Mode of action of hot-water dip in reducing decay of lemon fruit

A hot-water dip for 2 min at 52-53 degrees C prevented decay for at least one week in lemon fruit inoculated with Penicillium digitatum. The mode of action of hot water in reducing decay was investigated by studying the effects of this treatment on the pathogen and on the resistance mechanisms of the fruit. The hot-water dip had a transient inhibitory effect on the pathogen, arresting its growth for 24-48 h. During this lag period, the combined effects of the pathogen and the hot-water dip induced the build up of resistance in the peel. Lignin production in the inoculated sites began within 24 h after inoculation or wounding. When inoculation was followed by the hot-water dip, lignin accumulation continued for a week. Inoculated lemons that were not dipped in hot water rotted completely within 3 days after inoculation and their lignin content did not rise or even decreased. The scoparone concentration in the inoculated sites of hot dipped fruit started to rise 24 h after treatment and reached a level sufficient to inhibit the pathogen within 2 days after treatment. Parallel to scoparone accumulation, scopoletin was detected in inoculated and heat-treated lemons. Without the pathogen challenge or wounding, heat treatment by itself was not able to induce any of the above-mentioned defensive effects. Our data do not support the involvement of ethanol-extractable aldehydes, associated in the literature with wound gum, or of citral in decay inhibition in hot-water dipped lemons.

Ultraviolet C irradiation at 0.5 kJ.m(-)(2) reduces decay without causing damage or affecting postharvest quality of star ruby grapefruit (C. paradisi Macf.)

Star Ruby grapefruit [Citrus paradisi (Macf.)] were harvested in November, February, and May, treated with ultraviolet C (UV-C) light at 0.5, 1.5, or 3.0 kJ.m(-)(2), and then stored at 7 degrees C and 90-95% relative humidity (RH) for 4 weeks with 1 additional week at 20 degrees C and approximately 80% RH. Untreated fruits were used as control. UV-C irradiation at 0.5 kJ.m(-)(2) effectively reduced decay development as compared to nontreated fruit without causing damage. Irradiation at dosages >0.5 kJ.m(-)(2) did not further improve decay control and caused rind browning and necrotic peel, the extent of damage depending on treatment dosage and harvest date. The percentage of damaged fruit after irradiation at the higher UV-C dosages was significantly higher in fruit harvested in November; differences between fruits harvested in February and May were negligible. After UV-C irradiation, the phytoalexins scoparone and scopoletin accumulated in flavedo tissue, their amounts depending on harvest date and UV-C dosage. Both phytoalexins showed similar accumulation patterns, although the concentrations of scoparone were much lower than those of scopoletin. Phytoalexin levels increased in most samples as the treatment dosage increased. No detectable levels of scoparone and scopoletin could be found in nonirradiated fruit. The influence of UV-C treatments on soluble solids concentration and titratable acidity of juice was negligible.