The Science of Butter

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For a product made predominantly from one ingredient (cream), butter has a surprisingly interesting story. To really understand it, it’s helpful to delve into the physics – or at least touch on it – as well as run through the process of making butter.

First there’s the milk, ideally from a well-bred Jersey cow. I use Jersey and Guernsey herd milk, due to the breeds’ ability to produce an extremely high-fat and beta-carotene-rich milk. Beta carotene is naturally present in grass and it is what gives milk its fat-rich yellow pigment. The more grass a cow eats, the yellower the milk will appear – that’s why summer cheeses and butters are much more colorful than those made in winter. For me, butter should be yellow, so choosing a cow’s milk that has the capacity to produce a beautiful yellow butter has always been a priority.

Milk is a liquid with butterfat naturally emulsified within in it. The fat cells have a lower density to water, which is why we see cream rise to the top of a milk bottle – at least the unhomogenised milk the milkman used to deliver in glass bottles. (A favorite childhood story from my mum is the race between her and her brother to the front door to see who could get the cream from the milk on to their cornflakes.)

Separating the cream from the rest of the milk is the next part of the process. Historically (before industrialization), fresh milk would be left out in a large vat. After a day or two the cream will have naturally risen to the top of the vat and would then be skimmed (ladled) off from the top, leaving ‘skimmed’ milk left over.

Once centrifuges (a machine that spins at such a velocity that the cream rises higher than the milk) were introduced, the traditional techniques became obsolete.

You can make butter from fresh cream, but to make cultured butter (which has vastly more flavor), you need to ferment the cream. In Scandinavia, the very traditional technique would be to collect the cream (skimmed from the daily milk that was milked from the cows throughout summer) in wooden vessels. By constantly reusing these vessels, a dominant lactobacillus held in the grain of the wood that allowed the cultures in the cream to thrive and multiply, thereby fermenting the cream. Essentially, this is a process of lactic bacteria feeding off the lactose in the cream to produce lactic acid, which gives the cream a specific taste and acidity.

Home butter-makers today can mimic this process by adding a cultured dairy product to the cream, such as sour cream or crème fraiche – this is the starter for the butter. Bacteria has the ability to produce a pretty much infinite range and combination of flavors, so it’s worth experimenting with your favorites.

You can buy freeze-dried cultures online to make butter, but it’s more important to experiment with sour cream or crème fraiche as your starter to really experience the potential flavor possibilities.

For this newly fermented cream to become butter, it must be agitated. Agitating the fat globules that are emulsified within a liquid means that the membranes of the fat molecule (made of proteins and phospholipids) begin to break down, and the fat slowly begins to bond within the agitating liquid.

As the agitation progresses and fat globules begin to form, they group with more and more globules and can finally be seen visibly.

The liquid proteins released by splitting the emulsion is named ‘buttermilk’ and the visible globules of butter is known among butter-makers as ‘butter popcorn’ (my favorite time to taste). 

There’s a beautiful and historic selection of techniques used to agitate butter that range from wooden plungers that forced the cream to move to spinning beaters that applied even more agitation force. Then barrel churns (where the barrel is rotated allowing the cream to be in much a tumble dryer style motion) took over. 

Small paddle churns were a common feature in households from the nineteenth century onwards, but to be honest, shaking normal jam jar is just as good.  

It’s great fun (especially for children) to explore the physics of this and it’s very easy to replicate at home using a jam jar. Cream can be placed in a jar or sealable vessel, it’s important to only half fill the container as air space is very important for the motion to function properly. Then the jar needs to be vigorously shaken, making sure that as the cream thickens it falls away from the sides of the vessel. As long as there’s agitation and the temperature doesn’t rise over room temperature, it should work. I put this to the test once by half-filling a bottle with cream and hanging it off the back of a sailing boat: the constant motion of the waves hitting the bottle made us delicious butter within 45 minutes.

Once you have separated the buttermilk from the butter popcorn, it’s time to create the finished product. Cold hands are actually one of the most important elements from here on out as you need to hand knead the mixture to release the buttermilk. Buttermilk has such a delicious flavor and acidity you don’t want to wash it all out, but it’s also high in proteins which bacteria love, so if you leave in too much buttermilk, the butter could over produce bacteria and end up tasting overly sour and cheesy.

The butter needs to be hand kneaded until it no longer looks wet and only a few cold squeezes releases a few drops of buttermilk. For me, this is the perfect stage to stop and finally add salt, also by hand. The salt further draws moisture, so once the salt has drawn excess moisture, butter should be kneaded again to release any final excess of moisture.

And there you have it: with the noble help of physics, you can create the yellow gold that we consider a staple ingredient.


Excerpted with permission fromBread and Butter by Richard Snapes, Grant Harrington and Eve Hemingway, published by Quadrille October 2018, RRP $29.99 hardcover.

Inspiration for edible alchemy.