Ozone for control of post-harvest decay of table grapes caused by Rhizopus stolonifer

The effect of ozone on post-han est decay of table grapes was studied both with regard to its effectiveness and its possible mode of action. Ozone concentrations fell rapidly upon contact with organic matter and the amount which reacted with grape berries and the microflora on their surface was about 0.1 mg g(-1), when supplied at a rate of 8 mg min(-1) for 20 min. The dose applied could be increased by longer periods of exposure, but symptoms of toxicity were observed on certain cultivars. The number of colony forming units (cfu) of fungi, yeasts and bacteria naturally present on the berry surface was considerably reduced by a 20 min exposure to ozone. Ozone treatments significantly reduced the extent of berry decay caused by fungi following cold storage and increased shelf-life. A significant decrease in decay was observed in berries that were treated with ozone either before or after being inoculated with Rhizopus stolonifer. This finding indicates that, in addition to its sterilizing effect, ozone also induced resistance to post-harvest decay development. The phytoalexins resveratrol and pterostilbene were elicited by ozone treatments, at levels similar to those produced by uv-c irradiation. Resveratrol accumulated in greater quantities than pterostilbene. Inoculation with R. stolonifer in addition to ozone treatment, raised the levels of both stilbenes even more. Exposing berries to ozone was almost as effective as SO2 fumigation for the control of storage decay caused by R. stolonifer and no deleterious effects were observed on the appearance of the grape cluster. Ozone treatments can therefore be considered as a possible substitute for SO2 fumigation for the control of post-harvest fungal decay. (C) 1996 Academic Press Limited