Biotransformation of agro-industrial waste to produce lignocellulolytic enzymes and bioethanol with a zero waste

Abstract

The use of lignocellulosic wastes reduces dependence on fossil fuel resources, contributes to sustainable waste management, and reinforces the circular economy model of continual use of resources. Typically, the second generation of bioethanol production involves several steps to transform lignocellulosic material into bioethanol. The more complicated step of the overall process is to define a tailor-made to each lignocellulosic material available with a wide variety of complex structures of the lignin, hemicellulose, and cellulose. However, the thermochemicals are the most frequently reported pretreatments, and they have the bottleneck of producing an additional waste stream with a high charge of pollution owing to chemical products implicated. This problem harms on the zero waste policy that we must include in our technological process to be considered sustainable and ecological processes. Consolidated bioprocessing (CBP) is a viable alternative to produce bioethanol from lignocellulosic materials, using a single microorganism, in one-step, and no chemical products are implied. Three agro-industrial wastes with lignocellulosic characteristics were evaluated as a substrate for bioethanol production with a Mexican native white-rot fungus Trametes hirsuta CS5 in a one-step process. Qualitative and quantitative analyses of lignocellulolytic enzymes produced by native fungus were carried out. Instead, Trametes hirsuta is a lignin degrader white-rot fungus; it was capable to produce until 500 U/L of cellulase titers and a maximum xylanase activity of 45 U/mL when it was cultivated in orange peel substrate. Substantial ethanol yields were achieved using lignocellulosic materials like brewer’s spent grain (BSG), orange peel (OP), and wheat bran (WB) as a carbon source in fermentation with no chemicals, which represents a zero waste environment-friendly ethanol production system. Ethanol yield on wheat bran was the highest of all evaluated substrates, reaching value of 34.9% at 7 days, being T. hirsuta able to degrade other hexoses and pentoses present in the structural polymers of cellulose and hemicellulose.