Storage time effects on the soluble sugars concentration and pH of sweet pearl millet and sweet sorghum juice

Juice of sweet pearl millet and sweet sorghum can be extracted by mechanical pressing and then fermented to produce bioethanol. Since the juice contains a significant amount of soluble sugars, delays between the juice extraction and its transformation may be critical as sugars can degrade quickly. The main objective of this study was to determine the impact of storage time on sweet pearl millet and sweet sorghum juice pH and soluble sugars concentration. In 2013, the juice extracted from both crop species was stored at ambient temperature or under 4°C in a refrigerator up to 72 h. Juice was sampled every 24 h. In the 2014 experiment, the juice was stored at ambient temperature for 12 h and sampling was made after 0, 2, 4, 7, 8, 10, and 12 h. Obtained results showed that when the juice was refrigerated at 4°C, the pH and soluble sugars concentration remained stable throughout the 72 hours of storage. At ambient temperature, the total soluble sugars concentration significantly decreased during the first 24 h of storage of sweet sorghum juice (-17%) and between the first 24 to 48 h of sweet pearl millet juice (-29%). However, the total soluble sugars concentration did not vary in the juice of both crop species during the first 12 h of storage at ambient temperature. Producers have therefore up to a 12h-storage delay at ambient temperature after extraction before the need of a transformation process or the addition of preservation agents to avoid sugar degradation of the juice of both crop species.