Fermented milks
The types of fermented milk discussed here are those made by controlled fermentation. This is achieved by establishing the desired micro-organisms in the milk and by maintaining the milk at a temperature favourable to the fermentative organism.
A variety of fermented milks are made, each dithering markedly from the other. However, a number of steps are common to each manufacturing process, and these are outlined in Figure 19.Standardisation
Occasionally some fat is removed or milk SNF added. In some instances, the removal of moisture during heating increases the proportion of solids in the final product.
Heating
Milk is heated to kill pathogens and spoilage organisms and to provide a cleaner medium in which the desired micro-organisms can be established. Heating also removes air from the milk, resulting in a more favourable environment for the fermentative organisms, and denatures the whey proteins, which increases the viscosity of the product.
After heating, the milk must be cooled before it is inoculated with starter, otherwise the starter organisms will also be killed.
Inoculation with starter
Starter is the term used to describe the microbial culture that is used to produce the desired fermentation and to flavour the product. When preparing the starter, care must be taken to avoid contamination with other micro-organisms. Companies that supply starter cultures detail the precautions necessary. Care should also be taken to avoid contamination when inoculating the milk with starter.
Incubation
After inoculation the milk is incubated at the optimum temperature for the growth of the starter organism. Incubation is continued until the fermentation is complete, at which time the product is cooled. Additives may be added at this stage and the product packed. The manufacturing procedures for a number of fermented milks are given in Table 5.
Preparation of the fermentation vessel
The fermentation vessel is first washed to remove visible dirt. It is then dried and smoked by putting burning embers of Olea africana, wattle or acacia into the vessel and closing the lid. The vessel is then shaken vigorously and the lid opened to release the smoke. This procedure is repeated until the inside of the vessel is hot. Smoking flavours the product and is also thought to control the fermentation by retarding bacterial growth. While it is known that smoke contains compounds that retard bacterial growth, the precise effects of smoking on fermentation have not been investigated.
Once smoking is complete the vessel may be cleaned with a cloth to remove charcoal particles. However, in some areas the charcoal particles are retained to add colour to the product.
Milk treatment
In some processes the milk is boiled prior to fermentation. It is then allowed to cool and the surface cream removed. In other processes the milk is not given any prefermentation treatment.
Fermentation
The milk is placed in the smoked vessel and allowed to ferment slowly in a cool place at a temperature of about 16–18°C. The fermentation is almost complete after 2 days, but may be continued for a further 2 days, by which time the flavour is fully developed. The milk must ferment at low temperature, otherwise fermentation is too vigorous, with much wheying off and gas production.
The product has a storage stability of 15 to 20 days.