Nature, famously, abhors a vacuum. But some cooks have learned to feel differently. Step through the swinging doors at the back of a top restaurant like Alinea in Chicago, and you may find vacuum pumps being used to reduce cooking juices into concentrated sauces, to distill essential oils from fruits and vegetables, to dehydrate chips or to brew coffee.
Many of these techniques originated in chemistry laboratories or industrial food-processing operations, and the equipment involved still evokes the bench scientist more than the top chef. But with all those Erlenmeyer flasks, innovative cooks have discovered ways to achieve culinary feats that are impractical by any conventional means.
Consider the common problem of concentrating the flavors and aromas that are in a dilute liquid mixture, such as a broth. The old-fashioned method—a long stovetop simmer to boil off the water—allows many of the most piquant and fragrant compounds to escape with the steam. The kitchen may smell great—but at the cost of a duller sauce. A lengthy sit over the heat also chemically alters many of those compounds that remain, so they no longer taste or smell fresh. A vacuum-reduction setup does a better job because it uses low pressure, rather than high heat, to accelerate evaporation. Pour the liquid into a Pyrex flask that has a side port and connect the flask to a vacuum pump with a rubber hose. Then drop in a magnetic rod, stopper the flask and put it on a hot plate, which uses a spinning magnet to stir and gently warm the broth while the pump reduces the air pressure inside the flask. As the pressure drops, the boiling point of the liquid falls as well; the goal is to sustain a mild, low-temperature boil.
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That relatively simple setup greatly reduces chemical changes, but it still permits some aromatics to escape through the hose. A more expensive and complex bit of gear, called a rotary evaporator, can capture those vaporized essences and condense them back into liquid form. The cooks at our research kitchen in Bellevue, Wash., use this technique to concentrate apple juice, cabbage juice and vinegar to make a fantastic red coleslaw. Concentrated watermelon juice is also a delight.