Why There's Oil on Top of Some Peanut Butters - Separation Science (2024)

Ever opened a new pot of peanut butter only to find a thin liquid layer lying on top of the peanut butter?

If so, don’t worry! This is a completely natural thing to happen for unstabilized, natural peanut butter. That layer on top is peanut oil. Simply stir it back in and your peanut butter has been ‘restored’. To explain what’s going on, we need to study some physics and the science behind sedimentation.

The simplest peanut butters contain just peanuts

You can make peanut butter with just peanuts. No other ingredients necessary. To make the peanut butter, the peanuts are ground up into very small particles. This transforms crunchy peanuts into a smooth paste. Once the peanut particles are small enough, our tongue no longer tastes them, making a smooth eating experience.

Oils make peanut butter flow

Peanut butter can be spread and is quite liquid, despite the fact that it barely contains any water. But in order for something to be so soft and liquid, you do need some liquid ingredients. In peanut butter that liquid ingredient is supplied by the peanut oil.

Keep in mind that a little less than half the weight of a shelled peanut is made up of fat. In a whole peanut these fats are trapped within the peanut’s cellular structure. However, during grinding these structures are broken down and the fat is released. In peanut butter the fat is no longer trapped. Instead, the tiny particle float around in the oil, instead of the other way around.

Peanut pieces sink down

Whereas the liquid oil is crucial for making spreadable peanut butter, it doesn’t make for a very stable product. Over time, the peanut particles will sink down in the oil. They’ll sediment. This does take some time. The peanut pieces can’t move that fast. They’re in each other’s way and so will take a while to travel down. However, once they’ve gone down, they won’t travel up again by themselves. As a result, you’ll end up with a layer of clear oil on top of the peanut butter.

How to prevent peanut butter from separating

Scientists refer to the sinking of peanut pieces in liquid oil as sedimentation. It isn’t unique to peanut butter. Chocolate milk splits in a similar way over time, and so does orange juice with a lot of pulp.

A few basic physical principles and formulas can explain the science behind sedimentation. In short, they state that:

• Bigger particles sink faster
• Denser peanut particles sink faster
• Particles sink more slowly in a more viscous (thicker, gloopier) oil
• In a denser oil particles sink more slowly

We can’t truly control the density of the peanut particles. There might be some minor deviations between varieties, but generally speaking, there’s not much we can do here. Also, even though you can have some impact on the density of the oil, the effects will be minor compared to the other options. That leaves us with the size of peanut particles and the viscosity of the oil.

Controlling particle size is part of the solution

By making a smoother peanut butter, with smaller particles, manufacturers may slow down oil separation. There’s a limit here though. When particles get too small, the peanut butter may actually become too thick. There will simply be too many particles for all of them to be surrounded by oil. As a result, they don’t flow as easily anymore and get stuck. This is great from a separation prevention perspective. But, it might also make it hard to spread your peanut butter on a sandwich!

So yes, manufacturers can tweak things in particle size it’s often not the main solution. Most peanut butter manufacturers will instead look at tweaking the properties of the oils to prevent separation.

Controlling viscosity of the oil

The viscosity of an oil describes how the oil flows. A more viscous liquid is ‘thicker’, it flows less easily if you’d try to pour it from a glass. Honey for instance is more viscous than pure water. Peanut oil is not very viscous, it flows quite easily. To slow down separation, we need to make sure it flows less easily.

Why peanut fat is liquid

Peanut oils flow easily partially because they are liquid at room temperature. They have a low melting point. That is, they’ll only start to turn solid around 3°C (37°F). This can be explained by the composition of peanut oil. Peanut oil, like most other fats and oils, is a mixture of different molecules called triglycerides. The types of triglycerides in a fat determines its behavior.

An important characteristic of triglycerides is whether they contain saturated or unsaturated fatty acids. Unsaturated fatty acids have a so-called double bond in their structure. Whereas saturated fatty acids are linear molecules, unsaturated fatty acids are crooked. This ‘crookedness’ impacts how they can organize themselves. It makes it hard for them to pack tightly and as a result, it’s harder for them to turn solid and they’ll do so at lower temperatures only. In peanut oil over 80% of the fatty acids are unsaturated.

Adding solid fats to increase viscosity

To increase the viscosity of peanut butter it needs to turn ‘more solid’. One way to achieve this is by adding more solid fats. One way to do that is by adding hydrogenated peanut oils. Hydrogenated peanut oils have undergone a chemical reaction that saturates the unsaturated fatty acids. That is, they are now straight instead of crooked. As a result, without changing anything else, their melting point increases and they’ll likely be solid at room temperature.

By mixing in even a few percent of these hydrogenated fats, the overall composition of the fats changes enough for separation to slow down or even stop completely. Aside from using hydrogenated peanut oils, manufacturers may also mix in other types of vegetable fats that are (naturally) more solid than peanut fats such as (hydrogenated) rapeseed, palm or sunflower oils.

Storing peanut butter in the fridge

To slow down separation, we need to increase the viscosity. Recall that peanut oil starts to turn solid around 3°C (=37°F). That is, it will turn at least partially solid in the fridge, enough to stop separation. An added benefit can be that oxidation of the oils also slows down at lower temperatures. So, storing peanut butter made from just peanuts in the fridge slows down both separation & oxidation, a win-win!

Is the layer of oil on top a problem?

The layer of oil on top of peanut butter is completely natural and not harmful. In small pots of peanut butter especially it is very easy to mix the oil back in, giving you ‘normal’ peanut butter again. As such, it’s generally not a real problem. However, there are a few cases in which this layer can be problematic.

Varying composition throughout a container

In large containers of peanut butter the separation of the two phases can present some problems, especially if it the peanut butter has had a chance to settle for some time. The bottom layer may be very firm. It can be almost impossible to stir the liquid back in, even using stirring equipment. However, if you do not want to use the whole container in one go this will be a problem. The composition of the top layer of peanut butter will be very different from that of the bottom. If you use the two halves to make the same product you will probably find they turn out quite differently! One contains a lot more fat than the other which will impact the consistency and texture of whatever it is that you’re making.

In these cases it is best to:

• limit the storage time of peanut butter before consumption
• store the containers in the fridge to slow down separation
• purchase peanut butter in the required quantity for a specific application

Split oil can reduce shelf life

Another major disadvantage, no matter the size of your container, is the reduced shelf life of the peanut butter. Peanut butter with a layer of oil on top will turn rancid more quickly. Unsaturated oils especially turn rancid when they get into contact with oxygen due to a chemical reaction called oxidation. During this reaction, unappetizing flavors and smells are formed.

Nowadays, proper packaging can greatly limit oxidation. As long as you haven’t opened a jar chances are it won’t turn rancid. But, once you’ve let oxygen in, you can only store it for a limited period of time – which might still be a couple of months by the way.

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