Water is the most abundant and versatile substance on earth. Among its many uses in food preparation, its two most important functions are as a transfer medium for heat and as a universal solvent. In addition, it is important as an agent in chemical reactions, and is a factor in the perishability and preservation of foods.
Water both transfers and moderates the effects of heat. A potato heated by itself in a pan will burn. but surrounding that same potato with water ensures that the heat will be evenly distributed. Water also transfers heat more efficiently, which explains why a potato heats faster in boiling water than in the oven. Because water has a higher specific heat than other substances, it buffers changes in temperature. More energy is needed to increase the temperature of 1 gram of water than 1 gram of fat. For example, the specific heat of oil is 0.5; thus it heats twice as fast as water when given the same amount of heat.
Almost half of the methods used to prepare foods rely on water to transfer heat, and these are known collectively as moist-heat methods. The major moist-heat methods discussed in this book include boiling, simmering, steaming, stewing, and braising. Dry-heat methods use heat in the form of radiation and include baking, grilling, broiling, and frying. Microwaving uses both dry-and moist-heat methods; microwaves are actually a form of radiation that heats the water molecules in foods, which then heat the food itself. Microwaving techniques are discussed throughout this book under moist-heat preparation methods.
The many biochemical interactions occurring in living organisms—human, animal, and plant—could not occur in the absence of a solvent environment. Water is considered to be the earth’s universal solvent. The fluid substance, mostly water, within and around the cell is a solvent that contains many dissolved substances called solutes.
Combining a solvent and a solute results in either a solution, a colloidal dispersion, a suspension, or an emulsion. These mixtures differ from each other based on the size or solubility of their solutes.
In a solution, the molecules of the solute are so small that they completely dissolve and will not precipitate from their fluid medium. They cannot be separated by filtering, but can sometimes be removed by distillation. If a substance is able to enter into a solution by dissolving, it is considered to be soluble.
Much of what people perceive as the taste of foods depends on the formation of solutions with solutes in foods such as sugars, salts, acids, and other flavor compounds, and their resulting enhanced ability to attach to flavor receptors. Water also forms solutions with minerals and water-soluble vitamins (B complex and C). This increases the likelihood that these minerals and vitamins may leech out of foods into cooking water, which is often discarded, causing nutrients to be lost. To the delight of tea and coffee lovers, water can also dissolve caffeine and other flavorful compounds from tea leaves and coffee beans. Higher temperatures increase the amount of solute that will dissolve in the solvent, which explains why very hot water is used for making coffee and tea.
The solubility of a substance is measured by the amount of it in grams that will dissolve in 100 ml of solvent. Raising the temperature allows more solute to dissolve in the solvent, creating a saturated solution. Increasing the temperature even higher results in a supersaturated solution, which is very unstable and must be cooled very slowly to avoid having the solute precipitate out or crystallize. Many candies, including fudge, rely on the creation of supersaturated solutions.
Not all particles dissolve readily or homogeneously. Some particles, called colloids (e.g., proteins, starches, and fats) never truly dissolve in a solvent, but remain in an unstable colloidial dispersion. Unlike solutes in solutions, which completely dissolve, colloids do not due to their large size, but neither do they noticeably change the dispersion’s freezing or boiling point. Examples of different types of dispersions include a solid in a liquid, a liquid in another liquid (salad dressing) or solid (jam, gelatin, cheese, butter), and a gas that can be incorporated into either a liquid (egg white or whipped cream foams) or a solid (marshmallow). Two types of dispersions are suspensions and emulsions.
Suspension. Mixing cornstarch and water results in a suspension in which the starch grains float within the liquid.
Emulsion. Another type of colloidal dispersion involves water-in-oil (w/o) or oil-in-water (o/w) emulsions. Neither water nor fats will dissolve in each other, but they may become dispersed in each other, creating an emulsion. Examples of food emulsions include milk, cream, ice cream, egg yolk, mayonnaise, gravy, sauces, and salad dressings. These and other emulsions can be separated by freezing, high temperatures, agitation, and/or exposure to air.
Colloidal dispersions, which are unstable by nature, can be purposely or accidentally destabilized, causing the dispersed particles to aggregate out into a partial or full gel, a more-or-less rigid protein structure. An example of this is seen when milk is heated; its unstable water-soluble milk proteins precipitate out and end up coating the bottom of the pot, creating a flocculation. Full gels such as yogurt and cheese are also made possible by the colloidal nature of milk.