Integral use of pectin-rich by-products in a biorefinery context: A holistic approach

Abstract

There is no doubt that in recent years there has been a growing interest in the use of by-products from the agri-food industry with the aim of achieving zero waste. In this context, pectin is a valuable ingredient obtained from vegetable and fruit by-products whose extraction may be a problem rather than a solution. It was not until 2015 when studies began to be published on the need to avoid residues derived from pectin extraction. We show an exhaustive analysis of the existing bibliography on the possible ways for comprehensive use of these residues from a biorefinery approach. In addition, we have carried out a bibliometric analysis of 27 articles reporting by-product composition and high-value ingredient recovery, making a comparison of specific biorefinery applications described in selected articles. A general overview is provided on valorisation strategies, considering the carbohydrate composition (mono-/disaccharides, pectin, cellulose and hemicellulose) of fruit and vegetable by-products. Integrated utilisation strategies should start with the recovery of compounds soluble in organic solvents (essential oils, polyphenols, carotenoids, etc.), followed by the recovery of pectin and subsequent use of depectinised residues. Biofuels (bioethanol, biogas or solid biofuel) can be obtained by transforming the secondary flows by physicochemical or biological processes such as ethanolic fermentation or anaerobic digestion. Other fermentation processes allow organic acids such as succinic, lactic, butyric and mucic acids to be obtained, as well as other compounds such as bacterial cellulose and pullulan. The role of up to 40 microbial glycosidases in fermentation involved in biorefinery applications was also highlighted. Finally, other uses of waste biomass such as composting or animal feed are considered. Although most of the research has been carried out on citrus, this review points out that, despites the differences, studies with these by-products can help in the efficient utilisation of other types of agri-food wastes.