Effects of varying the method of applying alkaline emulsion on the drying of Sultana grapes

Abstract

Sultana berries were treated with alkaline oil-in-water emulsion by partial or total immersion, spotting or spraying. Their weight loss during dehyration at 40°C and the colour and extent of surface bloom of the resultant raisins were determined. Approximately 37 pl of emulsion adhere to the berry during immersion but about 85 % of this accumulates as a run-off droplet. Spread of emulsion from treated to untreated parts occurs on mature berries only; it increases with increasing maturity, as shown by its linear relationship with the concentration of total soluble solids in the fruit. Comparable amounts of emulsion are more effective when spotted uniformly instead of distally on the berry surface. Drying accelerates as berries receive more spots of emulsion. Approximately 9 pl of emulsion placed by spotting or part-immersion give raisins of similar quality to that of raisins dried after total immersion, despite slower drying rates during the first 36-48 h of dehydration. Any further reduction in the amount of emulsion leads to slower drying and darker raisins.The spray emitted by the nozzels used contained droplets of 190 or 440 pm mean diameter. Between 0.5 and 1.5 pl of emulsion was applied per berry. Small droplets were about 20% more effective than large ones. The efficacy of the best spraying treatment, 1.5 pl berry-l and small droplet size, was almost as good as that of immersion.It is concluded that the efficacy of emulsion treatment depends in part on the characteristics of the fruit to be dried. Comparisons between treatments should only be made from several measurements during the course of drying. The cost of drying may be reduced and the quality of the raisins may improve when the emulsion treatment is modified according to the type of Sultana berry to be dried.

Objective assessment of dried sultana grape quality using digital image analysis

“…It is common industry practice in Australia to trellis‐dry grape bunches in situ (∼80% of the Australian production) by spraying them with an alkaline drying solution to enhance the speed of drying (Clingeleffer ). However, uneven spray coverage may result in darkening of some berries on a bunch due to the activity of polyphenoloxidase (PPO) and some non‐enzymatic reactions (Aguilera et al , Lydakis et al ), and, consequently, uneven colouring (Grncarevic and Lewis , May et al ) and darkening of the fruit during drying. This inconsistency in coloration of berries within a batch requires grading upon receival at packinghouses; the grade assigned to a batch determines the price paid to the grower.…”