Susceptibility of strawberry varieties to breakdown in sulphite by endopolygalacturonase from zygomycete spoilage fungi

Differences were observed in the texture and susceptibility to enzymic breakdown of sulphited fruit of eight strawberry varieties. Cambridge Favourite was the softest variety in sulphite and was also the most susceptible to breakdown by pectolytic enzymes of Rhizopus and Mucor spp. Endopolygalacturonases (endo‐PG) from these species were more rapidly inactivated in sulphite liquor in the presence of fruit of the variety Harvester, which consequently showed the greatest resistance to breakdown. Isoelectric focusing indicated that a single major isoenzyme from each strain of R. sexualis, R. stolonifer and M. piriformis persisted in sulphite liquor and was, therefore, responsible for breakdown, although the major form of M. piriformis was less acid‐stable than those of the Rhizopus spp. Other minor isoenzymes produced by the fungi were inactivated within 24 h of sulphiting. Each species produced its major acid‐stable endo‐PG in the three varieties tested; thus varietal differences in susceptibility to breakdown may be related to the occurrence of natural enzyme inhibitors and/or fruit structure.