By Harold McGee
The cell-wall carbohydrates in purees and juices can be used to stabilize two otherwise fleeting physical structures, a foam of air bubbles and an emulsion of oil droplets, which are especially easy to prepare with modern electrical blenders and mixers. If a puree or juice is whipped to fill it with air bubbles, the cell-wall carbohydrates slow the flow of water out of the bubble walls, so the bubbles take longer to collapse. This allows the cook to make a foam or mousse that lasts long enough to be savored; foams from juice are especially ethereal. Similarly, when oil is whisked into a puree or juice, the plant carbohydrates insulate the oil droplets from each other, and the oil and water phases separate more slowly. The cook can therefore incorporate oil into a puree or juice to form a temporary emulsion, with richer dimensions of flavor and texture than the puree alone. The thicker the puree, the more stable and less delicate the foam or emulsion. The consistency of a thick preparation can be lightened by adding liquid (water, juice, stock).