Dairy products and milk-based food ingredients - Natural Food Additives, Ingredients and Flavourings - page 419

Pasteurised milk is processed by high temperature short time (HTST)
processing, which involves passing raw milk at 4°C through a plate or tubular
heat exchanger. The UK regulations require a heat treatment of 71.7°C for a
minimum of 15 seconds, although the Advisory Committee on the Microbiological
Safety of Food (ACMSF) recommends 72°C for at least 25 seconds to destroy
Micobacterium paratuberculosis (Dairy Council 2011). Pasteurisation destroys
vegetative pathogenic bacteria such as Micobacterium tuberculosis, Coxiella
burnetti , Salmonellas, Listeria monocytogenes , Campylobacters and Escherichia
coli O157:H7.
Prior to heat treatment raw milk may be clarifi ed by passing through a
centrifugal clarifi er to remove solid contaminants (dust, dirt and animal hairs).
Following heat treatment, milk is chilled and hygienically packaged in plastic
containers, laminated paper board cartons, glass bottles or polyethylene pouches.
Pasteurised milk packaged in plastic containers and chilled at 4° C will achieve a
shelf life of around 14 days. Pasteurisation itself does not destroy all milk-borne
bacteria. Thermoduric organisms (e.g. spore-forming Bacillus and Clostridium
species) can survive and some may grow slowly at low temperatures, spoiling
liquid milk through proteolytic and lipolytic action.
Extended shelf life (ESL) milk is becoming common. It can be produced using
different processing systems (e.g. pasteurisation) followed by the removal of
thermoduric bacteria using either microfi ltration (membrane separation) or
centrifugal separation. ESL milk has a shelf life of 20-30 days.
To produce liquid drinking milk with a shelf life of six months or more,
heat treatment at higher temperatures than pasteurisation is necessary to destroy
spore forming organisms. Ultra heat-treated (UHT) milk is manufactured by
means of ultra-high temperature processing where milk is heated to 135°C or
greater for not less than one second (usually 1-2 seconds). The temperature/
time conditions are suffi cient to sterilise the product, achieving F 0 values of
three or more. The F 0 value provides a means of comparing sterilisation
processes against the reference temperature of 121.1°C. A sterilisation process
equivalent to 121.1°C for three minutes is required to destroy Clostridium
botulinum spores.
UHT milk processing may be carried out using either direct or indirect heat
transfer systems. In direct heat transfer processing, raw milk at 4°C is preheated
to about 80°C using recovered heat. This stabilises the whey proteins against
precipitation during sterilisation by encouraging gelation. The milk is then taken
to the UHT temperature using either steam injection or steam infusion systems. In
the former live steam is mixed directly into the milk and in the latter milk is
sprayed into a vessel containing steam. In both methods the temperature of the
milk is raised instantaneously to the sterilisation temperature, but water is added.
This is removed by passing the hot milk through a vacuum vessel, which also
removes heat. In contrast to the direct heat transfer methods, indirect UHT milk
processing bears similarity to pasteurisation as milk is taken to the UHT processing
temperature using, for example, plate or tubular heat exchangers. The milk does
not come into contact with the heat transfer medium (i.e. steam) and so adulteration

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