Precooking Can Give a Persistent Firmness to Some Vegetables and Fruits

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.