APPROACHES TO OPTIMIZING SURFACE-COATINGS FOR FRUITS

A mathematical model predicted that final water vapour permeance in su rface-coated fruits would depend upon water permeance of the coating b ut not the proportion of pores blocked on the fruit surface. In contra st, predicted final oxygen (O-2) permeance depended upon numbers of po res blocked but not O-2 permeance of the coating. Predicted variation in internal atmosphere composition caused by coatings that blocked dif ferent 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 betw een internal carbon dioxide (CO2) and O-2 levels resulting from differ ent coating treatments. Two graphical approaches to assess surface coa tings for fresh fruits are presented. In the first, a plot of water va pour permeance against internal O-2 was used to identify the most suit able 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 l ower O-2 limit (LOLi), which lies just below the optimum internal O-2 level for modified atmosphere effects. Coatings containing different c oncentrations 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 tr eatments indicated that uniformity of response may be as important as average response in selection of coatings. Risks cannot be separated f rom benefits when using surface coatings to gain modified atmosphere b enefits, making their use to achieve modified atmosphere benefits more risk-laden than for other purposes.