3.3. Effect of pH on L-lactic acid production
In previous reports, the most commonly used alkaline neutralizing
reagent is CaCO3. This requires processing for the regeneration
of calcium lactate and result in by-product CaSO4. In order
to reduce the production cost, NaOH was used in this study. The
starting pH was adjusted to 7.2 and controlled at 7.2, 6.3, and 5.4
in the process, respectively.
As shown in Fig. 4, the best result was obtained at pH 6.3. At this
pH, 71 g l1 L-lactic acid was produced and only 1 g l1 glucose remained.
The yield of L-lactic acid was 88.75%. At pH 7.2, the consumption
rate of glucose became slow after 35 h. In the end,
59 g l1
L-lactic acid was produced and 15 g l1 glucose still remained.
The worst result was at pH 5.4 and the L-lactic acid production
rate has already slowed down at 10 h. After 70 h, the
final L-lactic acid concentration was only 50 g l1. The reason
may be due to the poor growth of Bacillus sp. NL01 strain at lower
(pH 5.4) or higher (pH 7.2) pH. As the pH in environment would
change the charge distribution on cell membrane and alter its permeability,
lower or higher pH hereby influenced the enzyme