The proteolytic activity of S. thermophilus isolates was determined
on agar plates prepared as described previously (Dandoy
et al., 2011) with the following modifications. The agar medium
contained 1.5 g/100 mL agar, 1.9 g/100 mL Sodium glycerophosphate,
and 5 g/100 mL skimmed milk powder. 0.01 g/1000mL
bromocresol purple was added as acidity indicator for growth. The
isolates were propagated inM17 broth at 42 C for overnight, plated
on agar plates, and incubated at 37 C for 72 h. If the colonies
appeared as opaque, big and yellow, the isolate was classified as
protease positive, however if the colonies appeared as small, white
and translucent, the isolatewas considered to be protease negative.