Saccharification of starchy food waste through thermochemical and enzymatic pretreatment, towards enhanced bioethanol production via newly isolated non-conventional yeast strains

A starchy food waste containing mainly cooked wasted rice (WR) was exploited for bioethanol production using novel yeast strains was investigated. Different pretreatment schemes of the waste at solids loading 10%-30% TS WR (w/v) i.e. enzymatic, thermochemical and combined thermochemical/enzymatic pretreatment, were eval-uated aiming to the maximum liberation of fermentable carbohydrates and their subsequent bioconversion to ethanol. Fermentation tests of the whole pretreated slurries were initially performed with the yeasts strains that were identified as Kluyveromyces marxianus isolate V3-19, Pichia kudriavzevii strain YF1702 and K. marxianus strain TTG-428, and their fermentation efficiencies (FE) were comparatively assessed. It was shown that the combined pretreatment led to the maximum saccharification, whereas FEs were higher for K. marxianus, V3-19, exceeding 90% of the theoretical maximum. In the case of the highest organic loading of WR, though, up to 25% of soluble carbohydrates remained unexploitable after 72 h of fermentation, indicating that kinetic restrictions occurred in the process. Further experiments with the hydrolysates that were recovered after combined pre-treatment, revealed that the removal of solids enhances the consumption of sugars and leads to complete uptake for the loading 20% TS WR (w/v).