Home cooks in this country generally opt for refined seed oils, such as canola, or, more recently, coconut oil, because of their high smoke point. However, growing evidence suggests there’s more to frying oil than the smoke point, and olive oil may have been unfairly maligned.
In 2018, Australian researchers published findings from a review of 10 commonly used cooking oils: high-quality extra virgin olive oil, virgin olive oil, olive oil, canola oil, rice bran oil, grapeseed oil, coconut oil, high-oleic peanut oil, sunflower oil and avocado oil.
The oils were subjected to two trials, one to simulate pan frying and a second to simulate deep-frying. They then assessed the oils’ free fatty acids, smoke point, oil stability index and presence of polar compounds.
When oil is heated at high temperatures, or for extended periods, it degrades, forming undesirable polar compounds such as aldehydes, alkyl benzenes and aromatic hydrocarbons. Some, but not all, of these compounds have been linked to cancers and neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease.
The link between these compounds and health is dependent on the level of exposure, and many countries limit the amount of total polar materials (TPM) allowed in frying oil. Typically, less than 24-27% is considered safe for food eaten immediately after frying. For fried food that is packaged or stored, the oil must have less than 10% TPM to ensure that it doesn’t exceed the 24% endpoint.
The Australian study found that refined seed oils create the most polar compounds, despite their higher smoke point. And, overall, the researchers noted that extra virgin olive oil was the most stable when heated, followed closely by coconut oil and other virgin oils such as those made from avocado and high-oleic seeds such as peanuts.
The New Zealand researchers recently assessed the stability of a range of oils available here: four New Zealand-grown extra virgin olive oils, an Australian and an Italian extra virgin olive oil and a popular canola oil.
To simulate extended frying, the oils were heated to about 180°C for 10 hours, then tested every two hours for various components, including TPMs.
Although all the extra virgin olive oils and the canola oil showed good stability, the Italian oil exceeded the recommended level of TPMs after 10 hours of heating.
This study found no correlation between the smoke point of an oil and its lifetime stability. Instead, it seems that a variety of other factors influence an oil’s stability, such as the fatty-acid composition, and the presence of minor components such as phenolics and tocopherols, which act as antioxidants.
Extra virgin olive oil, which has had minimal processing, typically contains significantly more phenolics than refined canola oil, for example.
The New Zealand research team concluded that high-quality extra virgin olive oils, which have the Olives New Zealand OliveMark, such as the New Zealand-grown olive oils in this study, were a better option for frying at home than canola or lesser-quality imported Italian oils.
This article was first published in the June 29, 2019 issue of the New Zealand Listener.