Problems, Control, And Opportunity Of Starch In The Large Scale Processing Of Sugarcane And Sweet Sorghum

Both sugarcane (Saccharum officinarum) and sweet sorghum (Sorghum bicolor) crops are members of the grass (Poaceae) family, and consist of stalks rich in soluble sugars. Sugarcane is primarily utilized for the manufacture of crystalline sucrose whereas sweet sorghum is mostly used for the manufacture of food-grade syrup and bio-products, and to a lesser extent forage and grain. The extracted juice from both of these crops contains insoluble starch, with much greater quantities occurring in sweet sorghum. Starch in sugarcane is regarded solely as a processing impurity. Insoluble starch can occur in products across both the sugarcane factory and refinery which can be detrimental to viscosity, amylase applications, and refinery filtration operations. Starch in sugarcane is mostly controlled enzymatically at the factory. In a factory trial, high-temperature (HT) stable amylases at 1 ppm hydrolyzed 15.8 to 55.5% soluble starch and 31.6 to 67.2% insoluble starch in clarified juice (96 degrees C), before substantial denaturation. HT amylases, however, can cause unwanted carry-over (residual) amylase activity in raw sugar even at 1 ppm. Three new technologies are available to prevent the occurrence of carry-over amylase in end-products: (i) the simultaneous addition of intermediate-temperature (IT) amylase to the next-to-the-last evaporator and last evaporator to hydrolyze both soluble and insoluble starch, (ii) the development of a new HT amylase with modified thermostability, that can still hydrolyze starch in clarified juice but not cause carry-over amylase, and (iii) the addition of powdered activated carbon to remove the residual amylase protein. In sweet sorghum processing, starch is a problem but can also be an opportunity if the fermentable sugars in starch can be utilized. Biorefinery trials have shown that sweet sorghum starch is best removed physically via (i) sedimentation of de-aerated juice, (ii) entrapment in juice surface flocs, and (iii) settling during juice clarification (80 degrees C; lime to pH 6.5; 5 ppm flocculant). By-products, rich in starch, are formed including juice sediment, heated juice surface scum, and clarification mud. These could be recycled into a fermentation tank to augment fermentation yields after appropriate treatment with heat and starch degrading enzymes, and research is now warranted.