39F-4

A. AMORSIN, Food Process Research and Development Laboratory, University of Georgia, Department of Food Science & Technology, Athens, GA 30602 and R. T. Toledo.

A continuous flow high pressure system which consists of a pressure intensifier and a throttling valve to induce very high rates of shear on the flowing fluid was found to effectively control microbial activity in citrus juices. Maximum fluid temperature during the process depends upon the pressure induced and the fluid initial temperature. This investigation was conducted to determine the long term storage stability of orange juice processed using the high pressure continuous flow system. In particular, the effectiveness of the system in inactivating pectinesterase and the cloud stability of the processed product during refrigerated storage, was investigated. Fresh squeezed orange juice (cv. Valencia, from Florida) was passed through a screen, homogenized in a standard Gaulin homogenizer at at 48.3 MPa before treatment in the high pressure system. The juice was pressurized from an initial temperature of either 4 or 20 ^oC, and with or without a holding tube prior to cooling after leaving the pressure reducing valve. The juice was subjected to pressures of 0(control), 137.9, 206.8, 275.8 MPa(gage) at a flow rate of 200 mL/min. The processed juice was cooled immediately after processing, packed in transparent glass bottles, and stored at 3± 1 ^oC for observations of cloud stability. The juice retained cloud stability at least 21 days with pressure treatments of 275.8 MPa when inlet temperature was 4 ^oC and no holding tube was used. If inlet temperature was 20 ^oC and a holding tube was used, cloud was stable at a lower pressure of 206.8 MPa and 38.4 s hold before cooling. The absorbance at 660 nm of 1.1 to 1.5 was used to indicate a stable citrus juice cloud. Stable cloud was obtained when the residual pectinesterase activity was 0.19 to 2.28 ´ 10^-3 PEU or 62 to 97% reduction from the PEU in unprocessed juice.