THE ARTICLE

Ex 4.27 Read the extract from a book. Check the meaning of the words in bold in the glossary (Appendix 2) if necessary.

WATER QUALITY AND PURITY

Pure water is an excellent solvent - indeed, it's sometimes called the universal solvent, because it dissolves more substances than any other liquid, including strong acids. That's due in part to its polarized structure and in part to its hydrogen bonds. Add a little carbon dioxide from the atmosphere, and water becomes an even better solvent, as the properties of carbonic acid augment its native abilities. Because water dissolves things so well, it's often full of minerals collected from its surroundings: particularly calcium and magnesium but also iron, copper, aluminum, manganese, bicarbonates, and sulfates, depending on the geographical location. Hard water is the term for water containing large quantities of dissolved minerals. Most kitchens use tap water for cooking, and recipes that call for water don't specify what kind to use. But the quality and purity of tap water can have a big impact on cooking processes. Hard water is a cooking variable that comes out of your faucet. Hard water toughens some vegetables cooked in it, for example, as the minerals in the water combine with the pectin in plant cell walls. Hard water can interfere with gelling and thickening processes, too, because the dissolved minerals are in the form of charged ions and the hydrocolloids used in these applications are very sensitive to ionic concentration. The minerals in hard water can also leave troublesome deposits on equipment that boils water, such as espresso machines and combi ovens. In addition to minerals, municipal tap water in most parts of the world contains both a form of chlorine to kill parasites and fluoride to prevent tooth decay. These compounds also can affect cooking processes, as well as the flavors and textures of cooked food.

How can you determine the quality of your water supply? Very hard water has an off-taste and a slippery or slimy feel. If you are on a municipal water system, you can contact your water provider to get a complete analysis of what's in your tap water. If you have a private supply, you can have your water tested or get a testing kit and do it yourself. Some manufacturers of water softeners will even give you a free kit. Once you know more about the contents of your water, you can pick the right strategy to purify it. There are a number of water-softening and purification methods, varying in cost, capacity, and the kinds of contaminants they remove. The simplest method is an ion-exchange filter, which uses special resins to capture the ions of dissolved minerals. Often referred to simply as "water softeners," these filters make deionized water, which works best for cooking vegetables and hydrating hydrocolloids. You may want an even higher level of purity if your water tests high for contaminants. Distillation removes impurities by boiling the water and condensing the steam in a separate container. Distilled water makes a fine substitute for deionized water, but it's more expensive. Reverse osmosis uses pressure to pass water through a membrane that screens out contaminants. It makes extremely pure water and is cheaper than distillation, but it generates a large volume of wastewater and doesn't remove chlorine or other dissolved gases. Carbon filtration, on the other hand, is the best way to remove chlorine and the dissolved organic compounds that can be a health issue in some areas. But it won't soften the water, so many household treatment systems utilize more than one approach: pressurized water passes through carbon filters and reverse-osmosis membranes before being irradiated with ultraviolet light to kill any lingering microorganisms. Microporous filtration yields water of the highest purity for use in laboratory experiments. But it's overkill for the kitchen. If you're overwhelmed by these options or don't want to spring for your own water purification system, you can always buy bottled water for critical cooking applications: deionized water and distilled water are widely available. A word of caution, however. Although very pure water may be appropriate for combining with food in cooking, it doesn't taste very good. We're used to water flavored by dissolved gases and minerals, and some of these substances contribute essential nutrients as well. Without them, the water tastes flat.