PRESENCE OF RESIDUAL PECTIN METHYLESTERASE ACTIVITY IN THERMALLY STABILIZED INDUSTRIAL FRUIT PREPARATIONS

Industrial fruit products were investigated for their content of pecti n methylesterase (PME) residual activity by a new procedure that allow s selective concentration by affinity chromatography of the active for m of the enzyme, when present. The technique utilizes a protein inhibi tor, purified from kiwifruit, covalently bound to a Sepharose resin. T his inhibitor, in fact, binds with high affinity the native active for m of PME that can be subsequently recovered by dissociating the comple x at alkaline pH with high ionic strength. In this way the enzyme is r ecovered at a concentration suitable for pi-I-stat assay. We examined fruit products from several industrial preparations of peach, pear, ap ple, apricot and pineapple. The analysed fruit purees showed PME activ ities between 10(-3) and 10(-5) U/mL, whereas the nectars did not show any detectable activity although cloud loss phenomenon was often obse rved. 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 clo ud loss on the levels of PME residual activity was found.