The water used in soft drinks must be free of objectionable tastes, odors, or colors. It must be safe to drink and low in organic and mineral content. Most soft drink bottlers employ extensive water treatment to assure this primary ingredient is pure and suitable for their products. Although the water must meet federal drinking water standards, most bottlers go well beyond those specifications to assure stability in a complex beverage system that must be shelf stable for six months or more.

Carbon dioxide is delivered to the bottling plant as a liquid compressed under high pressure. As the gas is needed, the liquid C02 is metered through a regulator where pressure is decreased and the liquid is vaporized to a gas. The gas is then dissolved in water in direct proportion to the pressure and temperature, following the universal gas laws. The amount of C02 in the product is measured in gas volumes, where one gas volume equals the volume of gas that will dissolve in water under standard conditions. The treated water is chilled to absorb more C02 and carbonated by exposing it to a countercurrent flow of C02 gas under pressure. Different flavors specify different levels of carbonation, up to a practical maximum of 5.0 gas volumes.

Most soft drinks are sweetened with sugars, predominantly in recent years by a liquid corn sweetener known as high fructose corn syrup (HFCS). Corn syrup is refined from cornstarch, which is a polymer of the simple sugar dextrose W-glucose). Starch is hydrolyzed to dextrose and then a portion of the dextrose is enzymatically converted to fructose to enhance sweetness. HFCS is transported across North America by rail tank car or by tank truck and delivered in bulk. Prior to the 1950s, granulated sugar (sucrose) was most common, delivered in bags or bulk, and this sweetener still predominates in most areas of the world.

Sugar, HFCS, and honey are nutritive sweeteners, meaning they supply some food energy or calories. A significant portion of the U.S. market, about 15%, prefers diet or sugar-free soft drinks made with nonnutritive sweeteners. These are typically intense sweeteners, hundreds of times sweeter than sugar, and are therefore used in small amounts measured in parts per million (ppm or figfL).

Where sugars may constitute 10 to 12% by weight of the beverage, an intense sweetener makes up a fraction of 1%. Sugars also contribute to the mouth-feel of the beverage and help carry and smooth flavors across the tongue. This presents a challenge to the flavor chemist formulating sugar-free products. The balance of flavors found in a sugar-sweetened beverage is difficult to duplicate in the diet version. Examples of nonnutritive sweeteners include aspartame, saccharin, sucralose acesulfame potassium, and alitame. Table 1 lists several of the sweeteners used in the beverage industry with relative sweetness compared with sucrose, solubility at 20°C, and approximate usage levels in beverages.

Another important ingredient is an acidulent or acid, typically citric acid or phosphoric acid, which serves several purposes. The primary function is to modify the flavor by contributing a bite or tang sensation while balancing the sweetness in a sweet-sour relationship. The acid also lowers the beverage pH (a measure of acidity or hydrogen ion concentration) increasing the solubility of some ingredients and helping to prevent spoilage. Carbon dioxide gas forms carbonic acid in solution and therefore is itself a minor acidulent. Other common acid ingredients include malic acid, found in apples and cherries; tartaric acid, primarily found in grapes; and acetic acid, found in vinegar.

The defining ingredient of soft drinks is the flavor system, which differentiates one product from another. Flavors can be simple or very complex combinations of ingredients blended together to achieve a desired balance. Some soft drink flavors have hundreds of ingredients, some of which may be added just to prevent analysis and unauthorized duplication. The secrecy of such formulas is legendary. Fruit juices or juice extractives are common in fruit flavors. Extracts of spices, nuts, and herbs characterize colas, ginger ales, and root beers.

Flavors are categorized by law as "natural" or "artificial" depending on their source and means of manufacture. The U.S. Code of Federal Regulations (21 CFR 101.22) states:

Natural flavors include: essential oils, oleoresins, essence or extractive. ... or product of roasting, heating or enzymolysis, which contains the flavoring constituents derived from a spice, fruit or fruit juice, vegetable or vegetable juice, edible yeast, herb, bark, bud, root, leaf. . . . meat, seafood or dairy product . . . whose significant function is flavoring rather than nutritional.

Artificial flavors are not derived from this list of foods, but typically are synthesized chemicals that may or may not be identical to natural flavor constituents. A great amount of research has been done and is continuing to find new flavoring materials in nature and in the laboratory. All in-

Table 1. Beverage Sweeteners
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