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Flavor 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 (Pyke, 1970), but recent research suggests that not only is their a fifth flavor sensation (umami), but that the tongue has more overlap in flavor detection than previously thought. 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) (Reineccius).
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 (Pyke).
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 (Pyke). Likewise, bitter sensations are difficult to classify, though many plant-derived molecules which interact with the brain such as caffeine and nicotine (Reineccius), as well as large molecular weight ionic salts such as cesium chloride, rubidium bromide and potassium iodide (Pyke) 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 (Reineccius, Pyke). 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 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 (Reineccius).
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 addition, the metabolic pathways involved in flavor production are often very complicated, which 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 plants 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 (Reineccius).
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. A proposed mechanism for one such mechanism is shown in Figure 2. Through this mechanism, linoleic acid is metabolized into a CoA derivative (linoteyl-CoA), which is converted into various decadienoate esters which are associated with the flavor of a pear (Reineccius). Figure 2: Mechanism for Flavor Formation in Pears (Reineccius)
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 (Reineccius).
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) (Reineccius). 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 (Pyke). 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).
One compound whose production methods are well known is vanilla flavoring. Vanilla is grown in very few places in the world (90% of the world’s vanilla is grown in Madagascar and Comoro and Reunion islands) (Reineccius). The principal flavor compound is vanillin (see Figure 3), but there are other important compounds, the composition of which varies depending on the region (see Figure 4). These composition differences are enough such that a difference in flavor can be detected when vanilla from different regions are tasted.
Natural vanilla flavor is created from the beans of vanilla plant. After a delicate growing and harvesting procedure the beans are treated with mild, moist heat in order to break down cell walls and promote enzymatic reactions. They are then sun, oven, and air dried for up to 6 months. During this curing process three types of glucosides are hydrolytically cleaved to produce phenols (one of which is vanillin, from glucovanillin) and glucose which give vanilla its characteristic flavor. After curing the beans crushed and mixed with sugar. They are then percolated in an ethanol/water mixture for several weeks to extract the oils from the beans. The solids are filtered or centrifuged out and the alcohol content is then adjusted to the 35% minimum required by law (Reineccius).
Since natural vanilla is very expensive, imitation vanilla is often used to mix with or completely replace natural vanilla. Imitation vanilla was produced from eugenol, which was itself produced by oil of clove, but now it is made from sulfite waste liquors from the paper industry (Reineccius).
References
Greater Cincinnati Chamber of Commerce (GCCC), “Cincinnati USA: Chemistry Industry Leadership.” 2006. < http://www.gccc.com/pdf/ind_profile/ChemEng.pdf> Accessed November 11, 2007.
Leffingwell & Associates, “2002 - 2006 Flavor & Fragrance Industry Leaders.” 2007. <http://www.leffingwell.com/top_10.htm> Accessed November 11, 2007.
Givaudan. <http://www.givaudan.com> Accessed November 11, 2007.
Pyke, Magnus. Synthetic Food. Spottiswoode, Ballanyne & Co: 1970.
Reineccuis, Gary. Flavor Chemistry and Technology. Taylor & Francis: 2006.
Vallod, D. et al. “Analysis of the off-flavor risk in carp production in ponds in Dombes and Forenz (France),” Aquaculture International. 15.3-4 (June 2007).