Wednesday, November 25, 2020

Statistical mechanics and Thanksgiving

Many books and articles have been written about the science of cooking, and why different cooking methods work the way that they do.  (An absolute favorite: J. Kenji López-Alt's work.  Make sure to watch his youtube videos.)  Often the answers involve chemistry, as many reactions take place during cooking, including the Maillard Reaction (the browning and caramelization of sugars and reactions with amino acids that gives enormous flavor) and denaturing of proteins (the reason that eggs hard-boil and firm up when scrambled over heat).  Sometimes the answers involve biology, as in fermentation.

Occasionally, though, the real hero of the story is physics, in particular statistical mechanics.  Tomorrow is the Thanksgiving holiday in the US, and this traditionally involves cooking a turkey.  A technique gaining popularity is dry brining.  This oxymoronic name really means applying salt (often mixed with sugar, pepper, or other spices) to the surfaces of a piece of meat (say a turkey) and letting the salted meat sit in a refrigerated environment for a day or two prior to cooking.  What does this do?

In statistical mechanics, we learn (roughly speaking) that systems approach equilibrium macroscopic states that correspond to the largest number of microscopic arrangements of the constituents.  Water is able to diffuse in and out of cells at some rate, as are solvated ions like Na+ and Cl-.  Once salt is on the turkey's surface, we have a non-equilibrium situation (well, at least a more severe on than before):  there are many more (by many orders of magnitude) ways to arrange the water molecules and ions now, such that some of the ions are inside the cells, and some of the water is outside, solvating the salt.  The result is osmosis, and over the timescale of the dry brining, the moisture and salt ions redistribute themselves.  (The salt also triggers reactions in the cells to break down some proteins, but that's chemistry not physics.)  After cooking, the result is supposed to be a more flavorful, tender meal.

So among the things for which to be thankful, consider the unlikely case of statistical mechanics.

(For a fun look at osmosis (!), try this short story if you can find it.)