Postharvest quality and processing of sugarcane and sweet sorghum for sugar and ethanol production
Location: Commodity Utilization Research
Title: Post-harvest changes in sweet sorghum II: pH, acidity, protein, starch, and mannitol
Submitted to: BioEnergy Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 24, 2012
Publication Date: January 1, 2013
Citation: Lingle, S.E., Tew, T.L., Rukavina, H., Boykin, D.L. 2013. Post-harvest changes in sweet sorghum II: pH, acidity, protein, starch, and mannitol. BioEnergy Research. 6(1):178-187.
Interpretive Summary: Sweet sorghum is a crop that stores a lot of sugar in the stalk. That sugar can be used to make many value added products such as ethanol. 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. In addition to the sugars, which were reported in an earlier paper (Lingle, S., Tew, T., Rukavina, H., Boykin, D., 2012. Post-harvest changes in sweet sorghum I: Brix and sugars. BioEnergy Research, 5(1): 158-167), we measured juice pH, acidity, starch, protein, and mannitol, a chemical made by microorganisms when they eat sugar. We showed that in the chopped sweet sorghum, the pH rapidly decreased and acidity increased because of the action of microorganisms, but there was little change in juice pH, acidity or mannitol in whole stalks and billets. The starch content of the juice increased as the harvest date increased, but even at the most mature, breaking the starch into glucose would not contribute more than 1% more sugar. We explain how the juice pH, mannitol, and starch would affect the processing of sweet sorghum. This research confirmed that sweet sorghum harvested as whole stalks or billets could wait a greater length of time before processing without losing quality than chopped sweet sorghum, which would need to be processed within a few hours of harvest.
This experiment was conducted to evaluate the effect of four harvesting methods on juice quality and storability in sweet sorghum. Three cultivars (Dale, Theis, and M81-E) were harvested at 90, 115, and 140 days after planting. Stalks were stripped of leaves and topped at the peduncle, then divided into four treatments (whole-stalk, 20-cm or 40-cm billets, or chopped). The sorghum was stored outside at ambient temperature, and juice was extracted from samples removed at 0, 1, 2, and 4 days after harvest. Changes in juice Brix and sugars were reported in an earlier paper (Lingle, S., Tew, T., Rukavina, H., Boykin, D. 2012. Post-harvest changes in sweet sorghum I: Brix and sugars. BioEnergy Research 5:158-167). In this paper we report changes in juice pH, titratable acidity (TA), protein, starch, and mannitol concentration. Juice pH dropped rapidly after harvest in chopped sorghum, but changed little during 4 days of storage in whole stalks or billets. Similarly, TA increased with storage time in chopped samples, but was unchanged in whole stalks and billets. Protein concentration was highly variable, and no pattern with treatment or storage time could be discerned. In whole stalks and billets, starch content slowly decreased during storage, while in chopped samples starch appeared to increase. This was most likely a result of an increase in dextran synthesized by microorganisms in those samples, which was also detected by the enzymatic starch assay. The concentration of mannitol, produced by Leuconostoc mesenteroides bacteria, increased with storage time in chopped samples, but not in whole stalks or billets. Within a harvest date, pH was highly correlated with total sugar but not Brix, while TA and mannitol were highly negatively correlated with total sugar but not Brix. Results confirm that whole stalks and billets were little changed over 4 days of storage, while chopped sorghum was badly deteriorated by 1 day after harvest. Changes in pH, TA or mannitol could be used to monitor deterioration in sweet sorghum after harvest.