Are nutrients destroyed, or altered in a harmful way, during the heating process for refined oils?
A key nutritional benefit of rapeseed oil is that is it low in saturated fat and high in unsaturated fats. This benefit applies to both cold pressed and refined versions of the oil. Replacing saturated fats with unsaturated fats in the diet has been shown to reduce blood cholesterol. High cholesterol is a risk factor in the development of coronary heart disease. All types of fat are high in energy though, so should be eaten sparingly.
Oils are refined to remove any unwanted taste, smell, colour or impurities. The extent of refining will depend on the desired usage (for example taste, appearance, or oil stability). Refined rapeseed oil (often labelled vegetable oil) has a beneficial high smoke point and is a great carrier of flavour as a result of the refining process. The oil doesn’t have a strong flavour of its own, so doesn’t mask other ingredients in baking/cooking. Unrefined oils, such as cold pressed rapeseed oil, tend to retain their polyphenol content so are likely to contain more than refined versions. Polyphenols comprise a very wide group (several thousands of plant compounds) with different biological activities, suggested to be beneficial for health. However there has been limited studies looking at the polyphenol content of cold pressed rapeseed oil and results are variable probably due to factors such as, for example, their cultivar and how and where they have been farmed, refined and stored. There is also limited research on whether baking effects the nutrient content of cold pressed rapeseed oil, although it has been suggested that polyphenols may be heat sensitive. Interestingly, studies have shown that baking fruit such as blueberries increases levels of certain polyphenols, decreases levels of others, with others being unaffected.