HortResearch Publication - Greasiness in Granny Smith and Royal Gala Apples
Granny Smith and Royal Gala are two cultivars which can become greasy in storage. Greasy fruit is very unappealing to the end consumer as shown by a HortResearch consumer panel which perceived fruit with a high level of surface grease to be over-ripe. For a very greasy apple, it was the greasiness more than any other attributes (softness and size of fruit) which made the consumers consider the fruit to be over-mature. The consumer panel considered the "ideal" level of greasiness on the fruit to be no grease at all or else a very low level of greasiness.
Skin greasiness can develop on the tree, particularly in the upper and outer zones of the tree, or during storage in fruit of late maturities. Greasiness develops due to changes in the waxes on the surface layers of the fruit. In particular, farnesyl ester has been determined as a major component in the surface wax of ‘Granny Smith’, ‘Fiesta’, ‘Royal Gala’ and ‘Braeburn’ apples.
It would be a huge advantage if we could predict whether lines of fruit were going to become greasy before they were put in storage. One way of predicting potential greasiness is to determine the levels of farnesyl ester on the fruit. Farnesyl esters are easily detectable as they turn a yellow/brown colour when exposed to iodine vapour. In an experiment where the surface waxes of the fruit were extracted and place in a chamber with iodine vapour for 5 minutes, the iodine staining was greatest on the wax from the greasy fruit. There was only a slight iodine staining on the wax from non-greasy fruit.
The iodine will stain the wax before the fruit even feels greasy. This means that the iodine test can be used to predict greasiness and could be useful for segregating apples for short, long-term and CA/MA storage on the basis of their potential to develop skin greasiness. Further work needs to be done to refine the test using a range of cultivars and locations.
Richardson, N. and Phelps T. 1996. Consumer evaluation of the tactile properties of Royal Gala apples varying in skin grease. HortResearch Internal Report.
Acknowledgment
HortResearch acknowledges ENZA New Zealand (International) for financial support.