Soured milk and yoghurt
Practical Action
3. Depending on the type of yoghurt required, the milk is then either kept in the pan (for stirred
yoghurt) or filled into individual retail pots (for set yoghurt).
4. Incubate the milk at 40-45 oC for 4-6 hours to allow the fermentation to take place.
5. For stirred yoghurt, the pan of curd is then stirred using a sterilised spoon or ladle until the
curd becomes a smooth cream that has a thick consistency without any lumps. This is then
either filled into retail pots that are sealed with lids, or sold into customers’ containers. Pots
of set yoghurt are sealed without disturbing the solid curd.
6. Both types of yoghurt are stored in a refrigerator at 4-8oC. If pots are to be distributed to
retailers, they should ideally be transported in an insulated box to keep them cool.
Flavoured or fruit yoghurt
Flavoured or fruit yoghurt is produced using the same method as plain yoghurt. For flavoured
yoghurt, the essence (e.g. vanilla, chocolate or a fruit flavour) with or without added colouring, is
added before the milk is fermented.
Yoghurt that contains fruit pulp is made in a similar way to plain yoghurt, and the fruit pulp is
added after fermentation. Less acidic fruits such as melon or pawpaw (papaya) are more
successful because they do not react with the milk. Acidic fruits (lemon, lime, pineapple etc.)
may cause excessive curdling and separation of the yoghurt.
To make fruit pulp:
1. Select fully ripe fruits.
2. Wash in clean water.
3. Remove the peel and seeds.
4. Pulp the fruit.
5. Heat the pulp for 15-20 minutes at 70-80 oC in a stainless steel pan, with constant stirring
to prevent it burning onto the pan. This softens the fruit and destroys any contaminating
micro-organisms and naturally occurring enzymes that would cause changes to the colour or
flavour of the fruit during storage.
6. Cool to room temperature.
7. Either mix the pulp into stirred yoghurt (e.g. 1 part pulp to between 3-5 parts yoghurt) or
place a layer of pulp at the bottom of pots before filling stirred yoghurt.
8. Store as for plain yoghurt.
Other product variations
Fruit or nut pieces can be added to each type of yoghurt, but care is needed to ensure that
they are thoroughly cleaned and blanched in boiling water or steam for approximately 5
minutes to prevent them introducing micro-organisms that would contaminate the product.
A layer of fruit syrup, maple syrup or treacle can be poured onto the top of set yoghurt.
If thicker stirred yoghurt is required, this can be made by mixing dried skimmed milk (at
approximately 50g/litre) into the milk before it is heated.
Some larger commercial yoghurt manufacturers use other thickeners such as starch or
pectin, but these are not necessary at a small scale of operation.
There is increasing interest in ‘probiotic’ bacteria that are reported to have health benefits.
To make probiotic yoghurt, the lactic acid bacteria Lactobacillus acidophilus and
Bifidobacterium sp. should be used (see ‘Yoghurt culture’ below).
Quality assurance
The quality of milk and the processing conditions that are used for making yoghurt should be
standardised so that consistent quality products are made each time. This involves control of
factors in the process that affect the quality or safety of the product. These are known as ‘control
points’ and are the points at which checks and measurements should be made (Table 1).
HACCP (Hazard Analysis Critical Control Point)
The specific potential hazards in making cultured milks and yoghurt are food poisoning bacteria
from the raw milk, poor hygiene and sanitation during processing, and incorrect processing
conditions. Other hazards that are common to all types of food processing (including
contamination of foods by insects, glass etc.) are prevented by correct quality assurance,
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