The changes in ACE-inhibitory activity, cell growth, peptide
content, pH, lactose concentration and ethanol content during
milk fermentation under optimal conditions are shown in Fig. 2.
Increase in ACE inhibition with an accompanying increase in
peptide content, began in the middle of log phase and
maximum activity was reached at the early stationary phase
to middle of stationary phase. The accumulation of peptides
in fermented milk is closely related to proteolysis by cell’s proteolytic
system. On the other hand, it also depends on
consumption of peptides required for cell growth. The concentration
of peptides in the medium is the result of interaction
between the two above mentioned effects. The OPA index increased
slowly at the early stage of log phase due to the leading
role of fast growing cells utilizing peptides rapidly. In the early
stationary phase, cell growth rate slowed down but proteolytic
enzymes still remained active and resulted in the
production and accumulation of peptides up to a higher level
that could be reflected by increased OPA. This result suggests
that the optimal fermentation time should be chosen at the
middle of stationary phase (48 h) to achieve the highest ACEinhibitory
activity (82.4 ± 3.12%). The decrease in the
concentration of lactose is correlated with the cell growth and
increase in ethanol content. After 48 h fermentation, lactose
concentration was 13.57 ± 1.06 g/L, which corresponds to 70.0%
lactose reduction, while ethanol content was 110.9 mM.