Abstract

Background: Peels from root and vegetable crops such as sweet potato (SP), elephant foot yam (EFY), tannia and ash gourd (AG) along with mixed vegetable wastes (MVW, comprising non-edible parts of common vegetables, rotten vegetables, seeds and pulp covering them etc.) discharged from households and restaurants constitute a major part of the biodegradable wastes. Considering the pollution hazards due to inadequate and non-judicious disposal of such wastes and the ever increasing demand for biofuel especially from the developing economies coupled with the very scanty information available on the effective valorization of such starch containing biowastes, a study was planned to tap their potential as biofuel feedstocks.

Methods: As different from the typical lignocellulosic biomass, the selected processing residues had high starch content coming along with them during peeling, necessitating different pretreatment strategies. The comparative compositional and ultrastructural changes in the lignocellulo-starch biomasses (LCSBs) during two pretretaments such as simple steam (100 °C) and dilute sulfuric acid (DSA) was investigated. Two moisture regimes such as 40% and 50% moisture content(MC) and three time periods (30, 45 and 60 min.) were adopted for the steam pretreatment, while the DSA pretreatment was done at 121 °C and 0.102 MPa for two time periods (30 and 60 min.).

Results: Compositional studies indicated that in addition to cellulose (11-18%) and hemicelluloses (12- 18%), LCSBs contained starch (20-32%) as a major polysaccharide (dry basis) and this could lead to differential effects during pretreatment. Lignin ranged from 7.0-10.7% in the biomasses, with the highest in ash gourd peel. Whilst steam pretreatment (40% MC; 60 min.) reduced hemicellulose, cellulose and starch by 51-53%, 27% and 25% respectively, DSA pretreatment hydrolyzed 85-94% starch, 42-48% hemicellulose and 3-15% cellulose. Delignification was more in steam (27-29%) than DSA pretreatment (10-19%). Large number of intact and deformed starch granules were seen in native biomasses and steam pretreatment resulted in gelatinization and adhesion of starch on the opened up pores. Surface morphology was altered in DSA pretreatment, with deformed/stretched pores coated with little starch.

Conclusion: The study showed that starch gelatinization and clogging of pores being a major change during steam pretreatment, preferential saccharification with amylases followed by cellulases might be needed to optimize the fermentable sugar yield. However, as starch and hemicelluloses are hydrolyzed to a high extent during DSA pretreatment, the amylase and xylanase loading could be reduced at the saccharification stage.

Keywords: Composition, pretreatment, steam, sulfuric acid, processing residues, ultrastructure.

Graphical Abstract