Learn the science behind:
If you’ve ever switched butter brands, and most of us probably have, you’ve probably noticed that one butter isn’t necessarily the same as another. Your new butter might be paler or yellower, harder or softer, more or less flavorful.
So, what makes butter a butter? How come not all butters are the same? And all those products that look like butter, why aren’t they butter and what’s different about them? That story, all starts with a cow.
What is butter and where does it come from?
To answer all those questions, we have to look at what butter is first. And every butter starts out with a cow, in a grass field or in a barn. That cow will eat, get calves and gives milk. That milk is what will be transformed into butter. You don’t need and other ingredient to make butter.
How butter is made
Assuming you don’t make your own butter, that milk goes to a large dairy factory. The first step in transforming it into butter is to get rid of a lot of that water in milk. Milk consists of mostly water with some proteins and sugar and a little (4%’ish) fat. Butter on the other hand is at least 80% fat. So there is a lot of water to remove.
Centrifuge – Using density to separate cream
The fat in milk has a lower lower density than water. As a result, if you leave fresh milk for long enough, that fat will float to the top. You can see a similar effect when you try to mix water with oil. Over time, the oil will float back to the top. The science behind this phenomenon is the same as that for the sedimentation of cocoa particles in chocolate milk. This same principle is also used to make clotted cream from regular cream (clotted cream contain even more fat).
There is a way to speed up this separation by using a centrifuge. In a centrifuge the milk is spun around really quickly. Due to the difference in density the water-rich and fat-rich portion can be separated. The fat-rich portion is cream, the water-rich portion is skimmed milk.
Transforming cream into butter – Churning
Cream has a high enough fat content to be transformed into butter. However, it’s fat content is only still 30-40% (the exact value depends a lot on the manufacturer and process used to make the butter). In other words there still is a lot of water that needs to be removed.
At this point you can’t use centrifugation anymore, instead, the cream is made into butter by a process called churning. During churning you agitate the cream in order to induce the individual fat droplets, floating in the milk, to clump together. These fat droplets will have a thin membrane on their outside. By breaking this, the fat particles will be able to find each other and clump together. In doing so, some water will still be caught within.
Over time, enough fat has clumped together and your left with butter with at least 80% fat. The rest of the cream contains barely any fat, but does contain a lot of moisture. This is what we call buttermilk.
The last step in butter manufacturing is to work the butter, kind of like ‘kneading’ the butter. During this phase additional ingredients may be added as well, salt being the most common. After that, it is ready for packaging, whether it’s wrapped in a pack or in a tub.
The structure of butter – an Emulsion
You may have been wondering how butter remains stable since it contains both fat (at least 80%) and water (and some minor ingredients). Why doesn’t it separate and why can’t you see the water and fat separately?
When you zoom in on butter you will notice that butter is a continuous phase of fat throughout which you will find water bubbles. This is what we would call a water-in-oil emulsion. Cream on the other hand is the opposite. In cream water is the continuous phase and fat droplets float throughout. It is therefore an oil-in-water emulsion. In these types of systems the droplets floating in the continuous phase can be quite small and dispersed evenly throughout which is why you don’t see them in butter and cream.
Butter doesn’t split over time as long as it is kept cool. At that low enough fridge temperature the fats in butter are solid. This is why you block of butter is quite hard. These solid fats prevent the water from moving within, so it stays put and won’t split.
Why you can’t turn molten butter back into its original form
If you’ve molten down butter before and left it to re-solidify in the fridge, you will have noticed that it never turns back to that same firm coherent piece of butter!
On the photo above you can see that the re-solidified butter (center) has a thin layer of moisture on part of the sample. This part was actually turned over, during solidification, the butter had raised to the top and the moisture to the bottom (thanks to gravity & density differences). This is because that delicate emulsion structure of the butter has been destroyed.
By warming up the butter you have molten down all the fats in the butter. As a result, the moisture and fat are free to go. As a result, you’re splitting the butter. Larger droplets of both fat and water will form. When it cools down, these two phases won’t merge seamlessly anymore. That doesn’t mean the butter can’t be used anymore though, for many applications (e.g. spreading it on hot toast) it doesn’t matter that much at all. For others (such as making a pie crust) is does matter.
Sign up to our weekly newsletter to be updated on new food science articles.
How butters may vary – Buttery nuances
Even though making butter is quite simple and really only uses one ingredient, there still still be a lot of differences between butters. This may be because of slight changes in the manufacturing process, additional ingredients, or, most importantly, because of differences between milks!
Changing the process
Cultured vs uncultured butter
You can make butter from fresh milk very quickly with the right machinery. If you do it as described above, it will be uncultured butter. However, it is possible to add an additional step: culturing of the butter. In cultured butter micro organisms have grown which provide extra flavour to the butter.
This step was a necessity in the past. You’d often would have to wait for the cream to separate from the milk. While doing so, micro organisms from the surroundings could end up in the cream. They would grow and in the worst case spoil the milk, but in the good scenario, add flavour to the butter.
Nowadays, adding cultures helps to enrich the flavour of butter, but it may also impact the hardness and even the colour of the butter.
Changing the milk
Cows are animals, not standardized machines. As a result, they won’t always produce the exact same milk. The fat content, protein content and mineral content can all vary. Within those, the exact composition of the types of fat and proteins can also vary.
Current day manufacturing processes are good at standardizing the overall content of fat and proteins in the milk. However, they can’t correct for a difference in the type of fat and protein in the milk.
Lactation cycle of the cow
Cow’s give milk because they’ve given birth to a calf. In the months that follow calving the volume as well as the composition of the milk will change. Most of these changes though can be standardized in the factory, not resulting in major changes noticeable to the consumer.
The cow’s DNA
There are a lot of factors at play here. A different breed of cows might make milk of a slightly different composition for instance. Within a breed differences may occur as well. The composition of milk is to some extent hereditary! In other words, a cow’s DNA has an impact of the composition of the milk it produces.
Nowadays, minor differences between cow’s won’t be seen in the end result though, since the milk from a lot of cow’s is blended together.
Feed of the cow
An important factor in this scenario, is the food that a cow gets to eat. A cow that has only (or mostly) eaten grass will produce milk (and butter) with a different composition than a cow that was mostly fed on grain or grass with plenty of clover for instance.
Research has shown that the types of fatty acids (those make up the fat in butter) changes with diet. As a result, the hardness of the butter will vary. But that’s not all, the colour can also change, grass fed diets tend to produce a butter with a more yellow colour. And last but not least, the flavour profile of the butter will change! Feed as such is hugely important.
Geography & seasonality
The location where the cow lives as well as the season you’re milking the cow impacts the cow’s milk composition. There is a lot of interaction here with the feed though, since most cows have a different diet during winter than during summer for instance.
Salted vs unsalted butter
Even though the general process is quite simple, there are a lot of different butters. The first main difference is unsalted vs salted butter. Apart from the presence of additional salt, these butters will otherwise be identical is made from the same milk, etc. Salt contributes flavour though and it helps extend shelf life of the butter (although with modern day cooling that is less of a concern nowadays).
If you use butter for baking, or cooking, it is almost always best to use unsalted butter. That way, you can control the amount of salt you add yourselves. Since not all salted butter contains the same amount of salt, it is hard to manage it well otherwise.
Choosing a butter
So, how do you choose a butter? If you like the butter you’re using, just stick with it. If you don’t, buy different types and decide which ones you like best. Do you want a more yellow or a softer butter? Or a harder one? It is not always possible to tell which butter will be which, without buying them and comparing them one on one!
O’Callaghan, T., Hennessy, D., Impact of cow feeding system on the composition and quality of milk and dairy products, Teagasc, Grass-fed dairy conference, 2018, link
Cheese Science Toolkit, Stage of lactation, link
Dairy processing handbook, Centrifugal separators and milk standardization, chapter 6.2, link; a great in-depth explanation of how a centrifuge works for separating cream from milk
Dunn, E.G., Irish Butter Kerrygold Has Conquered America’s Kitchens, Oct-2, 2019, Bloomberg business week, link
The cattle site, Managing Cow Lactation Cycles, May-18, 2015, link
Wagening University, Milk Genomics Initiative: Cow’s DNA determines composition of milk, link