Patterns of sulfite residue persistence in seedless grapes during three months of repeated sulfur dioxide fumigations

In 1989, the United States Environmental Protection Agency instituted a sulfite tolerance of 10 mu g SO2 per gram in grapes (Vitis vinifera L.). Accordingly, the residues resulting from repeated weekly SO2 fumigations of table grapes in storage, a typical industry practice, are a concern. In a simulation of commercial practices, grapes (cv. Thompson Seedless) were fumigated once a week with 2000 or 5000 mu L SO2 per liter of storage volume and aerated 30 minutes while stored at 0 degrees C for at least fourteen consecutive weeks. Each week, grapes were frozen immediately after aeration or stored at 0 degrees C, 10 degrees C, 20 degrees C, or 30 degrees C for 24 hours. The mean SO2 concentration for the treatment with the highest residues was 2.3 mu g/g (5000 mu L/L dose, seven fumigations). The maximum residue levels at each dose level were observed between the seventh and eleventh weekly fumigations. The influence of post-fumigation temperature on sulfite residue loss was minor and inconsistent. These data suggest that current industry practices leave sulfite residues well below the official legal tolerance.