In recent months E.F.S. has received many questions about oxidation: ‘How does it work?’, ‘Which antioxidants does E.F.S. use in its oils?’, ‘What happens if the oil is not stored properly?’. We will clarify these questions for you in this post.

What is oxidation?

Oxidation is a chemical process, also known as redox reaction, in which the reducing agent donates electrons to another substance called oxidising agent. A very well known form of oxidation is the rusting of iron (Fe2+ —> Fe3++ e), during which the iron donates an electron resulting in oxidation/rust. Fats, such as salmon oil can also oxidise. Due to this chemical process (salmon) oil/fat can become rancid. Overoxidised salmon oil/fat is no longer suitable for use in feed. This process can be postponed by antioxidants, hence extending the shelf life of the product.


Oxygen induces hydrolysis of fatty acids, causing fatty acids to detach from glycerol. These detached fatty acids may continue to oxidise as follows: By means of oxygen the free fatty acids are oxidised into peroxides, which are degraded into, among others, radicals. Radicals (e.g. free oxygen) are molecules or atoms that lack an electron and ‘steal’ these from another molecule. In turn these radicals react with other free fatty acids, causing the process to repeat itself and triggering a chain reaction. As the reaction keeps repeating itself fat/oil becomes rancid and is no longer suitable for use. Heating or exposure to uv-light or air can accelerate the chain reaction. Oxidised fat can be identified by its distinctive rancid smell. In order to prevent this an anti-oxidant can be added to fat. An antioxidant is able to scavenge free radicals, hence preventing oxidation and a subsequent chain reaction.

Salmon oil

The salmon oils from E.F.S. are fats with a high content of high-quality unsaturated fatty acids. Unsaturated fatty acids are more susceptible to hydrolysis and therefore more susceptible to oxidation. In order to prevent oxidation, all E.F.S. salmon oils are protected with an antioxidant.

Besides added antioxidants, salmon oil also naturally contains an antioxidant, i.e. astaxanthin. This natural antioxidant already offers a natural form of protection during salmon oil production. Astaxanthin gives salmon oil its distinctive colour (reddish brown) as well. After production, extra antioxidant is added as soon as possible to give salmon oil long-term protection.

The E.F.S. portfolio contains salmon oils from different origins and they are protected by various antioxidants. The antioxidants used are presented in the table below. Scottish salmon oil comes in two varieties, namely with BHA/BHT or with natural antioxidants. The natural antioxidants consist of various tocopherols (vitamin E).

Salmon oil Anti-oxidants (mg/kg)
Scottish salmon oil BHT 500
Scottish salmon oil Natural Antioxidants 375
Norwegian Salmon oil BHA 100
Propylegallate 10
E.F.S. Scottish fish oil BHA 50
BHT 150
Latvian salmon oil BHA 100
BHT 500

Table 1. Anti-oxidants per salmon oil

How to measure oxidation

Different methods are used to reflect the oxidation sequence. The most frequently used method is the peroxide value (PV), which measures the peroxide content. This is the most widely used method to reflect the level of oxidation in fresh oil. Its target value is <10 meq/kg.

Another oxidation assessment method is the anisidine value. This method measures aldehydes (peroxide degradation products). These aldehydes give oxidised oils/fats their distinctive rancid smell. Its target value is <30 meq/kg. The totox value is determined by multiplying the peroxide value by two and adding the anisdine value to this (totox = AV + 2*PV). The totox value provides a complete picture of the level of oxidation in fats. The iodine value is also known as a method to reflect oxidation, however, it only reflects the number of double bonds which reflects the susceptibility of fat to oxidation.

Storage & processing

In order to give the oil maximum protection it is important to process and store salmon oil correctly. Salmon oil requires protection from external influences (direct sunlight, high temperatures and oxygen) and it is im portant not to allow any contamination in the storage tank or IBC. This is why E.F.S. recommends keeping salmon oil packaging closed as much as possible, draining IBC’s from the bottom part and refraining from using an agitator in the tank. Also salmon oil does not require heated storage. Salmon oil remains liquid down to -10° Celcius!

Do you have any further questions? Please contact E.F.S.