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

The present study provides evidence for the involvement of protein in cloud instability of natural orange juice. No heat-coagulable proteins were found in the serum. Insoluble cloud matter (ICM) was heat-flocculated following enzymatic pectin degradation (EPD). The degree of flocculation depended on temperature (from approximately 50 to 75 degrees C) and was highest at pH 3.5. The fresh juice contained about 6.5 and 1.8 mg mL(-1) of ICM and alcohol-insoluble solids of the serum (AISS), respectively. The ICM and the AISS contained, respectively, proteins (182+/-14 and 119+/-3 microg mg(-1)), galacturonic acid (37+/-6.6 and 175+/-1 microg mg(-1)), and neutral sugars (350+/-44 and 338+/-22 microg mg(-1)). EPD 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. Under electrophoresis, proteins of the AISS included bands in the range of 20-52 kDa and 10-14 kDa and those of the ICM at 22 and 50 kDa.