chapter 3
bling strawberries (Gilmour and Rowe,
1990 ). Species of the genus Bacillus and
related Paenibacillus are Gram - positive
spore - forming bacteria that can survive pas-
teurization and therefore can be present in the
pasteurized product as a raw milk contami-
nant as well as through post - pasteurization
contamination. A sub - typing method based
on the rpoB gene can be used to track the
various strains through the milk processing
operation (Durak et al., 2006 ; Huck et al.,
2008 ).
Both genera have psychrotrophic strains
and their multiplication during refrigeration
leads to spoilage, which manifests as off
fl avors. The species B. cereus can produce a
defect known as “ bitty ” or “ broken ” cream
that appear as fl ecks of fat fl oating on the
surface of beverages to which the milk has
been added and is apparent mostly during the
summer months. This is caused by the pro-
duction of an extracellular phospholipase
enzyme that degrades the phospholipid milk
fat membrane surrounding fat globules,
causing them to lose their colloidal properties
and coalesce.
As a result of a move to a more health
conscious society, greater quantities of
reduced fat milk such as skim and semi - skim
are being consumed. The authors have been
unable to fi nd any published information
indicating that the nature or speed of spoil-
age of skim or semi - skim pasteurized milk is
signifi cantly different from that of whole
milk.Cream
Cream can be defi ned as “ that part of
milk rich in fat which has been separated by
skimming or otherwise ” (Robinson, 2002 ).
Creams are generally identifi ed by their fat
content: half and half (10.5% to 18%), single/
light cream (18% to 30%), whipping cream
(30% to 40%), double cream (more than
48%), and cultivated or sour cream (10% to
40%). Cream to be used as an ingredient in
processing contains 36% to 40% fat.and the sensitivity of the assay has been
called into question because of the possible
low relative transmissibility of the disease
from cows to mice as a result of the species
barrier effect (Tyshenko, 2007 ). Certainly
prions are known to be very heat stable and
would survive pasteurization or even steril-
ization processes, so emphasis lies in pre-
venting their access to milk rather than
applying a process to inactivate them (Asher
et al., 1986 ). The transmission of prions via
milk has not been reported from any country
with BSE. This lack of evidence suggests that
either milk does not readily transmit infective
prions or that its infectivity is too low.
Pasteurized Milk Spoilage
Normally shelf - life at 4 ° C or 7 ° C is defi ned
as the incubation time necessary for micro-
bial numbers to reach 10^7 to 10^8 cfu/ml,
which is the generally accepted threshold
for spoilage. The shelf life can range from
7 to 28 days or even longer, depending on
the quality of the raw milk, level of post -
pasteurization contamination, and tempera-
ture control, especially at packaging and
storage. Although post - pasteurization con-
tamination after the holding tube can occur
as a result of improperly cleaned and disin-
fected cooling sections, process lines, valves,
and tanks, the main point of entry, according
to a number of studies, is at the fi lling
machine.
Contamination is mainly by species of the
genera Pseudomonas and psychrotrophic
Bacillus, which eventually cause spoilage,
mainly due to their ability to multiply at
refrigeration temperatures and degrade the
protein and fat components in milk. In addi-
tion to pseudomonads, in particular Ps. fl uo-
rescens, other Gram - negative bacteria can be
detected after refrigerated storage such as
Flavobacterium spp., Enterobacteriaceae,
Alcaligenes spp., Aci netobacter spp., and
Aeromonas spp. Pseu domonas fragi, also
found in pasteurized milk, produces a char-
acteristic fruity aroma described as resem-