In their study, the Japanese research team examined seven fruits that are known to have high antioxidant content: gold kiwis, green kiwis, navel oranges, mandarin oranges, white grapefruits, ruby grapefruits and apples (the researchers did not specify the type of apple tested).In particular, the study team desired to know which of the fruits had the highest concentration of
polyphenols (powerful antioxidants that offset the effects of molecules known as free radicals), which were most effective in reducing lipid oxidation (the process whereby fatty acids are turned into free radicals that damage cells), and which were most effective in eliminating free-ranging hydrogen peroxide (another type of free radical produced as a byproduct of the body’s process to create and use energy at the cellular level).
People with high levels of unhealthy lipids (LDL cholesterol) in their blood serum (oxidized LDL in particular) are considered to be at high risk for cardiovascular conditions such as atherosclerosis, heart attacks, strokes and other maladies. By reducing the amount of oxidized lipids and the number of other free radicals in blood serum, physicians believe people can significantly reduce their risk of heart disease. A common way to reduce free radicals and oxidized lipids is to consume more antioxidant-rich food, hence the researchers’ interest in these fruits.
The short takeaway: Gold kiwi crushed the competing fruits on all measures. In fact, apples came in dead last on every metric!
For example, to assess polyphenol content, the researchers cut equal weight pieces of each fruit flesh and blended in a mixer for about 30 seconds. The blended fruit juices were then processed through a centrifuge for 10 minutes and subsequently strained through a filter. The strained juice was centrifuged again — this time for one hour — and then samples were taken of the remaining, centrifuged juice.
Using this method, the researchers found that gold kiwi’s polyphenols content was approximately 1.04 milligrams (mg) per milliliter (ml), green kiwi was second with 0.85 mg/ml, navel oranges were third at 0.80 mg/ml, while apples were dead last at 0.13 mg/ml.
To assess the antioxidant properties of these polyphenols, the researchers then mixed a 1% concentration of the fruit juice solutions with lipids from egg yolk and irradiated the mixture for various time intervals (irradiation via UV rays causes the lipids to oxidize). After irradiation, the study team examined the counts of oxidized lipid molecules remaining in the respective fruit juice mixtures. They found gold kiwi and navel orange both inhibited oxidation of 60% of the lipids. Again, apples were dead last at 23%.
To further evaluate the antioxidant properties of the various fruits, the researchers mixed a 5% concentration of the fruit juice solutions with hydrogen peroxide and let the mixture sit for two hours. Then the researchers measured the amount of hydrogen peroxide eliminated by the fruit juices. Once again, gold kiwi significantly outperformed the other fruits, eliminating over 60% of the hydrogen peroxide. No other fruit achieved greater than 30% elimination, and apples lagged the field at less than 10%.
The study authors concluded: “We propose the novel possibility that daily consumption of kiwifruit is effective on decrease of oxidative stress and further prevention of disease by excessive oxidation … All these indicators showed the highest activity for gold kiwi, demonstrating that
gold kiwi has strong anti-oxidant effects. Overall, green kiwi had lower anti-oxidant effects than gold kiwi, but had stronger effects than the other fruits.”
According to the prime producer of gold kiwi fruit, a New Zealand company called Zespri, the biggest difference between the green and gold kiwi is taste: “While green kiwifruit has a tangier, more tart flavor, gold kiwifruit is mellow and tropical, a mixture of mango, melon and citrus flavors. People who find green kiwifruit too tart usually love gold. As for other differences, the color is gold, instead of green, and it is tear-drop shaped, with a smooth skin and a crown on the top. Finally, while you might need to wait a few days for green to ripen, gold kiwifruit is always ready to eat.”
