SOURED MILK AND
YOGHURT
Introduction
Traditionally, soured milk is produced by the uncontrolled fermentation of milk using lactic acid
bacteria that occur naturally in the milk. It is a thick clotted product that has a stronger flavour
and a more acidic taste than yoghurt. It has a shelf life of 3-8 days and is used as a drink or as
an accompaniment to a meal. Cultured buttermilk is the fermented by-product of butter
manufacture (see Technical Brief: Butter and ghee), but it is also produced as a product from
skimmed milk or whole milk using a culture of Streptococcus lactis or Streptococcus cremoris.
There are a large number of other cultured milk products, including kefir, koumiss, labneh, and
others that are specific to particular regions, which each have different flavours and textures that
depend on the starter micro-organisms that are used.
Yoghurt is produced by the controlled fermentation of milk by two species of lactic acid bacteria
(Lactobacillus sp. and Streptococcus sp.). The acid they produce causes the milk to form a
characteristic curd and flavour. It can be kept for up to ten days under refrigerated storage (see
Technical Brief: Dairy processing - an overview for the principles of preservation, which is
intended to be read alongside this Technical Brief). The removal of lactose by lactic acid bacteria
also means that soured milks and yoghurt can be eaten by people who suffer from lactose
intolerance. Flavoured and fruit yoghurts have a similar shelf life to plain yoghurt, a similar
smooth creamy texture and a sweet-sour taste with the characteristic flavour and colour of the
essence or fruit that is added. These products are mainly used as desserts.
Methods of processing
Soured and cultured milks
These are produced without specialised equipment. The milk is filled into food-grade plastic,
aluminium or stainless steel containers (e.g. 5-litre) that have close-fitting lids, and allowed to
sour naturally in a warm ventilated room for 1-2 days. It can then be sold directly from the
container or dispensed into pots or jars for sale.
In the production of cultured buttermilk, milk is heated to 95 oC and cooled to 20-25 oC before
adding 1-2% starter culture. The fermentation time is 16-20 hours. This product may be used as
an ingredient in bakeries as well as being sold at retail outlets.
Yoghurt
There are two types of plain (unflavoured) yoghurt that are produced under more controlled
conditions than soured milk. These are ‘set’ yoghurt that has a firm curd, and ‘stirred’ yoghurt
that has a smooth creamy consistency, similar to a thick cream. The production method is:
1. Heat milk in a stainless steel (or less desirably an aluminium) pan to 80-85 oC and hold at
this temperature for 15-20 minutes with constant stirring. This destroys contaminating
micro-organisms.
2. Cool to 40-45 oC as quickly as possible and add a starter culture (or ‘inoculum’) of the mixed
lactic acid bacteria (see below).
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