There's nothing like the taste and smell of fresh-baked pizza taken right out of the oven. But when you heat up the leftover slices, the flavor often isn't quite the same. You'll find out why in this Science Update.
A mozzarella meltdown. I'm Bob Hirshon and this is Science Update.
A meal of leftover pizza prompted Justin Warner of New York City to call with this Why Is It? Question.
Why does the cheese taste different when you reheat pizza? It always doesn't taste quite right.
We consulted Carol Chen of the University of Wisconsin's Center for Dairy Research. She says you could be reacting to changes in the cheese's texture.
Cheese is mostly made up of water, protein, and fat. And when you put the cheese into the oven—of course, with the other goodies that are on pizzas—there's a lot of moisture loss from that cheese. And so the fact that you're losing moisture makes the other components of the cheese a little bit more concentrated."
And that tends to make the cheese denser and tougher to chew.
What's more, Chen speculates that a lot of tasty flavor compounds get cooked off the pizza the first time, and simply aren't there the next day.
But you know also, it's just the fact that there's less water really changes the whole chemistry of things. And flavor of course is all perception on your tongue, so the amount of water that's in there is important.
If you've got a cheesy science question, call us at 1-800-WHY-ISIT. If we use it on the show, you'll get a free Science Update mug. I'm Bob Hirshon for AAAS, the Science Society.
Making Sense of the Research
Flavor is a complex science. It depends on the interaction of a wide variety of physical and chemical properties in the food you're eating (not to mention the unique sensitivities of your nose and taste buds). You learned in elementary school that if you heat something, its physical and chemical properties may change. That's why cooked food tastes different from raw food, and re-heated food often tastes different from food that's been cooked only once.
In this Science Update, Chen reviews some of the reasons why re-heated pizza cheese may taste different from cheese on a freshly baked pie. One of these is a change in texture: when you heat cheese, you boil off some of the water and fat that separates the strands of protein inside it. What's left is a denser block of protein—more like a firm stick of low-fat mozzarella "string cheese" than a creamy, bubbly pizza topping. These proteins also rearrange themselves in response to heat, which also makes the cheese tougher. Re-heated cheese also tends to crumble into little bits in your mouth instead of forming nice chewy globs, and that difference in texture also affects your eating experience.
Cheese also contains volatile flavor chemicals that contribute to its familiar taste. "Volatile" means they escape easily into the air. If the cheese is freshly cooked, many of these chemicals escape into your nose through the back of your throat, which enhances the flavor experience. (In fact, many properties of what we call "flavor" are really smells, not tastes.) But if you let the pizza sit in the fridge overnight, and then re-heat it the next day, many of those chemicals just aren't there anymore—which can make the cheese taste bland.
Finally, the loss of water and fat can make what flavor chemicals are left harder to taste. Many flavor compounds are either water-soluble or fat-soluble, which means they dissolve in the droplets of water and fat in the cheese. When flavor compounds are water- or fat-soluble, they spread more easily around the mouth and release their vapors more readily, again contributing to the flavor. But if they're trapped in a mesh of solid protein, you might swallow them without even noticing they're there.
Now try and answer these questions:
- What are some physical and chemical properties that contribute to the flavor of pizza cheese?
- How does re-heating affect these properties? How do the changes affect the flavor?
- Can you think of foods that don't taste very different when re-heated? Why do you think that is? In what key ways do these foods differ from pizza cheese?
Dr. Chen works for the Wisconsin Center for Dairy Research.
The University of Guelph's Department of Food Science offers a detailed information page on dairy science and technology.
Making Sense of Taste, from Scientific American, is a detailed article explaining the biology and chemistry of taste sensation.