It's a phrase you hear all the time: "This [stew] [soup] [long-cooked-thing-X] will be even better the next day!" It's an appealing prospect. Most foods are best when they're at their freshest, but a dish you can make in one large batch that doesn't just hold up, but actually improves with time? That's a make-on-Sunday, eat-all-week recipe right there.
But is it true? Does stew really taste better when you reheat it the next day, or even days later? I did some testing to find out.
Long Story Short: Don't Worry About It
There's a bit of an intro before I get to the results, so I'll give you the quick version right here: Yes, there are some minor differences in flavor with soups and stews that have been allowed to rest overnight or longer, though the differences are subtle and difficult to tease apart. It's nothing worth altering your cooking schedule for.
Chili and other spicy, acidic dishes are the big exception: With time, their flavor becomes muted, losing brightness. But, again, if you were planning on making that big batch of chili on Sunday to eat through the week, that loss in flavor is nothing a little dash of hot sauce can't solve on Wednesday.
It's a straightforward question, but turns out it's not so straightforward to test. Sure, you can make a single batch of stew, pull it out of the fridge each day, reheat a portion, and taste it, but without a side-by-side comparison, it's impossible to tell whether those changes you think you're tasting really exist, or whether they're just in your mind. And if there are real changes, are they caused by the repeated cooling and reheating, or is time a factor?
Daniel and I put our heads together and came up with a series of experiments to try to really suss out what was going on.
The Effects of Cooling and Reheating
The first test was designed to see whether or not chilling and reheating has a major effect on flavor. I made a batch of my All-American Beef Stew. After completing it, I transferred a single portion to a bowl set in an ice bath and stirred it until it was very cold. The rest of the stew I left in the Dutch oven on the stovetop, where it stayed well above 160°F the entire time. I then reheated the chilled stew to a simmer, while also bringing the large pot of stew back to a simmer. (I figured that if chilling had any effect, stew chilled to near-freezing should taste different from stew that had only slightly cooled.) Tasted side by side, the two batches were indistinguishable.
In order to make sure that my findings would apply in a wide variety of situations, I repeated the test with two more recipes: my Texas Chile con Carne (to see how chilling and reheating might affect heat and brightness) and my 15-Minute Creamy Tomato Soup (a vegan dish that is fast and simple to make). The results were the same.
So reheating is not what causes changes in soups and stews, if those changes exist.
The Effects of Aging
Testing the effects of aging was a little more difficult. My initial thought was to simply make multiple batches of soup and stews. I'd make one batch on Monday, store it in the fridge, then make a second batch on Thursday and serve it alongside Monday's batch. The problem, of course, is that the stew I'm making on Thursday is not the same stew I made on Monday. Even if I carefully weigh everything out and use a timer to ensure that the cooking times are identical, there are inherent differences between batches of stew. As hard as the National Cattlemen's Association may try to deliver a uniform product, each steer is still a unique beast with slightly different flavors and textures. The onion I use to cook on Thursday is three days older than the onion I used on Monday. The kitchen might be a little warmer or cooler. You get the picture.
I considered a second option: make a single batch of stew on Monday, freeze half of it, then thaw that half on Thursday and taste them side by side. There are problems here, too: Freezing can alter the texture of meat and vegetables, and it can also break emulsions, turning a smooth, luscious sauce grainy or greasy. This method also presupposes that whatever may be happening to stew as it ages in the fridge completely halts in the freezer.
Neither method is perfect, but between the two of them, I figured I could get some answers. After a single round of testing, I had no real answers. It was very difficult to tell any difference. A couple of subsequent tests, with even more careful control over the cooking process, revealed some minor differences between the fresh and aged stews and soup.
With the stew, the beef itself tasted nearly identical. The carrots, potatoes, and onions did seem to take on a little more flavor from the broth and become a little sweeter, but it was a subtle difference. I'd also be hard-pressed to tell you if it was necessarily better. With the tomato soup, the aged soup again tasted a little bit sweeter, but it had lost some of the fresh brightness of a just-made pot. Was one better than the other? I'd give a very slight edge to the older batch.
The only case in which there was a noticeable difference was with the chili, where the older batches were distinctly more rounded and mellow in flavor. This isn't a good thing in chili, where I expect brightness and heat to stand up to the roasted flavor of the dried chilies and the richness of the thick stew.
The Causes of Change
Okay, so there are some minor differences. What's causing them? A stew sitting in the fridge might look stagnant, but on a molecular level, there is plenty of stuff going on in that pot, and I'm not talking about something abstract, like "marrying of flavors." According to Cook's Illustrated's food scientist, Guy Crosby, there are measurable changes in sweetness as complex carbohydrates (such as fructose from vegetables or lactose from dairy) and starches break down into sweeter-tasting simple sugars. Just as resting cookie dough, bread dough, or pizza dough overnight allows large proteins to break down into smaller chains, the same things happen in stews and soups.
This information correlated with my testing results: slightly sweeter vegetables in the beef stew, and a milder flavor in the chili. (Extra sweetness can dampen the impact of spicy and acidic flavors.)
I see my relationship with a pot of stew as being sort of like my marriage. My wife seems to get more beautiful as time passes. Just like a stew, she gets better with age. Yet if I were to somehow time-travel with her to, say, five years ago today, and if I compared my current wife with my five-years-ago wife, would I honestly be able to say that she's objectively more attractive now than before? I couldn't tell you.* Attraction is based on more than simple physical traits. Every interaction I've had with her over the last five years has altered my perception of her in ways that can't be un-altered.
* And frankly, for the sake of my marriage, I probably wouldn't even if I could.
Likewise, "taste" is not solely something we sense on the tongue or in the nose. It's a complex interaction of outside sensory stimuli and our own internal memories and perceptions. Simply thinking something should taste better can actually make it taste better. I'm not suggesting you get romantically involved with your beef stew, but you know what I mean.
So, after all this testing and tasting, I came to one conclusion: It doesn't matter. Without the context of a side-by-side taste test, nobody is ever going to tell you, "Hey, this stew is only three hours old. I prefer mine three days old. Take it back and age it for me, please." Even within the context of a tasting, you should consider whether or not the person who says that is someone you want to break stew with anyway.
Fact is, no matter what long-simmered stew or soup you're making, if it tastes good on the first day, it's gonna taste good on the second and third days as well. Conversely, if it tastes good on the third day, it probably would have tasted great on that first day, too. Make your soup and stew whenever it's convenient and, similarly, eat it whenever you want to eat it. The minor differences in eating it fresh versus aged are not worth fretting over.