peanut butter from the fridge - keeps its shape

Why Do Some Peanut Butters Split? – Science of Peanut Oil

Have you ever 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?

All you need is peanuts

All you need to make a simple peanut butter is, of course, shelled peanuts. Grind the peanuts into a fine paste and you’ve got peanut butter. In order to improve flavor most people will add some salt and roast the peanuts before grinding them.

About half the weight of a shelled peanut is fat. Peanuts also contain a considerable amount of protein, about a quarter of their weight. The rest is made up of some moisture and carbohydrates.

peanut shelled and unshelled

Why peanut fat is liquid

What makes peanuts so suitable for making them into peanut butter is that the peanut fats are mostly liquid at room temperature. The melting point of peanut oil is well below room temperature and is around 3°C (=37°F).

The melting temperature of fats depends on the composition of the fat or oil (we’ve discussed it before). The more unsaturated fatty acids a fat contains, the lower its melting temperature. Fats are unsaturated if the long fatty acid chains of the molecule contain a double bond between two carbon atoms. This double bond actually makes the fatty acid a little ‘crooked’. As a result, they can’t lie as compact next to one another and don’t solidify as easily.

In peanut oil over 80% of the fatty acids are unsaturated. The majority of those unsauturated fatty acids are oleic acid and linoleic.

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.

Grinding: releasing the fat

When you grind the peanuts into peanut butter you break up the peanuts in a lot of smaller pieces. In doing so, you release the fat that is trapped within the peanuts. Ground peanut butter literally consists of small peanut particles floating in their own 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.)

you can imagine that you need a lot of fat for all those peanut particles to be dispersed in, especially if the peanut butter is very smooth. It is why a high fat content nut such as peanut butter is so suitable for making into a smooth paste.

Why peanut butter splits

Even though you need a lot of fat and even though the liquid fat makes for a silky peanut butter, that high amount of liquid fat has a disadvantage. There is so much fat that the particles can float within it. They are not packed extremely tightly.

As a result, the peanut particles will sink down the oil over time due to a process called sedimentation. This is why some peanut butters in the store have a liquid layer on top. This layer is peanut oil!

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-flavors.

Apart from this shelf life reduction, split peanut butter is perfectly fine to eat. Just mix the particles back through the oil and the peanut butter is back to normal.

But, this reduction in shelf life was a problem, especially back when proper process control and easy transportation wasn’t as widely accessible. Peanut butter manufacturers were limited in their reach and efficiency. As such, manufacturers started to look for ways to prevent the splitting of peanut butter.

peanut in shell
A mighty peanut

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?

Hydrogenated fats

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 flavor) are used for this. Which hydrogenated fat is used depends on flavor, 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!

Sources

Coleman Collins, S., Why you shouldn’t be scared of oil in your peanut butter, National Peanut board, link

Engineering toolbox, Melting point of oils, link ; source for the melting temperature of peanut oil.

Gills, L.A., Resurreccion, A.V.A., Sensory and Physical Properties of Peanut Butter Treated with Palm Oil and Hydrogenated Vegetable Oil to Prevent Oil Separation, Journal of Food Science, 2000, link

McBride, J., No trans fats in peanut butter – contrary to current rumor, 2001, USDA, link

Michaud, J., A chunky history of peanut butter, 2012, The New Yorker, link

Procter and Gamble Co, Peanut butter Stabilizer, Patent US4341814A, 1980, link

Unilever, Peanut butter stabilizer, Patent US3766226A, 1969, link

USDA, FoodData Central, Unroasted peanuts, FDC ID: 784392, link

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