Cooking Meat: Thermodynamics and Biochemistry

Yesterday I attended Cooking For Geeks: Chemistry From the Kitchen, a talk given by Jeff Potter, at the New York Academy of Sciences. Mr. Potter is the author of Cooking for Geeks, a book about science and cooking. Although I haven't read it, I really enjoyed the talk, so I have every hope that the book will be just as funny and enlightening.

One topic addressed in the talk was the science behind cooking steak. As you may know, meat is mostly composed of muscle tissue from animals. In the case of steak, it's the muscle tissue of a cow. Here is a schematic of the type of muscle (skeletal muscle) that we most like to eat:

Schematic of skeletal muscle - what meat is made of!

As you can see, muscles are made up primarily of fibers made of two proteins called Actin and Myosin. Although opinions vary, most people prefer their steak to be cooked somewhere between rare and well done. In order to figure out why this is the case and what is it people enjoy about steak at the medium temperature (130-155F), we have to look at what is happening to this tissue at theses temperatures. Potter points out that at these temperatures, the Myosin in steaks has reached the temperature at which it denatures, or unfolds. Thea Actin, however, is a more thermostable protein and has not yet denatured. He concludes that, although this is just a correlation, it's too striking to be a coincidence. People, he believes, like the taste of denatured Mysoin and native (non-denatured) Actin.

Mmmm.... denatured Myosin.

However, as someone who is familiar with protein biochemistry, I have an alternative explanation. Could it be, instead, that people like the taste of meat that has been partially denatured because it affects the juiciness of the steak?

When proteins are in their native state, the long chain of amino acids that make up a protein cause it to fold up into a characteristic conformation. Some amino acids are hydrophillic (water-loving) and some are hydrophobic (water-hating). Like oil and water, these residues only get along with others of their kind. Generally, hydrophobic residues are folded into the inside of the protein, where they can interact with each other, while hydrophillic residues are on the outside of the protein, where they can interact with the surrounding liquid. When a protein denatures, this chain unfolds, exposing many hydrophobic residues. If there are other unfolded proteins around, the denatured proteins tend to stick together because their exposed hydrophobic residues interact with those on a neighboring protein. This often causes the proteins to become much less soluble, and they are no longer dissolved in solution, meaning that they no longer interact well with liquids.

If a steak is cooked so that some of the proteins are denatured, it becomes much less "chewy," partially because cells break down and proteins are denatured, allowing the muscle fibers to be more easily broken apart. However, if you cook a steak until it is "well done," people often complain that it is dry and tough. My hypothesis is that this dry, chewy texture occurs because the steak has reached a temperature where all of the proteins are denatured and are insoluble, so they can no longer hold on to moisture within the steak. This, combined with the fact that water evaporates faster at higher temperatures (and will convert to steam above 212F), results in massive moisture loss from the meat. A raw steak is too tough because the muscle is completely intact, but a steak that has been partially denatured (where, perhaps, Mysoin but not Actin proteins are denatured) is broken down somewhat, making it tender. Some of the proteins have been denatured, but there are still a sufficient number of native proteins to hold on to some moisture, resulting in a juicy piece of meat. At least, I think that's one good explanation. At this point, no one knows for sure - this hypothesis is fairly difficult to test, given the number of other variables. If one of my readers can think of a good experiment to test this, I'd love to hear about it in the comments!

What is the take home message of all of this? Don't under or over-cook your meat. Different meats (different cuts and species) have different forms and ratios of Actin and Myosin (and also different potential food borne pathogens) and need to be cooked differently. Chicken needs to be cooked to a higher temperature to be sure that there is no Salmonella contamination, but cooking it too long will also cause it to be dry for the same reason that overcooked steak is also dry - you have denatured all of the proteins and forced out all of the water from the meat.

For more interesting discussion of how to cook meat, also check out this timely post about frying Thanksgiving turkey (spoiler alert: don't do it!) from Food Lab.