Ultrasound application for the control of decay on apple at different stage of ripening

Ultrasonic technology is known for many years and is used for several purposes such as sonochemistry, extraction of natural compounds, degassing of solvent, inactivation of enzymes and microorganism. In postharvest ultrasound is applied in not destructive analysis of crop for the determination of the maturation stage. Until now, however, the potential of the sonication as a physical mean for not conventional postharvest treatment of fresh fruit has been little investigated. Here we report on the results obtained with apple fruit (Malus domestica Borkh. cv San Giovanni, from Sardinian germplasm) dipped in a solution of potassium sorbate with or without sonication. Treatment was carried out with an ultrasonic processor (1500 W, 20Khz) connected to a 25 mm phi probe immersed in 10 L of deionized water placed in a steel vessel. Fruit at different ripening stages, inoculated or not inoculated with Penicillium expansum, was sonicated before or after the inoculation. Following the treatment, fruit was left to dry, put into boxes and cold-stored. Results showed that ultrasound alone enhanced the natural resistance of ripe fruit when inoculated after sonication. No effect was observed when the ultrasound application was carried out after inoculation with P. expansum on un-ripe or ripe apples. Potassium sorbate showed to be ineffective in controlling the decay regardless of the ripening stage and the time of inoculation. On the contrary, a significant enhance in decay control was observed when the application of potassium sorbate was performed in the presence of ultrasound.