September 25, 2011
My favorite reaction
I suppose my favorite reaction isn’t necessarily the same as most other chemists. In the lab, the best reactions are ones that are well behaved and predictable. These reactions give high yields and can often bring about transformations from simple molecules to molecules of incredible complexity. And, to add icing to the cake, these reactions normally occur in one step.
You would think that my favorite reaction would involve these qualities.
But, no, my favorite reaction is different from what you might expect. My favorite reaction takes large, complex molecules and breaks them down into much smaller pieces. My favorite reaction has frighteningly low percent yield. And, in fact, the major products of this reaction can be detrimental. But, the side products make the search worth-while. Like any other reaction, though, finding the right products from my favorite reaction takes patience and a watchful eye.
And, I don’t run this reaction in a lab. This reaction is best run in a kitchen.
The Maillard Reaction
Methods Temper a steak by taking it out of the fridge and letting it sit at room temperature for about a half an hour. Heat up a pan containing a thin layer of oil on the stovetop. When the oil in the pan is smoking and the steak is tempered, place the steak in the pan. Make a note all of the changes that are occurring. Hear the sizzle of the meat in the oil. See the meat, where it touches the pan, start to change colors from deep red to gray to brown. But, most importantly, smell the new aromas emanating from the pan. The reaction is finished when the meat has a well browned layer of “crust” and no longer sticks to the pan. The reaction has run too long when the odors change from pleasant to acrid. The reaction has run too long when the brown “crust” turns to a black char.
Image highlighting the differences between an unseared (right) and seared (left) steak. Photo credit: Felicity Cloake as described her wonderful post on whether or not you should sear your steaks from the Guardian’s Word of Mouth Blog.
Are you hungry yet? I certainly am. A good sear can make a mediocre steak delightful. And, a bad sear can render a good steak disappointing. A sear, in this case, doesn’t just give texture to your food. It creates new flavors. It creates new aromas. A good sear is the realization of an uncooked steak’s hidden potential.
And all of this is mainly due to one reaction: The Maillard reaction.
The Maillard reaction is the reaction between a nitrogen containing molecule (particularly the amino acids lysine and proline, which are found in proteins) and a reducing sugar (glucose, for example). Louis-Camille Maillard was the first person to study this chemistry (in the early 1900s), which, fortunate for Maillard’s personal legacy, was much later found to be an important process in cooking. The color, flavor, and aroma development come about due to the primary fact that many of the chemicals that form, after the initial addition of the sugar to the amino acid, are unstable and decompose.
So, basically, the Maillard reaction can enhance the flavor of any food that contains proteins and sugars. While this description of any food containing protein and sugar, encompasses most of the things we eat, there are several types of food whose flavor profiles owe a lot to the Maillard reaction. Grilled meats. Roasted meats. Crusty bread. Dark beer. Roasted coffee. Chocolate. Toast. Cookies. Really, any food that you are cooking at temperatures above 250oF/120oC are going to have some Maillard components giving it color/texture/aroma.
The Specifics … (for the electron-pushing crowd) The set of reactions that take place under the general description of the Maillard reaction can be generalized as follows. (Please refer to the figure for more detail.) A sugar (1) combines with an amine (in this case, NHn-AminoAcid) to form 2. 2 rearranges into a glycosylamine (3), which is unstable in these conditions. The glycosylamine rearranges into an aminoketose (5) through an aminoenol (4) intermediate. The aminoketose is one of the main products of the Maillard reaction. It is called the Amadori component because, well, Amadori isolated these compounds from Maillard reaction products in the 1930s. And, while this is a primary component, it really isn’t very interesting. The tasty parts of the Maillard reaction come about when 4 is converted into a deoxy-hexosulose (7) or the Amadori product rearranges into an enediol (6), which is further converted into a deoxy-hexodiulose (8). 7 and 8 are the intermediates that ultimately lead to the small-molecule aroma, flavor, and color compounds that our senses recognize as the products of the Maillard reaction. The exact mechanism by which 7 and 8are converted into these small molecules is still not fully understood. I imagine that ANY number of reasonable or creative electron pushing descriptions have been used. (If you are interested in more of the detail about what we know of these types of reactions, please see the references at the end of the post.)
A schematic of the types of molecules created during the Maillard reaction. The lysine residues in meat products are the primary reactants and yield the aroma/flavor/color products shown at the bottom of the figure in black. Proline residues in cereal products (bread and beer) are the primary reactants and produce the aroma/flavor/color products shown at the bottom of the figure in red.
Before the reaction starts producing 7 and 8 (up to the point when the sugar is attached to a protein through an amino acid), the Maillard reaction isn’t making any molecules beneficial for humans. The protein is actually less nutritious than before the reaction. While our bodies recycle the amino acids that we consume, modified amino acids, like the Amadori product, contain little nutritional value. Because the Amadori product (5) is produced in higher amounts than other molecules, evolutionary arguments would suggest that humans should shy away from foods that have undergone the Maillard reaction. But our personal observations tells us that this is not the case. We recognize and hunger for the aroma/flavor/color molecules that the Maillard reaction produces in relatively low amounts. The simplistic argument is that we have developed the ability to sense these molecules in cooked food because cooking kills bacteria. And food with less bacteria is less likely to make us ill. A more developed and engaging set of arguments is laid out in Richard Wrangham’s book, Catching Fire: How Cooking Made Us Human.
Unfortunately, the health benefits are not so straight forward. Some of the molecules produced in the Maillard reaction are thought to be carcinogenic. Certainly, in charred meat, the black, carbon-dense molecules on the surface of the meat are thought to be carcinogenic. But other molecules, like acrylamide are formed during the course of the Maillard reaction are considered potential carcinogens. And, going further, these facts lead us to question to what extent cooking increases the health benefits of our food. While cooking food increases our life-span by killing off harmful bacteria before we ingest them, it also can hasten the onset of cancer as we age. How do we balance this information?
I, for one, plan on: refining my abilities in organic synthesis, trusting my analytical capabilities, and following where evolution has led me.
Translation: I’m going to keep searing my steaks, finding the right combination of temperature and trust my instincts telling me that what I am tasting IS good.
-mrh
Further reading on the Maillard reaction:
1. Food Chemistry by Belitz, Grosch, and Schieberle, 4th edition, Springer, 2009.
2. New aspects of the Maillard reaction in foods and in the human body Ledl and Schleicher Angewandte Chemie1990, 29(6), 565-594.
3. On Food and Cooking by Harold McGee, Scribner, 2004.