Development of an enzyme cocktail to bioconvert untapped starch in sweet sorghum processing by-products: Part I

Abstract

Soluble carbohydrates in sweet sorghum juices and syrups are the main sugars converted to ethanol during fermentation. Recently, it was found that sweet sorghum contains a substantial amount of insoluble starch in sweet sorghum by-products: juice sediment and clarification mud, which is an untapped source of fermentable sugars. In this study, a response surface method was used to optimize hydrolysis, liquefaction, and saccharification conditions and enzymes to customize a two-step process to convert starch in grain sorghum flour, as well as juice sediment, and clarification mud. Optimal starch liquefaction with the industrial α-amylase (Termamyl SC™) was best achieved at 80 °C in 90 min when <18% w/w flour was used, since the solid concentration significantly (P < 0.05) affected starch hydrolysis efficiency. Subsequent studies revealed that an industrial enzyme cocktail comprised of 63% SAN Extra™ (α-glucoamylase), 16% Promozyme D2™ (pullulanase), and 21% Viscozyme L™ (β-carbohydrase mixture) was most effective in improving the saccharification of starch, with particular emphasis on insoluble starch granules, to fermentable sugars at 60 °C in 90 min. Application of the optimal conditions tripled fermentable glucose and doubled total sugars in juice sediment; its application to clarification mud did not show much improvement (P < 0.05). Practical applications of this enzyme cocktail will also depend on cost effectiveness.