Stepped hot water treatment alleviates chilling injury of zucchini fruit by improving internal temperature uniformity

The effect of stepped hot water treatment (SHWT), involving sequential exposures to different temperatures for specific durations, on chilling injury (CI) tolerance of zucchini fruit during cold storage was evaluated in this study. Compared to hot water treatment (HWT) at 44 degrees C for 28 min, fruit subjected to SHWT exhibited lower reactive oxygen species accumulation, enhanced antioxidant enzyme activity, and improved cell membrane integrity. Notably, SHWT-treated fruit with 12 degrees C interval temperature variations (S12) showed up to a 24 % reduction in CI, a 14 % decrease in electrolyte leakage (EL), a 40 % reduction in superoxide radical content (O2 center dot-), and a 21 % increase in superoxide dismutase (SOD) activity compared to those treated with HWT. The stepwise increases in hot water treatment temperature with multiple small temperature differentials in SHWT groups caused a perturbation of the internal temperature distribution of the fruit, manifested as multiple increases in the center and average temperatures. This perturbation is the key factor contributing to the significantly higher temperature uniformity (TU) in SHWT compared to HWT, with S12 exhibiting 2.04 times the TU value of HWT. Orthogonal projection to latent structures-discriminant analysis identified O2 center dot-, EL, and SOD as the primary physiological traits contributing to the significantly greater CI resistance observed in SHWT-treated fruit compared to HWT-treated fruit, with these traits being significantly correlated with TU. Thus, we suggest that SHWT may enhance CI tolerance of zucchini fruit by improving TU within the fruit during heat treatment process.