3.3 Optimization of reaction conditions for whole-cell
biotransformation
Outstanding thermostability of SI has always been pursued in successful
industrial manufacturing bioprocess for isomaltulose, as even slight
enhancement can improve long-term activity under optimum conditions,
increase the ability to remain high activity in repeat batches of
whole-cell biotransformation. To develop an economically feasible
whole-cell biocatalysis process, the influence of temperature, pH, cell
density as variables were explored. As shown in Figure 3A, the optimum
pH of whole-cell activity was 6.0,
which was higher than that of pure enzyme expressed from C.
glutamicum . Wu’s report also showed the same shift in the optimal pH,
which may be attributed by the differences of
micro-environment.[23] Whereas no obvious changes
were detected in the optimum temperature between free enzyme and
whole-cell activity, all the maximum catalytic activity was at 30℃
(Figure 3B). Moreover, cell dosage also plays an important in the actual
production, and reaction time is related to the cell density during the
biotransformation process. Figure 3C shows that the concentration of
isomaltulose was highest in the first 1.5 h when the cell density was
OD600=30, while the yield of isomaltulose at
OD600=25 was significantly higher than
OD600=30 after 1.5 h. Therefore,
OD600=25 was selected as the optimal cell dosage for the
synthesis of isomaltulose.