Guy Crosby, Ph.D., joined us for a conversation on how understanding the science of food can make you a better home cook. He is the science editor at America’s Test Kitchen; an adjunct professor at Harvard University, T.H. Chan School of Public Health; and author of the New York Times best-seller “The Science of Good Cooking” and “Cook’s Science.” Crosby has been dedicated to food science from a young age.
This interview has been lightly edited for length and clarity.
EWG: Now that a lot of people are trying to avoid gluten for health reasons, celiac disease or other sensitivities, are there types of breads or other foods that might contain more or less gluten?
CROSBY: Yes, you could go for breads that have other types of flour in them – rye bread, pumpernickel bread and those types. They’re going to reduce the amount of wheat flour in a bread. They are certainly not going to eliminate the gluten entirely, and if people truly have an allergic reaction to gluten or sensitivity to celiac disease it certainly doesn’t take much in the way of gluten to trigger that reaction.
EWG: Are there differences in a how highly refined flour behaves compared to whole-wheat flour?
CROSBY: When you’re making whole-wheat bread, you’re using whole-wheat flour, which contains the bran and the germ in addition to the endosperm where all the starch is. The starch is tied up with some of the other components – like the bran, the fiber and the germ, and there are other factors going on in there – but your digestive system is going to break down the components more slowly from a whole-wheat bread or flour. The amount of glucose that goes into your body from the starch is going to be more gradual, so you’re not going to have a sudden burst of insulin to deal with that glucose. It’s a more gentle effect on your digestive system and your body when you’re using a whole-wheat flour as opposed to a refined flour, where the digestion is much faster of the starch to glucose. With refined flour you have a bigger, more sudden impact on insulin and on dealing with the surge of glucose in the blood, which can have impacts on people that might have a greater sensitivity to glucose and insulin than other people. Plus, of course, with whole-wheat flour you’ve got the advantages of the fiber that’s present for digestion and activity by the bacteria that are in the large intestine.
EWG: Would that also play into what is called a glycemic index?
CROSBY: Right, the glycemic index or glycemic load is the way you measure the effect on the body of the starch or other sources of sugars that are in the diet. The glycemic index is much stronger and more sudden if you’re eating a refined starch than if you’re eating a whole-grain product. It’s definitely tied into the body’s response to glucose insulin secretion, which is all part of the glycemic index.
EWG: I’ve been interested in how pH affects food. What is an acid doing to a food when you’re cooking with it? For instance, does it change the flavorful browning that occurs when you sear or bake foods through the Maillard reaction – the reaction of proteins and sugars? And are there certain flavors or dishes where you’d want to incorporate an acid?
CROSBY: There are a variety of things going on. Some people like to add what you call an acid – like lemon juice, vinegar, things like that – because it contrasts. The tartness of the acid contrasts to another savory flavor in the dish – balancing tartness with savory. The other effect is, as you mentioned, the pH. When you’re adding acid you’re lowering the pH, so you’re going to change the chemistry of the flavor development.
The example, you’re familiar with the Maillard reaction that produces many different flavors when you’re cooking food. That Maillard reaction speeds up 500 times at pH 9.0, which is a moderately alkaline system, versus pH 5.0, which is weakly acidic. So, when you go from a weak acid to a moderately alkaline environment the speed of that reaction increases 500-fold. You’re going to create not only more flavor, but also additional flavor molecules as you make it more alkaline, and you’ll reduce them as you make it more acidic.
EWG: Would baking soda be an example of how someone could make a more alkaline cooking environment?
CROSBY: Right. You can add baking soda. There are examples in baking cookies that tie this all together. If you add baking soda and make the dough a little more alkaline that weakens gluten because the strength of gluten is optimum between pH 5.0 and 6.0. So if you add a little baking soda you reduce the formation of gluten – you weaken it – and when you make cookies with a little tiny bit of baking soda they spread more because they don’t have so much gluten in there to hold their shape. Plus, they get darker because you’re getting a much faster Maillard reaction taking place, which not only leads to the flavor, but leads to the brown coloration as well.
Baking soda is one way to change the chemistry by changing the pH and it only takes tiny amounts see these effects. We find that it’s also true that you can marinate for relatively short periods of time meat, like pork that can be lean and tough, and its pH is typically around 5.5. If you marinate with just a very small amount of baking soda and water for 20 to 30 minutes and then rinse it off, you can alter the pH just enough that it becomes a little more tender and holds on to more moisture – it increases the water holding capacity as you raise the pH. So, you can also see those types of effects on meat and in a lot of systems where playing with the pH a little bit can alter things a fair amount – favorably or unfavorably.
EWG: Can you also encourage the Maillard reaction in vegetables? When roasting them, for example?
CROSBY: Yes, for sure. In fact, I did a webinar the other day where one of the things was roasting cruciferous vegetables. And although a lot of people throw the term out there when vegetables darken that they’re caramelizing – that’s true to a certain extent – but caramelization refers to only the reaction of sugars by themselves. But the Maillard reaction creates most of the color when you’re roasting because you’ve got proteins in plant foods and you’ve got sugars, and they react at an even lower temperature. The Maillard reaction produces a lot of the browning and a lot of the flavor. You can create a significant amount of flavor by roasting these vegetables and you’re doing it in a dry heat, which means you’re not leaching out all the nutrients as you would if you where boiling them. And in the dry heat you get pretty different conditions where you get the Maillard reaction taking place above 300 degrees pretty rapidly.
If you steam vegetables, which is a way of avoiding leaching a lot of nutrients, you’re getting completely different chemistry going on. If you steam Brussels sprouts you get a very different flavor compared to if you’re roasting them. Under the dry conditions you get a lot of the Maillard reaction and if you’re steaming them you don’t. You’ll notice a lot more of the sulfur notes of the flavor compounds in something like Brussels sprouts.
EWG: Yes, I’ve become a huge fan of roasting vegetables. And there are a lot of different cooking methods that transfer heat differently. How is, say cooking on the stovetop versus the conduction of an oven, going to impact how food cooks?
CROSBY: Certainly metal is a far greater conductor than the hot air in an oven. If you cut Brussels sprouts in half and put them in a skillet they’re going to blacken very quickly because the heat builds up and then the cast iron or steel transfers it to the Brussels sprouts well. They can burn and blacken very quickly. You can put them instead in the oven where they’re going to roast a lot slower. Even if you crank the oven up to high temperatures they’re still going to cook a lot slower because air is such a poor conductor of heat. So, you have more control over them and you’re not blasting them with such heat that you’re going to scorch part of it and then leave the rest of it raw almost. You can get a better balance of texture and browning of the vegetable if you roast it in an oven versus heating it up very high in a skillet. You may get more control and a better balance, nuance of flavor when you do it in the oven. And it doesn’t take long. I’ve done small Brussels sprouts in a 400 degree oven and they’re done in 18 minutes or so. I was promoting that for Thanksgiving. When you pull your turkey out and it’s resting you can roast a bunch of vegetables in the oven that’s still on and still hot. When cooking in a skillet, if you add oil to the skillet that’s going to help even out the distribution of the heat into the vegetables and they won’t burn as much. That’s better than just dry heating them in a skillet.
EWG: Speaking of oil, different oils have different properties – smoking points, for example. Are there any rules of thumb about which oil may work best in certain situations?
CROSBY: They do vary a little. Most of the oils are in the 400 degree Fahrenheit range. There are differences, but in some ways they’re not extreme differences. It’s good advice not to heat something up so hot that it’s smoking significantly. You don’t want to go overboard like that. They can afford to do that with a big stove, and hoods and everything in a restaurant. They’ll heat things up very quickly and very high.
But I think in the home kitchen it’s better to keep things under more control so you don’t get as much spattering, you don’t have the risk of anything igniting. Any of those vegetable oils – canola oil, soybean oil, corn oil, safflower oil – they’re all sort of in that range. Even olive oil, if you go to virgin olive oil, its smoke point is not so low that it can’t easily be used for sauteing. I use it all the time for sauteing and I’ve never had a problem with it smoking, burning, overreacting or anything.
EWG: Reading some of your work, I’ve learned frying at lower temperatures decreases the amount of oil absorption due to moisture loss.
CROSBY: It takes longer to fry at lower temperatures – you might think that gives more time for fat to be absorbed, but it’s just the reverse. You drive off less moisture at lower frying temperatures, so you absorb less fat into the food. And the other thing is oils contain less heat than a liquid like wine or water at the same temperature.
You can poach, the best example is any kind of fish, in oil and you’re going to be subjecting it to half the amount of heat as you would if you were poaching it in wine. The fact is fish is about 75 percent water, you don’t absorb any of the oil or almost none of the oil into the fish when it’s poached in oil. So, you can cook it more gently, so you’re not overdoing it, tightening up the muscle fibers, toughening it, while at the same time you’re not going to end up with a fish that’s full of fat, which most people are afraid would happen if you poach in oil. It doesn’t happen because they don’t mix, oil and water. And the water keeps the oil out of the fish. It’s a more gentle way of cooking things that need gentle heat, like fish.
EWG: Is it possible when cooking with oils that unsaturated fats can convert to trans fats?
CROSBY: First, it’s important to know that regardless of what labels say on fats and oils, they all contain very small amounts of trans fats. They only have to declare things that are greater than half a gram per serving and a serving of oil is only one tablespoon or 14 grams. Three percent or less of trans fat in an oil is essentially zero according to the label, so they can say zero grams of trans fat. They almost all contain a little bit due to the way they’re processed. I’d say 1 to 3 percent. As you heat them and use them over extended periods of time, like for a week – frying studies done seven to eight hours a day for five days to seven days – you will increase the amount of trans fat. It doesn’t go up dramatically, but if it started off at 1 or 2 percent you could go up to 4 or 4.5 percent of trans fat by the time you’re finished using the oil.
That situation occurs in lots of heavy duty frying as they do at fast food restaurants. No one really knows if that level of trans fat is an issue because it’s not like old margarines where they had 40 and 50 percent trans fats and it had a clear effect on cardiovascular disease. At the very low levels it’s not clear what effect it has and how much fat you’d have to be using to see an effect. It’s still controversial and undecided.
EWG: It sounds like if you’re just sauteing dishes it might not be that great of a concern.
CROSBY: Oh no, in a few minutes you’re not going to be forming trans fats. It takes maybe 375 degree temperatures for seven hours a day for a week, and then you’ll double the amount of trans fats. You can use olive oil – it’s one of the lowest in natural trans fats, maybe 0.5 percent, compared to some of the others like soybean or vegetable oils that are refined, bleached and deodorized by being heated to very high temperatures under a vacuum because American consumers don’t like smelly oils. The process ends up forming a little bit of trans fats, but not much – 1 or 2 percent.
EWG: Cooking can change flavors of foods, such as onions, and there is usually an order to cooking a dish in terms of when certain foods are added. Are there any guidelines to help judge when certain foods should be added in a cooking process, say for example making a soup or a stir-fry?
CROSBY: Onions are a case, especially if you chop them up, where the kind of chemistry that goes on to develop a lot of these very savory, meaty flavors comes on only with long, slow cooking. So, onions benefit from going in very early into a stock or a dish, and allowing them to cook for a fair amount of time to develop a lot of additional savory flavors.
There are other things, some herbs and spices, where they’re quite sensitive to heat. You want to throw them in at the very end because the heat will rapidly break down or vaporize the flavor molecules. So, a lot of seasonings should go in at the end as opposed to at the beginning. It all depends on the flavor chemistry that’s going on and the stability of the molecules that are being created in terms of whether you want to heat them for a long time or heat them very briefly, and when you would add them.
EWG: You’ve been teaching for a while. Are there certain topics or ideas that you feel really engaged your students and got them excited about the process of cooking?
CROSBY: Some people just love cooking, and want to do and get enjoyment out of it, and want to learn more about it. Some people really do enjoy digging into the science of it. I encounter all the time teachers that are interested in using cooking as a way of teaching science because it’s a wonderful way. Younger people love the idea of food and cooking, and learning science through that medium as opposed to just dry science by itself.
There are so many different aspects that I think it’s increasing people’s appreciation both for cooking and for the science behind the cooking. When they see a connection between the two it turns on some lights and they get more enjoyment out of it whether they do it in the kitchen or just read about it. I think people are appreciating cooking more and seeing that there is a lot of science in cooking.
EWG: True, I’ve been learning more about emulsions and had no idea how much they can affect flavor.
CROSBY: One thing to keep in mind, when you have an emulsion, if it’s water and oil, you can have oil droplets disperse in water or you can have the other way around, water droplets dispersed in oil. They both exist. The droplet is called the dispersed phase and the other is called the continuous phase. The thing is your mouth only detects the continuous phase. So, if you have an emulsion where you have oil droplets dispersed in water or vinegar, in your mouth it’s going to feel watery. If instead you have an emulsion where you have vinegar droplets dispersed in oil, it’s going to feel very oily and slippery in your mouth. You get a very different feeling and sensation, and also flavor delivery depending on which type of emulsion you have.
EWG: Are there ways – and I guess the examples would be contrasting a vinaigrette and mayonnaise – where you get a water-in-oil versus an oil-in-water emulsion?
CROSBY: If you make a classic vinaigrette of three parts oil to one part vinegar, and you disperse it and the oil is the continuous phase, it’s going to feel very oily and greasy; whereas if you add a little bit of mustard or mayonnaise, you add some emulsifier in there, you’ll switch it up the other way. The oil droplets disperse in water and it doesn’t feel greasy. In fact, even though you think about mayonnaise as 80 percent oil, the emulsifiers that are in there from the egg make it so that the oil is actually dispersed in the small amount of vinegar. The continuous phase is the vinegar. Mayonnaise doesn’t taste anywhere near as greasy in your mouth as butter, which is the reverse. It’s the same ratio of fat to water, but the butter is very greasy because the fat is the continuous phase. If you taste them side by side, you’ll see that the butter feels much greasier in your mouth than the mayonnaise does.
EWG: So the key is whether you add some emulsifying agent?
CROSBY: Correct, yes. The emulsifying agent is what determines whether you have water as the continuous phase or oil as the continuous phase. They call that an HLB, a hydrophobic-lipophobic balance. You get emulsifiers that span the range that will stabilize one type of emulsion versus a different emulsifier will stabilize a different type of emulsion.
EWG: You mentioned mustard and eggs. Are there other types of emulsifiers that people will commonly use around the kitchen?
CROSBY: Those are the most common in the kitchen. Eggs, mayonnaise, which contains eggs, mustard – those are the classic ones. You can make emulsions by using starch or flour, and using it as a thickening agent, where the thickening agent helps to stabilize the emulsion and prevent it from separating. When you make a gravy, for example, with flour or a rue, it’s really an emulsion of the fat and water. The thickening agent, once you beat it up and make the emulsion, keeps it from separating. So, they do stabilize emulsions differently from the way an emulsifier does.
EWG: You have a book out, “The Science of Good Cooking.” Do you have other books?
CROSBY: Yes, we have a book just released last month called “Cook’s Science.” The first one delved into the science behind cooking methods; this one goes into the science behind different foods and food ingredients.