Radiotracer Studies on the Uptake and Retention (Conversion) of 35S-Sulfur Dioxide in Table Grapes

The chemical forms of S incorporated into grapes as a result of single short-term SO2 fumigations and from the longer-term continuous use of sulfiting pads (i.e., SO2 chemical generators) were studied using 35S (87.2 d; 0.16 MeV beta rays) as a radiotracer. Several table grape varieties of commercial interest were exposed to SO2 for various times (minutes to weeks) and under different storage conditions (ambient and refrigerated temperatures). Sulfur dioxide uptake and retention (residue) levels were measured. The potential for the retained S to be chemically converted into other S-containing compounds was studied. Table grapes showed external (skin) and internal compartments for SO2 uptake and retention. External uptake (deposit) remains primarily in the form of sulfite and contributed in large proportion to the overall sulfite residue level found in treated grapes. Internalized SO2 is converted to sulfite which is mostly oxidized to sulfate. The oxidation capacity of grape berries is, however, limited, and this internal mechanism can be exhausted. Contrary to the case with sulfites, sulfates are natural components of table grapes and their presence has no toxicological (health) implications. The adsorption of SO2 in other packaging materials was also determined. The role of these materials as secondary SO2 supply pools (by rapid desorption) is also discussed. In addition to sulfites and sulfates, no other irreversibly bound SO2-derived S compounds were detected in this investigation. This study also concluded that, with the current practices for using SO2 in table grapes, the sulfite residue levels can be maintained well below acceptable tolerance levels (less than 10 µg/g or ppm).