The biochemical processing of lignocellulosic biomass (LCB) of sorghum plant (Sorghum bicolor (L.) Moench) to ethanol was investigated using ammonia and glycerol based pretreatments. Screening of sorghum LCB was performed for higher enzymatic digestibility using different varieties of sorghum (sweet sorghum, post rainy sorghum and hybrid sorghum variety). The LCB varieties were tested for enzymatic digestibility after pretreatment with glycerol and aqueous ammonia separately. The polysaccharide recovery and enzymatic digestibility of sorghum LCB varied with pretreatment method and plant variety. Among the sorghum varieties, ICSV 25333 yielded highest sugars during enzymatic hydrolysis after pretreatment. The polysaccharide recovery and digestibility of the selected LCB was further improved by a modified pretreatment method using a mixture of glycerol and ammonia. The pretreatment was studied using pure glycerol and crude-glycerol obtained from biodiesel industry. The crude-glycerol performed equivalent to pure glycerol indicating the absence of negative effect by the other components. Further, the conditions of ammoniacal crude-glycerol pretreatment were optimized by response surface methodology using central composite design (CCD). The optimal conditions of the pretreatment for obtaining maximum sugar yield in the enzymatic hydrolysis with cellulase CTec2 (10 U/g solid) were: 51.11% glycerol, 5.14% aq. ammonia, 120°C and 60 min residence time. The ammoniacal glycerol pretreatment of other feedstocks such as sugarcane bagasse, rice straw and wheat straw showed high recovery and hydrolysis of cellulose and xylan. When compared with glycerol pretreatment, the presence of ammonia as a catalyst improved the xylan recovery and partial lignin removal. It also reduced the temperature of the pretreatment process. The compatibility of the modified pretreatment with high-solids simultaneous hydrolysis and co-fermentation (SSCF) was investigated by fed-batch strategy using Candida tropicalis. SSCF with 20% solids of the pretreated sorghum biomass using 10 FPU/g CTec2 produced 36 g/L of ethanol. The loading of cellulase was reduced by 2 folds (4 FPU/g solid) by the addition of xylanase and polyethylene glycol (PEG) during SSCF. The recycling of pretreatment reagent or conversion of pretreatment liquor to value added products are desirable for low-emission and economic cellulosic ethanol process development. Since the waste liquor generated in the ammoniacal glycerol pretreatment contains both glycerol and lignin fractions, the pretreatment liquor (PTL) was used for the synthesis of polyester polymer. The PTL-polymer was very light and porous and had mechanical properties comparable to the other reported polymers. The major advantage of the PTL polymer was its inherent antioxidant activity and low water absorption capacity. The results showed that the novel pretreatment process using crude glycerol supported higher saccharification efficiency of sorghum biomass and the conversion of pretreatment effluent to a thermoset biodegradable polymer. The present approach would be useful for development of low-effluent and economic process for production of cellulosic ethanol.

Publications:
1. Joy SP, Kumar AA, Gorthy S, Jaganathan J, Kunappareddy A, Gaddameedi A, Krishnan C. Variations in structure and saccharification efficiency of biomass of different sorghum varieties subjected to aqueous ammonia and glycerol pretreatments. Industrial Crops and Products. 2021 Jan 1;159:113072.

2. Joy SP, Krishnan C. Modified organosolv pretreatment for improved cellulosic ethanol production from sorghum biomass. Industrial Crops and Products. 2022 Mar 1;177:114409.

3. Shereena P Joy, Chandraraj Krishnan.Method for pretreatment of biomass using ammoniacal glycerol. Patent granted with application no. 201841034434.