Reducing External Chilling Injury in Stored `Hass' Avocados with Dry Heat Treatments

Hass' avocados (Persea americana Mill.) were heated in air at 25 to 46 °C for 0.5 to 24 hours and stored at 0, 2, or 6 °C. After storage, fruit were ripened at 20 °C and their quality was evaluated. In unheated fruit, external chilling injury occurred in fruit stored at 0 or 2 °C, but not 6 °C. Chilling injury was also evident after storage at 2 °C in fruit heated at 34 °C, and to a lesser extent in fruit heated at 36 °C. A heat treatment (HT) of 38 °C for 3, 6, or 10 hours and 40 °C for 0.5 hour further reduced external chilling injury induced by storage at 2 °C. These HT's did not reduce internal fruit quality and resulted in more marketable fruit than unheated fruit stored at 6 °C. Low-temperature storage and HT slowed avocado ripening, resulting in longer shelf life after storage. In flesh tissue sampled directly after selected HT's, the levels of mRNA homologous to cDNA probes for two plant heat-shock protein (HSP) genes (HSP17 and HSP70) increased to a maximum at 40 °C and declined at higher temperatures. These increases in gene expression coincided with the extent to which HT's prevented chilling injury. Hot- air HT's confer significant protection against low-temperature damage to avocados.