Twelve forms of pectinesterase were detected in citrus fruits. Two forms, representing over 90% of the total pectinesterase activity in Navel oranges, were purified. These pectinesterases, named Pectinesterase I and II have isoelectric points of 10.05 and>11.0, respectively. Both pectinesterases have the same molecular weight of 36 200 but differ in amino acid composition. A third pectinesterase, with a higher molecular weight (about 54 000) was partly purified. The optimum pH of these three pectinesterases is about 7.5, but their pH-activity profiles differ and depend on the degree of esterification of the pectin. The high molecular weight pectinesterase is still active at pH 2.5. The <em>K</em><sub><font size="-1">m</font></sub> values of both purified pectinesterases decrease with decreasing degree of esterification of the pectin substrate and increase with decreasing pH. Pectinesterase II has a tenfold higher affinity for pectin than Pectinesterase I and is more strongly inhibited by pectate. The heat stabilities were determined: the <em>D</em><sub><font size="-1">90</font></sub> °C and Z values in orange juice are 0.00037 min and 6.5 °C for Pectinesterase 1, 0.0015 min and 11 °C for Pectinesterase II and 0.375 min and 6.5 °C for the high molecular weight pectinesterase. The orange juice cloud destabilizing properties of the pectinesterases at 5 °C and 30 °C are remarkably different. The activity measurements were done, amongst other methods, by an improved gas chromatographic methanol assay. The literature on pectinesterase was reviewed.
|Qualification||Doctor of Philosophy|
|Award date||28 Sep 1979|
|Place of Publication||Wageningen|
|Publication status||Published - 1979|
- orange juice