Evaluation of the use of sulfur dioxide to reduce postharvest losses on dark and green figs
Postharvest diseases limit the storage period and market life of fresh figs (Ficus carica L.). The objective of this work was to determine the effect of sulfur dioxide (SO2) applied by fumigation and/or by dual release SO2 generating pads on postharvest decay and quality retention of ‘Black Mission’ and ‘Brown Turkey’ (dark skin), and ‘Kadota’ and ‘Sierra’ (green skin) figs. A protocol for the computer-controlled application of gaseous SO2 has been developed which allows the application of very low specific concentration×time products of SO2 and simultaneous monitoring of the application progress. In vitro tests with important fungal, yeast and bacterial postharvest pathogens plated on Petri dishes and exposed to a SO2 concentration×time product (C×t) of 100 (µL/L) h at different temperatures showed fewer survived at 20 °C than at 0 °C. Therefore, fumigations were carried out at 20 °C in the rest of the experiments. The evaluation of different SO2 concentration×time products showed that a product of 25 (µL/L) h provided the best compromise between decay control and fruit injury. The performance of SO2 fumigations on warm or cold fruit, its combination with SO2 generating pads, and the use of repeated fumigations during cold storage were also evaluated. All the SO2 treatments tested reduced the percentage of decay, extending the market life of fresh figs. However, in some cases, the use of SO2 generating pads increased the incidence of skin bleaching. Fumigation of warm fruit at 25 (µL/L) h of SO2 reduced populations of Alternaria and Rhizopus spp. growing on the fig surface. The treatment was more effective against Rhizopus spp. than against Alternaria spp. Contamination of fruit by Botrytis spp. and Penicillium spp. was also reduced by SO2. In conclusion, results showed that SO2 can be a potential tool to control postharvest rots and therefore increase the market life of fresh figs.