requirements for safety, this has a substantial lethal effect on the micro-
flora. All but heat-resistant spores are eliminated so that the starter
culture encounters little by way of competition. The heat process also
improves the milk as a growth medium for the starter by inactivating
immunoglobulins, expulsion of oxygen to produce a microaerophilic
environment, and through the release of stimulatory levels of sulfydryl
groups. Excessive heating can however lead to the production of inhib-
itory levels of these compounds. Heating also promotes interactions
between whey or serum proteins and casein which increase the yoghurt
viscosity, stabilize the gel and limit syneresis (separation of whey).
The heat-treated milk is cooled to the fermentation temperature of
40–43 1 C which is a compromise between the optima of the two starter
organismsStrep. thermophilus(39 1 C) andLb. delbrueckiisubsp.bulga-
ricus(45 1 C). The starter culture is added at a level of about 2% by
volume to give an initial concentration of 10^6 –10^7 cfu ml^1 composed of
roughly equal numbers of the two organisms. The fermentation can be
conducted in the retail pack to produce a firm, continuous coagulum,
which is known as a set yoghurt, or in bulk tanks to produce a stirred
yoghurt where the gel has been broken by mixing in other ingredients
and by pumping into packs.
The fermentation takes about 4 h during which the starter bacteria
ferment lactose to lactic acid decreasing the pH from its initial level of
6.3–6.5. The lactic acid helps solubilize calcium and phosphate ions
which destabilize the complex of casein micelles and denatured whey
proteins. When the pH reaches 4.6–4.7, the isoelectric point of the casein,
the micelles aggregate to produce a continuous gel in which all the
components are entrapped with little or no ‘wheying-off ’.
During fermentation growth of the streptococci is fastest in the early
stages, but as the pH drops below 5.5 it slows and the lactobacilli tend to
predominate. By the end of fermentation the product has a total acidity
of 0.9–0.95% and the populations of the two starter organisms are
roughly in balance again with levels in excess of 10^8 cfu ml^1.
The relationship between the two starter organisms is one known as
protoco-operation, that is to say they have a mutually favourable inter-
action but are not completely interdependent. Both will grow on their
own in milk but will grow and acidify the product faster when present
together. Growth of the streptococcus in milk is limited by the avail-
ability of peptides and free amino acids which are present in relatively
low concentrations (E50 mg kg^1 ). The lactobacillus is slightly proteoly-
tic and liberates small amounts of these, particularly valine, which
stimulate streptococcal growth. In its turn the streptococcus produces
formate, pyruvate and carbon dioxide all of which stimulate the lacto-
bacillus. Formate is used in the biosynthesis of the purine base adenine, a
component of RNA and DNA and Lb. delbrueckiisubsp.bulgaricus
Chapter 9 325