Requirements of mixed tangerine (Citrus tangerine) and pineapple (Ananas comosus) powdered peel wastes fermentation for citric acid production

Abstract

In this study, mixed substrate submerged fermentation of fruit peel wastes for the production of citric acid is reported. Powders of the peel wastes of tangerine and pineapple were used as substrates. Methanol (1-3% w/w) and sucrose (5-15% w/w) were supplemented in the fermentation media (pH on the range of 2.0-6.0) with moisture content of 90% (wet basis). Effect of substrate mix, pH of the media, methanol and sucrose concentrations on the yield of citric acid was studied, and their optimal levels for the maximum yield of citric acid were identified based on Taguchi method.  Methanol as stimulant in the medium and ratio of substrates in the mix had the highest influence on the yield of citric acid.  Fermentation media with tangerine and pineapple peel wastes mixed in the ratio of 1:1, initial pH of 4, and supplemented with methanol and sucrose to the extent of 5 and 15% (w/w), respectively was found to be optimal, and that resulted in the maximum citric acid yield of 137.2±0.4 g kg^-1 of dry matter after 12 days of fermentation. The paper and thin layer chromatography, and spectrophotometric absorption spectra of the sample confirmed the production of citric acid. The fermentation kinetics of the citric acid revealed that the production of citric acid is a two stage process. In the first stage, slow production of citric acid at rapid biomass growth takes place, whereas in the second stage, very rapid citric acid production at declined biomass growth rate takes place. Valorisation of fruit peel waste for the production of citric acid reduces the environmental pollution, carbon emissions and greenhouse gas effects.