Applications of Sodium Metabisulfite and Oxalic Acid to Reduce the Pericarp Browning of Longan during Storage

Delay of pericarp browning of fresh longan is a real challenge in postharvest technology to replace sulfur dioxide fumigation. In the present experiment, ‘E-dor’ longan was trimmed to be individual fruit and then dipped in 1.0% oxalic acid (OA) containing 1.0% (1.0%SMS) or 2.5% sodium metabisulfite (2.5%SMS) for 5 min, compared to SO₂ fumigated and nontreated fruit.  All fruit was stored at 5°C, 90-95% RH.  After treatments of SO₂ and SMS+OA, fruit peels changed to brighter yellow colour compared to nontreated control. Peel of control turned brown quickly after storage, and SMS+OA fruit peels gradually changed to brown within 9 days, whereas SO₂ treated fruit remained bright yellow with slight changes in the peel clour throughout storage of 15 days.  Pulp soluble solid contents were stable in each treatment.  Furthermore, disease incidence was 8.0% in control, 5.6% in 1.0%SMS+OA, and 1.3% in 2.5%SMS+OA on day 15, while it was not found in SO₂ treatment. Nevertheless, sulfur-remaining compounds were detected in the peel of SO₂ fumigated fruit at 1200-1300 pmm throughout storage and in the pulp at 72.8 ppm at the first 6 days. On the other hand, it was less than 10 ppm in the peel and undatable in the pulp of SMS+OA treated fruit during storage.