Heat Causes Structural Changes
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.