Pectin methylesterase activity in juices from mandarins, oranges and hybrids

The pectin methylesterase (PME) activity of fresh and processed juices obtained from Valencia-late oranges, Clementine mandarins (cultivar Nules) and citrus hybrids (Ortanique) was studied. The PME activity was measured using a modification of the traditional method of Rouse and Atkins [1], based on the titration of carboxylic groups generated by PME during the hydrolysis of a commercial pectin solution. Instead of maintaining the pH at a given value by addition of NaOH, the maximum rate of pH decrease is used as measurement of the PME activity. An exponential decay function accurately represents the evolution of pH with time from a starting value of pH = 7.8 (optimum pH of PME activity for all assayed cultivars). This function allows an easy calculation of the PME activity. The total PME activity measured was similar in Clementine and Valencia-late juices, and approximately twice that of Ortanique juices. Ratios between thermostable and total PME activity were 4.0-8.4% for Clementine, 12.6-12.7% for Valencia-late and 24.2-34.2% for Ortanique juices. Soft heat treatments (70 degrees C, 30 s or 74 degrees C, 10 s) were enough to reduce the total PME activity of Clementine juices under the maximum residual level (10%) for commercial life of refrigerated juices, according to Irwe and Olson [2].