Bioethanol fermentation of clarified sweet sorghum (Sorghum bicolor (L.) Moench) syrups sealed and stored under vegetable oil

Authors: Klasson, K Thomas; Boone, Stephanie

Submitted to: Industrial Crops and Products
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/5/2021
Publication Date: 5/1/2021
Publication URL: https://handle.nal.usda.gov/10113/7284395
Citation: Klasson, K.T., Boone, S.A. 2021. Bioethanol fermentation of clarified sweet sorghum (Sorghum bicolor (L.) Moench) syrups sealed and stored under vegetable oil. Industrial Crops and Products. 163. https://doi.org/10.1016/j.indcrop.2021.113330.
DOI: https://doi.org/10.1016/j.indcrop.2021.113330

Interpretive Summary: Sugar crops such as sugar cane and sweet sorghum represent readily available renewable carbohydrate feedstocks that can be used for biofuel production. Generally, the season for sweet sorghum growing and window for processing is short and it is often desirable to store a sugar-rich material for future processing. Unfortunately, sweet sorghum juice deteriorates quickly and its sugar content is not optimal for direct fermentation. Therefore, we investigated the possibility of storing three different sweet sorghum sugar syrups under a layer of soybean oil; then investigated subsequent yeast bioethanol fermentation. The syrups, containing approximately 30, 40, and 50% of sugar, were stored for 10 weeks. Two-thirds of the different syrups were stored under a layer of soybean oil. During the 10-week investigation sugar and pH decreased in all syrups. All syrups examined showed evidence of sugar degradation by bacteria and yeasts. Sugar loss was reduced when more concentrated syrups were stored. A layer of vegetable oil on the surface of the syrup reduced sugar loss in 30 and 40% syrups, but the loss was still substantial (21-36%). Storage of 50% syrup for 10 weeks resulted in an 11-18% loss of sugars with or without a surface layer of oil. When sugar loss was mainly caused by ethanol-producing microorganisms, the stored syrups were still suitable for bioethanol production with yields of 83-91% of theoretical. When sugar loss was caused predominately by lactic and acetic acid producing microorganisms, the bioethanol yield was lower. Substantially elevated levels of lactic and acetic acids almost completely inhibited the yeast fermentation.

Technical Abstract: Sugar crops such as sugar cane and sweet sorghum represent readily available renewable carbohydrate feedstocks that can be used for biofuel production. Generally, the season for sweet sorghum growing and window for processing is short and it is often desirable to store a sugar-rich material for future processing. Unfortunately, sweet sorghum juice deteriorates quickly and its sugar content is not optimal for direct fermentation. Therefore, we investigated the possibility of storing three different sweet sorghum sugar syrups under a layer of soybean oil; then investigated subsequent yeast bioethanol fermentation. The syrups, containing 30, 40, and 50% dissolved solids (°Brix), were stored for 10 weeks. Two-thirds of the different clarified syrups were stored under a layer of soybean oil. During the 10-week investigation °Brix and pH decreased, and water activity increased in all syrups. All syrups examined showed evidence of sugar degradation by bacteria and yeasts, observed in the form of organic acids and ethanol. Sugar loss was reduced when more concentrated syrups were stored. A layer of vegetable oil on the surface of the syrup reduced sugar loss in 30 and 40 °Brix syrups, but the loss was still substantial (21-36%). Storage of 50 °Brix syrup for 10 weeks resulted in an 11-18% loss of sugars with or without a surface layer of oil. When sugar loss was mainly caused by ethanol-producing microorganisms, the stored syrups were still suitable for bioethanol production with yields of 83-91% of theoretical. When sugar loss was caused predominately by lactic and acetic acid producing microorganisms, the bioethanol yield was lower. Substantially elevated levels of lactic and acetic acids almost completely inhibited the yeast fermentation.