Effects of Ripening Stage, Temperature, and Film Wrapping on the Elasticity Index as Determined by a Nondestructive Resonance Vibration Method and Fruit Quality in ‘Irwin’ Mango

Abstract

We examined the effects of ripening stage, temperature, film wrapping, and storage period on the elasticity index (EI) and fruit quality of ‘Irwin’ mangos to develop a storage technique for improving the storage life of this fruit and to determine whether flesh firmness can be estimated using the EI. The second resonant frequency measured between the two sides of the fruit was nearly equal to measurements between the dorsal and ventral surfaces of the fruit. The EI declined as fruit ripening progressed and was not influenced by fruit size. Fruits harvested at the pre-natural fruit drop stage (pre-NFDS; 2 to 3 days before fruit drop) and the post-natural fruit drop stage (post-NFDS) were kept at 25 °C for 10 days. No significant differences were detected between fruits at these two stages in terms of the EI and flesh firmness, except that both variables at the pre-NFDS were higher than those at the post-NFDS after 0 and 4 days and after 0 day of storage, respectively. Similarly, no significant differences were found between fruits at these two stages in terms of total sugar and organic acid contents, regardless of the length of the storage period. Fruits harvested at pre-NFDS were individually wrapped in low-density polyethylene film (11 μm thick) and stored at 10, 15, 20, and 25 °C for 10 days. The EI of fruits kept at 10 °C was higher than that of fruits stored at 15 to 25 °C regardless of the presence of film wrapping. At 10 °C storage, the EI of film-wrapped fruits was higher than that of non-film-wrapped fruits. Film-wrapped fruits harvested at pre-NFDS were stored at 10 °C for 30 days. After 15 days in storage, relatively rapid and then gradual declines in both the EI and flesh firmness were observed. Sugar content was unaffected by the storage period, but organic acid content decreased after 30 days in storage. A strong positive correlation was found between the EI and flesh firmness. When the data set was limited to fruits less than 20 N·cm−2 in flesh firmness and more than 350 g in weight, the correlation coefficient was higher compared with a data set with all fruit combined. The results of this study indicate that flesh firmness in ‘Irwin’ mangos can be estimated by measuring the EI. Moreover, the ripening stage, storage temperature, and film wrapping affected the retention of fruit quality.