Effect of Planting Date and Maturity on Juice Quality of Rio Sweet Sorghum¹

Abstract

Economical production of granulated sugar from immature sweet sorghum stalks would extend the milling period for several weeks each year. ‘Rio’ sweet sorghum [Sorghum bicolorz (L.) Moench.] was planted April 1, May 1, and June 1 and was harvested in the flower, milk, dough, and ripe stages of maturity. Samples from each planting date and harvesting stage were stored openly on the ground for 0, 24, and 48 hours before milling. The juice was weighed and analyzed for Brix, sucrose, and purity to compare the effect of maturity and storage on juice quality. Yield of stalks from the April 1 and June 1 plantings were similar but significantly lower than from the May 1 planting date. Stage of maturity had no effect on yield of stalks. Extraction generally decreased with maturity, whereas Brix and starch increased. Sucrose increased with maturity through the dough stage and purity increased from flower to milk stage. Extraction and purity decreased during 24 hours in storage, and sucrose and starch decreased during 48 hours in storage. Sucrose inversion was highly significant in stored stalks harvested in the flower and milk stages, which resulted in a highly significant decrease in purity. Mature stalks showed little or no inversion during storage. Juice purity was similar for Rio stalks harvested in the milk, dough, and ripe stages of maturity and not stored. After 24 hours in storage purity of stalks harvested in the milk stage was significantly lower than that of those harvested in the dough and ripe stages.

Sweet Sorghum as a Bioenergy Crop for the US Great Plains

“…Sweet sorghum stalk yield was not much affected by growth stage between flowering to physiological maturity but juice extraction efficiency decreased, and Brix and starch increased, with advancing maturity (Broadhead, 1974); sucrose yield was maximized during the dough stage. In other studies, syrup yield was maximized by harvesting during the late milk to hard dough growth stage (Broadhead, 1972;Tarpley et al, 1994).…”

“…In other studies, syrup yield was maximized by harvesting during the late milk to hard dough growth stage (Broadhead, 1972;Tarpley et al, 1994). Stalk sugar concentration is often the lowest at boot stage and the highest at the soft dough stage (Lingle, 1987;Ricaud et al, 1979); the onset of sucrose accumulation was associated with the onset of the reproductive phase of growth and reduced acid invertase activity.…”

“…Sugar loss from heaped intact stalks was just 3% in four days (Ricaud et al, 1979) and no significant sucrose inversion occurred during 24 hours after cutting. In another study, juice extraction and purity decreased by 3% and 5%, respectively, during 24 hours following intact stalk harvest, but sucrose and starch decreased by less than 1% during 48 hours after harvest (Broadhead, 1974). Temperature during storage appears to be important to losses with 20% of fermentable sugars lost in 3 days of storage at room temperature but no loss with refrigeration .…”

“…In traditional harvest for syrup production, sweet sorghum stalks were topped to remove the panicle and stripped of leaves before crushing for juice extraction because of effects on syrup taste (Winberry, 1980). Farmers staggered plantings over four weeks to prolong the harvest period (Broadhead, 1974). Juice was extracted with simple wooden or metal roller presses, a labor intensive procedure (Lamb, 1982).…”

“…Chopped stalks can be stored as silage without appreciable sugar loss in fermentation if inhibited with an acrylic acid treatment (Hill et al, 1987), but a 20% loss in ensiled storage can occur without such treatment (Bennett & Anex, 2008). There is evidence of an interaction of harvest growth stage and stalk storage time (Broadhead, 1974). Juice purity was not affected by harvest growth stage if the stalks were not stored, but juice purity was 70 and 73% less at 24 hours of storage for stalks harvested in the milk compared with the dough and physiological maturity stages, respectively.…”