I’m a scientist who loves to cook, and that makes sense if you think about it because science and cooking have a lot of similarities. Both pursuits involve a protocol/recipe, ingredients, measurements and, in most cases, an incubation at a specific temperature for a specified time. If you’ve done everything correctly, you’ve got something really exciting to show for it, whether it is publication-quality data or the perfect lemon chiffon cake. That’s why I snapped up a copy of Harold McGee’s book On Food and Cooking: The Science and Lore of the Kitchen during a recent trip to a used-book store. Granted, the book is a bit old, but like many scientific treatises, age doesn’t make it any less of a classic.
In his book, McGee uses his knowledge of science to explain what is going on in my kitchen at the molecular level. In each chapter, I found examples of observations that I had made while cooking, but until I started reading this book, I had never really thought about them in detail.
For example, we have all probably seen the greenish gray outer layer of an egg yolk in a hard-boiled egg that has been overcooked. What causes that discoloration? Ferrous sulfide (FeS). An egg yolk contains a relatively high level of iron, and ovalbumen in the egg white contains a lot of sulfur, present as sulfur-containing side chains of amino acids. When an egg is heated, sulfur is liberated and reacts with hydrogen in the egg to form hydrogen sulfide (H2S). This gas diffuses throughout the egg, and at the interface between the egg white and yolk, H2S reacts with iron to form FeS, which has a dark color. This discoloration can be minimized by cooking eggs for the necessary amount of time only and plunging the hot eggs into cold water, which cools the egg’s outer layer. If we remember Gay-Lussac’s Law of P1/T1 = P2/T2, we know that as a closed system cools, pressure is reduced. The reduced pressure in the outer layer draws the H2S gas away from the egg yolk. Voila! No more sickly looking, unappetizing hard-boiled eggs.
McGee also explains culinary processes that cannot be observed by the naked eye. He publishes scanning electron micrographs of ripening cheese, complete with the responsible bacteria and mold, and describes how these ripening agents change the chemical composition, with more complex organic molecules being broken down to simpler ones. Lactose is converted to lactic acid and CO2, fats are metabolized into fatty acids, and proteins are broken down to peptides, single amino acids and even NH3. All of these products contribute to the final flavor of the cheese.
He discusses the aging and bleaching processes in flour and the positive effects of aging on the bonding characteristics of gluten, and thus, the elasticity of dough. Historically, freshly milled flour, which has a yellow tint due to a class of carotenoid pigments named xanthophylls, was aged and bleached naturally by oxygen in the air over the course of several weeks or months. Today, chlorine dioxide (ClO2), chlorine (Cl2) and other nasty gasses are used to speed the aging and bleaching processes. Unbleached flour escapes the chlorine treatment but is treated with potassium bromate or iodate to age the flour.
McGee devotes a whole section to a food that I always like to have in my kitchen: chocolate. Chocolate is derived from the fruit of the cocoa tree, which Linnaeus appropriately named Theobroma cacao—Theobroma meaning “food of the gods” in Greek. One of my favorite examples of applied science is the discussion of liquid-center chocolates. Prior to 1924, liquid-center chocolates were made by coating a mold with chocolate, filling it with the liquid, then sealing it with more chocolate. This process was made easier in 1924, when a government chemist, H.S. Paine, identified an enzyme that converts the disaccharide sucrose to glucose and fructose, which are more soluble than the equivalent amount of sucrose. A sucrose-containing filling has a fudge-like consistency and can be dipped in chocolate, avoiding the use of cumbersome molds. By adding Paine’s enzyme to the filling, the candy’s center is converted to a liquid over time through the reaction:
C12H22O11 + H2O → C6H12O6 + C6H12O6
sucrose glucose fructose
Because water is consumed in this reaction, the center has a creamy consistency rather than a watery one.
McGee’s book has more great examples of science in the kitchen, although I list only a handful here. While I’m learning a lot from the book, there is one question that still remains unanswered, a question that my college roommate and I would debate periodically after we had both taken a few advanced chemistry courses: As popcorn pops, is it undergoing a physical change (i.e., as popcorn pops, the volume simply changes) or a chemical change (i.e., as popcorn pops, chemical alterations are made to the proteins and other components)? We agreed that it must be both. Obviously the volume is changing, hence the term “popping”, and there is probably some cooking happening as the kernels are heated.
What are some of your favorite examples of science in the kitchen?
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