The kinetic basis of the role of Ca++ ions for higher yield of citric acid in a repeated-batch cultivation system

Abstract

The present investigation deals with role of Ca++ ions in increasing the yield of citric acid in a repeated-batch cultivation system (working volume 9-1) and its kinetic basis. Five different hyper-producing strains of Aspergillus niger were evaluated for citric acid production using clarified cane-molasses as basal substrate. Among the cultures, NGGCB101 (developed by u.v./chemical mutation in our labs) gave maximum production of citric acid i.e., 87.98 g/1, 6 days after mycelial inoculation. The addition of CaCl2 to the culture medium promoted the formation of small rounded fluffy pellets (1.55 mm, diameter), which were desirable for citric acid productivity. CaCl2 at a level of 2.0 μM, added during inoculation time, was optimized for commercial exploitation of molasses. During repeated-batch culturing, a yield of citric acid monohydrate of 128.68 g/1 was obtained when the sampling vs. substrate feeding was maintained at 4-1 (44.50% working volume). The incubation period was reduced from 6 to only 2 days. The values of kinetic parameters such as substrate consumption and product formation rates revealed the hyperproducibility of citric acid by the selected Aspergillus niger NGGCB101 (LSD = 0.456a, HS). Case studies are highly economical because of higher yield of product, lower energy consumption and the use of raw substrate without any additional supplementation.