The Biological RÔle of Taste

In considering the function of the sense of taste it is common to dismiss the topic in a summary manner by pointing out the fact, that in its original primitive conditions, at least, this sense enables the organism to discriminate between wholesome and deleterious food. This function is, of course, not to be neglected, especially if due credit be given to the rÔle played by smell in the same service. It is however true that, although the indications of taste and smell may be, for lower forms of animal life, fairly trustworthy guides in the selection of edible substances, such criteria as taste and odor can by no means be relied on by human beings. In a general way it is, of course, true that wholesome substances possess taste qualities which are agreeable and enjoyable, while foul, decaying and poisonous substances are often characterized by tastes and odors that arouse in us disgust and revulsion.

But in the complex lives of human beings, at least, this sort of natural adaptation is far from adequate to constitute a dietetic guide. Not only is it true that many substances accessible to human beings are injurious and unwholesome, in spite of their agreeable taste; it is equally true that many substances that are initially distasteful may be either nourishing or remedial. Human beings find it necessary to supplement, or even to supplant, the “beneficent guardianship” of taste by the introduction of various other sources of information and criteria of selection.

We do not find, however, that the sense of taste shows any evidence of deterioration as the result of such loss of function. Probably never before in the history of our race has there been such diligence and zeal in ministering to the demands and satisfactions of our appetite. In the preparation, marketing, and serving of food the appeal through tastefulness and flavor stands second only to that through purity and cleanliness. The situation is neatly stated by Jane Addams in the following words: “Perhaps the neighborhood estimate (of their New England kitchen) was summed up by the woman who frankly confessed that the food was certainly nutritious, but that she didn’t like to eat what was nutritious; that she liked to eat ‘what she’d ruther.’”

It is clear at once that the satisfactions of taste are pursued, either secretly or openly, for their own sake, and often in obvious disregard of their dietetic consequences. Shall we dismiss this pursuit as only an instance of the search for pleasure in its own right or wrong, or is there to be discerned a further function of taste experiences quite aside from their guidance to eating, their warning of danger, and their immediate sensory pleasure?

Taste and Digestion

Recent studies of the rÔle of taste in the economy of the organism show very decidedly that such an additional function, and a very important one, must be recognized. These investigations have revealed the fact that the pleasurable taste of food (along with its agreeable odor and appearance) is responsible for the initiation of the first stages of the process of digestion. Further, they have shown that disagreeable tastes and odors (along with pain, fear, and other emotional excitement) may effectually retard or even completely inhibit these very important processes. Not only does the mouth “water” at the smell, or sight, or thought, of delicious morsels, but the stomach itself responds, in an anticipatory fashion, to the taste of agreeable substances placed in the mouth. That “good digestion waits on appetite” is a physiological fact, as well as an ancient proverb.

In the middle of the last century two observers in Leipsic reported the pouring out of gastric juice at the mere sight or smell of a favorite food in the case of “a hungry dog which had a fistulous opening through the body wall into the stomach.” More recently it has been experimentally shown that similar flow of gastric juice follows upon the tasting of agreeable food, even if the food itself is not allowed to reach the stomach. So relevant are these findings to the point we are now considering that the following summary, by Cannon, of the work of Pavlow may well be quoted in detail:

Experimental Evidences

“The feelings or affective states favorable to the digestive functions have been studied fruitfully by Pavlow, of Petrograd, through ingenious experiments on dogs. By the use of careful surgical methods he was able to make a side pouch of a part of the stomach, the cavity of which was wholly separate from the main cavity in which the food was received. This pouch was supplied in a normal manner with nerves and blood vessels, and, as it opened to the surface of the body, the amount and character of the gastric juice secreted by it under various conditions could be accurately determined. Secretion by that part of the stomach wall which was included in the pouch was representative of the secreting activities of the entire stomach. The arrangement was particularly advantageous in providing the gastric juice unmixed with food. In some of the animals thus operated upon an opening was also made in the esophagus, so that when the food was swallowed it did not pass to the stomach, but dropped out on the way. All the pleasures of eating were thus experienced, and there was no necessity of stopping because of a sense of fullness. This process was called ‘sham feeding.’ The well-being of these animals was carefully attended to; they lived the normal life of dogs, and in the course of months and years became the pets of the laboratory.

Pavlow showed that the chewing and swallowing of food which the dogs relished resulted, after a delay of about five minutes, in a flow of natural gastric juice from the side pouch of the stomach—a flow which persisted as long as the dog chewed and swallowed the food and continued for some time after eating ceased. Evidently the presence of food in the stomach is not a prime condition for gastric secretions, and, since the flow occurred only when the dogs had an appetite and the material presented to them was agreeable, the conclusion was justified that this was a true psychic secretion.”

In several cases necessary operations on human beings have permitted of observations similar to these experiments on dogs. Thus, Richet, who had opportunity to observe such a case, “reported that whenever the girl chewed or tasted a highly sapid substance, such as sugar or lemon juice, while the stomach was empty, there flowed from the fistula a considerable quantity of gastric juice.” Another observer, Hornborg, “found that when the little boy whom he studied chewed agreeable food a more or less active secretion of gastric juice invariably started, whereas the chewing of an indifferent substance, as gutta-percha, was followed by no secretion.”

Carlson has reported numerous similar observations on an adult. In the case of this man the sight, smell, or thought of food, even when he was hungry, was inadequate to cause the gastric juice to flow. Moreover, “the mere act of chewing indifferent substances, and the stimulation of the nerve endings in the mouth by substances other than those related to food,” caused no secretion. But a few minutes after the taste organs were stimulated by edible substances it was seen that not only did the flow of gastric juice begin, but the “hunger contractions” of the stomach were inhibited. Further, the secretion of gastric juice in this patient was clearly seen to vary with the palatability of the food. The chewing of bread and butter yielded a smaller flow than did the mastication of meat, and the flow was always greatest during the chewing of desserts, or on occasions when the food was said to be “unusually fine.” Oranges, of which the patient was especially fond, produced a greater flow than did pies, puddings, and other fruits. Carlson, in reporting these observations, says: “There is no question but that the mastication of a palatable dessert at the end of a meal serves to augment and prolong the appetite secretion of the gastric juice.”

In referring to such cases Cannon has concluded: “All these observations clearly demonstrate that the normal flow of the first digestive fluids, the saliva and the gastric juice, is favored by the pleasurable feelings which accompany the taste and smell of food during mastication, or which are roused in anticipation of eating when choice morsels are seen or smelled. These facts are of fundamental importance in the serving of food, especially when, through illness, the appetite is fickle. The degree of daintiness with which nourishment is served, the little attentions to Æsthetic details—the arrangement of the dishes, the small portions of food, the flower beside the plate—all may help to render food pleasing to the eye and savory to the nostrils, and may be the deciding factors in determining whether the restoration of strength is to begin or not.”

The Function of Taste in the Organic Economy

The preliminary nature of these anticipatory secretions of the digestive juices is by no means an adequate measure of their ultimate importance. Studies of the mechanism of digestion show that each stage, as it occurs, either directly or indirectly, through its product, affords the appropriate stimulus which evokes the following stage. Thus, continued flow of gastric juice is provided for by the action of the preliminary flow or its products on the walls of the stomach; and other juices, such as the bile and the pancreatic, are in turn released by the action of this continued flow.

The pleasurable sensations of taste are thus the initial stimulus to the whole series of digestive processes. Even in the absence of hunger these sensations seem potent to initiate the digestive mechanism. Among the most interesting of recent physiological studies are those showing the very great sensitivity of the important organic mechanisms, especially those of secretion, to such experiences as shock, worry, fear, anger, grief, excitement, and pain. All these factors tend to retard the activity of the digestive system, while they may also be seen to reËnforce the activity of other mechanisms. As opposed to the effect of these factors, pleasurable experiences connected with food serve not only to guide the organism in its choice, but play an important part in its effective appropriation and assimilation, through their action in setting the digestive mechanisms in action, and in guaranteeing the continuation of this action after the completion of the act of eating.

Music and dance, jest and general merriment, genial conversation and cordial friendship, prosperity and individual success, fragrance, color, bodily ease, and a clear conscience—these and all the other joys of life play their part in promoting the bodily welfare of the organism. Conspicuous and potent among these favoring influences are the sensations of taste and the strongly toned feelings with which they are so closely associated—“the satisfactions of the palate.” Even the various “bitters” which are so commonly used as “appetizers” seem to owe such efficacy as they may possess to the influence of their taste on the preliminary flow of “appetite gastric.” Carlson has shown that these bitters, introduced directly into the stomach in medicinal doses, have no influence on the hunger mechanisms. In larger doses their effect is inhibition of hunger. Acting in the mouth, they also retard the hunger contractions of the stomach in proportion to their intensity as taste stimuli. In so far, then, as “bitters” are “appetizing,” it is by virtue of their taste qualities, rather than their medicinal properties, and the act of swallowing them would seem to be superfluous.