Why are Ripe Apples so Greasy?

🍎How do you recognize a sweet apple, when you go to the grocery store?

🍎I choose apples that are greasy if I am going to eat them soon. All the apples I choose this way are sweet and delicious. This wax is produced by the apples themselves and is a sign that they are ripe. For example, Jonagold, Tsugaru, Granny Smith, Cripps Pink, and Royal Gala apples all have this phenomenon. On the other hand, some varieties of apples are less greasy. As an example of varietal differences, the one study found that Jonagold rapidly became greasy during storage at 20°C for 20 days, while Red Delicious did not; Cripps Pink’ slowly became greasy after 70 days of storage.

🍏Surprisingly, the molecules that compose this apple wax have not been fully understood. Several studies have suggested that unsaturated fatty acids such as oleic and linoleic acids are involved. In fact, when linoleic acid was applied to non-sticky Jonagold peels, a sticky phenomenon very similar to naturally occurring stickiness appeared. They are liquid compounds that may dissolve the diverse long-alkyl chain alkanes, alcohols, and fatty acids present in the apple skin (in the image below, some examples were shown. Other long-alkyl chain molecules also exist).

🧪Recently, new compounds were proposed. Farnesene, a linear sesquiterpene, and its derived farnesol and farnesol esters were detected. These compounds are not found in apples (Royal Gala) early in the harvest and are liquid below freezing. They were considered as solvents for the other constituents of the surface wax. The major compounds may vary depending on the apple variety.

For apples, this wax serves many roles, including limiting water loss, limiting pathogen infection, protecting against UV light, acting as a self-cleaning surface, providing mechanical support, and preventing fruit cracking.

📖Reference

1. S. Noro, N. Kudo, T. Kitsuwa, “Changes of Lipids of ‘Jonagold’ Apple Peel in the Harvest Time” J. Japan. Soc. Hort. Sci, 1985, 54, 116.

2. JT Christeller, et al. “The novel esters farnesyl oleate and farnesyl linoleate are prominent in the surface wax of greasy apple fruit” NEW ZEALAND JOURNAL OF CROP AND HORTICULTURAL SCIENCE, 2016, 44, 164.

3. X. Ren, et al. “Relationships between cuticular waxes and skin greasiness of apples during storage” Postharvest Biology and Technology, 2017, 131, 55.

4. X. Zhang, et al. “Preharvest Application of 1-Methylcyclopropene and Ethephon Altered Cuticular Wax Biosynthesis and Fruit Quality of Apples at Harvest and During Cold Storage” Horticultural Plant Journal, 2022, 8, 143.

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