Preservation of Sweet Sorghum with the Help of Formic Acid under Anaerobic Conditions

Experiments were carried out to investigate the storage conditions of chopped sweet sorghum variety 'Top 76' to provide a low-loss supply for ethanol purposes. Therefor sweet sorghum was harvested with a common self-propelled-forage-chopper. At pilot plant scale two different particle sizes (length 10 mm and 20 mm, respectively) and three different loadings of 0.5 % (FM), 0.75 % (FM) and 1.0 % (FM) of formic acid were tested under anaerobic conditions in barrels, which is common practice for ensilage studies. The dry matter content of the raw material varied between 20.76 and 24.75 %. The initial sugar content was between 340.1 and 367.6 mg sugar (given as glucose) per gram dry matter. On predetermined dates, the barrels were drained with an integrated valve and the leachate was collected and analyzed for sugar content, pH value, formic acid, lactic acid, acetic acid and ethanol. After 90, 120 and 150 days the barrels were opened and samples were taken to analyze the remaining soluble sugar content. The addition of 0.75 % (FM) formic acid preserved the sugars for more than 150 days with a loss of less than 16 % of the initial sugar content. Besides the best results for sugar conservation, with this concentration of formic acid the material was also preferable for ethanol production at laboratory scale. Sugar content in the leachate allows to lower the loss of sugar to a level of 2.5 % to 7.5 %. The addition of 0.5 % (FM) formic acid suppresses the formation of lactic acid but not all activity of other microorganisms. This results in a loss of sugar content to a higher degree of 48 %. The addition of formic acid to chopped sweet sorghum initiated sugar conservation under anaerobic conditions for at least 150 days. Thereby the particle size has no influence on the effect of sugar conservation by using formic acid. Sugar losses due to leachate should be avoided by collecting and using it in the ethanol fermentation process.