Solid-state enzymatic hydrolysis of agro-industrial residues as alternative low-cost process for obtaining fermentable sugars

Abstract

Fermentable sugars (FS) are valuable intermediates used as source for obtaining a wide range of final products with higher value-added. Although FS can be obtained from different plants containing them such as from sugar cane or sugar beet, the use of second-generation sources such lignocellulosic biomass is of current interest. With such approach a two-fold aim is achieved. Producing FS economically while valorizing a waste stream. Among the lignocellulosic materials, the agro-industrial residues have been extensively studied as potential sources of diverse biotechnological applications. Since their structure contains significant cellulose and hemicellulose contents, they are prone to be source of FS after a hydrolysis step. Typically, hydrolysis of such materials is conducted using chemical (acids or basis) or enzymatic systems in liquid cultures. However, the use of solid-phase hydrolysis is gaining relevance due to their advantages over their liquid counterpart, such as a lower energy and water demand, and the reduced waste generation. In particular, the solid-state enzymatic hydrolysis (SSEH) has been used with several lignocellulosic materials to produced FS that can be further processed in both solid and liquid phases. In the Catalonian region different lignocellulosic waste are produced, but some of the main include those derived from the wine and olive oil industries. Although these wastes are disposed, composted or burnt to produced energy, their characteristics make them potential source of valuable bioproducts such as FS. Therefore, by using a suitable approach for using such local wastes would provide an alternative to the conventional disposal strategies, while gaining value from a strain considered waste. Thus, the aim of this study was to evaluate the potential of two local agro-industrial residues namely grape pomace (GP) and olive-mill solid waste (OMW) as raw materials for obtaining FS by using SSEH. Throughout the study, OMW and GP were hydrolyzed at different conditions. First, they were processed by a self-produced enzyme extract from Aspergillus niger that was previously obtained through solid-state fermentation. Then, they were hydrolyzed with a commercial extract (Viscozyme L). It was found that the highest FS contents were achieved using GP as a substrate. Besides, the commercial enzyme showed more effectiveness than the self-produced enzymes from Aspergillus niger. It was also demonstrated that the moisture content affected the hydrolysis process, but differently depending on the type of enzymes used. These results show that using SSEH of selected agro-industrial residues is a promising valorization strategy that follows the framework of the circular economy. This work was developed in the framework of the national project “Valorización de residuos agroindustriales para la producción de bioplásticos-VALORA” founded by the Spanish Ministerio de Economía y Competitividad (CTM2016-81038-R).