genes is assumed to control the acidification of the cytosolic environment
by decarboxylating an acid substrate (glutamate) into a
neutral compound (GABA) via incorporation of a proton (Cotter &
Hill, 2003). This consumption of the intracellular proton is responsible
for shifting the intracellular pH towards neutral, helping microbes
to survive in the acidic environment (Yuko, Kiyoshi, & Ueno,
2011). By adjusting the pH to 4.7 in the acid-base indicator method,
an acidic condition is achieved to activate the GAD enzyme. Moreover,
pH of 4.2 and 4.7 are the optimum pH for Lactobacillus and
Lactococcus GABA production (Yang et al., 2006). By these conditions
and throughout GAD gene expression, which is dependent on the
strain, GABA would then be exported into the extracellular environment,
thereby contributing to alkalinization and so changing the
medium’s colour.