It turns out that in certain vegetables and fruits— including potatoes, sweet potatoes, beets, carrots, beans, cauliflower, tomatoes, cherries, apples—the usual softening during cooking can be reduced by a low-temperature precooking step. If preheated to 130–140°F/55–60°C for 20–30 minutes, these foods develop a persistent firmness that survives prolonged final cooking. This can be valuable for vegetables meant to hold their shape in a long-cooked meat dish, or potatoes in a potato salad, or for foods to be preserved by canning. It’s also valuable for boiled whole potatoes and beets, whose outer regions are inevitably over-softened and may begin to disintegrate while the centers cook through. These and other long-cooked root vegetables are usually started in cold water, so that the outer regions will firm up during the slow temperature rise. Firm-able vegetables and fruits have an enzyme in their cell walls that becomes activated at around 120°F/ 50°C (and inactivated above 160°F/70°C), and alters the cell-wall pectins so that they’re more easily cross-linked by calcium ions. At the same time, calcium ions are being released as the cell contents leak through damaged membranes, and they cross-link the pectin so that it will be much more resistant to removal or breakdown at boiling temperatures.