A sustainable strategy for the production of bioethanol from agrifood waste

A sustainable strategy for the exploitation of citrus fruits processing waste aiming at the obtainment of bioethanol (BioEt) by fermentation, has been designed and implemented. Process innovation mainly consisted of maximizing both eco-efficiency and yield. The experimental plan has been applied to samples of citrus waste (CW), preliminarily deterpenated (i.e. limonene-free), and coming from Sicilian citrus processing plants. The conventional process applied to the production of BioEt has been redesigned with a “green vision”, thus overcoming its weakest points such as low yield and high environmental impact1. In particular, key steps of our approach have been focused on: i) selection of optimal bacterial strains; ii) substitution of acidic hydrolysis treatment with physical and mechanical methods; iii) increase of final bioethanol yield through circular recovery of residual dry matter. 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 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: 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.