If you ever find yourself in my kitchen in the middle of the week, there are a few things you are almost always guaranteed to find. There’d be Tupperware scattered across the floor, the work of my busy toddler. Toast crumbs would be littered embarrassingly from one end of the kitchen to the next, a product of a full life with three young kids. My refrigerator would be full, with a pretty even toss-up of uncooked foods for the rest of the week and forgotten leftovers, packed up in guilt after a weeknight dinner but never touched again. Leftovers are funny. Some foods I look forward to reheating and eating again since they only seem to get better with time. Others get shoved in the back of the fridge, where I can hardly look at them in their sad state of mush and mediocre flavors. I’m not typically a picky eater, so I feel guilty that so much gets ignored until it’s time to toss.
Still, there seems to be a definite pattern to leftovers as to what tastes good and what is unbearable on day two. As it turns out, this isn’t just my personal preference. The real reason some leftovers taste so good, and why some taste just plain bad, is all about the chemistry of the flavors.
The Real Reason Some Leftovers Taste So Good
Everyone has their preferences, but there is actually a science to which foods taste better with time. Asking around, I hear a lot of the same opinions. Soups usually taste great on day two, red sauce only gets better with time, and chilis and stews are favorite foods to pack up to be reheated at work the next day. “Soups and chilis work the best the second day,” says Dustin Green, senior executive chef from the Weber Grill Restaurant in Chicago. “The flavors tend to blend. Once they relax and they have time to cool down, the flavors tend to mesh a little bit better.” And when it comes to soups, chilis, and similar foods, it isn’t just the flavors that change—the consistency transforms as well. Taking these foods from hot to cool and then allowing them to sit in the fridge overnight thickens these foods, according to Green. The science of the flavor changes has a lot to do with the fats that are in foods, which are called lipids, says Devin Peterson, PhD, professor and director of the Flavor Research and Education Center at Ohio State University. The lipid molecules in the food are always changing. Typically, time gives foods a less desirable flavor, but in some cases, the effect is much different. [pullquote align=”center”]… the lipids continue to break down the molecules we perceive, and that’s part of the new flavor we experience the next day.[/pullquote] “When you heat them, those lipids form things you smell, and that’s a large part of where the flavors are coming from,” he explains. “That reaction is faster when you do it at cooking temperatures, say in a stew, but it still happens at room temperature and even in the fridge.” The constant changing influences the flavor. In some foods, it isn’t a great result, while it creates a more enjoyable flavor profile in others, according to Peterson. Another factor to consider is that iron causes lipid oxidation in foods. Lipid oxidation is a chemical reaction, and it changes various characteristics of foods. When a food product is high in iron, like turkey, lipid oxidation speeds up and this can influence the flavor, according to Peterson. “By heating it, you kind of allow things to mix more effectively,” he says. “That’s a big reason why, even when you cook a turkey and you eat out of the oven, when you put it in the fridge and eat it the next day, the lipids continue to break down the molecules we perceive, and that’s part of the new flavor we experience the next day.” Lastly, there is another chemical reaction that happens in foods, according to Berkeley Wellness. In some cases, the protein in the food breaks down further, releasing amino acids. Some amino acids enhance the savory taste in foods, while others create new flavors through their interactions with sugar in the foods.
The Reason Some Leftovers Taste So Bad
Some leftovers are better thrown in the trash than reheated the next. Asking various friends and family, there are a lot of different opinions, but I also hear a lot of the same answers. Anything with pasta in it is better consumed right away, rice takes a lot of tender, loving care to bring it back to life on day two, and fried chicken is just awful after a night in the fridge. Negative changes to food take place for a variety of reasons, including the result of the processes mentioned above, which simply have a difference result in different foods. Additionally, there is the issue of temperature and moisture, according to Thomas Bowman, director of product development at Hampton Creek, who offered fried chicken as an example of food that tastes amazing on day one but awful after sitting in the fridge overnight. “What has happened here? This is something that happens to all leftovers, but some foods deal with it a little better than others,” he explains. “It’s called equilibrium relative humidity, or ERH, for short. This is where moisture equals out to be more level with the environment around it.” This is what causes the once-crispy outside of chicken to go soggy overnight, and the juicy meat inside to get tough, according to Bowman. An added component is how fat and collagen change with temperature changes. “Animal fats in general take on a waxy texture when cold, and collagen congeals into a savory, rubbery jello,” he says.
Why Foods Taste Better—or Worse—Cold
Most foods that are meant to be eaten warm only taste good that way. There’s a reason for this, and it isn’t about personal preference. Temperature and flavor go hand in hand, according to Peterson. “The temperature of your food will affect how much flavor reaches your receptors,” he explains. “When it’s colder, you could look at it as the molecules being less active and going to be less received in your mouth.” When you heat that same food up, the flavor changes. Specifically, when you heat food up, more of the flavor ends up in the air around the food, and when you eat it, you receive the flavor to a higher degree. In some cases, like eating cold pizza, this is an enjoyable change, but the majority of leftovers will taste better after a couple minutes in the microwave.
Getting the Most From Your Leftovers
In our family, there are differing opinions on when leftovers should be eaten. My husband, who generally isn’t that into to reheated foods, prefers to toss leftovers after a day or two. If it’s food that keeps well, I tend to keep reheating and eating until they’re a week old. As it turns out, the time to throw out leftovers tends to fall somewhere between the two. Green suggests no more than four days. Bowman, however, is willing to give certain foods until day seven but says it really is dependent on the food. “The length of time you keep your leftovers around also depends a lot on equilibrium relative humidity,” he says, admitting that calculating the spoilage rates of specific foods is a lot of work, suggesting that home chefs simply pay close attention to how their food changes over time. And all foods should be pitched once they hit day seven. “It’s not a perfect science. There are some things that will last longer than and other shorter depending on moisture, preservative acids, and the temperature inside the fridge. Use your best judgement and repurpose those leftovers!” How you store leftovers matters, too. Specifically, the quicker you can cool down your food, the better. Foods need to spend as little time between 125 and 70 degrees, which is a window where growth of microorganisms happens most quickly, according to the Food Safety Extension of The University of Minnesota. Within four hours, hot food needs to drop below 41 degrees, and Green offers a tip for making that happen. “If I was doing it at home, the best way is an ice wand, so I could put that into the batch of what I’m making and put that in the refrigerator,” he says. “Depending on how thick something is, if I need to cool it down, I may need to separate it into smaller batches.”
It’s not all about science.
It’s fascinating to learn that there is a scientific explanation for why some foods taste so good on day two or three, but it isn’t all about science. It may seem obvious, but preference still plays a huge role in taste. It’s the reason I can’t help but indulge in off-brand iced oatmeal cookies, which are objectively not that great but remind me of the endless childhood afternoons I spent snacking at the kitchen table with my three siblings. When it comes to what type of food people prefer and how they prefer it cooked, it is heavily influenced by their past experiences. “I think in the end, preference is largely related to a context,” says Peterson. “So if I’ve grown up a certain way and I’m used to preparing my foods a certain way, that’s usually a part of my preference views. And so a lot of what we like is based on our prior exposure.”