Pilot Scale Freeze-drying of Pomegranate Juices and Roselle Extracts

Abstract

A series of experimental test runs were conducted to investigate the possibility of freezedrying single strength (17Brix) and concentrated (34Brix) pomegranate juices and similar concentrations of roselle extracts using a pilot scale freeze-dryer. The investigation included determining suitable process conditions and relevant chemical composition and physical properties of the original liquid products and the produced freeze-dried powders. The time required to reach a freezing temperature of -25C for both pomegranate juices and roselle extracts was about 80 min. which is considered as a slow freezing process resulting in large ice crystals. Temperature of the primary freeze-drying phase was kept constant at -40C because of the high sugar content of the pomegranate juices. The primary freeze-drying times were 40 and 48 h for single and concentrated pomegranate juices, and 5 and 8 h for the single and concentrated roselle extracts, respectively. Secondary drying of juices and extracts was carried out by raising the platen temperature from -40C to +25C. Attempts were successful to produce freeze-dried powders from single strength pomegranate juice and both concentrations of roselle extracts. The high sugar content of the 34Brix pomegranate juice was primarily responsible for the difficulties to freeze-dry it. The results of chemical analysis indicated that roselle extract of 17Brix is a very good source of vitamin C as having a four times concentration of ascorbic acid as compared to the 17Brix pomegranate juice. The bulk density of powder produced from 34Brix roselle extract was higher than the one obtained from the 17Brix. Hygroscopicity data showed tendency of the 34Brix roselle powder to absorb more moisture than the 17Brix roselle powder. Two models describing the hygroscopic behavior of freeze-dried pomegranate and roselle were given.