organisms present in a culture are chosen for their ability to produce acid under various
conditions. The organisms mentioned are active at room temperature. At the higher
temperatures needed in the manufacture of cheese, other strains are used. For high
yields of lactic acid, organisms such as Lactobacillus bulgaricus are used. For a
combination of acidity, flavor, and aroma, the cultures contain Streptococcus citrovorum
or Streptococcus paracitrovorum.
b. Other Effects. Starter cultures fulfill a number of functions in the preparation
of fermented milks and in the manufacture of some varieties of cheese. The acidity
these cultures produce helps rennet to coagulate the milk in cheesemaking. Cultures
containing Streptococcus thermophilus and other special types of bacteria are used in
the manufacture of Swiss and other kinds of cheese. Propioni-bacterium shermanii is
important because it uses the lactic acid produced by other organisms to form propionic
acid, acetic acid, and the carbon dioxide gas which give Swiss cheese its characteristic
flavor and forms the "eyes."
c. Types of Fresh Cultured Products. Some of the types of fresh cultured
dairy products are:
(1) Sour cream. This is the fluid or semifluid cream resulting from the
souring by lactic acid-producing bacteria or similar culture, and of pasteurized cream,
which contains not less than 0.20% acidity expressed as lactic acid. Other names for
sour cream are salad cream, Hampshire cream, or cream dressing.
(2) Cultured buttermilk. This is the fermentation of milk or skim milk with
lactic acid bacteria. The pasteurized milk used for the preparation of cultured buttermilk
is inoculated with up to 1% of starter and held at a temperature around 70F (21C) until
the desired acidity is reached. (0.7-0.9% acidity expressed as lactic acid.)
(3) Yogurt. This is the fermented milk of the lactic acid type. The starter is
prepared by growing separate cultures of Streptococcus thermophilus and Lactobacillus
bulgaricus and mixing them just before adding them to the milk that is to be fermented.
A number of different fruits and flavors may be added to the yogurt.
(4) Cottage cheese. This is a lactic acid, unripened, soft-curd cheese.
Cottage cheese is made commercially from skim milk, reconstituted skim milk, or nonfat
dry milk. The most common starters used are Streptococcus lactis or cremoris.
d. Steps in Manufacture. The following steps are applicable in the
manufacture of most fresh cultured dairy products.
(2) Cooling the milk.
(3) Inoculation or introducing the bacteria.