March, 1952]
H.
LAXMINARAYANA
AND
K.
K.
IYA
5
In recent years special fermented milks under the name ' cultured milks'
have become very popular in Europe and America. The most important of these
are 'Bulgarian butter milk' and 'Acidophilus milk'. Bulgarian butter milk is
prepared by souring skimmed or whole milk with a pure culture of Lactobacillus
bulgaricus and the product is viscous and highly sour [Burke, 1938]. In the pre-
paration of Acidophilus milk pure cultures of Lactobacillus acidophilus are used and
the product is not as sour as Bulgarian butter milk [Knaysi, 1932]. It is greatly
valued for its reputed therapeutic properties. Pure cultures of Streptococcus
paracitrovorous are sometimes used for the development of desirable flavour in
cultured milks. In order to improve the taste and nutritive value of the products
various fruit juices and special flavouring substances are sometimes incorporated.
Several modifications, involving the use of skimmed milk, whole milk, cream,
condensed and dried milks or whey, and the manufacture of refrigerated or iced
butter milk and dried butter milks constitute some of the latest developments in
this field of dairy industry [Burke, 1938; Cronshaw, 1947].
It is evident that in the various fermented milk preparations lactic acid bacteria
are the principal agents taking part in the fermentative processes. The fermenta-
tion of lactose into lactic acid is the primary chemical change sometimes accompanied
by the production of volatile acids and gas or a mild alcoholic fermentation brought
about by yeasts. There may be a slight increase in the soluble nitrogen content
due to the action of microorganisms on proteins. Van Slyke [1928] and recently
Khambatta and Dastur [1950] investigated the various changes in the constituents
of milk occurring during the process of souring. The lactic acid bacteria occurring
in fermented milks can be divided into three main groups according to the latest
taxonomical classification of Bergey [Breed and others, 1948]. The first group
includes the common milk souring organism, Streptococcus lactis ; the high tempera-
ture milk souring streptococcus, L. thermophilus; and S. cremoris, used for the
ripening of cream. They produce mainly lactic acid by the fermentation of lactose,
the final acidity reached being one per cent calculated as lactic acid. The second
group includes the high acid producing lactobacilli, L. caucasicum, L. bulgaricus,
L. acidophilus. L. casei and L. plantarum. The first two species are most common-
ly associated with the sour milks, while L. acidophilus is primarily an intestinal
organism, whose presence in fermented milks is considered of therapeutic signi-
ficance. All these lactobacilli ferment lactose mainly to lactic acid, usually up to
2.5 per cent or rarely 3.5 per cent titratable acidity, and on account of their higher
acid tolerance they always predominate over streptococci in course of time. The
hetero-fermentative lactobacilli (Betabacteria), which grow slowly and produce
acetic acid, propionic acid and other volatile compounds as well as carbon dioxide
and hydrogen in addition to lactic acid, come under this group. The third group
includes the so-called ' aroma bacteria'—S. citrovorous and S. paracitrovorous,
which are slow acid producers and can only grow in association with other lactic
streptococci. They produce lactic acid together with volatile acids and gas but
their main importance consists in their ability to form acetyl methyl carbinol
(acetoin) and its oxidation product diacetyl by the fermentation of citric acid or
citrates in milk. The formation of diacetyl is considered to be responsible for the
characteristic aroma associated with many fermented milks. Next in importance
to lactic acid bacteria are the yeasts (Saccharomycetes and Torulae, as well as