Flavor is a combination of taste and smell. It is a property of materials (particularly edible materials), which is associated with the sense of taste. School children learn that the tongue can detect four distinct flavors (sweet, sour, salty, bitter) and each flavor has dedicated taste buds in different regions of the tongue (the tip, the sides, the center and the back, respectively). This 4-distinct-flavor view of tastes was held as late as the 70's, but it was derived from a mistranslation of an early 1900s German study which was later misproven. More modern research has shown that, not only are there not distinct "regions" on the tongue, but a fifth flavor, umami, also exists.
Tactile sensations such as heat and cool as well as aroma (from volatile flavor compounds) are now also considered part of the flavor properties of a food. Despite the seemingly limited combination of flavor properties, extremely large numbers of flavors are able to be sensed by the tongue.
The Five Flavors
Sour flavors are generally produced by acids, and different acids produce different acidic flavors. Some familiar sour flavors are vinegar (acetic acid), sour milk (lactic acid), citrus fruit (citric acid), apples (malic acid) and wine (tartaric acid).
Salty flavors are most commonly associated with the mineral of the same name (sodium chloride), however most low molecular weight salts (ammonium chloride, potassium chloride, lithium chloride, sodium bromide, ammonium bromide, sodium iodide and lithium iodide) have a salty taste.
Less is known about sweet and bitter substances. There seems to be no relationship between organic functional groups and sweet flavor (as almost every class of compound has a member which tastes sweet), yet at the same time the chemistry is very specific. For example, saccharine is 300 times sweeter than table sugar (sucrose), however if the hydrogen bonded to the nitrogen is replaced with a methyl group, all flavor sensation disappears. Likewise, bitter sensations are difficult to classify, though many plant-derived molecules which interact with the brain such as caffeine and nicotine, as well as large molecular weight ionic salts such as cesium chloride, rubidium bromide and potassium iodide produce bitter tastes.
Umami (Japanese for "delicious", "savory" or "meaty") is a newcomer to the basic flavor group. Umami is associated with L-glutamate and is found in meat broths, sea weed broth and aged cheese. It can also be used in solid form, usually monosodium glutamate, MSG. Unlike the other four flavors, umami is not thought to have a flavor of its own, but rather it intensifies other flavors already present, hence the common use of MSG in many prepackaged food items.
Little is known about the mechanisms involved in tasting foods, but it is thought that sour and salty tastes utilize channels which allow inward proton transfer combined with H+-gated channels, while sweet, bitter and umami flavors utilize surface binding pathways to transmit flavor information. All five flavors depolarize the taste cell (which initially has a net negative internal charge) by causing the transfer of calcium ions (Ca2+) into the cell. This forms an electrical signal which is then sent to the brain and interpreted as flavor. When umami binds to a flavor cell, not only is there an influx of Ca2+ into the cell, but the cell also become more sensitive to the effects of other flavor compounds.
Flavor Production in Nature
Historically, most science in the field of flavor has been dedicated to determining what flavor is rather than how it is produced in nature. In addition, the metabolic pathways involved in flavor production are often very complicated, which makes their determination difficult. Recently, however, due to the demand for man-made flavors (especially the production of flavors able to be labeled “natural flavors”) more research has been focused in this area of study.
While some compounds are produced as part of normal biological processes (sugars and acids, for example), volatile aroma components seem to have no biological function for the plant. Most aroma compounds are actually formed through break down of the cell; in fruits the cells soften and degrade during ripening, and in vegetables the cell walls are ruptured by an outside force, such as a tooth or a knife. It is not until the cells break down that enzymatic reactions take place which rapidly form aromas.
Flavor in fruit is created by various linked metabolic pathways. Fatty acids, amino acids, carbohydrates and proteins are enzymatically converted to various flavor and aroma compounds associated with the fruit.
Unlike fruits, vegetables to not develop flavor over a long period of time, but rather through rapid enzymatic reactions after cellular disruption. Similar to fruit, fatty acids, amino acids, carbohydrates and proteins are the substrates for the flavor reactions, however sulfur-containing volatiles are much more important.
Flavor Production in Industry
Flavors are manufactured and extracted for use in food and beverage by hundreds of different companies. They are primarily used in prepackaged food items, but they can also be sold to home consumers (in products such as imitation vanilla). Due to the difficulty (and sometimes impossibility) of patenting flavor compounds, many details of flavor manufacture are closely guarded secrets, however some general principles are known.
Flavor manufacture falls into two categories, natural and artificial. Legally speaking, natural flavors are those which are produced by nature or through an engineered biological reaction, while artificial flavors are created by man through chemical reactions. From a production standpoint, artificial flavors are more appealing, since natural flavors tend to be in low concentrations (especially fruit flavors), are susceptible to seasonal fluctuations, and have a high material cost (since natural flavors are generally derived from a whole spice or fruit). Consumers generally prefer natural flavors over artificial, however. One reason for this is that natural flavors are not simply one or two compounds, which could be easily synthesized, but a large number of compounds in a characteristic distribution, called a flavor profile. For example, natural pineapple has at least 17 chemical compounds which make up its flavor profile. Any imitation pineapple will not taste exactly like real pineapple without this same profile. Generally cost and quality dictate whether the expensive, but accurate natural flavors will be used in a product instead of the less costly, but also less accurate, artificial flavor.
The flavor industry also classifies flavors by phase (liquid, emulsion, paste and solid). The phase in which a flavoring product is produced is determined by the application (liquid phase flavors are produced for beverages, for example) and by the physical properties of the compounds involved. Emulsions are typically broken down further into beverage emulsions, which are used to add flavor or turbidity to beverages, and baker's emulsions, which are viscous macroemulsions (Reineccius).