Soured milk and yoghurt
Practical Action
including the design and operation of the processing facilities, staff training in hygiene and
production methods, and correct cleaning and maintenance procedures.
Hygiene
Technical Brief: Dairy processing - an overview gives details of hygiene and sanitation, the design
of a dairy and the use of correct cleaning procedures. Hygiene requirements are also described in
Technical Brief: Hygiene and safety rules in food processing.
Avoiding spoilage
Unclean equipment, contaminated milk, poor hygiene of production staff or incorrect processing
and storage conditions will each cause spoilage of soured milk and yoghurts. All equipment
should be thoroughly cleaned after use and checked before production starts again. The
temperature and time of heating milk should be monitored and controlled to ensure that it is not
over- or under-heated. The temperature and time of incubation should be monitored and
controlled to ensure that the fermentation takes place correctly.
Raw material control
The milk used for yoghurt and cultured milk production should be fresh, good quality and free
from dirt and excessive contamination by bacteria. Older milk may impart an unpleasant flavour
to the final product. The Technical Brief Dairy processing - an overview gives details of the
methods needed to ensure that good quality milk is used.
Yoghurt culture
In yoghurt making, the correct amount of inoculum and the correct proportions of the two lactic
acid bacteria are both important to produce good quality yoghurt. Commercially produced dried
mixed cultures of inoculum (Streptococcus thermophilus and Lactobacillus delbrueckii
subspecies bulgaricus) can be obtained from many large towns/cities or from suppliers
elsewhere, and these suppliers also supply probiotic lactic acid bacteria. The dried culture is
grown in pasteurised milk (see Technical Brief: Pasteurised milk) and then kept in a refrigerator.
A part of this ‘master culture’ (1-3% of the weight of the batch of milk) is then used each day for
a week. The last part is inoculated into pasteurised milk to form a new master culture. This
method can be continued for several months, provided that good hygiene is practised, but
eventually undesirable bacteria will contaminate the culture and it must be replaced.
It is also possible to use commercial yoghurt - ie. ‘live’ yoghurt that has not been pasteurised - as
a starter culture. New yoghurt should be used each day at between 20 and 50g per litre of milk.
Alternatively, part of the yoghurt produced can be held overnight in a refrigerator and added the
following day to a new batch of milk at 20-50g per litre of milk. There is a greater risk of
contamination using this method and it is not recommended unless the other methods described
are not possible.
Process control
A process control schedule should be prepared for each product. Table 1 is an example of a
process control schedule for yoghurt production.
Stage in process
Activity
Process control points
Heat
Cool
Ferment
Fill pots (stirred
yoghurt)
Heat milk to destroy micro-
organisms and enzymes.
By immersing vessel in cold water
and stirring milk
Add yoghurt starter culture and
ferment for 4-6 hours.
Stir the yoghurt to break the gel
and then pour liquid yoghurt into
pots
Check temperature and time (80-
85 oC for 15-20 minutes).
To 44 oC +/- 2 oC within 20
minutes
Check that starter culture is active
and check weight of culture
added. Maintain temperature at
44 oC +/- 2 oC. Check texture of
yoghurt to determine end of
fermentation time (5 hours +/- 1
hour)
Check the weight of yoghurt in
each pot (e.g. 200g +/- 2g net
weight)
3