Pectolytic enzymes and degradation of pectin associated with breakdown of sulphited strawberries

Both pectate‐based culture filtrates and ripe strawberries, artificially inoculated with Rhizopus stolonifer, Rhizopus sexualis, Mucor piriformis or Aureobasidium pullulans , contained endo‐polygalacturonase and pectin esterase. Uninfected strawberries contained traces of exo‐polygalacturonase together with pectin esterase. No other pectin‐degrading enzymes were detected in either sound or infected fruit. Naturally infected strawberries from commercial sources contained pectin esterase and variable amounts of polygalacturonase, the greatest amounts being associated with fruit carrying the heaviest microbial contamination. Disintegration of strawberries in sulphite liquor was associated with pectin degradation as judged by loss of liquor viscosity, reduction in pectic substance degree of polymerisation and the appearance in liquors of galacturonic acid. Polygalacturonase was detected in liquors during the early stages of disintegration but activity subsequently disappeared due to instability of the enzyme at low pH. Addition of culture filtrates to sound sulphited fruit to give polygalacturonase levels similar to those detected in some commercial liquors reproduced the fruit breakdown symptoms. Disintegration could be substantially reduced if polygalacturonase was inhibited either by lowering the pH, or by addition of anionic surfactant. It is concluded that polygalacturonase secreted by various decay fungi and present in fruit at the time of sulphiting is primarily responsible for breakdown. The possible involvement of non‐enzymic synergists is discussed.