temperatures up to 260 1 C at the top of a tower. The droplets dry very
rapidly and fall to the base of the tower where they are collected. In drum
drying, the milk is spread on the surface of slowly rotating metal drums
which are heated inside by steam to a temperature of about 150 1 C. The
film dries as the drum rotates and is scraped off as a continuous sheet by
a fixed blade close to the surface of the drum. Although it uses a lower
temperature, drum drying gives greater lethality since the milk is not
subject to the same degree of pre-concentration used with spray-dried
milk and the product spends longer at high temperatures in a wet state.
Spray drying is however now widely used for milk drying because it
produces a whiter product which is easier to reconstitute and has less of a
cooked flavour. Milk is pasteurized before drying although there are
opportunities for contamination during intervening stages. Most of the
organisms which survive drying are thermoduric but Gram-negatives
may survive and have on occasion been the cause of food poisoning
outbreaks.
The limited lethality of drying processes and the long storage life of
dried products means that manufacturers are not exempt from the
stringent hygiene requirements of other aspects of food processing. Good
quality raw materials and hygienic handling prior to drying are essential.
Outbreaks ofStaph. aureusfood poisoning have been caused by dried
foods which were stored at growth temperatures for too long prior to
drying allowing the production of heat resistant toxin which persisted
through to the final product. The dried product must also be protected
from moisture by correct packaging and storage in a suitable environ-
ment otherwise pockets of relatively high awmay be created where
microbial growth can occur.
4.8 Compartmentalization
Butter is an interesting example of a rather special form of food pres-
ervation where microbial growth is limited by compartmentalization
within the product.
Essentially there are two types of butter: sweet cream butters, which
are often salted, and ripened cream butters. In ripened-cream butters, the
cream has been fermented by lactic acid bacteria to produceinter alia
diacetyl from the fermentation of citrate which gives a characteristically
buttery flavour to the product. They have a stronger flavour than sweet-
cream butters but are subject to faster chemical deterioration. Sweet-
cream butter is most popular in the United States, Ireland, the UK,
Australia and New Zealand whereas the ripened cream variey is more
popular in continental Europe.
Butter is an emulsion of water droplets in a continuous fat phase in
contrast to milk which is an emulsion of fat globules in a continuous
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