|By goGreen | September 21, 2011|
Yoghurt is produced by the controlled fermentation of milk by two species of bacteria(Lactobacillus sp. and Streptococcus sp.). The sugar in milk (called lactose) is fermented to acid (lactic acid) and it is this that causes the characteristic curd to form. The acid also restricts the growth of food poisoning bacteria and some spoilage bacteria. So, whereas milk is a potential source of food poisoning and only has a shelf life of a few days, yoghurt is safer and can be kept for up to ten days, under proper storage conditions.
Yoghurt can be easily produced at the small-scale. The procedure is as follows: Collect the milk in carefully cleaned, covered vessels. Pasteurise the milk at 80-85C for 15-20 minutes. This is especially important iftuberculosis bacteria are thought to infect animals locally. Cool the milk to 40-45C as quickly as possible and add a starter culture of theyoghurt bacteria. Keep the milk at this temperature for 3-4 hours while thefermentation takes place. If possible, then cool the yoghurt in a refrigerator until it iseaten or sold.
There are three potential problems in yoghurt making:
Spoilage by bacteria or molds
This is due to unclean equipment, contaminated milk or poor hygiene of the production staff.Ensure that all equipment is thoroughly scrubbed, sterilised with diluted bleach (twotablespoons of bleach per gallon of water) and thoroughly rinsed in CLEAN water beforeproduction starts. Pasteurisation should ensure that fresh milk is not contaminated, but donot use old milk. Make sure operators wash their hands before starting work and do notallow anyone with stomach complaints, coughs or skin infections (eg boils) to work with the milk.
Maintenance of correct incubation temperature
A commercial yoghurt maker of kitchen size may be purchased, but these tend to be ratherexpensive for what they are. You could easily get something made locally from a shallowwater bath with a small electrical element, keeping the water warm and the whole thingcontrolled by a simple variable temperature thermostat. An alternative way would be to fillthe yoghurt mix at 40-45C into a large commercial thermos flask. Finally, you can use ablock of 4 polystyrene into which indentations are made of such a size that small creamcontainers fit comfortably. The warm yoghurt mixture is thus filled into the containers insidethe block of polystyrene, and a polystyrene lid placed on top. The insulating effect of theblock will then prevent the loss of heat sufficiently to maintain the temperature of the productat the required 40-45 C. A similar idea consists of a hollow polystyrene box approximately.75m3 fitted with a 40W electric light bulb. The heat from the bulb maintains the temperaturewithin the required range.
The correct balance of the two Lactobacillus bacteria is important for good quality yoghurt . Inpractice, a dried culture can be obtained from most large towns/cities and this can be grownup on pasteurised milk and kept in a refrigerator. A part of this master culture can then beused each day for a week and the last part re-inoculated into milk to form a new masterculture. This method can be continued for several months, provided good hygiene is used,but eventually undesirable bacteria will contaminate the culture and it must be replaced.If a refrigerator is not available, it is possible to add one or two teaspoonfuls of commercialyoghurt (which has not been pasteurised after the fermentation) as the starter culture foreach pint of milk. This can be done each day. Finally, it is possible to add part of your yoghurt production to a new batch of milk the following day. There is a greater risk ofcontamination using this method and it is not recommended unless the other methodsdescribed are not possible.
Yoghurt can be either stirred or set. Stirred yoghurt is fermented in bulk, stirred and thendispensed into pots or sold into customers containers. Set yoghurt is made by pouring theinoculated milk into pots and fermenting it in the pot. Fruit and nuts can be added to eachtype but care is needed to ensure that they are thoroughly cleaned and blanched to avoidcontamination.
In some countries a layer of fruit syrup on the top of set yoghurt is a popular alternative. In other places a thicker stirred yoghurt is preferred. This can be made by adding driedskimmed milk (at approximately 50g/l) to the milk before pasteurising. The use of otherthickeners such as starch and pectin is also possible, but generally unnecessary.
The main quality control points to consider concern hygiene and are described above. Potsand other containers must also be absolutely clean before use.
Milk churns or similar containers.Pasteurising pan (e.g. 101) preferably made from stainless steel, but aluminium is adequate.Gas ring or other source of heat. Thermometer (0-100C).Incubation box/flask.Refrigerator (optional).
Note: This is a selective list of suppliers and does not imply ITDG endorsement
Cultures and equipment
Contempra Industries Inc
371 Essex Road
New Jersey 07753
Cumberland General Store
Braun Natural Yoghurt Maker
3269 American Drive
Chr Hansens Laboratory Inc
Horan-Lally Company Limited
1146 Aerowood Drive
Marika Dairy Cultures
Natalex Corp. Box 3060
New Jersey 07087, USA
Source: Practical Answers to Poverty