22.4.1 THE YOGURT BACTERIA
22.4.1.1 Growth
The yogurt bacteria, Streptococcus thermophilus and Lactobacillus delbrueckii
ssp. bulgaricus, grow in milk better when present together than each alone
(protocooperation). The proteolytic rods enhance growth of the streptococci by
forming small peptides and amino acids, the main amino acid being valine. Milk
contains too little of these amino acids and the cocci, which are only weakly
proteolytic and form the acids too slowly. The cocci enhance the growth of the
rods by forming formic acid out of pyruvic acid under anaerobic conditions and
by a rapid production of CO2 (see Section 13.1). The stimulatory effect of formic
acid remains unnoticed in intensely heated milk because in this milk formic acid
has been formed by decomposition of lactose. The production of formic acid by
the cocci is, however, essential in industrial practice, where more moderate heat
treatments of yogurt milk are applied, e.g., 5 to 10 min at 85°C. Due to mutual
stimulation during combined growth of the yogurt bacteria in milk, lactic acid is
produced much faster than would be expected on the basis of the acid production
by the individual pure cultures. Some antibiosis also occurs in yogurt in that the
cocci cannot grow after a certain acidity has been reached. The rods are less
susceptible to acid and continue to grow. Protocooperation and antibiosis are of
great importance in the growth of the yogurt bacteria as well as for the quality
of yogurt (see also Figure 22.1).