Dairy processing – an overview
Practical Action
Other tests
Measuring the fat content of milk requires chemicals and laboratory apparatus that are not likely
to be affordable in a small dairy, and samples should be taken to a Bureau of Standards or
university Food Science department for testing. A low fat measurement indicates that some of
the cream has been skimmed from the milk or that it has been diluted with water.
Methods of processing
There are four main methods used to process milk that are suitable for small-scale operation:
Cooling fresh milk to extend the shelf life by a day or two, or freezing it (also making
ice cream).
Heating milk to destroy both contaminating micro-organisms and naturally occurring
enzymes that change the flavour of milk.
Making the milk acidic to slow down or prevent the growth of spoilage bacteria or food
poisoning bacteria (this also changes the milk to a curd).
Reducing the amount of water in milk products to slow down or prevent the growth of
bacteria. This may be combined with adding sugar (to make milk confectionery) or
salt (in cheese or butter production), both of which also prevent bacteria from
growing.
Cooling
Cooling does not destroy bacteria or enzymes but it slows down their activity. Cooled raw milk
keeps its quality for a few days before it is processed. Milk products such as yoghurt, cheese,
butter and pasteurised milk are also cooled to ensure they have the required shelf life for
distribution to shops and retail storage. At the smallest (micro-) scale of operation, a refrigerator
set at 4-5oC can be used to cool milk, but most dairy processors use a milk cooler (Figure 3) or
cold store to cool milk in bulk before it is processed. Finished products should be stored in a
separate dispatch store at 4oC +/- 2oC, or for frozen milk and ice cream, frozen in a freezer
operating at below -18oC.
Figure 3: Milk cooler
Photo: Courtesy of Everest
Instruments Pvt.
Heating
There are regulations in most countries that specify the time and temperature that milk should
be heated to pasteurise it. Most specify that milk should be heated to 63oC for 30 minutes (see
Technical Brief: Pasteurisation of milk). Higher temperatures and shorter times are used in
larger commercial operations but the equipment needed to do this is more expensive.
Acidifying
Acid is produced in milk by the growth of certain types of harmless bacteria called 'lactic acid
bacteria'. They are normally present in milk and are also used as starter cultures in the
production of yoghurt (Technical Brief: Soured milk and yoghurt). Lactic acid bacteria convert
milk sugar (lactose) into lactic acid, which increases the acidity of the milk and prevents the
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