Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 28, 1999
Publication Date: N/A
Interpretive Summary: Juice was extracted from Valencia 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 was 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 either pH 4.5 or 7.5, 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 Soft Extracted Juice had the most stable cloud, followed by Medium and the Hard Extracted Juice, (2) using PME activity estimates made at pH 4.5 and adding equal activities to pasteurized, reconstituted FCOJ produced results similar to raw juice incubated at 30 degree C, (3) using pH 7.5 PME activity estimates resulted in contradictory results suggesting that predicting consequences of PME activity estimated at pH 7.5 are unreliable.
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 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 and 7.5. 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 Soft extraction. At 4 C the effect was not as great. When pH 7.5 activity estimates were used the Soft extract destabilized cloud most rapidly. When pH 4.5 estimates were used the Hard and Medium extracts destabilized cloud more rapidly. Activity estimates were reduced at pH 4.5 compared to 7.5 but not equally, with Soft extract retaining more activity at pH 4.5 than Medium or Hard extracts. Results indicate that extraction pressure affects cloud stability. Extracts from juice obtained at higher pressures destabilized cloud more rapidly than extracts from the lowest pressure.