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NEW ENGLAND ASSOCIATION of CHEMISTRY TEACHERS The Chemistrv of Flavor' ERNEST C . CROCKER Arthur D. Little, Inc., Cambridge, Massachusetts
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VERYONE is familiar with flavor-intimately famihar-and has been so all his lie. Everyone has .. noticed how foods figuratively speak to us through flavor, suggesting that we do or do not eat them. A few animals choose food by sight. Birds generally do, and many fishes. Sometimes trout or bass regret that they snap a t almost anything that suggests food. Most animals are attracted to food by its aroma. When presented with fobd, they appraise i t carefully by smell, and only if i t stimulates the appetite, will they eat it. The human race is gradually making food its servant. People eat what they suppose is good for them. However, this does not prevent the appeal of fine foods in good condition. No doubt, in the pre-refrigerator days, appraisal of foods by odor prolonged the lives of our ancestors. Another angle of food flavor is that it may be used as a part of enjoyment or even of indulgence, giving rise to gourmets and gourmands. The principal value of flavor is in its appeal to and stimulation of the digestive system. Flavor registers through several senses, but most notably through smell and taste. Our awareness of it is always chemically excited, whereas sight is stimulated by light, the coolness and warmth factors of feeling by temperature, the contact sense by pressure, and hearing by sound. Always, some chemical snbstance must impress itself upon us, diffused in the air we breathe or dissolved in a juice or in our saliva. We need not shudder a t that word "chemical," for even the best of foods are necessarily chemical in constitution. Flavor may be defined as that property of food and beverage that enables i t to register on the senses when the food is placed in the mouth. We think naturally first of the sense of taste, which permits us to detect the sweetness of sugar, the saltiness of sodium chloride, the sourness due to the hydrogen ion, and the bitterness due to a wide variety of chemicals, some ionized and some not. This sense operates through the taste buds located on the tongue
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Presented at the Seventh Summer Conference of the New England Association of Chemistry Teachers, Amherst, Massachusetts, August 10. 1945.
and adjacent tissues. How it operates is not known. We do know, however, that there are four distinct kinds of taste buds and that each kind responds to only one component of taste, as sweetness, saltiness, sourness, or bitterness. The implication is that four kinds of nerves lead from these sensitive detector cells, each capable of giving rise to a particular kind of sensation when stimulated. Physiologists have located areas on the tongue that are sensitive to each of the respective elemental kinds of taste sensation, for instance bitterness chiefly a t the back of the tongue and sweetness prominently a t the tip. The beer drinker knows what he is doing when he tilts back his head to enjoy the bitter flavor as the liquid laves the back of his tongue, and the youngster is wise in licking an ice cream cone, for the end of his tongue best responds to sweetness. Also, the slow eating of ice cream allows it to warm up a bit, thereby giving off more aroma. Besides the four true tastes of sweetness, saltiness, sourness, and bitterness, there is the intense sensation of coolness produced when we eat a peppermint patty or other composition containing menthol. Menthol produces an icy cold sensation that is a prominent part of flavor. It can do this anywhere on the skin of the body but works fastest in the mouth or nose. Spices, such as ginger or pepper, taste hot. Oil of cloves tastes hot for a moment and then numbs the tongue. Alcohol, in various dilutions, also has heating and numbing action when in the mouth and if used as a depth bomb. Besides stimulation of warmth, coolness, and numbness, there can also be pain, as from too much red pepper, and textural consciousness. Thus all of the factors of touch contribute to flavor. Still more important is smell. All of us are aware of the aroma that greets our noses when we enter a house where food is being prepared. Of far greaterimportance is the aroma that is released within the mouth whiie food is being eaten. The aroma courses up to the nose by the b+ck way, inside the head. Under these very favorable conditions of moisture and temperature, even substances such as fats and oils, that seem odorless, produce enough aroma that it becomes an important
part of the flavor. Almost all the flavor of fruits, and much of that of most other kinds of food is smell. Food workers soon learn that smelling is an effective way of judging much of the flavor of food, and smelling doesn't put on weight. The significance of this whole matter is that volatile flavorings are the most important of all. The sense of smell is very sensitive and is extraordinarily analytical and discriminating. Whereas there are only a comparatively few distinctively different tastes, there are many thousands of diierent odors. Odors differ in many ways besides the obvious smell differences, such as in elusiveness or persistency, gentleness or pungency, and in their diierent tendencies to dissolve in water or in oil. This does not mean, however, that there exist any great number of kinds of odor response cells, each wlth appropriate nerve connections. Odors have been classified on the basis of the stimulation of only six or even of four kinds2 of nerves, each stimulated to relatively different degrees by any particular odor. Besides the ability to detect odor, the nose also is responsive to certain feelings, such as coolness, contact, and pain. Menthol feels cool on smelling, and most so-called "strong" odors cause pain in the nose. Pungency probably is only a pain in the nose. Summarizing, what we smell is true smell plus feeling, and what we taste is true taste plus feeling, involving thus the three senses of smell, taste, and feeling for flavor detection. Nearly all flavors are organic in the chemical sense. The only important inorganic flavor is salt. Even the so-called chlorine flavor of water is not that of the element chlorine, but is due mostly to chlorinated organic substances. Of the organic flavoring substances, some, such as citric, tartaric, and a few other fruit acids and most of the alkaloids and other bitter substances, are nonvolatile. The great majority, however, of flavoring substances are volatile and depend upon stimulation of the sense of smell for their activity and value. Flavor chemicals generally contain carbon, hydrogen, and oxygen only, hut some important ones also contain sulfur or nitrogen, or both. Almost never is any other chemical element involved. With these few elements, nature has produced an abundance of odors, many of which are of interest in food flavors. It is hardly a requirement for a cook to have studied organic chemistry, but i t might sometimes be a help. To most of us, food flavors are another role for chemicals, some of which we would never ordinarily associate with edibles. Let us survey the various classes of organic compounds, to see what manner of substances we eat, and like. Among the hydrocarbons in foods the terpene dipentine is perhaps the most conspicuous. Oils of orange, lemon, and lime contain about 90 per cent of this substance. Many other essential oils contain terpenes,
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'Refer to Crocker-Henderson system of odor classification. presented in the McGraw-Hill Co. hook. "Flavor." by the author.
too. These are considered not especially desirable, for most of them become varnishy flavored, or so-called "terpy," on oxidation. Some of them, as in oils of ginger and peppenhint, may act as carminatives. There are times when we may wish to belch, but generally a burpless flavor is desired. To prevent excessive tendency either to terpiness or to burpiness, some of the terpene-rich oils are deprived of their terpenes. It must be said, however, that these deterpinated oils, while smooth in flavor and useful, are considerably short of the realism and complete flavor of the terpene-containing oil. Alcohols are found in fermentation products and in some essential oils, but their oxidation products, the aldehydes, and the combinations of alcohols with acids, the esters, are much more common. Flavor alcohols, of course, include ethyl alcohol and the so-called fuse1 oil alcohols, chieily isohutyl and isoamyl. The alcohols present in oils of spices, herbs, and flowers are mostly terpene alcohols with the empirical formula CloHaO, and include the geraniol and citronellol of roses and the terpineols of many coniferous oils. A few, however, are nonterpene in character, as octyl alcohol and phenylethyl alcohol. Ethers are comparatively rare, and those that exist are mostly the methyl ethers of phenols, known as methoxyl compounds. The so-called "fruit ethers" are chemically not ethers a t all but are esters, frequently the ethyl esters of acetic, hutyric, and other volatile acids. Esters are very common in the flavors of fruits and spices, and in some instances, of flowers as well. Linalyl acetate, for instance, is typical of lavender, methyl salicylate of wintergreen and birch oils, and ethyl acetate is the principal substance besides acetic acid in vinegar. It is prominent in the bouquet of all liquors. Isoamyl acetate, often called banana oil, smells like bananas. The fats, whether of vegetable or animal origin, are the glyceryl esters, principally of oleic, palmitic, and stearic acids. They have little or no true taste and hut slight odor, when pure, but contribute greatly to the textural part of flavor. Acids are very common food flavor ingredients. The volatile ones range from the acetic acid of vinegar, through the propionic acid of Swiss cheese to the bntyric acid of butter and the traces of caprylic and caproic acids of the stronger cheeses. The nonvolatile acids include the citric of oranges and lemons, the malic of apples, and the oxalic of rhubarb. Acid anhydrides exist in flavors produced by heating and by the action of certain bacteria. Most important among these is diacetyl, which is a sort of anhydride of acetic anhydride. Diacetyl is characteristic of the odor of butter but is also to be found in such other diverse flavors as cheese, beer, tobacco smoke, honey, and maple syrup. Ketene, another anhydride, smells of the pickle barrel. Aldehydes add vigor and pungency to many flavors. ~h~~~ of interest include the acetaldehyde of wine, the henzaldehyde of wild cherry flavor, cinnamic
aldehyde of cinnamon and cassia, and the substance vanillin. These are commonly present in large proportions, while a wide variety of other aliphatic and aromatic aldehydes are present only in traces. Ketones produce camphoric and weedy notes in lavender, eucalyptus, sage, and many other herbs. Ketones also produce odors such as violet and oms, which are sometimes used in candy. Phenols are found in food flavors. Two examples are the guaiacols or creosotes which are absorbed by ham, turkey, fish, or frankforts from hardwood smoke, and the eugenol of oil of cloves. The phenols and their methyl ethers are very common in spice oils. Nitrogen compounds in great variety exist in food flavors and contribute a great deal of distinctiveness. They are the methyl amines, which produce the flavors of fish and of shellfish and the butyl and amyl amines which have vegetable character, especially beaniness. Canraloupe melon is almost unique among fruits in that i t has a relatively high pH, sometimes being even markedly alkaline, and gives off an amine that has unusual ability to contaminate other foods stored with it in a refrigerator. The amino acids of foods contain nitrogen. When coupled together as proteins, amino acids have very little flavor, but a few of the free acids, especially glutamic, have pronounced taste. The pyridines and qninolines are present in tobacco smoke and in the browned crusts of bread and meats. The ester
methyl anthranilate, that produces the distinctive character of Concord and Delaware grapes, contains nitrogen. Minute traces of the bases indole and scatole are released when proteins decompose and sometimes get into food flavors. Sulfur compounds tend to produce sharp, harsh, and pungent flavors, such as the odors of onions and garlic. They are responsible for all the odors and the tingle of products of the cabbage family, such as turnips, sauerkraut, mustard, and radishes. Eggs release some of their sulfur on boiling. Sulfur compounds exist in traces in some fruits, such as tangerines, temple oranges, and the durian fruit, and contribute to the tang. This recitation of the various chemical substances that produce food flavors should produce mixed emotions. On the one hand, some of us may be almost horrified a t the thought of some of the chemicals that we eat, and which taste good. Apparently, in the quantities present in foods these substances are relatively harmless. If they do a little damage we quickly recover therefrom, for such slight toxicities as are present are usually not cumulative, as are some of the mineral poisons. On the other hand, i t is evident that there is only one chemistry, for all purposes. Nature draws upon many of the substances for flavoring purposes which we think of as belonging only in the laboratory. We should have more appreciation of both the chemicals and the flavors to which they contribute.
