The research, published in the Journal of the Science of Food and Agriculture , compared kiwi fruit grown on the same soil type and environmental conditions.
Its findings could re-ignite the debate between conventional and organic fruit that has raged with claim and counter-claim from both sides.
According to background information in the article, the world market for certified organic foods was estimated at $23-25 bn (€17.3-18.8 bn) in 2003 with annual growth of about 19 per cent.
The researchers, led by Maria Amodio from the University of California, Davis, measured post-harvest performance of the organic and conventionally grown Hayward kiwifruit grown on the same farm, and harvested at the same maturity stage.
The authors report that ascorbic acid (vitamin C) levels were higher for the organically grown fruit during storage at zero degrees Celsius (0.33 versus 0.29 grams per kilogram) but no significant difference was observed after storage at 20 degrees for seven days.
Total phenolics were also higher for the organically grown fruit, both for storage at zero degrees (0.56 versus 0.48 grams per kilogram for organic versus conventional fruit, respectively) and after seven days at 20 degrees (0.61 versus 0.51 grams per kilogram for organic versus conventional fruit, respectively).
"It is possible that conventional growing practices utilize levels of pesticides that can result in a disruption of phenolic metabolites in the plant, that have a protective role in plant defense mechanisms," wrote Amodio.
The organic fruit also performed better for total antioxidant activity, measured using the DPPH assay, reported Amodio and co-workers.
For storage at zero degrees the organic fruit had an antioxidant activity of 12.35 mol per litre per kg, compared to 9.69 for the conventional fruit, and, after seven days at 20 degrees, 13.84 versus 10.09 mol per litre per kg, respectively.
"All the main mineral constituents were more concentrated in the organic kiwifruits, which also had higher ascorbic acid and total phenolic contents, resulting in a higher antioxidant activity," concluded the researchers.
However, not everyone is convinced by the research.
Carl Winter, director of the FoodSafe Program at UC, Davis, told Chemistry & Industry magazine that such increased nutrient and antioxidant levels may not result in any health benefits.
"The authors also did not look for any secondary metabolites of potential toxicological impact," he said.
Source: Journal of the Science of Food and Agriculture Published on-line ahead of print, doi:10.1002/jsfa.2820 "A comparative study of composition and postharvest performance of organically and conventionally grown kiwifruits" Authors: M.L. Amodio, G. Colelli, J.K. Hasey, A.A. Kader Chemistry & Industry 26 March 2007, Page 8 "Organic kiwis win out" Author: Lisa Richards