Why do (some) peanut butters split?

A peanut butter with cucumber sandwich, is that a very weird combination? It’s how I like to eat my peanut butter sandwich though. Peanut butter seems to be one of those foods that are combined with just about anything. There’s peanut butter cookies (= photo on top of this post) and pies, it’s part of dressings and sauces and it’s a favorite on sandwiches.

And as goes for anything popular there are loads of different types and brands of peanut butter. There’s differences in taste and texture, but also a very practical differences: some split, the oil will sit on top of the peanut butter, whereas others are stable and will never have to be stirred to mix again! Why do some peanut butters split whereas others don’t?

How peanut butter is made

There are lots of peanut butters, but all have in common that their mostly made from ground peanuts. Peanuts (just like most nuts) contain quite a lot of fat. When the nuts are ground into fine particles this fat is set free. As a result, you will end up with small peanut particles floating around in oil.

In the most simple peanut butters, nothing else is added, except maybe for some salt. However, there are plenty of peanut butters which contain other ingredients. You will commonly find sugars (either regular, or in some sort of syrup) and other fats in the peanut butter.

A short history of peanut butter

Peanuts have been around for centuries and are used in a variety of cuisines, either whole or ground into pastes (e.g in Asian cuisine). Eating peanut butter as such though is a more recent invention. In the 19th century it was a luxury product instead of a staple. It can considered to be very beneficial for someone’s health. Towards the end of the 19th century peanut butter becomes a more common product, however, at the time, peanut butter couldn’t be transported well. The shelf life was limited, partly because of that separation of the fat layer.

The invention of a way to stabilize peanut butter seems to have really ramped up peanut butter production. It could be stored and transported more easily. Also, manufacturers started adding sugars to make the product more appealing to most consumers (even though I personally strongly prefer one without added sugar, I like the salty taste better than the sweet).

When browsing through patents of the beginning of the 20th century, there are plenty of references to peanut butter for candies or for equipment to make peanut butter. Now, peanut butter can be found everywhere, especially in the US, other countries seem to be slightly less big fans of the nut paste. But how did they manage to overcome that separation of fats?

Why peanut butter splits

As we discussed at the start, peanut butter consists of finely ground peanut particles, floating around in peanut oil. Peanut oil is liquid at room temperature. It has quite a low solidification temperature. As a result, the particles can slowly float down through the peanut butter. This process is called sedimentation and is the same as happens in home made chocolate milk.

Simply mixing the split peanut butter will overcome the problem, no harm is done. However, the shelf life of peanut butter that splits tends to be shorter than that of non-splitting peanut butter. The reason for this lies with the peanut oils. These oils can oxidize when they come into contact with air. This will create off smells and off flavours.

Why not all peanut butters split

In order to prevent splitting the peanut butter the particles shouldn’t be able to float down anymore. This can be done by solidifying the fat phase. If the fat is solid the particles will stay where they are.

In the 20th century a new process was introduced to do this: hydrogenation of fats. In order to understand what this process does let’s have a quick look at fats and oils (find a more extensive discussion on fats here). The chemical structures of importance here are the so called triglycerides. These are large molecules with three long chains attached together. Each of these chains is a fatty acid. It’s the combination of fatty acids that determines the characteristics of the fats and oils.

Fatty acids
Examples of several fatty acids (not necessarily those that occur in peanut butter).

There are a lot of fatty acids. All of them are a chain of carbon atoms, but they may differ in length and in the way the carbon atoms are attached to each other as you can see above. If a double bond is present that tends to bend up the fatty acid, taking up more space. Because these bended fatty acids take up more space, their melting points are higher. In other words, they are more likely to be liquid at room temperature than without those double bonds.

That’s where hydrogenation comes in. Hydrogenation is a chemical process in which the double bonds are transformed into single bonds. In the case of peanut butter the use of hydrogenated fats will give more solid fats at room temperature. More solid fats translates into less chance of separation of the fats.

Most peanut butter manufacturers use hydrogenated peanut oils to solidify the peanut butter and prevent the separation. As you can see in the ‘sources’ at the end of this post, improvements and changes to this stabilization have been come up with.

Slowing down separation without hydrogenation

If you have a peanut butter that does split there is quite a simple trick to prevent it from occurring: place the peanut butter in the fridge. At these low temperatures part of the liquid peanut oils will solidify. Solid oils will prevent the particles from slowly travelling down and thus will prevent the separation.


Article looking into the different in separation of peanut butters with or without the addition of other fats.

The New Yorker wrote a nice article about the history of the peanut butter, but I haven’t been able to verify all the patents they mention,

Incomplete hydrogenation of fats can lead to trans fats which are thought to be bad for human health when consumed in too high amounts. The USA peanut board (as well as the USDA)wrote about this, it seems as if currently only/mostly fully hydrogenated oils are used.

There exist a lot of patents on peanut butter, several looking into better ways to stabilize the peanut, by changing the hydrogenated fat fraction or by using different hydrogenated fats (not peanuts).

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