71 C and the only organisms capable of growing will be psychrotrophs.
There are many psychrotrophic species, but those most commonly found
in raw milk include Gram-negative rods of the genera Pseudomonas,
Acinetobacter, Alcaligenes, Flavobacterium, psychrotrophic coliforms,
predominantlyAerobacterspp., and Gram-positiveBacillusspp.
One consequence of the current extensive use of refrigeration and the
change to a microflora dominated by psychrotrophs is that traditional
tests for the microbiological quality of milk based on the reduction of a
redox dye such as methylene blue or resazurin have become obsolete.
Psychrotrophs tend to reduce these dyes poorly and the tests are not very
sensitive to low numbers of bacteria.
5.2.3 Heat Treatment of Milk
Proposals for the heat treatment of milk were made as early as 1824,
forty years before Pasteur’s work on the thermal destruction of micro-
organisms in wine and beer. When milk pasteurization was introduced
by the dairy industry around 1890, it was as much to retard souring as to
prevent the spread of disease. This had become an important commercial
requirement since large quantities of milk were now being transported by
rail into the large cities rather than being produced locally in cramped
and insanitary cowhouses.
Milk has long been recognized as an agent in the spread of human
disease and within a few years it was appreciated that pasteurization was
also providing protection against milk-borne disease. Nowadays it is
safety rather than spoilage considerations which determine the minimum
legal requirements for pasteurization.
Originally the main health concerns associated with milk were tuber-
culosis caused byMycobacterium bovisandMycobacterium tuberculosis
(see Section 7.10) and brucellosis caused byBrucellaspp. (see Section
7.3). In some parts of the world milk is still a significant source of these
infections but in the UK and some other countries they have now been
effectively eliminated from the national dairy herd by a programme of
regular testing and culling of infected animals. Such programmes must
be constantly maintained to be effective and there have been occasional
problems. Initiatives such as the culling of badgers, thought to be a
reservoir ofM. bovis, have been the subject of some controversy and in
2002 there was an outbreak of brucellosis in a dairy herd in Cornwall,
although this was the first recorded in England for ten years. Enteric
pathogens such as Salmonella and Campylobacterare still however
prevalent in raw milk and pasteurization remains the most effective
measure for their control.
The four types of heat treatment applied to milk are described in Table
5.4. Specification of pasteurization temperatures to the first decimal
Chapter 5 127