It’s been said that everything is potentially toxic and only the dose and level of exposure are the determining factors. Oxygen is no exception. The amounts we normally breathe are life sustaining. When it comes to food though, oxygen is what is at least partially responsible for it spoiling and losing its nutritional value, leading to rancidity.
When it comes to fresh pressed poly and monounsaturated oils, oxygen is the main cause of rancidity. Oxygen creates free radicals in the oils and this results in broken bonds in the fatty acids; volatile aldehydes and ketones are released and these cause the bad smells and off-flavors we call rancidity. Rancid oils may have sharp, musty, sour, mildewy tastes and smells. Some have described turpentine-like qualities. There may be some bitterness but certain oils, like flax, often have bitter notes even when completely fresh, depending on harvest and climatic conditions.
Naturally occurring antioxidants in the oils (like vitamin E and carotenoids) can slow this process down and nitrogen flushing the bottles before sealing them can remove any oxygen. Once opened though, with enough time, the oils will go rancid.
At Flora we do a number of quality tests on our fresh oils before releasing them and in follow up shelf life studies to see how they fare “in the real world”. One of these important tests is for Peroxide Value (PV).
A PV test will show how much active oxygen is in the oil and how much oxidative rancidity has occurred. There are not a lot of 3rd party standards available for what constitutes a “healthy” level. The World Health Organization (WHO)’s standards for cold pressed/virgin oils are 15 millequivalents (mEq) of active oxygen / kg oil. Flora’s internal standard is 5 mEq/kg by the end of the oil’s shelf life. Typical results are around 1 mEq/kg at release with very little change after 2 months of being open and refrigerated.
Sometimes customers do confuse the normal bitter notes in flax oil (US/CA) with rancidity. Hopefully these additional testing details will be reassurance that actual rancidity is not something that generally occurs under normal conditions and within recommended use periods for our oil products.