Post-harvest Changes in Sweet Sorghum II: pH, Acidity, Protein, Starch, and Mannitol

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- or 40-cm billets, or chopped). The sorghum was stored outside at ambient temperature in a shade tent, 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, Tew, Rukavina, Boykin, Post-harvest changes in sweet sorghum I: Brix and sugars, BioEnergy Research 5:158–167, 2012). In this paper, we report changes in juice pH, titratable acidity (TA), and protein, starch, and mannitol concentrations. 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 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, while TA and mannitol were highly negatively correlated with total sugar. The results confirm that whole stalks and billets were little changed over 4 days of storage, while chopped sorghum was badly deteriorated 1 day after harvest. Changes in pH, TA, or mannitol could be used to measure deterioration in sweet sorghum after harvest.