Using Hexanal Vapor to Inhibit Activity of Penicillium expansum in Inoculated Whole Apple Fruit

Abstract

Hexanal vapor is a natural, metabolizable fungicide that inhibits fungal activity and enhances the aroma biosynthesis in sliced apple fruit. Whole apple fruit were inoculated at two points per fruit with Penicillium expansum at a concentration of 0.5 × 105 spore/ml and treated with hexanal vapors. Inoculated fruit were exposed to hexanal for 48 hr and kept for another 72 hr in hexanal-free air at 22°C. Treatments included 8.2–12.3 μmol·L–1 (200–300 ppm), 14.5-18.6 μmol·L–1 (350–450 ppm), and 24.8-28.9 μmol·L–1 (600–700 ppm), each with an air control. At a concentration of 200–300 ppm hexanal, there was no fungal growth during treatment, but lesion development was evident on 100% of the treated fruit following cessation of treatment. After 72 hr holding in air, lesion diameter was significantly smaller for treated fruit. When inoculated apple fruit were exposed to 350–450 ppm and 600–700 ppm hexanal vapors, the decay rate was 44.7% and 23.9%, respectively, while the decay rate of inoculated control apple fruit was 100% and 98%, respectively, after 72 hr holding in air. The development of aroma volatiles was investigated for both treated and untreated whole apple fruit. Hexanal was actively converted to aroma volatiles by `Golden Delicious' fruit and there was no detectable hexanal emanations. The amount of hexylacetate, hexylbutanoate, hexylhexanoate, hexylpropionate, butylhexanoate, and hexyl-2-methybutanoate were about 2- to 4-fold higher in treated apple fruit than in untreated apple fruit. `Mutsu' apple fruit were treated with 350–450 ppm hexanal for 48 hr and processed into apple sauce within 4 hr. An informal sensory evaluation for processed `Mutsu' apple revealed no apparent flavor difference between treated and control fruit sauce.