Potential reduction of nonstructural carbohydrate losses by juice expression prior to ensiling sweet sorghum

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Both the nonstructural carbohydrate (sugar) and structural carbohydrate (fiber) components of sweet sorghum must be collected and utilized, if it is to be competitive as an ethanol feedstock. Sugar collected in juice expressed from chopped whole stalks equaled 42% of whole-stalk sugar. Potential increase in press performance resulting from removal of a rind-leaf function was investigated. A fractionation ratio of 0.25 (rind-leaf fraction equal to 25% whole-stalk mass separated prior to juice expression) gave maximum collection of sugar in the juice. Collected sugar was 52% of whole-stalk sugar. Extraction ratio (mass of juice divided by input mass) increased from 0.36 for chopped whole stalks to 0.60 for the 0.25 fractionation ratio ( F r = 0.25) sorghum. For higher fractionation ratios, the increase in extraction ratio was not sufficient to offset sugar loss due to rind-leaf removal. Assuming press capacity (Mg h −1) is the same for both chopped whole-stalk and F r = 0.25 sorghum, press performance (juice sugar collected per h) will increase by 67%. Removal of juice sugar reduced expected sugar conversion during ensiling from 92% of initial sugar (chopped whole stalk) to 40% (combination silage produced from residues from F r = 0.25 processing). An expression was derived to calculate nonstructural carbohydrate (NC) content of combination silage produced by combining the pith presscake and rind-leaf fractions. Minimum NC occurred for F r = 0.25.