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Title: Post-harvest changes in sweet sorghum I: brix and sugars

item Lingle, Sarah
item Tew, Thomas
item Rukavina, Hrvoje
item Boykin, Deborah

Submitted to: BioEnergy Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/4/2011
Publication Date: 3/31/2012
Citation: Lingle, S.E., Tew, T.L., Rukavina, H., Boykin, D.L. 2012. Post-harvest changes in sweet sorghum I: brix and sugars. BioEnergy Research. 5:158-167.

Interpretive Summary: Sweet sorghum is a crop that stores a lot of sugar in the stalk. That sugar can be used to make ethanol for fuel and other products. We grew three varieties of sweet sorghum in Louisiana, and harvested it at three times: 90 days after planting (DAP), 115 DAP, and 140 DAP. We then removed the leaves and seed head, and divided the harvested stalks into four treatments: stalks left whole, stalks cut into 20-cm or 40-cm pieces called billets, or chopped into small pieces. The sorghum was then stored for up to 4 days before removing the juice. We measured juice Brix, which measures the total amount of dissolved solids in the juice, and the sugars glucose, fructose and sucrose. We showed that in the chopped sorghum, the sugars decreased rapidly after harvest. We observed significant differences after only one day of storage. In the other treatments, sugar did not decrease rapidly. We also showed that juice Brix was not a good way to monitor sugar deterioration in sweet sorghum, because it didn’t always decrease with the sugar. We concluded that harvesting methods that cut either whole stalks or billets would be acceptable for the production of ethanol from sweet sorghum.

Technical Abstract: This experiment was done to measure the deterioration of sweet sorghum (Sorghum bicolor L. Moench) after harvest. Three varieties of sorghum were grown in Louisiana and harvested at 90, 115, and 140 days after planting (DAP). Whole stalks were cut from the field at soil level, stripped, and topped. There were four treatments designed to approximate possible harvesting methods: stalks left whole (soldier harvester), cut into 20 or 40-cm billets with a chop saw (sugarcane harvester), and chopped using a hammer and knife mill (forage harvester). Samples were placed in individual plastic bins, and stored outside in a shade tent for up to four days. At 0, 1, 2 and 4 days after harvest, subsamples were removed from each bin, and juice expressed. Aliquots of juice were immediately placed on dry ice, then stored at -80°C until analyzed. Juice was analyzed for Brix (total soluble solids) and the simple sugars glucose, fructose and sucrose. In most of the treatments, juice Brix tended to increase with time. Sucrose decreased in all treatments, while glucose and fructose increased in the whole stalk, 20-cm billet, and 40-cm billet treatments. In those treatments, total sugar did not change much over the 4-day storage period. In chopped sorghum, decreases in glucose were obvious after 1 day of storage. Glucose and then fructose decreased to very low levels by 2 days after harvest. When the chopped treatment was included, the correlation between juice Brix and total sugar was 0.714. When the chopped treatment was omitted, the correlation between juice Brix and total sugar was 0.971. Juice Brix cannot be used to evaluate juice for deterioration. We conclude that whole stalk and billet harvesting are superior to harvesting by forage harvester, which would have to be processed within hours to maintain the sugars.