Something really weird happens between the time you put your dinner in the fridge and the time you heat it up the next day. Once heavenly fried chicken deflates into a soggy mess; perfect salad wilts; even basic pasta turns into dense sludge.
This kind of wtf moment happens to us a lot. So we started to wonder, fork hovering over a lackluster Tupperware full o' gunk: What happens to leftovers in that dark, cold box? Demons? In a manner of speaking, yes.
The first bit of devilry is a crazy fact: Your food is in a constant state of chemical change. The reactions that cook your food do not stop after the chicken is pulled from the oven. There is still a lot of stuff going on. When you cook meat, for instance, twisted or clumped proteins unravel. During this forced relaxation, the proteins are more open to the chemical reactions that cause them to break down. The process also causes the proteins to release iron, which behaves like a little magnet, explains Chris Loss, director of the department of menu R&D at the Culinary Institute of America. "With its positive charge, the iron goes around breaking down other nutrients in the food." When lipids interact with the released iron, for instance, the taste turns cardboard-y.
Water also contributes to the ickiness of certain types of leftovers. Think about that fried chicken for a moment: "Deep-fried foods are only good right from the deep fryer because once the moisture from the inside migrates to outside, you lose the contrast of textures," explains Loss. The redistribution is just a consequence of time, and it happens with most everything. It also waterlogs pasta and soggifies salad.
Enzymes are the third evil force responsible for our ever-changing foods. These little molecular machines, as Loss calls them, are reaction pushers, and they come in lots of different varieties. Lipases, for instance, act on lipids, to turn fats rancid. And proteinaces can cause a bitter flavor after they've interacted with a protein. And you know how your lettuce gets slimy after a while? Enzymes are behind that, too.
Now leftovers are not always a bad thing. About a week and a half from now, the whole nation is going to be stuffing themselves with leftover turkey sandwiches. The reason? For all the gross things that can happen, there's also a bunch of taste-improving reactions that take place as well.
The first reaction is your own. Turns out, "your individual psychology plays more of a role than the actual chemicals in the turkey itself," says Loss. When you grow up eating turkey sandwiches after Thanksgiving, that three-day old taste becomes what you think of as good.
But the chemicals work some delicious magic, too. The flavor called umami, which you may know if you've watched enough Top Chef, is used to describe a food's savory flavor. And sometimes when food ages, this umami flavor will appear. It happens because, over time, proteins in food break down into amino acid chunks, and those chunks taste good. "We're hardwired to associate their presence with healthfulness," says Loss.
Speaking of which, after taking one bite, there's no way I'm eating this leftover eggplant.
Rachel Swaby is a freelance writer living in San Francisco. Check her out on Twitter.
Giz Explains is where we break down whatever science or tech questions are scratching at the backs of our noggins. Got questions of your own? Email them to us at firstname.lastname@example.org and we'll see about answering them.