610 Part VI: Fermented Foods
growth and should be present in approximately
equal numbers for optimal flavor development. Lb.
delbrueckiissp. bulgaricushas important protease
activity and hydrolyzes the milk proteins to small
peptides and amino acids. These peptides and amino
acids enhance the growth of S. thermophilus,which
has limited proteolytic activity. S. thermophilusme-
tabolizes pyruvic acid to formic acid and carbon
dioxoide, which in turn, stimulates the growth of Lb.
delbrueckii.Initially, S. thermophilusgrows faster
than Lb. delbrueckii; however, at the later stages
of the fermentation process, the growth of the S.
thermophilusis inhibited by the reduced pH of the
yogurt. The mutual stimulation of the yogurt cul-
tures through their metabolic activity significantly
increases the formation of lactic acid to a rate
greater than would be possible by the individual cul-
tures.
The acid production by the yogurt cultures results
in the aggregation of casein micelles and gel forma-Figure 26.5.Processing scheme for set- and stirred-style yogurts.