30G-6 |
Floc formation and changes in serum soluble cloud components of fresh Valencia orange juice |
J. L. ACKERLEY, Dept. Food Science and Technology, University of Georgia, Food Science Building, Athens, GA 30602 and L. Wicker. Juice clarification originates from de-esterification of pectin by pectinmethylesterase (PME). Subsequent formation of insoluble calcium pectate removes "cloud". PME is a cell wall bound enzyme that forms complexes of variable activity with pectin. Floc formation in the serum parallels clarification in juices. Clarification is increased by serum soluble factors, and is inhibited by soluble pectin. Our objective was to compare floc formation in juice and changes in serum soluble composition during storage of fresh, unheated juice. Juice was extracted from Valencia oranges in August 2000 and centrifuged at 8000g, 20 min to remove settling pulp. A portion of the pulp free juice (PFJ) was ultracentrifuged at 150,000g, 30 min. The pellet (UCP) was freeze dried for later analysis. The serum of the ultracentrifuge (UCS) and the PFJ were stored at 4C for 11 days. Analysis for PME, Brix, pH, % transmittance (%T), particle size and SDS-PAGE was performed at selected times. The Brix and pH of UCS and PFJ were approximately 11.6 and 3.7, respectively, and did not change during storage. The UCS and PFJ had approximately the same enzyme activity (0.07 U/ml), which is low relative to typical values for fresh juice. The PFJ had a high initial %T (20%), and increased significantly by day 4 to 40-60%. Floc (UCF) in the UCS appeared approximately the same time as the %T increase in PFJ. The UCP, UCF and UCS were analyzed by SDS-PAGE. Predominant bands at 13, 27 and 36 kDa were found in both UCF and UCP. The UCS had no detectable protein. The 36 kDa band is presumptively PME. Soluble (in UCS) and insoluble (in PFJ) PME are involved in clarification. Proteins at 13 and 27 kDa are presumptively involved in clarification and precipitate during floc formation.
Session 30G, Fruit & Vegetable Product: Processed Fruits and Vegetables
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