Starch in sweet sorghum and sugarcane: Problems, opportunities, and control

Authors: Eggleston, Gillian; MONTES, BELISARIO - Alma Plantation, Llc; HECKEMEYER, MATTHEW - Heckemeyer Mill; Cole, Marsha; Triplett, Alexa; STEWART, DAVID - Alma Plantation, Llc; Lima, Isabel

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 4/1/2017
Publication Date: 6/22/2017
Citation: Eggleston, G., Montes, B., Heckemeyer, M., Cole, M., Triplett, A., Stewart, D., Lima, I. 2017. Starch in sweet sorghum and sugarcane: Problems, opportunities, and control. In: Proceedings for the Advances in Sugar Crop Processing and Conversion Conference, March 15-18, 2016, New Orleans, Louisiana. p. 177-192.

Interpretive Summary: Both sweet sorghum (Sorghum bicolor) and sugarcane (Saccharum) crops are members of the grass (Poaceae) family, and consist of stalks rich in soluble sugars. Sweet sorghum is primarily utilized for the manufacture of food-grade syrup and bioproducts, whereas sugarcane is mostly utilized for the manufacture of sugar. The extracted juice from both of these crops contains insoluble starch, with much greater quantities occurring in sweet sorghum. In sweet sorghum processing, starch is a problem but can also be an opportunity if the fermentable sugars in starch are utilized. Sweet sorghum starch is best removed at the biorefinery physically by de-aerated juice sedimentation followed by clarification (80 °C; lime to pH 6.5; 5 ppm flocculant). At the same time, soluble and insoluble starch are concentrated in juice sediment and clarification mud, which can then be recycled into a fermentation tank after appropriate treatment with heat and starch degrading enzymes. In contrast, starch in sugarcane is regarded solely as a processing impurity. Considerable amounts of insoluble starch can occur in products across both the factory and refinery, which can be detrimental to viscosity, amylase applications, and refinery filtration operations. Due to lower amounts of starch in sugarcane, it tends to be controlled enzymatically at the factory. Both high-temperature (HT) and intermediate-temperature (IT) stable amylases can hydrolyze starch in clarified juice at 96 °C during the first 10 min before substantial denaturation but, even at 1 ppm, HT amylases can cause unwanted carry-over (residual) amylase activity in the raw sugar. Three new technologies are now available to prevent the occurrence of carry-over amylase in end-products: (i) the simultaneous addition of 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 with the added benefits of removing cane-derived phenolic/flavonoid colorants and insoluble starch.

Technical Abstract: Both sweet sorghum and sugarcane crops are grasses and consist of stalks rich in soluble sugars that also contain insoluble starch, with much greater quantities occurring in sweet sorghum. In sweet sorghum processing, starch is a problem but can also be an opportunity if the fermentable sugars in starch are utilized. Sweet sorghum starch is best removed at the biorefinery physically by de-aerated juice sedimentation followed by clarification. In contrast, starch in sugarcane is regarded solely as a processing impurity that can be detrimental to viscosity, amylase applications, and refinery filtration operations. Due to lower amounts of starch in sugarcane, it tends to be controlled enzymatically at the factory. The optimized application of amylase in a sugarcane factory is described.