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Dairy processing (Practical Action Brief)
Milk is a valuable nutritious food which, if untreated, has a shelf life of only a few days before it spoils. There are however, a variety of preservation techniques applied to milk to extend its shelf life to several weeks or months and also to change its qualities by developing different flavours and textures. For small-scale processing, it is not possible to attempt production of dried milk powder or UHT (Ultra-High Temperature) sterilised milk, but other dairy products are potentially suitable.
To preserve milk it is necessary to destroy or inhibit the action of enzymes and contaminating bacteria. Milk is a low-acid food which contains all of the nutrients required for bacteria to grow. It is therefore a potential cause of food poisoning if not adequately processed. In all dairy processing it is essential that full and proper hygiene precautions are taken to ensure the safety of the products.
The four main methods of preservation suitable for small-scale operation are:
• Cooling - to extend the shelf life of fresh milk by a day or two.
• Heating - (pasteurisation, sterilisation or concentration) to destroy enzymes and micro- organisms.
• Acidification - to inhibit spoilage or food poisoning bacteria from growing and also change the physical characteristics of milk.
• Separation - of the milk components.
Refrigeration is expensive and milk coolers based on refrigerated brine or other coolants are unlikely to be affordable on a very small-scale. However, for cottage/village or medium-scale processing this equipment is often essential to maintain the quality of raw milk before processing and to cool processed products such as yoghurt, cheese, butter and pasteurised milk for temporary storage before distribution. This operation does not destroy bacteria or enzymes but slows down their activity to extend the shelf life of the products.
Milk should be heated to 63°C for 30 minutes to adequately pasteurise it or, if in bottles, to 121°C for 15-20 minutes to adequately sterilise it. Higher temperatures and shorter times are used in larger commercial operations but this type of equipment is very expensive and is not considered to be 'small-scale'.
Pasteurisation can be done in open pans with continuous stirring, then filling the milk into pre-sterilised bottles (100°C for ten minutes in steam or water) and sealing immediately. Sterilisation requires the use of a pressure cooker to achieve 121°C at 104 kPa (15 psi ) which increases the capital cost of processing. Milk is filled into bottles which are then sealed and placed in the pressure cooker. The temperature and pressure are gradually raised and lowered to give the correct processing time. Sterilised milk has a shelf life of several weeks/months if unopened but extreme care is needed to ensure adequate heating in order to prevent food poisoning. This is not recommended for inexperienced processors.
Boiling milk to evaporate some of the water and produce a brownish gel is a low technology process which produces a snack food/sweet popular in some parts of Asia. The product has a shelf life of a few weeks and may be used in other foods or have ingredients such as sugar, colour, spices, fruits and nuts added to give variety.
The natural acidification of milk occurs by the presence and growth of certain types of harmless bacteria called 'lactic acid bacteria'. These bacteria convert milk sugar (lactose) into lactic acid, this increases the acidity of the milk and causes the formation of the characteristic gel of yoghurt (or curd) and inhibits bacterial growth. The shelf life is extended by several days and the changes in flavour and texture make this a popular product in most regions. The technology involved can be very simple eg inoculating milk, in a loosely covered pan, with some of yesterday's batch and allowing it to ferment at room temperature for several hours. More sophisticated developments include the use of new starter cultures for each batch and packaging in plastic pots sealed with hand operated foil lid sealers.
Cheese making requires more skill and experience than yoghurt production and with the exception of cottage cheese or simple curd cheese; it is recommended that training is obtained from an experienced cheese maker. There are more than 700 recognised types of cheese in the world and care should be taken to find which ones are likely to be popular in your region before contemplating production. Two excellent books on this subject are: 'Traditional Cheesemaking' by Josef Dubach, published by ITDG Publishing/SKAT, and 'Traditional Cheesemaking Manual' by Charles O'Conner of the International Livestock Centre for Africa (ILCA). Contact details for these organisations are shown at the end of this technical brief.
Milk contains fat and a complex mixture of water, proteins and vitamins/minerals. By separating the fat from the watery part it is possible to obtain cream. This product is extremely susceptible to food poisoning and food spoilage. It is not recommended for all except the most experienced small-scale dairies.
By churning cream, it is changed to butter which, if prepared and stored correctly, can have a shelf life of several weeks. Clarified butter (ghee) has a shelf life of several months. Both are high value products for which there is often a ready market.
In summary, yoghurt, milk gel and ghee are highly suitable for small-scale operation. Cheese, cream and pasteurised milk require greater care. Sterilised milk requires considerable care and experience to produce a safe, high quality product. Dried milk and UHT milk are not suitable for most small-scale operations.
References and further reading
Small-scale Butter Making (Practical Action Technical Brief)
Yoghurt Practical Action Technical Brief 
Yoghurt Incubator Practical Action Technical Brief 
Ghee Practical Action Technical Brief
Dairy Processing: Food Cycle Technology Source Book, UNIFEM 1996
Preparation of Dairy Products: Agrodok 36, Agromisa 1991
Tools for Agriculture: A Guide to Appropriate Equipment for Smallholder Farmers, ITDG Publishing, CTA, & GRET 1992
Rural Dairy Technology, ILRI Training Manual, International Livestock Research Institute 1995
Traditional Cheesmaking, Josef Dubach, SKAT/IT Publishers, 1989
Useful organisations and contacts
Dairy & Meat Officer (Institutional Support & Training)
Animal Production & Health Division
Food and Agricultural Organization (FAO)
Website: http://www.fao.org/ag/aga/agap/lps/dairy/SD E/SDE.htm
CH-9000 St. Gallen
Tel: +41 71 228 54 54
Fax: +41 71 228 54 55
International Livestock Research Institute
P.O. Box 5689 Addis Ababa, Ethiopia
Tel: (251-1) 613215
Fax: (251-1) 611-892
P.O. Box 30709
Tel: 254-2 630743
Fax: 254-2 631499
Note: This is a selective list of suppliers and does not imply endorsement by Practical Action.
Tel: +44 (0) 1691 622391
Fax: +44 (0) 1691 622355
• Packo Batch Pasteuriser. Specially developed for smaller quantities of milk, cream, yoghurt, cheese and cultured cream. Capacity: 150-500 litres/hour Power: Electric Email: email@example.com
• Mini Mobile Milking Machine. Easy to cans and pails manoeuvre unit. Capacity: 8 litre tank.
C-230,Ghatkopar Industrial Estate
Mumbai - 400 086
Tel: +91 (0)22 2517 1636 / 2517 1960
Fax: +91 (0) 22 2517 0878
• Milk testing equipment & utensils. The equipment includes hand operated and electric centrifuge machines, funnels, beakers, flasks and milk collecting trays,cans and pails
. Sealing and Fillinf machines. Semi automatic machine for packing liquids such as milk, oil, gheeautomatic machine for packing liquids in pillow packs. Capacity: 300 pack/hour Power: Electric
APV Unit Systems Pasteursvej
Tel: +45 8922 8922
Fax: +45 8922 8901
Pasteurising Units designed for small-scale heat treatment of liquid products. The unit is used for the pasteurisation of milk and cream, but is also applicable for other products. The unit is provided with an electrical water heater and can be used where steam or hot water is not available. Capacity: 200-1,000 litres/hour Power: Manual