*Correspondent: Fax: +39 81 921921. e-mail: giovane@ds.unina.it The detection of residual pectin methylesterase activity in pasteurized tomato juices Domenico Castaldo 1* , Luigi Servillo 2 , Roberto Loiudice 1 , Ciro Balestrieri 2 , Bruna Laratta 1 , Lucio Quagliuolo 2 & Alfonso Giovane 2 1 Department of Fruit Juices and Tomato products, Stazione Sperimentale Industria Conserve Alimentari, via Nazionale 121/123 84012 Angri (Salerno) and 2 Department of Biochemistry and Biophysics, II University of Naples, Italy Summary The enzyme pectin methylesterase (PME) is believed to be involved in the destabilization and cloud loss of vegetable juices through the de-esterification of pectin followed by the successive coprecipitation of the pectate with insoluble materials present in the juices. Cloud destabilization is often observed even when the vegetable products have been sub- jected to thermal treatment to produce sterile products and also to stabilize the cloud. Therefore it is possible that loss of cloud may be because of residual enzymatic activities surviving the thermal treatments. However, so far no evidence demonstrating the exis- tence of residual PME activity in pasteurized juices has been published. In this paper it is reported that in industrial tomato products showing cloud loss residual PME activity is present. It has been possible to detect this very low activity by an affinity chromatog- raphy procedure. The method is based on a cyanogen bromide-activated resin which is coupled with a pectin methylesterase inhibitor protein purified from kiwi fruit. This resin binds native PME with high selectivity and the enzyme can be concentrated from the product in a single step. Thus, the very low PME residual activity present in pasteurized juice and generally not detectable with common techniques can be detected and easily determined with the method described in this paper. Keywords Affinity chromatography, cloud stability, food control, thermal processing, trace analysis. Introduction It is known that the liquid or half-liquid products from fruits or vegetables can be considered bipha- sic systems made up of a dispersing phase, called ‘serum’, in which all the water-soluble compo- nents are dissolved, and of a disperse phase, called ‘cloud’, composed of solid particles of different size and shape that originate from fruit tissue mechanically crushed for the puree prepa- ration. One of the main objectives of vegetable juice technology is to achieve a product which is ‘cloud-stable’ over long storage periods. Vegetables contain varying amounts of the enzyme pectin methylesterase (PME EC 3. 1. 1. 11) which is usually bound to the cell wall. During the juice extraction step, PME activity de- esterifies the pectin. As a consequence, the low methoxyl pectin produced interacts with calcium ions forming insoluble pectates which coprecipi- tate with the pulp particles thus causing cloud loss (Joslyn & Pilnik, 1961). Results of chemical and physical analyses have demonstrated that juice stability is independent from the serum composition but depends on the charge and hydration of the pulp-particles in the suspension (Stahle & Gierschner, 1989). If the pulp-particle charge changes by the action of PME, the hydra- tion also changes and the particles may form International Journal of Food Science and Technology 1996, 31, 313–318 © 1996 Blackwell Science Ltd 313