Got into an interesting conversation over the weekend about butter, and why inexpensive butter can make cookie dough greasy and hard to cut. It all has to do with melting points, which tend to be sharper with cheap butters that with higher quality butters. Allow me to explain. A melting point can be “sharp” or “broad”, a sharp melting point being defined as a rapid shift from solid to liquid. Cocoa butter is a great example of a substance with a sharp melting point. It’s solid as a rock at room temperature, then melts almost instantantly when it reaches 98 or so degrees Fahrenheit. Substances with a broad melting points, by contrast, change from one to the other more slowly.
The melting point of butter can be sharper or broader depending on its composition. If you were to zoom in and examine butter on a microscopic scale you’d find that it has quite a lot of variation in it. That is to say, it’s made up of many different types of lipid molecules — dozens of them, and each one has its own melting point. Some melt at relatively low temperatures (in the 60′s) and others at relatively high temperatures (in the upper 90′s). It’s this variation the keeps butter pliable at room temperature, not rock hard like cocoa butter, which contains just two main types of fats.
However depending on what the cow that gave the milk that made the butter ate, you can have very different combinations of fats. If the dairy cow ate a very limited diet, just a few fats will dominate the mix. If none of them have very high melt point the butter will tend to go liquid easily, the result being a greasy cookie dough that’s limp and falls apart when you try to cut it. However if the dairy cow was fed a highly varied diet — if for example it was grazed — then the butter will have a greater variety of fats, some of which will have higher melt points. That butter will tend to stay firmer longer and give you a more workable dough. Just another reason to buy decent quality butter. It need not be the best, mind you, just decent. Mid-tier will almost always deliver acceptable if not always sterling results in cookies.