Tiny hairs could explain apple storage issues

The Fuji apple was bred in Japan, and is a cross between two American varieties - Ralls Janet and Red Delicious.

They are appreciated for their crisp texture, sweet flavour, and keeping qualities.

However late-harvested apples have a tendency to turn brown on the inside.

Dr Mary Parker of the Institute of Food Research in the UK believes this may be due to the hairs she identified - dubbed 'callus' hairs - and says the discovery could enable apple breeders to develop varieties with less hairs.

Moreover, she said that testing for the presence or absence of callus hairs could help identify food fraud, where dried apples are labelled as Fuji but may, in fact, be some other variety.

Finally, while initial investigations have shown the callus hairs to be rich in phytonutrients, they could also contain allergens.

Finding ways to control callus hair growth could, therefore, in reducing the risk of allergic reactions.

For her study, published in the Elsevier journal Postharvest Biology and Technology , Dr Parker sourced mature Fuji apples and Fuji sports (clones) from around the world.

She examined them using light microscopy and scanning electron microscopy, and found that there were clumps of multicellular, branched hairs in the outer 17mm of the outer cortex.

These hairs, which occur in the intercellular spaces between the parachyma cells and also in larger air lacunae in the apple flesh, have not been reported before.

"The reason there hairs have not been spotted before is probable because the full extent of their growth can only be appreciated in 3D," said Dr Parker.

She believes that these hairs could restrict gas flow through the fruit, which is vital for long term storage of the fruit in modified atmospheres, since the hairs need oxygen to grow and emit carbon dioxide.

Other factors that tend to affect his flow though apples include respiration rate of the tissue, density and diffusivity of the outer cortex and hypodermis, and thickness and waxiness of the skin.

There was a difference in the volume of hairs seen in apples of different origin.

The clumps were "particularly well developed" in Fuji Suprema apples from Brazil, and "moderately developed" in Fuji Kiku apples from Italy and in Fuji from South Africa, Chile, New Zealand and the USA.

Fuji apples from China had the least developed callus hairs.

Dr Parker concluded that more work is needed to quantify the effect of callus hair growth and storage problems, using fully replicated samples of apples, of known origin.

"Factors such as the cultivar, position on the tree, geographical location and altitude of the orchard, type of rootstock and/or interstem, agronomic management of orchards including practices such as bagging, and harvesting and storage regimes will all need to be taken into consideration," she said.

Source Postharvest Biology and Technology 48 (2008) 192-198 DOI: 10.1016/j.postharvbio.2007.10.2007 "Occurrence and implications for postharvest quality of intercellular callus hair growth in the outer cortex of apples of 'Fuji' and 'Fuji' sports."

Authors: Mary L Parker, Walter Guerra