Browning of sultana grape berries during drying

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Abstract
Berries of Vitis vinifera L. cv. Sultanina (Sultana, Thompson Seedless) were partly immersed in commercial oil emulsion before drying in an oven at 40°c. The treated part dried to a yellow colour without visible surface bloom, which is characteristic of fully treated berries, while the untreated part dried to a bluish‐violet colour with surface bloom characteristic of untreated berries. Colour development was prevented in partly treated berries if they were fully immersed in oil emulsion before they had lost about 50% of their fresh weight. The browning reaction was inhibited in isolated grape skins by sodium diethyldithiocarbamate. The results support the hypothesis that browning of sultanas during drying under mild conditions is caused by o‐diphenol oxidase (E.C.1.10.3.1) acting on substrates released from cell compartments when cell integrity is lost. Uneven colour development in commercially produced, dried sultanas can be prevented by a second spraying with oil emulsion within 7 days of the first.

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