Chemical and enzymatic hydrolysis of waste wheat bran to sugars and their simultaneous biocatalytic conversion to valuable carotenoids and lipids

Defatted wheat bran, an industrial waste of the food chain, represents a strategic renewable material for modern biorefinery schemes. Through a combination of chemical and biological catalysis, a cascade process was developed to produce high-value fine chemicals, such as carotenoids and lipids, from polysaccharide fraction. Due to the low lignin content and suitable particle size of defatted wheat bran, pretreatment steps are unnecessary, allowing the direct enzymatic or chemical hydrolysis of polysaccharide components (glucan, xylan, and arabinan) to give fermentable sugars. Regarding the biocatalytic approach, the optimisation of the main reaction parameters, such as enzyme dosage (15, 30, 45, 60 FPU Cellic® CTec 3 HS/g glucan) and biomass loading (5, 10, 15, 20 wt%), was performed to improve the monosaccharide yield. Regarding the chemical route, a microwave-assisted FeCl₃-catalysed approach was optimised in terms of catalyst amount (1.0, 1.3, 1.6 wt%) and reaction time (2.5, 5, 10 min) to maximise the sugar yield, minimizing the formation of furanic derivatives which are strong inhibitors for the subsequent fermentation step. The biological conversion of sugars obtained by both enzymatic and chemical routes into carotenoids and lipids was then performed by adopting the commercial yeast Rhodosporidium toruloides DSM 4444. The simultaneous production of carotenoids and lipids was optimised by investigating the effect of the C/N ratio in the fermentation medium. Under the optimised process conditions (C/N 60), by fermenting hydrolysate obtained by chemical and enzymatic routes, carotenoid productions of 120 and 180 mg/L and lipids productions of 5.2 and 3.5 g/L were achieved, respectively. The highest carotenoids cell content achieved in this study (14.8 mg/g) is about 5 times higher than the maximum value reported in the literature to date for this yeast. Moreover, Rhodosporidium toruloides achieved the complete conversion of sugars into desired bioproducts for both the biomass hydrolysates demonstrating the effectiveness of the two different catalytic approaches adopted for biomass hydrolysis.