Statistical optimization of cassava fibrous waste hydrolysis by response surface methodology and use of hydrolysate based media for the production of optically pure d-lactic acid

•First report on d-lactic acid (DLA) production from cassava fibrous waste (CFW).•Highest DLA productivity reported for anaerobic fermentation of renewable feedstock.•High optically pure DLA was produced from CFW.

Cassava fibrous waste (CFW), generated as a solid waste by the sago industries in India, is rich in starch. CFW may cause environmental pollution when disposed improperly. Such a waste is used as feedstock, on hydrolysis, for production of optically pure d-lactic acid (DLA) by lactic acid bacteria (LAB). Statistical optimization of hydrolysis (acid and enzymatic hydrolysis) of CFW by the response surface methodology showed enzymatic hydrolysis to be the best method to obtain a maximum glucose yield (69.9 g/L) compared to that obtainable from acid hydrolysis (24.7 g/L). Lactobacillus delbrueckii subsp. delbrueckii (NBRC 3202) (DLA concentration = 2.9 g/L, optical purity = 97.87%, YP/S = 0.35, YX/S = 0.30) is selected from eight strains of the homo-fermentative LAB with the shake flask experimental studies in MRS medium. Results from the shake flask experimental studies confirm that the CFW enzyme-hydrolysate is a suitable carbon source (DLA concentration = 4.5 g/L, optical purity = 98.02%, YP/S = 0.61, YX/S = 0.42) and yeast extract is a suitable nitrogen source (DLA concentration = 3.16 g/L, optical purity = 97.33%, YP/S = 0.71, YX/S = 0.151) for the production of DLA by L. delbrueckii subsp. delbrueckii. Results from the bioreactor experimental studies for production of DLA by L. delbrueckii subsp. delbrueckii in the media with the CFW enzyme-hydrolysate as the carbon source, yeast extract as the nitrogen source, show higher productivity on anaerobic fermentation (rP = 0.90 g/L/h) than aerobic fermentation (rP = 0.78 g/L/h).