Further Studies on Control of Decay of Table Grapes by Two-Stage Generation of Sulfur Dioxide within Unvented Containers

Abstract

An improved two-stage system was developed for controlled generation of SO₂ in closed containers to retard the spread of decay in packed table grapes from <i>Botrytis cinerea</i> Pers Two sheets of polyethylene-coateded Kraft paper, heat-sealed together, entrapped granular NaHSO₃ in 12 rectangular pockets for the second (slow-SO₂-generation) stage. A third sheet of uncoated Kraft paper was glued to one side, forming pockets holding additional NaHSO₃ for the first (fast-generation) stage. The dose of NaHSO₃ per laminated sheet ranged from 0.5 to 2.5 grams for the first stage, and 1.9 to 7.7 grams for the second stage. The sheets were packed, one below and one above the fruit, each backed by an excelsior pad. About 26 pounds of fruit and the pads were enclosed by the water-vapor-barrier liner of the container. The highest dose combinations provided decay control equal to, or better than, standard weekly SO₂ fumigation of vented containers during 2 months of storage for Thompson Seedless and 4½months for Emperor grapes at 31°F: however, there was appreciably more bleaching. The lowest doses caused little bleaching but did not control decay adequately. Over all protection was best from the intermediate-dose combinations. Emperor grapes packed in unvented containers with SO₂-generating sheets and held 0, 1, and 2 days at 86°F before being precooled to 31°F, shorted little deterioration in decay, bleaching, or stem condition at 40 days. Vented fumigated controls, in contrast, had a drastic deterioration in fruit quality from such delay in precooling. Although cooling fruit to 31°F caused higher initial concentrations of SO₂ than cooling to 39°F, there was no consistent effect of temperature on the length of the SO₂-generation period. The SO₂-generating sheets caused very litte localized fruit bleaching, and commercial manufacture should be feasible.