Winterization is a commercial process used on many food oils. It is a process by which waxes and fatty acids with a high melting point are removed from the oil. It is often done for aesthetic reasons, because it makes the oil look very clear, but there are other reasons too. People have claimed that winterization of cod liver oil is a bad thing, and damages the oil, so in this article we will have an objective look at the process of winterization, and how it impacts oils.


How does winterization work?

The aim of winterization is to remove the stearin (solid) fraction of oil from the olein (liquid). These are broad terms, and the composition of each fraction will depend on the oil. However, in all oils, the sterin will contain the longer chain fatty acids and saturated fatty acids whereas the olein will contain the poly-unsaturated fats and short chain fatty acids.

This separation is done by slowly lowering the temperature of the oil. This causes the saturated fats/ long chain fatty acids to crystallise and form stearin. These solids can then be filtered out/ removed. You can see a similar process if you warm up butter and slowly cool it – the parts of butter that solidify first are the saturated and long chain fatty acids.

The temperature that the oil gets lowered to will determine which fats become solid. A lower temperature will cause more fatty acids to solidify for example, which may include mono-unsaturated fats too. Some commercial oils will also use solvents such as methanol to help with the extraction process, which will be removed after winterization. This solvent stage is not always used, and winterization can be done easily without it.

You can tell if an oil has been winterized by conducting a ‘cold test‘. To do this, store the oil between -1°C and 5°C (a fridge is perfect) for 1-3 hours, and see if any solids appear. (The time may need to be a little longer depending on the oil). If the oil remains clear then it has undergone winterization.


How does winterization affect cod liver oil?

Use of solvents

It is important to note that solvents are not needed for the winterization process. Some manufacturers don’t use it, where as others will, and it is not clear which ones do and don’t (you can make a fairly safe bet that some of the large commercial brand will). The solvents will always be removed, and their presence will be tested in the oil. Tests on oil that have used solvents have shown ‘pharmaceutical grade cod liver oil’1. This means it is very safe, although there is undoubtedly tiny tiny trace amounts in the oil. However, the process of using solvents and removing them can damage the fatty acids. Steps can be taken to reduce the damage, and the tiny amounts of solvents probably have no noticeable impact on health. Knowing if manufacturers take these steps is impossible to find out, and most people don’t want tiny amounts of these solvents in their supplements. So although the use of solvents is accepted as being safe, it still leaves you feeling a little concerned about their use in oils.


Omega-3 fatty acid content

The important omega-3 fatty acids EPA and DHA are both polyunsaturated fats, and so do not get removed by winterization. In fact, the concentrations of both EPA and DHA increase because the saturated/ long chain fatty acids get removed. The concentration of these omega-3 fatty acids can increase by as much as 20%2 although the increase will be dependant on the conditions used, and may be less. This can only been seen as a beneficial result of the winterization process.


Vitamin Content

Cod liver oil is known for its high levels of vitamin A and D. Winterization has no significant impact on vitamin A concentrations (in fact, it shows a slight increase in concentration)3. The study makes no mention on vitamin D levels, and research on this is sparse. However, it is quite a reasonable assumption to make that that vitamin D levels would remain similar during winterization too.

There are some sources (without any figures, research or references) which claim that winterization can reduce vitamin A and D concentrations. Although I’m not dismissing these claims, they aren’t supported with any evidence, and possible based on belief that any kind of processing must damage the oils4 (which it often does). However, the research, limited though it is, suggests that vitamin concentrations in cod liver oil remain unchanged after winterization.


Oil stability

Winterization is a cold process, and so the oil will remain stable. Assuming care is taken, there is no reason to believe that the oils will oxidise or the vitamins will degrade, and the low temperatures should help maintain the stability. However, if solvents are used there is a possibility that small amounts of fatty acids will become damaged.


Winterized VS non-winterised cod liver oil

Green Pastures cod liver oil is a great example of a non-winterized cod liver oil, and Dropi’s Icelandic Cod liver oil is a great example of a gently winterised product. However, there are other factors that have a greater impact on their nutritional profile than winterization. For example, Dropi has about 8% more DHA than Green Pastures, but Green Pastures has about 55% more EPA than Dropi. This variation is due to where the fish were caught, the season, their age, and how the oil is extracted. Winterization is a very small step in the extraction process, and although it can help to concentrate the omega-3 fatty acids, it doesn’t necessarily mean they are more concentrated than other products.

You can read more about how these 2 products compare, as well as many others, on the Fish Oil Showdown.



Winterization is a process that removes saturated fats and longer chain fatty acids from oil. It is done to produce an aesthetically pleasing oil, but also has the benefit of concentrating important omega-3 fatty acids and producing an oil with an even consistency. If done without the use of solvents, it can produce a clean, concentrated and undamaged oil that is very healthy. Even if solvents are used, the oil produced is of good quality, but a small amount of the fatty acids may have been damaged, and there very small amounts (undetectable) of solvent may be present.

Identifying which oils have been winterized and which haven’t can easily be done with a ‘cold test’. However, the details of how the winterization is conducted is not easy to find.

In  determining the quality of a cod liver oil, winterization is not a big priority. Sourcing, extraction and other processing methods have a much larger impact on quality.



1. Qiong Lei. (2016). Enrichment of omega-3 fatty acids in cod liver oil via alternate solvent winterization and enzymatic interesterification. Food Chemistry. 199 (5), 364-371.

2. Ana Mendes. (2007). DHA Concentration and Purification from the Marine Heterotrophic Microalga Crypthecodinium cohnii CCMP 316 by Winterization and Urea Complexation. Food Technol. Biotechnol.. 45 (1), 38-44.

3. George E. Éawe. (1993). Significance of Stearin Content of Cod Liver Oil. The Journal of the American Pharmaceutical Association . 22 (2), 109-112.

4. DAVID WETZEL. (2006). Cod Liver Oil Manufacturing. Available: Last accessed 25/5/2017.

Image courtesy of Denis Collette.

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