Soured milk and yoghurt
Practical Action
Seal
Label
Seal pots with a foil lid or clip-on
plastic lid
Attach label to pots
Refrigerate
Hold at 4-8 oC
Table 1. Process control points in yoghurt production
Check that the seal is properly
formed
Check that correct label is used,
that they are correctly positioned,
and show the correct use-by date
and batch number.
Check refrigerator temperature
The temperature and time of heating and cooling the milk should be controlled. Over-heating
and/or slow cooling causes changes to the flavour, colour and nutritional value of the milk,
whereas under-heating may lead to survival of undesirable micro-organisms, causing spoilage of
the product. The correct yoghurt incubation temperature is required to allow rapid production of
lactic acid by lactic acid bacteria. If the temperature is too high the bacteria will be killed; if it is
too low there may be insufficient acid production to form the yoghurt.
The main controls for processing the fruit pulp for fruit-flavoured yoghurt are:
Fruit should be ripe or slightly over-ripe, free from insect damage or mould/yeast growth,
and free from bruises, cuts or splits. Correct sorting, cleaning and peeling procedures are
needed to remove unwanted parts of the fruit and any contaminated or spoiled fruit.
Correct pulping to produce a uniform fine pulp without large pieces that would be
inadequately heated during pasteurisation.
Adequate time and temperature of pasteurisation to destroy contaminating micro-organisms
(especially yeasts and moulds that are able to grow in the yoghurt).
Mixing of pulp and yoghurt in the correct proportions to retain the thick creamy consistency
while giving the required flavour and colour in the final product.
Maintenance of correct incubation temperature
A commercially produced domestic scale yoghurt maker may be purchased, but they are
relatively expensive. A local engineering company may be able to fabricate a shallow water bath
with a small electric heating element and thermostat to keep the water at the correct
temperature. The fermentation pan is placed in the water. Alternatively, at a micro-scale of
production the inoculated milk at 40-45 oC is filled into large thermos flasks, which retain the
heat for the required time. Similar insulation can be achieved using a block of 10 cm thick
polystyrene, which has indentations to fit small containers. The warm inoculated milk is filled
into the containers in the block of polystyrene, and a polystyrene lid is placed on top. A similar
incubator consists of a hollow polystyrene box approximately 0.75 m3 fitted with a 40 W electric
light bulb. The heat from the lightbulb maintains the temperature within the required range.
Typically these incubators can accommodate up to 250 pots per batch. Practical Action South
Asia with the Cathy Rich Memorial Food Processing Centre have designed improved low cost
incubators for yoghurt that have a greater capacity, with either manual and semi-automatic
operation. The incubators have aluminium frames with ‘rigid foam’ insulation. There are two
chambers with 6 removable shelves in each chamber. 40W light bulbs are fitted at the base of
the incubator to supply the required heat. The incubator has a glass panel to view the yoghurt
without opening the incubator. The manual model requires the incubator temperature to be
monitored and the light bulbs to be turned on and off as required, whereas the semi-automatic
version uses a thermostat to control the temperature. There is an optional internal fan to produce
more uniform temperatures throughout the incubator.
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