The kinetics of ascorbic acid loss and nonenzymatic browning in clarified orange juice (serum) were investigated in an anaerobic environment from 70.3 to 97.6"C and from 11.7 to 80.6"Brix. Data were fitted to firstorder kinetic models. Rate constants of ascorbic acid degradation in serum were not different from rate constants in whole juice. Activation energies were -30 kcal/mol and largely independent of solids concentration. Rate constants of browning pigment formation were 30-50% greater in serum. Activation energies were 19-25 kcal/mol and increased slightly with solids concentration.

The enzyme pectinesterase (PE) reduces the quality of citrus juices. Current inactivation of the enzyme is accomplished by heat, resulting in a loss of fresh fruit flavor in the juice. We explored the use of pressurized treatments of orange and grapefruit juices to bypass the use of extreme heat during processing. PE inactivation using isostatic high pressure in the range of 500-900 MPa was accomplished in orange and grapefruit juices. The higher pressures (>600 MPa) caused instantaneous inactivation of the heat labile form of the enzyme but did not inactivate the heat stable form of PE. Treatment times caused significantly different (R ) 0.05) total PE activity losses in orange but not in grapefruit juices, and PE inactivation at different pressures was significantly different in both juices. Heat labile grapefruit PE was also more sensitive than orange to pressure. D p values for orange PE inactivation at 500 and 600 MPa were 83.3 and 2.4 min, respectively; while the z p value between 500 and 600 MPa was 65 MPa. Orange juice pressurized at 700 MPa for 1 min had no cloud loss for >50 days.

High pressure inactivation kinetics (D and z values) of Saccharomyces cerevisiae ascospores were determined in fruit juices and a model juice buffer at pH 3.5 to 5.0. Approximately 0.5 to 1.0 3 10 6 ascospores/mL were pressurized at 300 to 500 MPa in juice or buffer. D-values ranged from 8 sec to 10.8 min at 500 and 300 MPa, respectively. The range for z-values was 115 to 121 MPa. No differences (P$0.05) in D (at constant pressure) or z-values among buffers or juices at any pH were determined, indicating little influence of pH in this range and absence of protective or detrimental effects of juice constituents.