The cocci as well as the rods contribute significantly to the properties of
yogurt. The properties of the bacterial strains used should be matched to each
other because not every combination of strains is suitable. Furthermore, both
species should be present in large numbers in the product, and hence in the starter. The mass ratio of the two species depends on the properties of the strains
and is often approximately 1:1. This ratio between the yogurt bacteria is best
maintained if the inoculum percentage is, say, 2.5, the incubation time is 2.5 h
at 45°C, and the final acidity is approximately 90 to 100 mM (pH ≈ 4.2). The
growth of cocci and rods in yogurt incubated under these conditions is depicted
in Figure 22.2. The ratio between the species keeps changing. Initially, the
streptococci grow faster due to the formation of growth factors by the rods and
probably also due to the latter compounds being added via the inoculum
(especially in the manufacture of set yogurt). Afterwards, the cocci are slowed
down by the acid produced. Meanwhile, the rods have started to grow faster
because of the growth factors (CO2 and formic acid) formed by the cocci. As
a result, the original ratio is regained. The yogurt should then have attained the
desired acidity. Continuing incubation or inadequate cooling causes the rods to
become preponderant.