Valorisation of stalks from different grape cultivars for sugars recovery

This study has been realized under a national project Valorvitis focused on the valorisation of wine making by-products for the production of high added-value compounds. The aim of this work was to investigate the possibility of recovering sugars as hemicelluloses hydrolysates suitable for bioethanol production. The production of hemicelluloses hydrolysates from stalks of six different grape cultivars was studied to assess: the cultivar influence on the recovery of non-structural sugars in the washing pre-treatment step; the influence of the cultivar and of the liquid/solid ratio in the acid hydrolysis step on the recovery of structural sugars; and the cultivar influence on the content of sugars degradation products(5-HMF, furfural and 5-MF), total phenols (based on Folin index), phenolic acids (by HPLC) and minerals in the hydrolysates (since these compounds could positively or negatively influence fermentation processes). The applied process consisted in a washing pre-treatment, followed by a dilute acid hydrolysis step (2 % sulfuric acid with two different solvent-to-solid ratios (10:1 and 20:1). The pre-treatment allowed to recover from 4.7 to 6.8 gGlucose+Fructose)/100 g dm depending on the cultivar, but in very diluted liquors (< 5 g/L as Glu+Fru). The 10:1 ratio was selected for the hydrolysis, since allowing for almost the same xylose yield as for the 20:1 ratio (from 4 to 7.4 g/100 g dm depending on the cultivar), the liquors contained lower amounts of phenolic compounds and sugar degradation products. The antioxidant activity (based on the radical ABTS test) of the hydrolysates (which could have an inhibitory effect in fermentation) was slightly influenced by the cultivar but not by the solvent to solid ratio. For all the cultivars, mineral analysis showed a certain content in micro and macronutrients that could positively influence a hypothetical fermentation process for bioethanol production. Therefore, the applied process could be optimized in order to recovery higher quantity of sugars with a reduced generation of inhibitory substances, to be used in fermentation processes.