Radiotracer Studies on the Uptake and Retention (Conversion) of35S-Sulfur Dioxide in Table Grapes

Abstract

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>i.e.</i>, 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).