Involvement of proteins in cloud instability of valencia orange [Citrus sinensis (L.) Osbeck] juice.

The present study examined the involvement of proteins in cloud flocculation of Valencia orange juice. Marked differences in cloud instability were found between juices of different harvest dates. Heating of enzymatic pectin degraded juice from April and June harvests resulted in development of clumps and their precipitation. Although the juice from both harvesting dates remained hazy, the juice of April harvest was more turbid than that from June. Usually clarification increases as the temperature increases from ambient to 125 degrees C. Clarification occurred at pH 2.5-4.5 and was maximal at pH 3.5. The clarification of the April harvest juice was markedly lower than that of the June harvest. The fresh juice contained about 5.2 and 1.7 mg mL(-1) insoluble cloud matter (ICM) and alcohol-insoluble serum solids (AISS), respectively. The ICM and the AISS, respectively, contained: proteins (244.5+/-8.7 and 132+/-1.8 microg mg(-1)), galacturonic acid (40+/-0 and 120+/-0 microg mg(-1)) and neutral sugars (270+/-39 and 329+/-23 microg mg(-1)). Enzymatic pectin degradation resulted in removal of a marked portion of the pectin, and was accompanied by partial removal of neutral sugars (mainly glucose and galactose) and some proteins from the pectic polymer in both AISS and ICM. Proteins of the AISS included major bands at 10-14, 20, and 28 kDa and those of the ICM bands at 22, 24, 26, and 45 kDa.