3.1.2. Effect of pH of reaction and incubation time
It has been reported that the activity of cellulase depends
on pH [37]. According to the manufacturer’s recommendation,
the enzyme preparations are effective between pH 3 and 7
and the optimum pH generally lies between 4 and 5. Nevertheless,
the most appropriate pH which gave the maximum
extraction yield of genipin was needed to be investigated in
our case since the reactor was complicated by side reaction
occurring between genipin formed and the enzyme at certain
pH to produce the unwanted blue products, leading to the
loss of yield. Furthermore, genipin itself could be hydrolyzed
in acidic condition [38]. Accordingly, the proper operating pH
must be deliberately selected to achieve both the high activity
of the enzyme and the low degradation of the product.
In this work, the effects of pH and of incubation time were
simultaneously studied by keeping the constant enzyme concentration
at 10 mg/mL as previously optimized. As shown in
Fig. 3, the high yields of genipin were generally obtained from
the reactions having pH 3.5–4.5, depending on the incubation
time used. The concentrations of geniposide tended to decrease
while those of genipin raised with time for all pH tested due to
the ongoing conversion of glycoside to aglycone. Since the maximum
yield was achieved at about 35 mg genipin per gram of