Thermal properties of ‘Gala’ apples during growing season for predicting harvest time

Apples are one of the most widely cultivated fruits worldwide and are processed into many products. The harvest time significantly impacts the quality and shelf life of the fruit. An apple picked early or late is more sensitive to physiological disorders and has shorter storage life than fruit harvested at the proper time. Many physical, biochemical, and physiological properties have been assessed to determine the harvest time. However, no study on the measurement and use of thermal properties to predict fruit harvest time has been reported in the literature. Therefore, the objectives of this study were to measure the thermal properties, i.e., thermal conductivity (k) and thermal diffusivity (D), of the ‘Gala’ apple cultivar during the 2017 growing season in Pennsylvania using a dual-needle heated probe (DNHP) and to investigate the relationships of these properties with harvest fruit quality and maturity indices. The measured weekly average k and D values of ‘Gala’ apples during the growing season were between 0.454 ±0.015 and 0.411 ±0.015 W m·K^-1 and between 0.152 ±0.006 and 0.137 ±0.004 mm² s^-1, respectively. The k and D values decreased significantly during the growing season (p < 0.05). Overall, statistically significant (p < 0.05) changes in k and D occurred during the last two weeks of the growing season, which corresponds to the time when major biochemical changes are known to occur. The correlation among the thermal property changes and the fruit quality and maturity indices, including firmness, soluble solids content (SSC), starch index (SI), and flesh stain percentage, were determined based on Pearson correlation coefficients (r). Thermal conductivity (k) had better moderate correlations with firmness (r = 0.44), SSC (r = -0.62), SI (r = -0.53), and flesh stain (r = 0.50) than thermal diffusivity (D) during the growing season. Hence, thermal conductivity can detect the weekly quality changes during the growing season and determine the harvest time of apples.