9.5.2 Other Fermented Milks
The popularity of acidophilus milk is largely due to health-promoting
effects which are claimed to stem from the ability of Lactobacillus
acidophilusto colonize the gut. It is a thermophilic homofermenter but
is slow fermenting and a poor competitor and is easily outgrown. As a
result, the fermentation takes longer than for yoghurt and great care
must be taken to avoid contamination. In the original process whole or
skimmed milk was sterilized prior to fermentation by a Tyndallization
process. This involved two heating stages of 90–95 1 C for up to an hour
separated by a holding period of 3–4 h to allow spore germination to
occur. Nowadays the same effect can be achieved more swiftly and
economically by UHT processing. The milk is then homogenized, cooled
to the fermentation temperature of 37–40 1 C and inoculated with 2–5%
of starter culture. It can take as long as 24 h to produce the required
acidity of about 0.7%, after which the product is cooled to 5 1 C.
In addition to the extra care required in its production, acidophilus
milk suffers from a number of other drawbacks. In particular, it lacks the
sensory appeal of yoghurt, being restricted to a rather sour, acidic taste.
Also, theLb. acidophiluscells do not survive well in the acid product,
dying out after about a week’s storage at 5 1 C. To avoid these problems,
a non-fermented sweet acidophilus milk is produced in the United States
where large numbers ofLb. acidophilusare simply added to pasteurized
milk without incubation.
In an attempt to combine the supposed virtues of acidophilus milk
with those of yoghurt, a number of ‘bio-yoghurts’ are now produced.
These contain a mixture of organisms, those able to colonize the gut such
asLb. acidophilusandBifidobacteriumspp. withStrep. thermophilusto
provide the characteristic yoghurt flavour. However, because of their
poor survival at acid pH, it is likely that the strains used are chosen for
their ability to survive in the product as much as for any benefit they may
havein vivo.
Kefir and koumiss are distinctive fermented milks produced by a
mixed lactic acid bacterial fermentation and an alcoholic yeast fermen-
tation. Kefir is further distinguished by the fact that the microflora
responsible is not dispersed uniformly throughout the milk but is added
as discrete kefir ‘grains’. These are in fact sheets composed largely of a
strong polysaccharide material, kefiran, which folds upon itself to pro-
duce globular structures resembling cauliflower florets. The outside of
the sheets is smooth and is populated by lactobacilli while the inner,
rougher side of the sheet carries a mixed population of yeasts and lactic
acid bacteria. A large variety of different organisms have been reported
as being associated with the fermentation, probably reflecting the wide-
spread and small-scale nature of production. The morphology of the
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