Using tools from chromatography to genetic engineering, flavor technologists work to improve our dining experience.

Flavor is made up of the chemical interaction of molecules from food and drink with the taste and aroma receptors in the human mouth and nose. Taste, in fact, is often the least important in differentiating between flavors. There are only five major taste receptor types—bitter, sweet, sour, salty, and umami (the unique flavor typified by MSG). The vast number of flavors we perceive are the result of the complex mixture of these few taste flavors acting on our taste receptors and on the interactions of chemical volatiles in food and drink that react with our even more discriminating scent receptors (of which there appear to be nearly a thousand types).

The identification, purification, synthesis, and transformation of these chemicals to improve food and beverage flavor represents chemistry at its finest. Changing or maximizing the flavor of foods to enhance the sensory experience has become ever more sophisticated. The food processing industry is even using enzymes from genetically modified organisms (GMOs) to boost flavor. But it remains the interaction of specific chemicals with receptors that serves up the courses of this increasingly mutable feast.

Patterns of Palette
So what is the basis of the chemistry of flavor? Improving flavor involves enhancing the “good” tasting and smelling molecules and diminishing or covering up the bad. Determining what these molecules are is a combination of analytical chemistry and human testing. Chromatography and spectroscopy are key to the isolation and identification of molecules (see box, “The Fingerprint of a Flavor”) for analyzing, improving, and commercially monitoring food flavors. Although the human nose and palate are the ultimate arbiters of “good taste”, once that connection has been made, instrumentation can take over the process.

In addition, once flavor-specific compounds are determined, there are several ways to incorporate them into food products as

• simple chemical additives—whether synthesized or purified;
• genetically selected (or genetically engineered) “natural” accessories to the raw product; or
• the result of processing modifications of the raw product using enzyme treatments or biological fermentation (such as with yeasts in beer, wine, and bread, as well as bacteria and molds in dairy products).

The Matter of Taste
The five basic taste receptor types are finally being elucidated using molecular biology. In the past few years, putative receptors have been identified for the bitter and sweet tastes perceived by humans. More recently, researchers at the Howard Hughes Medical Institute (Chevy Chase, MD) identified the receptor for umami. (For more information, visit www.hhmi.org/news/zuker3.html)

Chemically, the other components of flavor due to taste are also fairly simple. Sweet tastes are generally associated with organic molecules such as sugars and alcohols. Salty tastes are the response to ionic solutions in which the cation dominates the degree of saltiness—the larger the anion, the less saltiness detected at the same concentration. Sour tastes such as that of vinegar are caused by the presence of hydrogen ions, but the anion affects the intensity of sourness that is detected. Bitter tastes are associated with organics, many of them poisonous, and the disagreeability is thought to be an evolutionary protection against many naturally occurring toxins. Specific, unique tastes are thought to be created by different patterns of firing of the five types of taste receptors in response to a complex food.

Scents and Scents-ability
But the true subtleties of flavor are primarily found in the aromatics, sensed through smell rather than taste. By definition, aromatics are volatile chemical molecules that diffuse to the olfactory epithelium in the upper part of the nose. The number of uniquely discriminated odors (in the low thousands) is more than an order of magnitude larger than the number of tastes (in the low hundreds).

Because aroma compounds are volatile, they disappear quickly and thus have to be stored in some fashion and released only when needed, as in flowering plants that attract insects for pollination and fruiting plants that attract animals to spread seed. Typically, many aroma compounds occur as glycosylated moieties that only become volatilized when the sugar components are removed by the activity of a variety of glycosidase enzymes. This is readily apparent in fruit species in which the sugar-free moieties liberated by glycosidase activity volatilize to provide the classic fruit scents.

TABLE 1: Some Character Impact Compounds Chemical Associated Food Eugenol Cloves 4-Pentyl isothiocyanate Horseradish Ethyl-2-methyl butyrate Apple 4-(p-Hydroxyphenyl)-2-butanone Raspberry 1-Octen-3-ol Mushroom (Z)-3-Hexanol Tomato (fresh) 2-Ethyl-6-vinylpyrazine Potato (baked) 2-Ethyl-3,5-dimethylpyrazine Potato (chip) 2-Furfurylthiol Coffee 4-Methyloctanoic acid Lamb Bis(2-methyl-3-furyl) disulfide Aged prime rib Mercaptopropanone dimer Chicken broth Propionic acid Swiss cheese 1-Nonen-3-one Milk

But how is it that we can detect one food uniquely from another? Is it all because of the components of flavor generally, since there can be dozens in a complex natural food, or is it due to what researchers refer to as “character impact compounds”—the one or few molecular species that seem to overwhelmingly define the flavor to the human palate? In fact, character impact compounds are at the heart of the flavor industry—the tastes or scents that define for us “butter” (2,3-butanedione), “banana” (isoamyl acetate), “blue cheese” (2-heptanone), and a host of other foods and flavors we seem to recognize instantly (see Table 1). It is here that the chemist is critical in providing structural analysis of the molecules in question, as well as in providing synthetic methods for producing the compound or variations that may have improved or altered flavor properties.

Often, however, chemical impact compounds are proving necessary, yet insufficient, to satisfy the growing sophistication of modern consumers. Indeed, vanillin (synthesized for use as imitation vanilla) is the chief character impact compound in vanilla beans and natural vanilla extract, but the accessory flavor compounds in a variety of configurations add the subtleties that appeal to true gourmets.

Small quantities of otherwise objectionable flavors can also have major impact on food acceptability. One of the most striking examples of this is cited by R. J. McGorrin of Oregon University: “At high concentrations, 4-mercapto-4-methyl-2-pentanone ('cat ketone´) has an off-odor associated with cat urine, but in the context of Cabernet Sauvignon wine, it provides the typical flavor impression of the Sauvignon grape.” And this is not to mention those authentic “off-flavors” that humans shy away from—presumably as a natural evolutionary protection against eating spoiled foods—that usually result from enzymatic breakdown of natural compounds, especially proteins, and have the effect of making food taste unacceptable.

Enzymes and Eats
A wide variety of enzymes are used to develop food and flavor additives and to modify the taste and consistency of foods themselves. These enzymes have traditionally been derived from naturally isolated or bred plant, animal, or microbial sources and have been added to various stages of food processing from beverage production to cheese making and from tomato canning to bread baking.

The basic enzyme types used in food production are hydrolases, isomerases, ligases, lyases, oxidoreductases, and transferases. Under these general categories are a host of enzymes fulfilling every function in food processing from juice extraction (pectinases) to dough structure control (certain proteases) to flavor release (glycosidases).

Since the mid-1980s, many of the enzymes used in food production have been derived from genetically engineered microbes. These specially selected enzymes from optimal flavor-producing sources are made more efficiently (and cheaply) by GMOs, from which they are purified and used as reagents. One of the most interesting cases is that of the enzyme chymosin, which helps to clot milk protein to make cheese. Traditionally, chymosin for cheese making was purified from the stomach walls of slaughtered calves. Researchers cloned the calf enzyme and inserted it into bacteria where it could be mass-produced in fermentation vats and subsequently purified for food production at vastly improved cost savings, especially as veal consumption is significantly down in the United States. The use of such chymosin is theoretically an animal rights breakthrough, but it is often decried because genetically engineered materials are anathema to many of the same constituencies as the most vocal of the animal rights groups. However, for true vegetarians, as well as for people desirous of having what many consider a Kosher hard cheese, the product serves a valuable function above and beyond cost savings. So popular has the product become among dairy manufacturers that more than 60% of hard cheese production in the United States uses the engineered enzyme.

According to the U.S. Food and Drug Administration (Code of Federal Regulations, 21, Section 101, 22 (a) (3)), natural flavors are defined as “the essential oil, oleoresin, essence or extractive, protein hydrolysate, distillate, of any product of roasting, heating, or enzymolysis, which contains the flavoring constituents derived from a spice, fruit juice, vegetable or vegetable juice, edible yeast, herb, bud, bark, root, leaf or similar plant material, meat, seafood, poultry, eggs, dairy products, or fermentation products thereof whose significant function in food is flavoring rather than nutrition.”

Because of this definition, in the United States and in other countries with similar regulations, if the result of using an enzyme from a GM microorganism is a “natural” product, then the process involved in producing it is considered irrelevant. This helps to maintain the designation of “natural flavors” even for biotechnology-derived products. Many people battling the development of GMOs find the use of the term “natural” an outrage when they discover it can be used for foods that use products derived from GMOs.

In some cases, microbial fermentation is used to produce a variety of flavors. Everyone is aware of the role of yeast in beers and wines and of lactobacilli in yogurt. But microorganisms are also used to produce specific natural flavors that are then purified for use as food amendments. For example, certain yeasts are used to produce isovaleraldehyde (a chocolate flavor base).

From Palette to Palate
Ultimately, flavor compounds are the final step of a significant series of chemical modifications of natural plant or animal products. Flavor production can now occur equally in nature or in the industrial fermenter where modifications can be made using purified enzymes (from natural or genetically engineered sources) or via synthetic reactions. And, although in nature metabolic pathways of individual organisms limit the variety of flavors, in the lab, all bets are off. For the first time in human history, an expanding palette of natural and designed tastes and aromas is becoming available to food scientists, setting the stage for a feast for the senses of potentially infinite mutability. And that is even before the foods reach the kitchen or the cook.

There does remain one caveat, however: whatever the optimism of our increasing understanding and control of tastes and smells, none of this can explain why flavored cough syrup still tastes like it does.

Suggested Reading

• Flavor Analysis: Developments in Isolation and Characterization; Mussinan, C. J., Morello, M. J., Eds.; American Chemical Society: Washington, DC, 1998.

• Flavor Chemistry: Industrial and Academic Research; Risch, S. J., Ho, C-T., Eds.; American Chemical Society: Washington, DC, 2000.

• Flavor, Fragrance and Odor Analysis; Marsili, R., Ed.; Marcel Dekker: New York, 2002.

• GE enzymes and microorganisms: www.comm.cornell.edu/gmo/crops/enzymes.html.

• Homing in on a Receptor for the Fifth Taste: www.hhmi.org/news/zuker3.html.

• Takeoka, G. R.; Guntert, M.; Engel, K.-H. Aroma Active Compounds in Foods: Chemistry and Sensory Properties; American Chemical Society: Washington, DC, 2001.

Mark S. Lesney is a senior associate editor of Today´s Chemist at Work. Send your comments or questions regarding this article to tcaw@acs.org or the Editorial Office, 1155 16th St N.W., Washington, DC 20036.