39F-5

M. CORREDIG and L. WICKER. Department of Food Science, University of Georgia, Athens, GA 30602

Pectinmethylesterase plays a major role in citrus juice clarification. Various forms of this enzyme, differing in their isoelectric points, molecular weights or catalytic properties have been reported. Thermostable pectinmethylesterase (TS-PME) maintains its activity after heating (70°C for 5 min), and rapidly clarifies citrus juice at refrigeration temperatures.

The present research investigated orange juice clarification by isolated fractions of TS-PME.

TS-PME was isolated from grapefruit pulp by ammonium sulfate precipitation, ion-exchange chromatography and affinity chromatography.

Two fractions of TS-PME, isolated as two distinct elution peaks and characterized by differences in specific activity and polypeptide composition (as determined by SDS-PAGE), showed different clarification behavior. When enzyme was added to reconstituted orange juice at a concentration of 2 Units/ml, all juices clarified within a few days. Values of percent transmittance (%T, 650 nm) for juice supernatants after centrifugation showed that the TS-PME fraction with the lowest specific activity clarified at the fastest rate. Analysis by light scattering of particle size distribution of the cloud fraction (Mastersizer X, Malvern Instruments, MA) showed that upon addition of TS-PME the average particle size increased from 0.7 µm to 1 µm, even while maintaining its monomodal distribution. After 10 days of storage at 4°C, an increasing population of larger particles was observed. Small cloud particles were still present in the juice, even in samples that clearly showed clarification (%T values > 35%). A combination of analysis of particle size distribution, % T, pectin and protein analyses of the alcohol insoluble solids fractions, reveal the complexity of the clarification of juice cloud caused by TS-PME.