Cows' milk often does not support extensive growth of
bacteria because of the lack of free amino acids. Yogurt starter
cultures therefore contain bacteria with proteolytic activity, such
as L. delbrueckii subsp. bulgaricus, that break down protein to
produce amino acids that support the growth of nonproteolytic
bacteria. In traditional yogurt, S. thermophilus is generally the
beneficiary of this proteolysis, but in our fermentations the
probiotics could also have benefited from it.