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Great grapes! Getting rid of sulfite residues.

Keeping grapes free of unwanted sulfite residues should be easier, thanks to a new idea from an ARS scientist.

Packinghouse managers can now use slender glass tubes resembling small thermometers to accurately measure how much sulfite-producing sulfur dioxide reaches boxes of grapes stacked in cold storage, says Joseph L. Smilanick. He is with ARS at Fresno, California.

Applied as a gas, sulfur dioxide protects grapes from botrytis, an unattractive gray mold. Because some consumers are allergic to sulfites however, federal law stipulates that sulfite residues on grapes must not exceed 10 parts per million - the equivalent of 10 ounces in 31 tons.

With routine sulfur dioxide fumigation, grapes like Thompson Seedless, Flame Seedless, or Emperor can stay fresh in refrigerated storage for as long as 5 months. "That's why you can buy late-summer grapes from California in December," Smilanick says. Without fumigation, grapes could be stored for only about 3 weeks before succumbing to rot.

Sulfur dioxide gas dissipates quickly, so warehouse managers usually need to fumigate grapes at least once a week, typically for about half an hour. In a cold storage room packages with hundreds of boxes of grapes, though, tracking the fruit's exposure to the fumigant is difficult.

Packers who now rely on the glass tubes for the task find them cheaper and more convenient to use than more sophisticated instruments.

Known as dosimeter tubes, the devices cost about $4. Tubes placed at the top, middle, or bottom of randomly selected stacks of crates or cartons of grapes will record the amount of fumigant that circulates through the storage room. This monitoring helps protect grapes from getting too little fumigant and consumers from getting too much.

The tubes are easy to use and read. They're packed with chemically treated beads of silica gel. The chemical turns the beads yellow when it comes in contact with sulfur dioxide.

"You snap off the tops of the tubes so the fumigant can get inside," says Smilanick. "Then you put the tubes in the cartoons. After you've finished fumigating, you remove all the tubes and read them just like a thermometer. But instead of looking for a red or silver column, you're reading a yellow band."

Smilanick's new idea is just one spin-off from a 3-year study by team of ARS and university researchers, growers, and Cooperative Extension specialists who are scrutinizing each step of grape fumigation. The group wants to ensure worker and consumer safety,slash cost, and limit escape of sulfur dioxide into the environment. They've run test in some two dozen commercial cold storage houses in central California. "Fumigation hasn't changed much since the 1930's" says Smilanick. "We're looking for ways to streamline it."

In earlier recommendations, the team pinpointed adjustments that storehouse managers need to make to accommodate differences in type of crates or cartons grapes are stored in. Traditional wooden crates, for example, absorb fumigant differently than the newer polystyrene foam cartons. Until the team's studies, however, most packinghouses used guidelines based on the wooden crates.

The California Table Grape Commission funded part of the research.
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Author:Wood, Marcia
Publication:Agricultural Research
Date:Aug 1, 1992
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