Biorefinery approach for an integrated valorisation of grape pomace in distilleries

Abstract

Grape processing generates around 30\% of solid by-products, composed mainly of grape pomace stalks and lees. Distilleries usually collect red grape pomace and lees to recover the embedded ethanol and produce tartrates or colorants. The exhausted grape pomace is generally composted or dried to be used as fuel for energy recovery. As these processes present a low profitability, the development of alternatives to improve grape pomace valorisation is therefore of great interest. In this study, several valorisation strategies were assessed following an integrative approach within a biorefinery scheme to maximise ethanol production, to obtain valuable phenolic extracts from peels and seeds and to produce a protein enriched biomass by fungal solid-state fermentation from exhausted peels. The combined heat and enzymatic treatment of grape pomace at 34\% (v/w) enzyme to grape pomace led to a very significant increase in reducing sugar release with the subsequent increase of up to 28-fold reducing sugar concentration and, therefore, increase in theoretical ethanol production. Furthermore, fermentation of exhausted peels with a Rhizopus oryzae ROR004 strain after application of the different pretreatments led to an increase in protein content of the final biomass in all the cases. Interestingly, heat pretreatment followed by partial removal of phenolics was the most effective pretreatment with up to 55\% in protein increase. Extracts from non-treated peels and seeds contained up to 3.33\% anthocyanins and 29.11\% proanthocyanidins, respectively. Moreover, the antioxidant activity was enriched about 16 and 10 times in the seed and peel extracts, respectively. This study highlights the importance of following a biorefinery approach for the integrated valorisation of the by-products and process streams which could be implemented by a distillery to improve its competitiveness and profitability.