Effect of acid pre-treatment and two-stage oxygen-assisted fermentation on the production of vinegar from lignocellulose biomass peel of pineapple

The valorization of lignocellulosic waste stands as a promising avenue to bolster sustainable food production and consumption within a circular economy framework. This study centered on the production of vinegar from pineapple peels through a two-stage fermentation process aided by oxygen. The pineapple peels underwent sorting, washing, drying, and subsequent grinding into a powder. This powder was subjected to hydrolysis using dilute sulphuric acid, followed by primary alcoholic fermentation utilizing Saccharomyces cerevisiae. The resulting fermented must was then subjected to oxidation in a second stage, facilitated by Acetobacter aceti, with varying concentrations of oxygen. A central composite design involving three factors, fermentation time, bacteria inoculum, and oxygen was employed to investigate the impact of these process parameters on the physicochemical attributes (pH, specific gravity, total soluble solids, titratable acidity) and the ferric reducing antioxidant power (FRAP) of the produced vinegar. The acid hydrolysis phase led to a notable rise in total soluble sugars (6 to 11.5 oBrix) and glucose concentration (300 to 580 mg/dL). Primary fermentation resulted in significant reductions in pH (7.02 to 5.38), total soluble solids (11.5 to 6 oBrix), and glucose concentration (580 to 62 mg/dL), accompanied by marked increases in titratable acidity (g/100 ml) and alcohol content (0.6 to 7%). The volume of oxygen demonstrated significant effects on acetic acid content, pH, and specific gravity, with the highest values (4.68 g/100 ml, 4.02, and 1.004, respectively) achieved at the maximum oxygen volume of 100 ml. The FRAP values ranged from 16.7 to 24.97 mg Fe2+ / mg, with the sample lacking oxygen displaying the highest FRAP. Furthermore, fermentation time and bacteria inoculum exerted significant effects on acetic acid content, with an optimal value of 4.43 g/100 ml. Interaction between bacteria inoculum, oxygen volume, and fermentation time also had significant effects on specific gravity.