BIOPROCESS DEVELOPMENT OF CITRIC ACID PRODUCTION

Abstract

A high performance fermentation process for the continuous production of citric acid from sugarcane molasses using the combination of submerged calcium alginate-immobilized and surface stabilized cultures of Aspergillus"niger KCU 520 in a continuous flow horizontal bioreactor is described. The citric acid productivity was dependent on the dilution rate with an optimum value of 0.015 h1. Presaturation of fermentation medium with sterile air, in addition to surface aeration, before feeding into the bioreactor enhanced the citric acid productivity. The highest productivity, citric acid product concentration and citric acid yield obtained were 1.7 KgM-3h"1, 110 KgM'3 and 91%, respectively. The fermentation was continuously operated for 30 days without any apparent loss in citric acid productivity. Anew mutant strain, Aspergillus nigerGS-\ 11, showing resistance to manganese ions inhibition of citric acid fermentation on a sugarcane molasses containing medium was isolated from Aspergillus niger KCU 520, a high citric acid-yielding strain. In submerged, surface or continuous cultures in the presence of manganese ions concentration upto 1.5 ppm the mutant strain yielded citric acid about 90 KgM-3. The citric acid yield was comparable to that obtained with theparental strain KCU 520 inthe absence ofmanganese ions, but it was 3-fold higher than that obtained by the latter in the presence of manganese ions. The mutant strain immobilized in calcium alginate beads was used in combination with surface stabilized cultures for about 36 days in a continuous flow horizontal fermenter without any apparent loss in citric acid productivity. These results indicate that the manganese resistant mutant is stable and may be used in the presence of sufficient manganese ions concentration (1.5 ppm) in the fermentation medium. This capability of the mutant strain Aspergillus nigerGS-\\\ has been correlated with greatly reduced levels (about (i) one-third) of the NADP+ -isocitric dehydrogenase, one of the control enzymes for citric acid accumulation. Citric acid fermentation waste, after the removal of mycelium and citric acid, was used forthe production of single cell protein (SCP), asthiswastecontained about 20 g/L of total sugars. The yield of SCP (yeast) obtained from the citric acid spent liquor was higher than that obtained using the normal growth medium, YEPD, and the composition of SCP was comparable with the standard SCP reported in the literature. It is recommended that SCP may be an important byproduct of the citric acid fermentation industry using sugarcane molasses as the substrate.