Lactobacilli are extremely fastidious, adapted to complex organic substrates, and
for energy, they require carbohydrate and carbon sources, as well as amino acids,
nucleotides, and vitamins.131 The composition of milk satisfies some of the growth
requirements of lactobacilli, containing more than 87% water, ~4.7% lactose, ~3.8%
fat, ~3.3% protein, ~0.2% citrate, and ~0.6% minerals.15 Typically, lactobacilli can
be cultivated quite successfully in milk, and will reach maximum numbers after
24 h incubation at 37°C.132,133 Lactobacilli grown in milk can usually reach levels of
up to 108 or 109 CFU/mL, by which time they will also have entered the stationary
phase.132 During the fermentation of milk with lactobacilli, the pH is typically in the
range of 3.9 to 4.4. Thus, in some cases, it is the acidity of the final fermentate that
can prove inhibitory to lactobacilli when cultured in milk-based media, whereas it
does not affect the survival of the aciduric or acid-tolerant species of lactobacilli.131
At this point, fermentation is usually stopped by cooling and/or neutralization, which
will prevent acid injury132 during subsequent processing or storage