Selection of Neutralizing Agent
It is now a well-known fact that in Rhizopus-mediated FA production, the pH value of FA
production medium drops down (e.g., from 5 to 2) quickly in the first 20 to 24 h after
inoculation due to production of FA. The consequence is the strong inhibitory effect on the
growth of R. oryzae and FA production. This necessitates the addition of a neutralizing agent
that will make complex with FA and thus maintaining the pH level at optimum for the growth
of R. oryzae and FA production. After decades of research on different neutralizing agents
(e.g., CaCO3, Na2CO3, NaHCO3, (NH4)2CO3, and Ca (OH)2), calcium carbonate was found to
be the most efficient neutralizing agent in the commercial level production of FA [1]. The
justifications made were (a) FA yield and volumetric productivity were found to be lower for
other neutralizing agents than CaCO3; (b) accumulation of byproducts, such as malic acid and
ethanol, was higher with other neutralizing agents; and (c) CaCO3 can supply CO2 that can be
used for the formation of oxaloacetate in the tricarboxylic acid (TCA) cycle.
Thus, selection of CaCO3 as a neutralizing agent makes sure that the present study does not
compromise with the growth of R. oryzae and production of FA.