Improved Accuracy in Determining Optimal Harvest Time for Pitaya (Hylocereus undatus) Using the Elasticity Index

Abstract

This study was conducted to determine whether the optimal harvest time of pitaya (Hylocereus undatus) could be identified more accurately using the elasticity index as determined by a nondestructive resonant-vibration method. Seven-year-old pitaya grown in beds filled with sand in an unheated greenhouse were used, and pollinated fruit in July (Jul.-PF) and September (Sep.-PF) were harvested at intervals of 4 days during days 20–36 or 20–40 days, respectively, after anthesis. The second resonant frequency, the elasticity index, and flesh firmness decreased with ripening; however, values for Jul.-PF decreased faster than those for Sep.-PF. During the final harvest time, both groups of pollinated fruits had similar values for second resonant frequency (330 Hz), the elasticity index (60×10⁵), and flesh firmness (7 N·cm⁻²). The a-value of peel color for Jul.-PF and Sep.-PF increased rapidly during days 24–32 and 28–36, respectively, after anthesis and thereafter showed almost constant values. Total sugar content in Jul.-PF increased rapidly until 28 days after anthesis and then increased slightly. Total sugar content in Sep.-PF increased later than in Jul.-PF. Organic acid in Jul.-PF and Sep.-PF decreased rapidly until 28 and 36 days, respectively, after anthesis and thereafter decreased slowly. In Sep.-PF that were harvested at the optimal time, the second resonant frequency declined as fruit size increased, but the elasticity index was not affected by fruit size. High positive correlations were found among the second resonant frequency, the elasticity index, and flesh firmness in both groups of pollinated fruit. Considering a comprehensive suite of parameters including pulp rate, peel color, sugar, organic acid, flesh firmness, and occurrence of cracking, the optimal harvest time was estimated to be 36 and 40 days after anthesis for Jul.-PF and Sep.-PF, respectively, and the elasticity index of fruit at optimal harvest time was 62 × 10⁵ and 73 × 10⁵, respectively. This study showed that the optimal harvest time of pitaya could be determined more accurately by measuring the elasticity index of on-tree fruit.