The proneness of butter to turn rancid is dependent on what the butter is made of, how it has been made and how it is stored.

The variables include:

·       exposure to oxygen, light, and heat;

·   the presence of antioxidants, microbes, and catalysts (e.g. metals such as iron and copper, which kitchen implements might be made of);

·       the water and salt content of the butter;

·       and the saturation of the fats in the butter.

For example, the oxygen in semi-solid room-temperature butter can move through the butter more freely (has higher mobility) than in colder, more solid butter: room temperature butter will therefore, all else being equal, oxidise more quickly than refrigerated butter. Furthermore, butters with higher concentrations of water can spoil more quickly, predominantly because free metal ions in the water can catalyse oxidation reactions.

Conversely, butters with a higher level of saturated fats will be more stable with respect to rancidification. Similarly, butters that contain more antioxidants - occurring naturally in the diet of the cows or added at some point during processing - remain stable for longer.

Antioxidants typically scavenge oxygen by being sacrificially oxidised, thus preventing the formation of primary oxidation products. Once antioxidants are exhausted, the level of primary oxidation products increases. However, these are not detectable to the human palate. Therefore, how quickly something turns rancid can be deceptive.

For example, two samples, one with a much higher concentration of primary oxidation products, may taste, look and smell the same. It has also been found that some antioxidants - most notably for this project, some species of seaweeds found on the Danish coast - serve the extra function of actually reducing secondary oxidation products to their original oxidation states.

Modern manufacturing processes for many foods involve addition of a panoply of synthetic antioxidants, each with various strengths and weaknesses: some common ones include propyl gallate, BHA and BHT. However, traditionally, many naturally occurring antioxidants have been used to preserve fats. For example, studding a ham with cloves, which contain antioxidative phenolic compounds, serves to add flavour but also protect its fats from free radical oxidation; and slippery elm bark has been used to preserve bear fat. Furthermore, oats - a good source of phenolics - might serve an antioxidative effect in the double cream cheese Caboc (reputedly Scotland’s oldest cheese, dating from the 15th Century), which is rolled in toasted oats.

With the advent of more rapid processing and distribution methods, modern butters and oils typically reach consumers in a less oxidised state and / or with more antioxidants intact.

For example, in many places, olives were traditionally harvested in batches and stored, perhaps even for months, until enough had been collected to press. As a result, oxidation and fermentation would take place, and polyphenolic compounds, which impart the oil with spiciness and bitterness, would be sacrificially consumed. (Leaving them to rot slightly was also said to facilitate extraction of the oil, leading to greater yields.) The resulting oil would be milder and smoother than the ones that are made today, in which olives tend to go from tree to oil in under 12 – 24 hours.