Okay, this was something I went way overboard on when writing the traditional cooking chapter of MC (see the subchapter on boiling if you own it). The only advantage of the lid is to allow the water to come back to a rolling boil faster. It works because escaping water vapor is removing a huge amount of heat energy, and putting a lid on the pot keeps that energy in the pot, where it raises the temperature of the water faster.
But why should this matter? How couldblanching in water a degree or two cooler than theboiling point possibly make a difference? After all,heat transfer is proportional to the difference intemperature between the food and the heatsource, so a couple of degrees shouldn’t makemuch of a difference—yet experience shows that itclearly does.
Boiling water may be only slight hotter thannear-boiling water, but it is far more turbulent. Because boiling water moves chaotically, itconducts heat into the surface of the food two tothree times as fast as stagnant water a few degreescooler does. If that seems counterintuitive, thinkabout what you do when you get into a really hotbath. You try to stay as still as possible becausestirring the water makes it unbearably hot. Theoverall temperature of the water is nearly the samein either case, but flowing water feels much hotterbecause it hasn’t had time to cool off against yourskin. The greater rate at which boiling watertransfers heat makes it worth the extra effort inmany cases to blanch food in small batches and ina big pot so that the water maintains its roilingboil. The food will cook faster and will retain moreof its flavor, color, and nutrients.
On salted water: it’s sometimes said that salted water boils at a higher temperature and therefore cooks vegetables faster and preserves color. Not true: it takes way too much salt to make a significant difference in the boiling point. However, try the experiment of cooking two batches of vegetables in two pots side by side, one with salted water, one unsalted. The salted water ends up less discolored. That’s because plain water is an osmotically unbalanced cooking medium. Since there’s nothing dissolved in it, it draws substances dissolved in the plant cell fluids—salts, sugars, amino acids—out of the cells, and the water itself flows into the cells, diluting what’s left. With salt pre-dissolved in the cooking water, the fluids inside and outside are more balanced, and less of the vegetable’s substance is drawn out into the water. So cooking vegetables in salted water helps retain more of the vegetables’ nutrients and flavor. Are the differences significant or easily noticed? It probably depends on the vegetable and how it’s subsequently prepared. I’m not aware of any careful studies of this question.
