Food Science: Expert Q&A
Professor Michael Brenner answers questions on the structure of proteins, how to make eggs stay together in egg drop soup, how fruit ripens, and more. If your question wasn't answered here, try watching the NOVA scienceNOW program, "Can I Eat That?"
Q: Many people asked questions here about the book that was mentioned in the "Can I Eat That?" program. That book is "Modernist Cuisine at Home," by Nathan Myhrvold. Our expert Professor Brenner also had some reading recommendations. NOVA, Boston, MA
Michael Brenner: I'd like to start by recommending two wonderful books, which contain the answers to many, many questions about science and cooking. The first is Harold McGee's classic "On Food and Cooking." This book is beautifully written, and contains discussions of many of the scientific questions you might ask about food. The second book, "The Science of Good Cooking," by America's Test Kitchen, was just published in October. This book contains highly readable scientific explanations and results of experiments interspersed with recipes illustrating the basic principles. I've tried several of the recipes and they are quite tasty! Several others we used as illustrations of scientific principles in our Harvard Science and Cooking class.
Q: Could you explain the changes in the structures of meat proteins when different temperature and time are applied (e.g. sous vide steak vs. slow braised beef)? And, how that is reflected on texture? Many thanks! Terry Huang, Manchester, UK
Brenner: There are critical temperatures that must be reached for the structure of meat to achieve a given texture. For example, for meat to be cooked rare, usually you want to hit about 55°C (120°F); for meat to be cooked medium, you need 60°C (140°F); well done meat is usually about 70°C (160°F). The structure of the meat is very different depending on the temperature you cook it at because of transformations among the proteins in the meat. Sous vide cooking involves placing meat in a plastic bag and then immersing it in a constant temperature water bath at the target temperature. In this way, the entire inside of the meat is cooked to the target temperature that you want, and the texture is uniform and consistent.
If this were all there was to cooking meat it would be easy. Unfortunately it is necessary to also do one other thing: you know from your experience that for a steak to taste good, it must be brown on the outside. This will not happen if you just dunk the steak in a pot of boiling water (or a sous vide bath)! Indeed, the browning reactions require a temperature that is much higher, above 120°C (250°F). This is why that after a sous vide bath, it is necessary to sear the outside of the meat (or fish), in order to brown it and release the flavor compounds. If you are braising beef or cooking it in a different way, you need the temperature of the stove or oven to be above the temperature for browning. Otherwise the meat won't brown! But since this temperature is higher than the temperatures I've listed above for getting the texture of the inside of the steak right, there is a real danger of overcooking the inside when getting the outside to brown appropriately!
An interesting illustration of the challenges is given in this video from Heston Blumenthal, who shows you how he cooks a steak.
Q: When I try to make egg drop soup, sometimes the egg stays together in that nice swirly film, and sometimes it totally separates out into little granules. Any idea why, or how to make it right every time? Caroline, MA
Brenner: Cooking an egg is also much more difficult than it seems at first sight. The critical temperature for the proteins in an egg to unfold and coagulate is around 60-70°C, much below the boiling point of water. The different components of egg—the yolk and the white—cook at different temperatures. When we cook a hard-boiled egg in boiling water we lose these distinctions because the temperature is so much higher than the critical temperatures.
On the other hand, egg drop soup is interesting because essentially you are trying to cook the egg, outside of the yolk, directly in the boiling water. The eggs are typically beaten so that the yolk and egg white are mixed. Depending on how long it has been when you take it out of the refrigerator, the egg is potentially much colder than even room temperature, and much, much colder than the boiling water. What happens is complex: the heat makes the egg want to solidify, but the fact that the egg is so much colder than the water cools down the water, and can cause rapid fluid flows that can break down the egg into pieces.
My advice would be to try the following experiment: when you make the soup, warm the eggs up before you add them to the boiling water—not so hot that the eggs "set"—but hot enough that they don't substantially cool down the boiling liquid. Then pour them in and stir carefully. I would think that if the original temperature of the egg were reasonably close to the temperature where the egg "sets" (around 60°C) the outside will solidify and it will stay together.
Having said this, I must admit that I have not tried this experiment! It sounds like an interesting thing to attempt. I think I will try to convince some of the students in our class to study this recipe—it is very interesting.
Q: Why does food ripen, how can I slow it down, and does it affect nutritional values? tommy c., Long Island, NY
Brenner: There is a great discussion of this in On Food and Cooking, from which I learned enough to answer this question. Ripening is caused by proteins in the fruit and the reactions that they cause. The primary cause of ripening in many fruits is ethylene, which happens to be a gas. This is why fruit always ripens faster if it is near other fruit in a later stage of ripening—or rotting. The ripening fruit gives off the gas and accelerates the process in the other fruit. (Putting ripening fruit in a closed container so the ethylene can't escape accelerates the ripening even further!) Fruits whose ripening is triggered by ethylene include pears, tomatoes, bananas, etcetera.
There are other types of fruits such as melons and berries that don't ripen on their own. It depends more on how much they ripen when they are still on the plant. This is why picking melons at the grocery store is such a difficult business. If it were picked at the wrong time, there is basically nothing you can do—or at least I don't know of anything you can do—to fix it.
There is one way to slow down ripening that we all use constantly: refrigeration. For all fruits, ripening is a chemical reaction, and it is a reaction that slows down this rate at lower temperatures. So the colder the better! Of course you can't make it too cold because then the fruit will freeze and there will be damage from the solidification and melting of the water.
A nice experiment to try at home is the following: take some bananas, one batch which is basically ripe and one that is still green. Put a ripe banana in a closed bag with a green one and place another green one in a bag on its own. You should see that the green one together with the ripe one ripens much faster—this is enough to deduce that there is some ripening agent from the first that affects the second—a scientific discovery in your kitchen!
Q: Every time I whip up egg whites to use in a batter, my cake doesn't rise. Is there a scientific explanation as to why egg whites are so delicate and hard to use? Also, any tips? Chris, NJ
Brenner: As I mentioned above in the discussion of egg drop soup, the chemical reactions that cause egg to solidify are complex. There are actually several different protein components in egg white, each of which solidifies (forms a gel) at different temperatures.
When you use egg in a batter the situation is even more complicated: now the egg proteins are mixed with the elements of the batter (flour, sugar, etc.)–and this affects the temperatures when the solidify.
I'm not completely sure why you are seeing a difference in the rising of your cake when you whip the egg—but here is a guess: when you whip the egg, you introduce air bubbles—so now what you are doing is mixing egg with air into the batter. It is possible that the air bubbles impede the expansion of the cake. I don't see why this would happen off hand, but the process is complex and it is possible. I might also give this question to our undergraduate students—by systematically changing the whipped egg to non-whipped and examining under a microscope what is happening as the cake is baking, it is possible to figure this out!