The author of “CookWise” and “BakeWise” writes about the science behind various aspects of cooking and/or baking. For some food items, its all about the physics. For biscuits, it is about the steam generation. For butter, it is all about the agitation and breaking of the suspension.
I have recently started looking into the physics of making butter – and while there are great hands-on science activities that relate to making butter – there is not really a good explanation of what is happening on a microscopic level. (Here is a very good making butter hands-on activity from the Scientific American) But even this experiment doesn’t really get down to the basic science of what is happening. One of the Dairy Science pages comes out and says “exactly how churning works is unknown”.
So, while there is no definitive reference for exactly what is happening, here are a couple of aspects of the overall process:
1) Whole milk – whether from goats, cows, sheep or other mammal – is a complex mixture of water, proteins and fats. In addition, the mother is also providing other essential items including vitamins, minerals and enzymes. (You can go to the Milk Composition Website to learn more.)
2) Milk that you purchase in today’s grocery stores have been pasteurized and homogenized. The pasteurization process requires the heating of the milk to kill the “bad” bacteria, i.e. those bacteria that cause illness in humans. Homogenization is a physical process, by which the larger molecules, primarily fats, are broken down to allow them to remain in suspension. If you can purchase milk from a local dairy, you may be able to find non-homogenized milk. Non-homogenized milk will separate into layers, i.e. a cream layer and a milk layer. (This is a physical separation using gravity. A commercial dairy uses a centrifuge to perform this separation and provides a milk with a consistent fat content. It is still a physical process based upon the density of the material.)
3) Milk can be considered a colloidal mixture. A colloidal mixture is a fluid in which “particles” are suspended in a liquid, or dispersed throughout. You can think of milk as being a mixture of water, butter fat particles, protein particles, etc. suspended and floating around in the container. It is essentially, a liquid with very small solid particles floating in suspension. This is a bit different than an emulsion. An emulsion refers to two separate liquids, with droplets of one liquid floating in another liquid, for example oil and vinegar salad dressing.
So, what is happening when we make butter?
First, making butter requires “churning” or mixing of the cream. We have started with a physical separation of the butter fat into the cream layer and now have begun agitating it. As the churning progresses, air is mixed with the cream to form a foam, i.e. air is trapped among the butter fat particles forming a stable suspension. As the churning process continues, the “whipped cream foam” falls, i.e. the foam is no longer stable, because the butter fat particles have now begun to aggregate into larger particles and are no longer able to form the foam lattice. The churning continues until large clumps of butter can be seen and collected using a strainer.
The entire process appears to be based on aggregation of the the butter fat particles by increasing the amount of individual interactions between the particles through physical agitation. It is apparent that the collisions between butter fat particles under these conditions is inelastic, hence the aggregation of the individual butter fat particles.
Hopefully, this brief explanation will allow you and your aspiring kitchen helpers – to play with physics and enjoy the tasty result!