5.1.14
POST-HARVEST DECAY PREVENTED BY VINEGAR VAPOUR

PL SHOLBERG

Agriculture and Agri-Food Canada, Pacific Agri-Food Research Centre, Summerland, BC VOH 1ZO, Canada

Background and objectives
Acetic acid is extremely effective as a vapour in killing spores of a wide range of post-harvest fungi [1]. For example, acetic acid vapour tested on the fruit of apples, grapes, kiwifruit, pears, oranges and tomatoes killed conidia of Botrytis cinerea and Penicillium spp. In preliminary experiments, using small samples of various seeds (canola, corn, peas, soyabean and wheat) inoculated with Aspergillus spp. and Penicillium expansum, acetic acid vapour eliminated both naturally occurring and inoculated moulds in several instances [2]. Vinegar usually contains 4-6% acetic acid, although concentrated solutions can be made to contain 10-12.5%. Vinegar may be made from a wide variety of substances as long as sufficient alcohol or sugar is available for fermentation. The vinegar is produced in two steps: firstly, a suitable strain of yeast converts the sugar into alcohol, followed by a species of Acetobacter which converts the alcohol into acetic acid. The object of this study was to determine if the vapour of several commercially available vinegars containing 4-6% acetic acid would be effective in preventing decay of fruit when inoculated with post-harvest fungi.

Materials and methods
Apple cider (5.0% acetic acid, AA), balsamic (6.0% AA), brown rice (4.2% AA), malt (5.0% AA), raspberry (6.0% AA), red wine (5.0% AA), white wine (5.0% AA) and white (5.0% AA) vinegars were purchased from a local merchant for use in this study. Stone fruit, strawberries and apples were inoculated with conidia of Monilinia fructicola, B. cinerea and P. expansum, respectively, by placing a 20 l drop of suspension (1x103 c.f.u.) at three equidistant points on the fruit surface. Due to the soft nature of strawberries they were sprayed with conidia. After the suspension had dried the fruit were placed in fumigation chambers made from cans (12.7 l) lined with aluminium. Each chamber contained a fan and a filter paper wick to evaporate 79-236 l/l vinegar. Fruit sealed in the chamber was exposed to vinegar vapour for 0.5-16 h at 10C. Chambers were opened in a laminar flow hood and fruit were injured with a sterile glass rod (2 mm diameter) where conidia had been placed. After injuring, the fruit were incubated at 20C for 5-7 days, after which decay diameter was recorded.

Results and conclusions
Fumigation of stone fruit with four different vinegars (apple cider, white wine, red wine and white) at a concentration of 79-119 l/l reduced infection by M. fructicola to zero from 100% in the control. White vinegar at 79 l/l for 16 h was very effective in reducing decay of strawberries caused by B. cinerea from 46.9 to 4.2%. Fumigation of Jonagold apples inoculated with P. expansum for 30 min with 236 l/l vinegar showed that red wine, white and apple cider vinegars were more effective than malt and balsamic vinegars in reducing decay. In a more extensive experiment with Golden Delicious, Red Delicious and Spartan apples, white vinegar was more effective than any other vinegar tested. In conclusion, vinegar in the vapour form can be used to eliminate fungal spores contaminating the surface of fruit and thus reduce the potential for decay.

References
1. Sholberg PL, Gaunce AP, 1995. HortScience 30, 1271-1275.
2. Sholberg PL, Gaunce AP, 1996. Canadian Journal of Plant Science 76, 551-555.