Factors Influencing Sulfite Residues in Table Grapes After Sulfur Dioxide Fumigation

¹ Horticultural Crops Research Laboratory, USDA-ARS, 2021 S. Peach Ave., Fresno, CA 93727.

Sulfur dioxide (SO₂) content of fresh table grapes after SO₂ fumigation was determined by ion exchange chromatography (IC) and pararosaniline colorimetry (PRAC). Results of these methods were not significantly different when residues of five lots of fumigated Thompson Seedless grapes were compared. The coefficient of variation for PRAC (28.7%) was higher than IC (17.1%). Table grapes contained 5 to 20 µg/g total SO₂ immediately after fumigation for 30 minutes with 0.5% SO₂. Residues decreased rapidly after fumigation; the SO₂ half-life was 24 to 36 hours at 0°C. Grapes fumigated at warmer temperatures accumulated more SO₂, but residues were less persistent. Immature grapes accumulated more SO₂ than mature grapes in three of four cultivars evaluated. Grapes injured by cuts or infected with Botrytis cinerea accumulated up to seven times more sulfite than sound grapes. Sulfur dust applied preharvest left no detectable sulfite residues. Residues can be minimized by the selection of mature grapes free from rot and injuries, use of minimal SO₂ fumigant doses, and post-fumigation storage of several days duration so that residue can diminish before marketing.

Key words: sulfur dioxide, fumigation, sulfur dioxide residues, decay, Botrytis cinerea, table grapes, ion chromatography, tetrachloromercurate, pararosaniline

Submitted on September 1, 1989