Approaches to optimising surface coatings for fruits

A mathematical model predicted that final water vapour permeance in surface-coated fruits would depend upon water permeance of the coating but not the proportion of pores blocked on the fruit surface. In contrast, predicted final oxygen (O-2) permeance depended upon numbers of pores blocked but not O-2 permeance of the coating. Predicted variation in internal atmosphere composition caused by coatings that blocked different proportions of pores on the model fruit surface was consistent with data from two experiments on coated apples (Malus domestica Borkh.). A new equation was developed to characterise the relationship between internal carbon dioxide (CO2) and O-2 levels resulting from different coating treatments. Two graphical approaches to assess surface coatings for fresh fruits are presented. In the first, a plot of water vapour permeance against internal O-2 was used to identify the most suitable of three surface coatings for reducing water loss in 'Royal Gala' apples at 20 degrees C. The second method used a plot of internal CO2 versus internal O-2 in coated fruit to identify the crop's internal lower O-2 limit (LOLi), which lies just below the optimum internal O-2 level for modified atmosphere effects. Coatings containing different concentrations of carboxymethyl cellulose produced internal O-2 levels ranging from almost 0 to 16 kPa in 'Granny Smith' apples at 20 degrees C. The LOLi of these fruit was estimated using the new equation to be c. 0.8 kPa O-2. Large fruit-to-fruit variability with some coating treatments indicated that uniformity of response may be as important as average response in selection of coatings. Risks cannot be separated from benefits when using surface coatings to gain modified atmosphere benefits, making their use to achieve modified atmosphere benefits more risk-laden than for other purposes.