High pressure (HP) processing has been emerging rapidly as a novel technique for non-thermal preservation of foods. Application of HP processing for shelf life extension of orange juice was the principal objective of the current research. To accomplish this objective, and to establish a scientific basis for HP processing of orange juice, a systemic approach was used which included the evaluation of: (a) HP inactivation kinetics of pectin methyl esterase (PME, the key enzyme in orange juice implicated with respect to quality changes), (b) destruction of spoilage microorganisms and changes in product quality, (c) HP process verification and finally, (d) storage studies on HP treated orange juice. / In preliminary studies, the effect of HP treatment on indigenous microorganisms, texture and color of selected fresh fruits and vegetables were evaluated. Results showed that HP had a significant effect on the destruction of microorganisms. Product texture and color were mildly affected, often resembling the appearance of mildly heat-treated products. / Pressure induced inactivation kinetics of pectin methyl esterase (PME) was investigated at pH 3.7 and 3.2 in freshly squeezed single strength (12.6°Brix) and concentrated (10--40° Brix) orange juice. Results showed a biphasic nature of pressure induced inactivation of PME in both juices. The first phase consisted of rapid change in inactivation of enzyme, designated as instantaneous pressure kill (IPK), due to pulse pressurization, followed by gradual inactivation of enzyme, characterized by a first order rate of inactivation during pressure hold-time. / Combination treatment involving pressure cycle, pressure level and pressure hold-time was then evaluated for inactivation of PME using a response surface methodology. Overall, pressure pulse had a lower effect on inactivation of PME compared to other factors. / Pressure destruction kinetics of Leuconostoc mesenteroides and Saccharomyces cerevisiae the spoilage organisms in orange juice, were then investigated. Pressure destruction kinetics followed the same dual effect behavior, as observed with PME inactivation. IPK effect increased with pressure cycles and was more pronounced with S. cerevisiae that Leu. mesenteroides. / Storage studies of HP treated single strength and concentrated orange juice were conducted at selected temperatures (4, 10 and 20°C). Results showed that treated juice was microbiologically stable from a few days to several weeks depending on type of juice, storage temperature and processing conditions. (Abstract shortened by UMI.)

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