Led by Dr Rong Tsao of the Agriculture and Agri-Food Canada, the research team used high-performance liquid chromatography (HPLC) to study the polyphenol profiles in eight common apple cultivars: Red Delicious, McIntosh, Cortland, Northern Spy, Ida Red, Golden Delicious, Mutsu and Empire. All the samples were grown on the same farm under the same conditions.
Their aim was to identify the varieties with the most polyphenol compounds and the parts of the apple where the compounds are most concentrated. The findings are published in the June 29 issue of the Journal of Agricultural and Food Chemistry, now online.
When the apple peel was examined, Red Delicious had the highest total polyphenol load and antioxidant activity. Northern Spy's antioxidant activity was almost as high, but its polyphenol load was around half, which could suggest that the polyphenols it did have were more active.
In the apple flesh, Northern Spy was the best performer both in terms of total polyphenol load and antioxidant activity, and Red Delicious came in second.
Different polyphenols are believed to have various biological functions, and the researchers were also able to identify the 16 compounds belonging to the five main polyphenol groups, which are responsible for the antioxidant activity of apples.
The predominant group occurring in both peel and flesh was procyanidins, and procyanidin B2 and epicatechin made the greatest overall contribution to antioxidant activity. Dihydroxycinnamic acid esters, phloretin glycosides and flavan-3-ols were also found in both peel and flesh, whereas quercetin glycosides were found only in the peel and hydroxycinnamic acid esters only in the flesh.
Cyanidin 3-galactoside was found only in the peel of red apples, and 3-Hydroxyphloretin 2'-xyloglucoside was identified in apples for the first time.
Although the study did not include certain other types of apple popular in North American, such as Gala, Granny Smith, Jonathan, York, Stayman and Rome, it highlights a gulf between the nutritional values offered by different varieties.
"When taste and texture do not matter, choosing an apple with a high proportion of polyphenols in the flesh and skin can potentially produce more health benefits," said Tsao.
By interbreeding the best performing fruits, scientists could create a hybrid with the highest possible antioxidant content and disease fighting potential.