Postharvest methods to reduce sulfur dioxide residues in fresh longan.

摘要

Fumigating fresh longan with sulfur dioxide (SO₂) is commonly used in Thailand as an effective postharvest method for pericarp bleaching and rot control. However, SO₂ residues in both pericarp and arils after fumigation are a major concern for customers. Results from 14 sampling sites in Chiang Mai and Lumpoon provinces showed that more than 80% of the samples had SO₂ residues in the pericarp higher than 1,700 ppm and all of the samples had residues in the arils exceeding 10 ppm. The SO₂ residues in off-season longan were higher than in in-season fruit. Washing longan with running water was more effective in reducing SO₂ residues than soaking in water. Washing longan for 5-10 min. within 24 h after fumigation reduced SO₂ residues by up to 36% in pericarp and about 48% in arils. Water temperature (0, 5, and 23 degrees C) did not change the effectiveness of washing for reducing SO₂ residues in both pericarp and arils. Washing fresh longan for 5 min. had no effect on postharvest longan quality and overall acceptability as determined by a taste panel. Postharvest shelf life of washed longan was not significantly different from that of unwashed longan, which was about 18 days compared to 11 days for non-fumigated longan.