Utilization of sorghum bagasse hydrolysates for producing microbial lipids

Pretreatment of sweet sorghum bagasse at 100 °C at three lime doses (g per g bagasse), 0, 0.05, and 0.1; two water contents (ml per g bagasse), 7 or 10; and two treatment durations (h), 1 or 2 were evaluated. Composition analysis of pretreated samples revealed that the higher the lime dose, the more loss of lignin and xylan occurred. From the combined effects of pretreatment and enzymatic hydrolysis, one pretreatment condition was identified to be optimal. Under this condition of 0.1 g lime/g bagasse, 10 ml water/g bagasse, and 2 h treatment time, the total reducing sugar (TRS) recovery was 44.6 g TRS/100 g bagasse, which was equivalent to 73.6% of maximum sugar available. Unwashed hydrolysates, either the supernatant or the whole slurry promoted excellent growth of Cryptococcus curvatus, an oleaginous yeast. Maximal cell biomass dry weight as 6 g/l was achieved in 5 days. Maximal neutral lipid content as 2.6 g/l was observed by day 3. Neutral lipid yield (g neutral lipid/g sugar) was calculated as 0.19 which is close to the theoretical value. In summary, converting sorghum bagasse to biodiesel-suitable lipids represents a valuable pathway for utilizing an agricultural by-product for generating renewable biodiesel feedstocks.

► Lime assisted pretreatment is effective in disrupting structure of sorghum bagasse.
► Cryptococcus curvatus grows well on hydrolysate derived after pretreatment and hydrolysis.
► Nile red can quantify neutral lipid content in yeast cells and hydrolysate.
► Sorghum bagasse can serve as an excellent feedstock for producing microbial lipids.