Why Do Some Foods Freeze Better Than Others?
In his new column, Ask Kenji, the cookbook author Kenji López-Alt answers your questions. First up: why smaller is better in the freezer.
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Welcome to my new column, Ask Kenji. I’ve spent the last decade and a half of my career rigorously researching and testing recipes, techniques and widely accepted kitchen wisdom to figure out the whys of cooking. I’ll try to answer the questions you’d answer yourself — if you had the resources and time, diving into research, tapping writers, chefs and scientists, and designing kitchen experiments to get to the bottom of your burning cooking questions. This week, we look at a reader question and figure out why, when it comes to the freezer, smaller (and faster) is better.
Q: I’m curious why quickly freezing peas would affect how much cell walls break. I would have guessed the water expands just as much regardless of how quickly they freeze, but maybe I’m missing something?
The cookbook author J. Kenji López-Alt has spent a decade and a half of his career rigorously researching and testing recipes. Now, he’s answering reader questions.Credit...Adam Centrella/The New York Times
A: Whether scattered into a beef stew, stirred into a stovetop macaroni and cheese or simply sautéed with butter, frozen peas are an easy last-minute addition to many meals. Moreover, they’re often sweeter and more consistent than my spring pea hauls from the farmers’ markets.
One reason is freshness. Over time, enzymes will naturally convert some of the sugars in peas into starches, making them dull and mealy. Frozen peas — and many other frozen foods — are typically blanched soon after harvest when they’re at their best, destroying those enzymes, and then frozen immediately after, locking those sugars in place until you defrost them.
Peas also seem to suffer far less from the textural issues that can plague other frozen vegetables. To understand why, you’ll want to look at two things: their structure and how ice crystals form.Like all plant matter, peas are made up of cells bound into rigid structures. Plants maintain their firmness and crunch through the water pressure inside those cells, like tautly filled water balloons taped together. Let some liquid out of those balloons, and the overall structure loses form, going limp or mushy.
Water is necessary for crispness — think of every time you forget to water a houseplant, or find a droopy carrot in the crisper. But this very water can also wreak havoc on food’s microscopic structure when frozen into large, jagged ice crystals that puncture vegetable cell walls or rip at animal flesh. Vegetables that appear intact and firm when frozen can rapidly turn soft and mushy upon thawing, as intracellular liquid seeps out through ice-damaged cell walls.
For water to freeze, it must first be super-chilled — that is, chilled below 32 degrees Fahrenheit. Only then does it undergo a phase change from liquid to solid: Its structure transforms from a loosely bound mass of flowing molecules into crystalline aggregates with a strict, regular alignment. These aggregate crystals in frozen foods can vary in size, from microscopic and smooth on the palate to plainly visible with a grainy texture (the velvety texture of well-churned ice cream compared to the sharp shards of a granita, for example).
What determines the size of those crystals?
I like to think of water as a room scattered with Tinkertoys and solid ice as the structures my daughter’s first-grade classmates might erect out of them. If you ask them to put pieces together as fast as they can, very little collaboration will take place. Your room will end up full of many small structures. If, on the other hand, you give them more time, they may start haphazardly, building a few small projects here and there, but eventually the allure of extra-large towers will have them working together, cannibalizing the smaller structures to add to the bigger ones. You wind up with fewer, larger buildings.
So it is with ice formation. Ice crystals, like toy skyscrapers, take time to build. And just like those toy skyscrapers, the rate at which they’re formed makes a big difference. As described in a May 2020 article in Food Engineering Reviews, a journal covering food science and the food industry, when water freezes slowly, smaller ice crystals have a tendency to spontaneously remelt in a process known as recalescence, and merge into larger crystals, which continue to grow like amoebas, absorbing smaller, less-stable crystals.
Put simply, when it comes to the size of ice crystals, the longer it takes food to freeze, the larger the crystals will be. The larger the ice crystals, the more cellular damage they can cause. The more cellular damage caused, the more moisture will leak out during defrosting. And the more liquid leaked out, the limper your thawed produce will be.
Thus, if limiting cellular damage is the goal, then you want to freeze food as quickly as possible. In a commercial kitchen, one might rely on extremely cold liquids like liquid nitrogen, or use blast chillers, freezers kept at 40 degrees below zero or colder and outfitted with powerful fans designed to rapidly draw heat away from foods. Like an air-fryer, but in reverse.
But you can also encourage food to freeze faster by playing with its geometry. Larger objects freeze far more slowly than small ones, giving ice crystals more time to aggregate.
The experiment
To demonstrate this, I weighed and froze two batches of apples. The first batch, I cut into ½-inch dice and spread it on an aluminum tray in a single layer. The second, I froze whole. The diced apples took around 45 minutes to fully freeze, while the whole apples took more than 5 ½ hours. The next day, I thawed both batches, dicing the whole apples only after they had fully thawed. I then let all of the apples drain in a colander for 15 minutes before weighing them again. The results were telling.
On defrosting, the diced apples lost on average around 10 percent of their weight in water, while the whole apples lost a whopping 30 percent, turning a near applesauce-like consistency in the process. This moisture loss and mushiness is why most frozen foods are chopped into smaller pieces before freezing, and it’s why small things like peas freeze so well. They directly benefit from being pea-sized.
The takeaway
I don’t have a blast freezer at home, but I can still take lessons from commercial operations. For most fruit or vegetables, dicing followed by a quick blanch in rapidly boiling water (since deactivated enzymes can cause discoloration), a rest in an ice bath and a thorough drying will best set them up for the freezer. (Sweeter, softer fruit like mangoes and plums can be frozen without blanching, and smaller fruits like berries can be frozen whole without blanching.)
To freeze ground meat, I pack it into freezer bags, squeeze out the air, then seal the bags and press the meat into a single flat layer that freezes and thaws rapidly. Whole cuts of meat intended for rapid cooking should be separated into individual steaks or chops and frozen solid in a single layer on a parchment-lined baking sheet before being transferred to a freezer bag for long-term storage.
When spreading food onto a tray to freeze, I try to leave as much space as possible between pieces to encourage air circulation. The material the tray is made of can also make a big difference. Aluminum is effective, as it is highly conductive and thus great at wicking heat away from food as it chills.
If you’re buying frozen produce, you’ll want to consider how you’re using it. Brassicas, like broccoli, cauliflower or brussels sprouts, are great roasted at high heat until they’re charred at the edges. Berries (blueberries, raspberries, cherries) work best in baked goods, like pies and muffins, and in smoothies. And generally, all frozen foods are their best in anything that will end up soft, like a soup, a stew or a purée. As for peas, they’re good in everything.
Because they’re so small, frozen peas retain a lot of the characteristics of their fresh counterparts and can easily bolster weeknight pastas.Credit...Armando Rafael for The New York Times. Food Stylist: Cyd Raftus McDowell.
How to cook with frozen peas
There’s virtually no limit to how many peas I can add to a box of my stovetop mac and cheese, but if I want to put in a little more effort, I turn to this recipe I call macaroni and peas. It’s a riff on Italian American-style pasta carbonara that starts with a bag of dried pasta and a cup of frozen peas. I combine them in a creamy, cheesy and garlicky egg-based sauce studded with bits of rendered bacon or pancetta and finish everything with a handful of chopped mint. It’s just another way to get more mileage out of something so small.
