grain itself suggests that the lactic acid bacteria are responsible for its
production and a capsular, homofermenterLactobacillus kefiranofaciens
has been shown to produce kefiran. A heterofermentative lactobacillus
Lb. kefiris numerically very important in many grains and plays a key
role in the fermentation, probably among other things contributing to
the required effervescence in the product. Although less significant
numerically, several yeasts have been reported includingCandida kefir,
Saccharomyces cerevisiaeandSacc. exiguus. The latter is particularly
interesting because it was shown to utilize galactose preferentially in the
presence of glucose and this may confer an advantage when growing in a
mixed culture of organisms most of which will preferentially metabolize
the glucose portion of lactose.
Kefir is produced commercially in a number of countries, most
importantly in Russia and those states which comprised the old Soviet
Union. In the mid-1980s production of kefir reached 12 million tonnes
representing 80% of all dairy products, excluding soft cheese and sour
cream. In commercial practice, milk for kefir production is homogenized
and heated to 85–95 1 C for between 3 and 10 min. It is cooled to 22 1 C
before addition of kefir grains at a level of up to 5%. The fermentation
itself lasts for 8–12 h but is sometimes followed by slow cooling to
around 8 1 C over 10–12 h to allow for the required flavour development.
Kefir has an acidity of about 0.8% and an alcohol content which has
been reported as varying between 0.01% and 1%. Ethanol levels tend to
be lower in commercial products than domestically produced kefir and
increase with the age of the product. In addition to the character
imparted by the ethanol, lactic acid and carbon dioxide, acetaldehyde
(ethanal) and diacetyl are also present as flavour components.
Koumiss is a fizzy, greyish white drink produced traditionally from
mare’s milk in eastern Europe and central Asia. It can have an acidity up
to 1.4% and an ethanol content up to 2.5%. A mixed yeast/LAB flora is
responsible for the fermentation comprising Lb. delbrueckii subsp.
bulgaricusand a number of lactose fermenting yeasts. These are dis-
persed throughout the product and do not form discrete particles as in
kefir. Cow’s milk is a more convenient raw material to use nowadays and
this is usually modified to resemble more closely the composition of
mare’s milk which has a lower fat content and higher carbohydrate
levels.
Strictly speaking, buttermilk is the liquid which separates from cream
during the churning of butter (see Chapter 5). However, to achieve a
consistent quality product most buttermilk today is produced directly by
the fermentation of skimmed or partially skimmed milk. Cultured but-
termilk is an acidic refreshing drink with a distinctive buttery flavour. A
mixture of starter organisms is required to produce these attributes;
Lactococcus lactisproduces most of the lactic acid, while the buttery
328 Fermented and Microbial Foods