A Sustainable Strategy for the Conversion of Industrial Citrus Fruit Waste into Bioethanol

Abstract

The present study focused on the development of a methodology for the production of ethanol from the fermentation of waste resulting from citrus fruit processing. The experimental design included a number of steps, each optimized to make the whole process cost-effective, energy-saving, and ecofriendly. Particular emphasis was given to the pretreatment of citrus waste (CW), which was carried out through a combination of physical means, namely milling, heating, sonication, and microwave irradiation. Following this, an enzymatic hydrolysis was performed by loading a mix of enzymes, i.e., cellulase, pectinase, and β-glucosidase. Different combinations and concentrations were assayed with respect to the effective degree of saccharification. Afterwards, the hydrolysate was transferred to a bioreactor, added with nutrients and inoculated with two yeast strains, i.e., Saccharomyces cerevisiae and Saccharomyces bayanus. Fermentation lasted 48 h, leading to an amount of 40.1 g·L−1 ethanol. The process involved an extra step of fed batch that allowed the entire potential productivity of CW to be exploited by yielding 52.3 g·L−1 ethanol at a rate of 1.09 g·L−1·h−1. In accord with previously published data, this investigation has proven to be successful in reaching its prefixed objectives of sustainability.