HOT WATER DIP AND PRECONDITIONING TREATMENTS TO REDUCE CHILLING INJURY AND MAINTAIN POST-HARVEST QUALITY OF VALENCIA AND NAVEL ORANGES DURING COLD QUARANTINE

Abstract

The potential of hot water dipping (HWD) at 41±1°C for 20 min or at 50±1°C for 5 min and pre-storage conditioning (6 days at 16-18°C and 45-65% RH) treatments to control chilling injury (CI) and decay of W. Navel and Valencia Late oranges during cold quarantine at 1°C and 85–90% RH for 20 days, subsequent storage at 10°C and 85–90% RH for 20 days (as a transit period) and an additional 20 days of simulated marketing period (SMP) at 20±2°C and 40-65% RH was investigated over 2 harvest seasons (2006/2007 & 2007/2008). Untreated fruits were used as control. We studied their effects on various other postharvest quality parameters such as weight loss, juice %, soluble solids content (SSC), total acidity (TA), ascorbic acid (VC), and reducing sugars, free phenols, peroxidase (POX) and catalase (CAT) activities, and ascorbic acid oxidase (ASAO). After SMP, HWD treatments reduced the incidence of CI in W. Navel (4.2-13% versus 29–33% in control and 12-21% in conditioning treatments) and Valencia (0–27% against 60-63% in control and 60–67% in conditioning treatments). HWD treatments effectively prevented the incidence of decay in both cultivars in all storage stages, while conditioning one reduced it in W. Navel to 17-13%, against 25% in the control, although in Valencia, a very little insignificant decay % was found (3-7%). HWD treatments reduced the rate of weight loss in both cultivars, while conditioning one reduced it in Valencia, only. In addition, HWD treatments mostly increased VC content, especially in W. Navel, while reduced ASAO activity. HWD treatments increased free phenols content in both cultivars, as compared with control and conditioning treatments, although the last increased it as compared with control. HWD at 41°C for 20 min was more effective than at 50°C for 5 min in this respect. All treatments significantly increased POX and CAT activities, whether in fruit peel or juice of both cultivars as compared with control. HWD at 41°C for 20 min treatment was more effective in this respect. Generally, the reduction in chilling injury % was paralleled to higher POX and CAT activities in fruit peel and juice, and free phenols in juice, this may explain how and why the treatments reduced CI incidence. Overall, it is concluded that short postharvest HWD treatment is preferable, since it effectively induced tolerance to cold temperatures in W. Navel and Valencia Late oranges without impairing any other postharvest qualities.