“One of the main findings was that total flavanols and, in particular, two quercetin glycosides were significantly higher in organic, compared to conventional, onions”, said Dr. Kim Reilly, the researcher who set up the 6-year field trial conducted by Teagasc Food Research Centre, Dublin.
Using fully organic management, both ‘Hyskin’ and ‘Red Baron’ varieties show higher antioxidant activity, total flavonol content and levels of specific quercetin glucosides than in conventional management.
“Differences were primarily due to different soil management practices used in organic agriculture rather than pesticide/ herbicide application,” wrote Dilip Rai, senior author the Journal of Agricultural and Food Chemistry.
Flavonols, including quercetin are widely regarded to have preventive benefits for a number of health conditions.
The 6-year trial is the longest running study of conventional versus organic crop phytochemical content. The extended trial duration enabled inclusion of weather as an influencing factor in phytochemical content.
Anthocyanin content of the onions was higher in conditions of abundant sunlight and warm temperature. “There seemed to be a marked influence of weather conditions,” commented Reilly.
However, choice of crop management system (organic or conventional), had no consistent effect on anthocyanins.
Earlier meta-analyses examining nutritional differences between organic and conventional crops mostly concentrated on wider nutritional measures such as vitamin and mineral content, rather than phytochemical and antioxidant content. This may partially explain previous conflicting results. The authors suggest that shorter study length and exclusion of weather may also be significant.
Importance of soil treatment
The trial was also novel in examining both elements of cultivation method (soil management and herbicide/ pesticide application) separately. Findings suggest that soil management, rather than pesticide treatment is the driving factor for higher phytochemical content.
“It seems to be the soil treatment aspect that is bringing out these effects,” suggests Reilly.
This hypothesis has interesting implications for future research into improving crop nutrition.
“This opens up massive possibilities for looking into how the soil microbiome can change, plus how it affects the plant and the food that we eat,” added Reilly.
The study also examines variations in crop yield under organic and conventional conditions. ‘Hyskin’ onions grow bigger using conventional cultivation, but larger “Red Baron” onions result from organic management.
This result surprised the researchers. Reilly hypothesised “there may be crop varieties which are better suited to different production systems.”
While findings are preliminary, they may prompt larger trials examining whether variety-specific organic crops can match conventional yields. Achieving equal yields may alleviate doubts that organic cultivation is able to contribute meaningful quantities towards world food supply.
On the wider issue of consumer choice of organic products, Reilly concludes, “people may choose to eat organic for various different reasons. They may be scared of pesticide residues; they may think it’s nutritionally better which in some aspects it is, but in other aspects, it isn’t.”
Source: Journal of Agricultural and Food Chemistry
Published online ahead of print: doi: 10.1021/acs.jafc.7b01352
“Higher Antioxidant Activity, Total Flavonols, and Specific Quercetin Glucosides in Two Different Onion (Allium cepa L.) Varieties Grown under Organic Production: Results from a 6‑Year Field Study”
Authors: Dilip K. Rai, Kim Reilly et al