peanut butter from the fridge - keeps its shape

Why do some peanut butters split?

Have you even noticed in stores that some  peanut butters have a layer of clear liquid on top of the peanut butter? It doesn’t seem to matter whether it’s chunky or not, some brands just do. Other brands though do not have this layer and seem more homogeneous.

There is nothing wrong though with those peanut butter with a layer on top. It’s just the natural peanut oil coming out and lying on top. you can just stir it through and it’s fine. But why, you might be wondering, don’t all peanut butters show this phenomenon?

How peanut butter is made

Peanut butter starts, of course, with peanuts. You could make peanut butter with nothing else but peanuts, that’s what peanut butter is all about. You will find though that most brands of peanut butter add a few (or quite a lot of) other ingredients.

Making peanut butter often starts by removing the shells from the peanuts and cleaning them. Next, the peanuts are often roasted. Roasting enhances and strengthens the flavour of the peanuts. After the peanuts have been cooled and after the skins have been removed it is time to transform the peanuts into peanut butter.

Grinding the peanuts

This is done by grinding those peanuts until they form a paste. Which is essentially peanut particles floating in peanut oil. The smoother the peanut butter, the more the peanuts have been ground and broken up into smaller particles. (You could do this at home as well, using a strong food processor or blender.)

Peanuts contain a large amount of fat, almost 50%. Therefore, there is a lot of fat available to make a smooth paste instead of a dry powdery/floury consistency. For the simplest peanut butters, this is where the production process finishes.

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.

Why peanut butter splits

So peanut butter contains a lot of peanut oil. That layer that you see on top of your peanut butter, that is peanut oil. Peanut oil is liquid at room temperature (if you’ve ever bought peanut oil, you will have noticed). As a result, it can slowly move to the top while all the peanut particles settle down, they sediment (this can happen to chocolate milk as well!).

Why peanut oil is liquid

Whether a fat or oil is liquid at room temperature depends on its melting point. If the melting point (which is the temperature at which the fat melts from solid to fat) is below room temperature, it will be liquid. For peanut oil it is well below room temperature, being around 3C (=37F).

This temperature again depends on the composition of the fat or oil (we’ve discussed it before). The more unsaturated a fat, the lower it’s melting temperature, so the more likely it is to be liquid at room temperature. This is because unsaturated fatty acids tend to be more clunky and less linear so as a result they can’t snuggle pu as well.

Unsaturated refers to the molecular structure of the fat. The (fatty acid) chains of a fat are long chains of carbon molecules. Most of these are connected through a single bond. However, some are connected through a double bond (imagine them holding two hands instead of just one). This makes a fat unsaturated.

Fatty acids
Oils, including peanut oils, are made up of triglycerides. Each triglyceride is made up of a glycerol molecule (not shown here) and three fatty acid chains. This graphic shows a example of 4 of such fatty acid chains. The top two are saturated fatty acids (no double bonds), whereas the lower two are unsaturated.

In peanut oil over 80% of the fats is actually unsaturated. As a result, it is liquid at room temperature.

Split oil can reduce shelf life

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 that fat layer on top, the particles in the 
peanut butter shouldn’t be able to float down anymore. They won’t do that anymore if the peanut oil weren’t fluid, but solid! So how to introduce some solid fats that would still make peanut butter taste like peanut butter?

In the 20th century a new process was introduced to do exactly this: hydrogenation of fats. Remember those unsaturated and saturated fats we discussed before (or find a more extensive discussion on fats here)? Hydrogenation is a chemical process that converts unsaturated chains into saturated ones. As a result, their melting temperatures go up and become more solid.

Most peanut butter manufacturers use hydrogenated oils to solidify the peanut butter and prevent the separation of oil and particles. You will notice that they don’t necessarily use peanut oils, often other vegetable oils (e.g palm which has a very neutral flavour) are used for this. Which hydrogenated fat is used depends on flavour, regulation and availability and costs of materials.

Slowing down separation without hydrogenation

So what if you do have a peanut butter that splits and you don’t want to stir your peanut every time you want some?

Luckily, there’s another way: just store the peanut butter in the fridge. Remember, we want to have more solid fats. Instead of making the fat solid by changing its composition, you can do so by decreasing the temperature. 

But, you might think, the melting temperature of peanut oil is 3C, so in a fridge it will still be liquid? That is partially true. Oils like this don’t necessarily have one temperature at which all molecules go from solid to liquid (as is the case for water). Instead, there is a temperature range over which this occurs. So in the fridge, just above 3C (27F), some of the fats have solidified and some are still liquid. That is enough though to stop the particles in the peanut butter from moving through the oil!

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 at the time.

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, partially thanks to the invention of hydrogenation of fats.


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).

Nutritional database from the USDA for peanuts, giving the fat content and its composition. Of ourse, these numbers are always averages as nature varies.

The engineering toolbox was our source for the melting temperature of peanut oil.

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