A study of sorption isotherm and quality changes during storage of powdered longan cube

Abstract

This research studied the sorption isotherm and quality changes during storage of powdered longan cube. Longan powder was produced using 40oBrix concentrated longan juice and longan flesh at ratio of 1:1. Sugar was added at 30% w/w of the total weight of the concentrated longan juice and flesh. The obtained solution was evaporated with continuous stirring to become a thick viscous paste and become glassy when cooled. The longan glass was grinded into powder. The longan powder was formed into a cube using a hydraulic press at 1.5 tons of pressure. The longan cubes were subjected to sorption isotherm study by
keeping the samples at 7 relative humidity conditions (11-75%). Quality changes during 12 weeks storage were investigated. The sorption isotherm of the sample was adsorption isotherm. Moisture content of the sample increased as the relative humidity increased. It was also found that the longan cube was sensitive to adsorption of water from the environment. There were significant quality changes during storage. The sample stored at 43% relative humidity had darker colour. At 52% relative humidity and above, the samples became gel-like and had darker colour. As the storage relative humidity increased, the longan cube expanded and was covered with liquid film on the surface. L* value decreased as the relative humidity increased. Rapid decreases in L* value was found at relative humidity above 32%. The a* and b* values significantly increased at the relative humidity above 22%. Increase in a* value significantly increased in redness of the sample. Moisture content and water activity of the longan cube significantly increased during storage. The moisture content increased from 3.36±0.41 to 4.51±0.42%, and water activity increased from 0.260±0.02 to 0.420±0.05 during storage. Solubility of the sample reduced throughout the storage time from 85.5±0.47 to 75.12±0.34%. Therefore, the longan cube should be packed in a low moisture permeable packaging and stored at relative humidity of 32-43%.