Heat treatment effects on chilling injury and disease incidence in sweet orange fruits stored at chilling and non-chilling temperatures

Citrus chilling injury causes 25% postharvest losses by shortening its storage life and lowers its market value in developed countries. In citrus fruits, chilling injury is characterized by increased moisture loss and disease susceptibility. The present study was conducted with the objective to evaluate the effects of pre-storage heat treatments (HTS: control, 5, 10, 15 and 20 min), heat treatment methods (HTM: wet heat treatment [WHT], vapor heat treatment [VHT]), and exposure temperatures (ET: chilled [2.5 °C], non-chilled [10 °C]) on citrus fruits (cv. Blood Red having sour orange rootstock) during 2015–2016 at the University of Agriculture, Peshawar, Pakistan. Sweet orange fruits and fruit discs of 10 mm diameter were simultaneously treated with WHT and VHT. The heat treated fruits and fruit discs were separated into two lots. One lot was held at non-chilling temperature (10 ± 1 °C), while the other lot was exposed to chilling temperature (2.5 ± 1 °C) for 75 days and packed in fiber board packages. The fruits and fruit discs after 75 days storage at both storage temperatures (10 ± 1 °C and 2.5 ± 1 °C) were incubated at ambient temperature for 7 days (simulated marketing time) and analyzed for different chilling injury parameters. Heat treatment periods (HTS), exposure temperature (ET) and their interactions (HTS × ET) showed significant (p ≤ 0.05) differences for the majority of the traits. The interaction of HTS × HTM was significant (p ≤ 0.05) for weight loss and surface pitting. However, heat treatment methods (HTM) and interaction of ET × HTM × HTS was significant for surface pitting and weight loss, respectively. The non-chilled (10 ± 1 °C) fruits revealed lower weight loss (6.80%), surface pitting (4.40%), disease incidence (7.07%), disease expansion (3.97 mm), ion leakage (15.77%) while the ascorbic acid increased (32.38 mg 100 g⁻¹). However, chilling (2.5 °C) triggered the increase in weight loss (8.32%), surface pitting (16.13%), disease incidence (14.59%), lesion diameter (10.37 mm) and ion leakage (27.96%) in sweet orange. Heat treatments with WHT and VHT for 5–10 min resulted in reduced weight loss (6.07%), surface pitting (3.33%), disease incidence (2.00%), disease expansion (5.00 mm) and ion leakage (20.23%) in sweet oranges. Heat treatments with WHT and VHT for 15–20 min, accelerated the fruit senescence by increasing the weight loss (8.98%), surface pitting (15.67%), disease incidence (15.33%), disease expansion (8.33 mm) and ion leakage (23.62%). In comparison to heat treatment methods, highest weight loss (7.57%), surface pitting (11.53%), ion leakage (21.94%) and disease incidence (10.86%) were recorded in VHT, whereas WHT treatments had lowest weight loss (7.55%), surface pitting (9.00%), ions leakage (21.80%) and disease incidence (10.79%). Therefore, modest WHT pre-storage heat treatment (5–10 min) is recommended for storage of sweet orange.