Studies on the Sulfur Dioxide Fumigation of Table Grapes

¹ Departments of Viticulture and Enology and of Mathematics, respectively, University of California, Davis.

Table grapes were packed in standard lugs with a given number of botrytis infected berries and fumigated with different concentrations of SO₂ for 20 minutes before precooling. This was followed by different concentrations of SO₂ for 30 minutes every 7, 10 and 14 days in storage. The fruit was stored at 85, 90 and 95% relative humidity; all at O°C.

Grapes at 95% RH required 15-20% more SO₂ for a given exposure than those at 85%. Dosage had to be adjusted to storage RH and amount of fruit treated to maintain given exposure levels owing to container absorption.

The amount of botrytis decay which developed in 3 months was inversely related both to the initial and storage SO₂ treatments. It was directly related to the length of the interval between storage treatments. Fumigant injury to the fruit was directly related to the initial treatment and inversely to the interval between treatments. Best decay control with minimum injury was obtained with the 7-day interval.

A formula and figure for calculating the dosage of fumigant are included which take into account the fruit-space ratio (absorption factor) and storage RH as well as the SO₂ concentration.