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Alkaline pH is a critical component of the drying emulsion used in Australia to enhance the drying rate of Sultana grapes. Weight loss of grapes treated with 0.18M K2CO3 solution increased approximately linearly from pH 4 to 14.
Minimal weight loss after 24 hours of drying occurred between pH 2 and pH 6. It was higher at pH 2 than at pH 4, while at pH values above 4 the weight loss became progressively greater, indicating an isoelectric point in the pH range 2 to 6.
Alkali metal carbonate solutions of high pH were required for maximum drying rate, with the effectiveness of the alkali metal ions increasing with decreasing hydrated ion radius. Cation exchange capacity was also higher at high pH.
The cuticle reacted comparably to a highly cross-linked cation exchanger of the weakly acidic type and exhibited similarities to leaf and fruit cuticles of other species in its response to pH.
The light colour of the dried sultanas was highly correlated with the rate of weight loss.
Mode of Action of the Drying Emulsion used in Dried Vine Fruit Production. II. The Effect of Emulsion pH
1996, Australian Journal of Grape and Wine Research, vol 2, pages 91-96