Science: Why Sous Vide is Perfect for Cooking Vegetables

• Plants are made up of microscopic cells
• The cell walls break down with heat
• We cook most sous vide vegetables at 185°F/85°C or higher to ensure tenderness

Just like animals (including us), plants are made of microscopic cells. These cells contain, among other important ingredients, starches, sugars, and water, which are all held in place by sturdy cell walls. In a crisp fresh fruit or vegetable, the cells are full of water, which causes them to inflate like a balloon. As the fruit or vegetable ages, some of that water leaves the cells, causing them to deflate, and thus the plant becomes progressively more limp over time.

When a plant is ready to eat, it shouldn’t be too hard to bite into or unpleasantly bland or starchy. It should have an appealing aroma and flavor and maybe some sweetness. Some plants (or plant parts) get there on their own: A ripening fruit gets soft, sweet, and fragrant as its own enzymes convert tasteless starch to sugar and break down the pectin that strengthens its cell walls.

Other plants, however, we need to cook before they can reach that point. Specifically, vegetables don’t ripen the way fruits do, but we can use heat to soften those cell walls and transform a tough and dense artichoke or potato into dinner. Even ripe fruits can benefit from the softening force of heat.

Sous vide has advantages when it comes to fruit and vegetable cookery. While roasting does a good job of softening vegetables, it causes significant water loss in the process—hello, leathery roasted carrots. Sous vide also has advantages over boiling or steaming these plants; the sealed bag means you’re not washing away delicate vegetable flavor. In addition, the precision of sous vide cooking allows us to easily home in on the ideal texture for cooked vegetables, cooking them until appropriately softened but before they turn to mush.

When plant cells are heated to about 140°F/60°C, a series of structural transformations starts to occur. The hotter the temperature, the faster they happen.

First, the balloon-like membranes inside the cells that hold onto water rupture. The vegetable starts to release some moisture and its cells start to deflate, making the vegetable limper, though its structure is still intact.

With continued cooking, the cell walls start to break down. Cell walls are rigid enclosures made up of three different types of structural molecules that have different properties: cellulose, hemicellulose, and pectin. Cellulose—think of the strings in a celery stalk—doesn’t break down readily in cooking, but the other two do. Gradually, hemicellulose and pectin begin to dissolve in the surrounding liquid, which lets the cells separate from each other and collapse. With continued cooking, the plant becomes softer and softer until it is mush.

Almost all of our vegetable recipes call for a water bath temperature of at least 180°F/82°C. This is the temperature at which cell walls begin to weaken significantly, which translates to tender but still toothsome vegetables. We like this temperature for more delicate and quick-cooking produce like asparagus, fresh corn, and bok choy. For sturdier, starchier vegetables, we had success raising the sous vide temperature further. Pectin and hemicellulose start to dissolve between 190°F/88°C and 198°F/92°C. In this range, a carrot softens from snappy to bendy, the perfect texture.

 All plants have cell walls, but some plants have a significant amount of starch as well, which they store inside their cells as a source of energy. Cooking transforms these starchy plants (think potatoes and winter squash) too. In the raw plant, starch is stored in the form of microscopic, tightly clumped starch molecules.

Two ingredients are needed to soften starch molecules: heat and water. If you soak potatoes in cool water, nothing dramatic happens, but as soon as they heat up, the granules start to absorb water. They swell and soften as the water penetrates, and ultimately the granules burst apart. When they do, the starch molecules stretch out and tangle up with each other to create a mesh that holds onto water: the result is a starch gel. Starch in this form is tender and edible.

The starches in different plants have different characteristics, and they burst at different temperatures. Fluffy baking potatoes have starch that bursts and gels around 136°F/57°C, while waxier boiling potatoes don’t unclump their starch until they reach 158°F/70°C. We found that cooking starchy vegetables at relatively high sous vide temperatures rendered them fork-tender, but not mushy. Small red potatoes cooked at 194°F/90°C along with broth or water for 60 to 75 minutes exited the bath tender from end to end, perfect for potato salad or for smashing and crisping up in the oven. Butternut squash cooked at 200°F/93°C for 1½ to 2 hours came out sweet and soft and easily pureed into a silky-smooth side dish.

Whether you’re cooking bok choy, potato puree, or butternut squash, just remember: It all comes down to the cell walls.

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