Wait, Nuts and Flour Should Go in the Fridge?

My refrigerator has a chronic real-estate problem. The issue isn’t leftovers, but condiments. Jars and bottles have filled the door and taken over the main shelves. There’s so little room between the chili crisp, maple syrup, oyster sauce, gochujang, spicy mustard, several kinds of hot sauce, and numerous other condiments that I’ve started stacking containers. Squeezing in new items is like simultaneously playing Tetris and Jenga. And it’s all because of three little words on their labels: refrigerate after opening.

But a lot of the time, these instructions seem confusing, if not just unnecessary. Pickles are usually kept cold after opening, but the whole point of pickling is preservation. The same is true of fermented things, such as sauerkraut, kimchi, and certain hot sauces. Ketchup bottles are a fixture of diner counters, and vessels of chili oil and soy sauce sit out on the tables at Chinese restaurants. So why must they take up valuable fridge space at home?

Meanwhile, foods languish in the pantry when they would do better in the fridge. Nuts develop an off taste after a few months; spices fade to dust in roughly the same time span. Recently, a bag of flaxseed I’d bought just a few weeks earlier went rancid and began to smell like paint thinner. A lot of commonly unrefrigerated foods could benefit from cold storage, Kasiviswanathan Muthukumarappan, a refrigeration expert at South Dakota State University, told me. Yet maddeningly, they aren’t labeled as such, while so many shelf-stable foods are refrigerated by default. The conventions for food storage are full of inconsistencies, wasting not only precious refrigerator space, but sometimes also food itself.

Judging by a trip to the grocery store, there are two kinds of foods: fridge foods and pantry foods. Pasta and granola bars, for example, are kept at room temperature, whereas fresh foods such as meat, dairy, and produce are kept cold. These types of highly perishable items are defined by the FDA as “temperature control for safety” foods, and keeping them below 40 degrees Fahrenheit slows the growth of many harmful microbes, which can cause food poisoning. Outside the fridge, pathogenic microbes grow rapidly: According to the U.S. Department of Agriculture, these foods shouldn’t be left unrefrigerated for even just two hours.

But the binary—fridge foods and pantry foods—is too simplistic. Many condiments, for example, exist in a murky middle ground. Some mustards can sit out on a counter, whereas others are prone to mold, Karen Schaich, a food-science professor at Rutgers University, told me. Relishes, which are usually chopped pickled vegetables or fruits, can also develop mold or yeast fermentation if not refrigerated. In part, it comes down to their sugar content: Microbes don’t thrive in acidic conditions, but they generally do like some sugar. A broad rule of thumb is that “extremely tart or sour” condiments are usually safe to leave on the counter, as long as they aren’t also sweet, Schaich said.

Proper food storage just can’t be boiled down to a single question—to chill or not to chill?—because the effects of refrigeration are twofold. Beyond safety, the fridge helps maintain a food’s flavor. It does this in part by slowing the growth of spoilage microbes, which usually aren’t harmful but produce revolting flavors and odors. The fridge also slows natural processes that degrade quality. Once safety is controlled for, “chemistry takes over,” Schaich said, referring to reactions that cause food to develop weird or gross flavors over months or even years.

The big one is oxidation, which is responsible for many foul odors, tastes, and textures in food, such as stale Cheerios and oil that smells like Play-Doh. It’s caused by exposure to oxygen and accelerated by factors including time, moisture, bacteria, light, and, crucially, heat. Refrigeration keeps food tasting fresh by controlling for the latter. That’s why products like Heinz ketchup and Kikkoman soy sauce have labels saying they should be stored in the fridge: Not for safety, but for flavor. Put them in your pantry, and they’re unlikely to make you sick.

When it comes to maintaining flavor, one molecule is more consequential than others. “It’s the fat that matters,” Muthukumarappan said. Fatty foods—certain nuts such as pecans and walnuts, some kinds  of oil—oxidize and go rancid, usually developing sour or bitter flavors and, sometimes, the tangy smell of metal or the waxy one of crayons. It makes sense to refrigerate peanut butter, and nuts in general, Muthukumarappan said. Better yet, store them in the freezer if you plan on keeping them for years. Grains are likewise vulnerable to rancidity: Hemp seeds have a high oil content and can oxidize within months, and so can some types of flour, Schaich said—in particular, whole-grain flours such as rye and spelt. Storing them in the refrigerator is better than in the cupboard, she said, but vacuum-sealing them to remove oxygen, then putting it in the freezer, is best for long-term storage.

There are other reasons you might want to put things in the fridge. Spices don’t usually become rancid, but their potency fades. A milk-carton-size container of smoked paprika I ordered about a year ago is now basically red sawdust. Old cumin smells dull, like pencil shavings. The flavor and pungency of spices comes from volatile oils, which too are vulnerable to oxidation. Staleness, Muthukumarappan said, is usually caused by repeated exposure to the air—as in, regularly opening and closing a spice jar. Keeping it near heat and light can accelerate the process. The freezer is useful if you plan to store spices long term, provided that they’re kept in airtight containers. But if they’re going to be used frequently, it’s best for them to stay at room temperature. Keeping them cold risks condensation forming every time the container is opened, potentially leading to clumps, off flavors, or even microbial growth, Luke LaBorde, a food-science professor at Penn State, told me.

In all my years of cooking, I can’t remember seeing a ketchup bottle that said it was okay to store at room temperature, just as I’ve never come across a spice jar that was meant to be kept in the freezer. Storage instructions on foods, or lack thereof, manifest a different reality, one where proper storage techniques aren’t general knowledge but insider information: There probably won’t be any refrigeration instructions on a bag of pine nuts, but if you know, you know. Expecting every product to have detailed instructions is unrealistic. A simpler storage system, if a more space-intensive one, might be to keep everything cold by default. That way, at least most foods would be safer, and presumably stay fresher. When I asked Muthukumarappan whether any foods would taste better if stored at room temperature, he said he couldn’t think of any. Yet there is still lively debate over whether tomatoes, bread, eggs, butter, and olive oil taste best at room temperature.

The fridge-pantry dichotomy will never fully encompass the murky science of food safety, and the experts don’t always agree. Even the rules for produce aren’t totally clear-cut: All sliced fruit, but not all whole fruit, should be kept cold—especially sliced melons. Unlike most fruits, melons aren’t very acidic, making them more hospitable to pathogenic microbes, LaBorde said. Garlic is safe for several months when kept at room temperature, but homemade garlic-in-oil carries the risk of botulism unless refrigerated.

There’s only one way to reclaim our fridge space and avoid rancid nuts, stale oats, and moldy jellies: thinking beyond the fridge-pantry binary. In particular, factor in how long and where you intend on storing food. It’s not always easy.:Buy in bulk from Costco, where you can get a five-pound bag of walnuts and a gallon of mayo, and food can easily linger—or be forgotten—in a humid pantry for months, even years. Still, if a bottle of ketchup is going to get used up in a week of summer barbecues, you can let it hang out on the counter. Went nuts when the walnuts went on sale? Freeze some for future you.

The science of food storage was widely known several generations ago because it was taught in American schools, Schaich told me. Now we’re on our own. Although we’re unlikely to ever grasp all of its complexities, understanding it just a little more has some advantages. Disregarding the recommendation to refrigerate an open jar of capers gave me a frisson of excitement—not just because it felt like breaking an imperfect rule, but because of the space it opened up in my fridge.