Presence of Residual Pectin Methylesterase Activity in Thermally Stabilized Industrial Fruit Preparations

Industrial fruit products were investigated for their content of pectin methylesterase (PME) residual activity by a new procedure that allows selective concentration by affinity chromatography of the active form of the enzyme, when present. The technique utilizes a protein inhibitor, purified from kiwifruit, covalently bound to a Sepharose resin. This inhibitor, in fact, binds with high affinity the native active form of PME that can be subsequently recovered by dissociating the complex at alkaline pH with high ionic strength. In this way the enzyme is recovered at a concentration suitable for pH-stat assay. We examined fruit products from several industrial preparations of peach, pear, apple, apricot and pineapple. The analysed fruit purees showed PME activities between 10 ‐3 and 10 ‐5 U/mL, whereas the nectars did not show any detectable activity although cloud loss phenomenon was often observed. The diced fruit samples showed activities between 10 ‐2 and 10 ‐5 U/g. The highest values were found in samples of chi grape (10 ‐2 U/g) and pineapple (10 ‐3 U/g). A dependence of the intensity of cloud loss on the levels of PME residual activity was found.