Effects of Hot Air and Freeze Dnying Methods on Physicochemical Properties of Thai Hom Mali Rice Grass Juice Powder

Abstract

Rice grass juice powders were using hot air drying at the temperatures of 40º and 60ºC and freeze drying methods. Their physicochemical properties (freezing point, chlorophyll content, water activity (a_w) and colour values (L*,a*,b*) were measured and compared with fresh rice grass juice and rehydration rice grass juice powders. The study has demonstrated  that the qualities of freeze-dried powder were higher than that of hot air dried powder. Moreover, the rehydration freeze-dried powder has the similar qualities to the fresh ones. The freezing point of fresh rice grass juice was approximately -1ºC which contained the lowest value of total chlorophyll content (0.080 mg/g.). The total chlorophyll content of freeze-dried powder increassd from 0.910 to 0.990 mg/g. It was significantly higher than that of hot air dried powder which gave 0.398 and 0.999 mg/g at the temperatures of 60ºC and 40ºC, respectively. The results revealed that the total chlorophyll content (0.116 mg/g) of rice grass juice powder decreassd after rehydration. Water activity value (a_w) of freeze=decreased and was lower than 0.6 which is suitable for powder products. The whiteness, redness and yellowness values of hot air dried powder were increased after drying at the temperature of 40º and 60ºC as compared to that of fresh rice grass juice. The gradual decrease of L*b*and a* values was observed after freeze-drying process. As such, this study indicates that rehydration rice grass juice powder which drying at low temperatures using hot air at 40ºC and treeze drying methods, could maintain chlorophyll content and colour quality similar to fresh ones.