3.1. Cheese strains with GAD activity
The GAD activity of 580 colonies (10 colonies per cheese) was
qualitatively assessed. Changes in colour from magenta to green
observed in L-glutamate medium due to pH values above 5.4
allowed strains with high or low GAD activity to be distinguished.
Rapid changes (24 h or less) showed high GAD activity, since slow
(48 h or more) or diffuse (grey) change in colour means low activity.
If more than 72 h was required to distinguish colour, the strainwas
discarded. Results showed that a total of 65 colonies belonging to
23 cheeses resulted positive in the screening test, which represents
11.2% of the total colonies analyzed. A total of 10 colonies out of
these 65 (Table 1) showed their GAD activity in less than 24 h, like
the control strain. The rapid increase in pH and the colour change in
the L-glutamate medium of the ten strains corresponded to irreversible
decarboxylation of L-glutamate to GABA molecule by the
GAD enzyme. Cheese milk’s biological origin may be related to high
GAD activity, since only one strain was isolated from cow cheese,
whereas 4 strains were isolated from sheep cheese and 4 from goat
cheese. The most promising strain for GABA production was from a
mixture of milks. In all cases, all cheeses were cured and fatty.
The 10 high-potential strains were selected for the next experimental
step.