Submitted to: Journal of Food Processing and Preservation Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 3, 2000
Publication Date: December 13, 2000
Interpretive Summary: Juice was extracted from Hamlin oranges using a standard industrial juice extractor. The extractor was set to express the juice at conditions equivalent to a Hard, Medium or Soft juice extraction. The stability of the juice from each extraction was monitored at periodic time intervals. Protein extracts also were obtained from each juice. The activity of the enzyme pectinmethylesterase (PME) was estimated in each extract at both pH 4.5 (near the pH of Valencia orange juice) and 7.5 (optimal pH for PME activity). PME initiates the sequence of events that results in juice cloud destabilization. A measured amount of PME activity, as estimated at pH 4.5, was added to pasteurized, reconstituted frozen concentrated orange juice (FCOJ) and samples were incubated at 30 degree C and 4 degree C. At periodic time intervals the juice was sampled for cloud stability. Results indicate: (1) with raw juice samples at 30 degree C the Hard Extracted Juice had the most stable cloud, followed by Medium and the Soft Extracted Juice, (2) using PME activity estimates made at pH 4.5 and adding equal activities to pasteurized, reconstituted FCOJ produced results different from those obtained with raw juice. PME containing extracts from Soft Extracted Juice caused the most rapid cloud loss.
Technical Abstract: Pectinmethylesterase (PME) causes cloud destabilization in citrus juices. PME extracted from orange fruit peel destabilizes cloud more rapidly than PME from rag or hand-expressed juice. Two PME's from peel destabilized cloud within a seven day period at either 4 or 30 C. This suggests that increasing the amount of peel PME in juice may destabilize cloud more rapidly than lower peel PME levels. One variable which may effect the amount of peel PME present in juice is extraction pressure. Juice was extracted from field-run Hamlin fruit at three pressures, Soft, Medium and Hard. Cloud stability in the raw juice was determined at 4 and 30 C. Total salt extractable proteins were isolated from juice obtained at each setting. PME activity was estimated at pH 4.5. Pasteurized Frozen Concentrated Orange Juice was reconstituted to include a determined amount of PME activity. At 30 C raw juice cloud stability was highest in the Medium and Soft extraction. When pH 4.5 estimates were used the Soft and Medium extracts destabilized cloud more rapidly. Results indicate that extraction pressure affects cloud stability. Extracts from juice obtained at lower pressures destabilized cloud more rapidly than extracts from the highest pressure.