By Robert L. Wolke
Wednesday, September 13, 2006
Why does a pot roast brown in a crockpot? It seems to be steaming in the pot, which one would think would create a blanched and pale cut of meat, but it comes out as browned as if we had seared it on the stovetop (not that I'm complaining).
As many people do, you're confusing two kinds of browning.
All red meat will turn brown when cooked by any method. As its temperature rises above 170 degrees, its red pigment, myoglobin, changes into metmyoglobin, which is brown. And that's all there is to that.
But the browning obtained by searing meat is created by a different high-temperature process called the Maillard reaction. And thanks for giving me the opportunity to shoot down a widespread misunderstanding about that.
When we sear a steak, for example, certain parts of the protein molecules (the so-called amino parts) interact with so-called carbonyl groups, which are parts of sugar molecules. When that happens, dozens of complex chemical reactions take place, producing a hodgepodge of dark brown, very flavorful compounds. That's why it's always good to sear meat before braising or slow-cooking it.
Now, did I say the Maillard browning reaction involves parts of sugar molecules?
Yes, I did.
Does that mean there are sugars in the meat?
Then what the. . . .
Easy, now. Let me explain.
A carbonyl group is indeed a certain grouping of atoms found in sugar molecules. But it also is found in many other kinds of molecules, including the meat's very own fats and proteins. The Maillard browning process can use the carbonyl groups that are inherent in the meat; it does not require sugars. And that's fortunate, because there are no sugars in meat, beyond perhaps traces of glycogen, a source of glucose that fades away following the animal's death.
This may be more chemistry than you bargained for, but I needed a soapbox from which to rail against a pervasive flaw in food literature. Book after book casually informs us that seared meats brown by the Maillard reaction between proteins and sugars. But they are mysteriously silent on the question of what the heck sugars are doing in meats in the first place. That's because -- in the vernacular -- there ain't no sugars in meat.
Why do so many otherwise knowledgeable food writers skip over that gap in reasoning? I think there are two sources of confusion.
One is that when pure sugar is heated, it turns brown by a process called caramelization. That leads suggestible types to call all browning caramelization. (Moreover, it's a more impressive word than browning.) But meats don't caramelize; only sugar does. Once a person has crossed that mental fault line, there is sugar on the brain, even when the subject is meats.
The second source of confusion is that in many other types of foods -- such as toasted bread and roasted coffee and cocoa beans -- the Maillard reaction does involve sugar. Those foods contain starches, which are made up of sugars.
All food writers, please take note.
I'm a food and nutrition educator, and I often answer cooking questions from visitors to our Web site, but I cannot find a satisfactory answer to this one. Years ago I read somewhere that adding lemon juice when making a stock with bones will pull the calcium out of the bones and into the stock. Is that true? I know acid will cause deterioration of tooth enamel and probably would do the same to bone, but do you have any information to add?
Yes, I do. Come with me into the lab.
As you have surmised, the mineral part of bones and tooth enamel, a form of calcium phosphate called hydroxyapatite, does dissolve in acids. But to what extent?
I located a research paper by M. Eisenburger and M. Addy (Clinical Oral Investigations, 2001, Vol. 5), who soaked dental enamel in various concentrations of citric acid and measured the depth of the erosion caused by the acid. I then made an acidified mock stock by adding one fluid ounce (one lemon's worth) of lemon juice to three quarts of water and measured its acidity.
I'll spare you the details, but using the Eisenburger-Addy data, I was able to estimate the amount of calcium that would be extracted from a reasonable number of bones immersed for four hours in a stock of that acidity. It turned out to be roughly 500 milligrams. So to get your recommended daily allowance of 1,000 milligrams of calcium by consuming acidified stock, you'd have to drink 1 1/2 gallons of it.
Don't bother adding that lemon juice.
Would you please address the subject of preheating? I know there are times when it makes sense to preheat ovens in baking, roasting, etc. But I have seen plenty of recipes and cooking shows where they instruct you to preheat a broiler or grill for 15 minutes or more. To me, that seems unnecessary. Broiling and grilling are direct-heat methods. Once the broiler heating element, or the flame and cooking grid, are hot, waiting any longer won't make any difference, since the ambient air isn't doing the cooking.
You're right. The air is not doing the cooking. In an oven, the hot walls and hot air surround the food, whereas a broiler or grill has but a single heat source, and the cooking is done almost entirely by infrared radiation coming from that source, whether it's a gas flame, an electric heating element or charcoal.
The main difference between broiling and grilling is that in grilling, the heat source is below the food, whereas in broiling it is above the food. That's of little consequence, because infrared radiation doesn't know up from down. Like light, it isn't affected by gravity. (Okay, Professor Einstein, unless the gravitational force is extremely high, as from a nearby star.)
And there is another difference between grills and broilers. If you don't preheat a grill, the grid won't get hot enough to brand those mouthwatering grill marks into the meat. In broiling, there is no grid, so there is no need to preheat. In restaurant broilers, on the other hand, the food often rests on a grid instead of a broiler pan, so the cook keeps the broiler hot in order to burn those marks into the meat.
Why do many home broiling recipes specify preheating? Sorry, but I'm as mystified by that as you are.
Robert L. Wolke (http://www.robertwolke.com) is professor emeritus of chemistry at the University of Pittsburgh. His latest book is "What Einstein Told His Cook 2, the Sequel: Further Adventures in Kitchen Science" (W.W. Norton, 2005). He can be reached email@example.com.