The first study examining virgin coconut oil production methods was published in 2008. This study was conducted in Malaysia and published in the International Journal of Food Science Nutrition. It was the first study reporting that the wet-milling fermentation method produced the highest amounts of antioxidants. (Study abstract here.)
In 2011, studies conducted in Sri Lanka by Professor Kapila Seneviratne of the University of Kelaniya also showed that traditionally made wet-milling virgin coconut oils have the highest levels of antioxidants.
What was surprising in this study, was the discovery that high levels of heat actually increased the amount of antioxidants in the coconut oils. This was surprising because there was a misunderstanding that heat supposedly was detrimental to coconut oil processing, as many virgin coconut oils in the market were claiming they were “raw” or “cold pressed” and did not use any heat. Here is what they wrote in The Sunday Times of Sri Lanka:
More surprises awaited the research team. The general impression is that cooking at high temperatures would degrade the quality of the oil. However, it is not applicable since coconut oil is thermally stable, it is learnt. “Fortunately, most of the phenolic anti-oxidants present in coconut oil are also thermally highly stable,” he pointed out, explaining that the reason for a greater composition of anti-oxidants is that simmering for a long time at a high temperature dissolved more anti-oxidants into the oil. (Published in The Sunday Times of Sri Lanka, October 16, 2011 – “Coconut Oil: It’s good for your after all,” by Kumudini Hettiarachchi and Shaveen Jeewandara)
In 2013, another study conducted in India also showed that virgin coconut oils produced by wet-milling and using heat produce higher levels of antioxidants. The study compared “cold extracted virgin coconut oil” (CEVCO) with “hot extracted virgin coconut oil” (HEVCO) and standard refined coconut oil (CCO) and was published in the journal Food Science and Biotechnology. Their testing showed that the “antioxidant activity in the HEVCO group was 80-87%, 65-70% in CEVCO, and 35-45% in CCO.” The researchers went on to comment why heat is necessary to produce the highest amounts of antioxidants in virgin coconut oil:
The increased polyphenol level in the HEVCO group may be due to increased release of bound polyphenols by heating. Coconut milk is an emulsion of oil and water that is stabilized by protein. To recover the oil from coconut milk, the protein bond has to be broken by heat in a double walled boiler known as a VCO cooker (steam jacket vessel developed by Central Plantation Crops Research Institute) under slow heating to allow the protein to coagulate and release the oil.”
The abstract of the study is here.