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Mixtures can often be tasted when all components are too weak to be tasted separately. Such mixtures are said to be integrative. Integration was demonstrated by mixing compounds in concentrations proportional to their separate detection thresholds and then measuring the detection threshold of the mixture as a whole by forced choice with plain water. Mixtures of 3, 6, 12 and 24 compounds were thus evaluated. With earlier data on 2-, 3- and 4-component mixtures, the results show that the concentration of any constituent compound goes down in approximate proportion to the number of compounds with which it is in mixture. This nearly complete integration describes mixtures of like-quality compounds, of unlike-quality compounds, and of both like- and unlike-quality compounds together. Integrative mixtures provide a model for the detection of the ultracomplex stimuli of everyday life, such as foods and drinking waters. Although the degree of integration may trail off slightly with mixtures of high complexity, the results proffer no limit on the number of compounds that can be at least partially integrated. In principle, integration permits detection of natural substances whose myriad components could all be far below threshold. The mechanism of taste integration is speculative, but the facts are congenial to the hypothesis of multiple parallel channels for the processing of intensity and quality.

Mixtures of compounds can often be tasted even when all of their components are too weak to be tasted separately. Such mixtures are said to be integrative. Integration was demonstrated by mixing compounds in concentrations proportional to their separate detection thresholds and then measuring the detection threshold of the mixture as a whole by forced choice with plain water. Mixtures of 3, 6, 12, and 24 compounds were thus evaluated. With earlier data on two-, three-, and four-component mixtures (Stevens, J. C. Detection of taste in mixture with other tastes: Issues of masking and aging. Chem. Senses 21:211-221; 1996.), the results show that the concentration of any constituent compound goes down in approximate proportion to the number of compounds with which it is in mixture. This nearly complete integration seems to describe mixtures of like-quality compounds, of unlike-quality compounds, and of both like- and unlike-quality compounds. Integrative mixtures of the sort studied here provide a model for the detection of the ultracomplex stimuli of everyday life, such as foods and drinking waters. Although the degree of integration may trail off slightly with mixtures of high complexity, the present result proffers no limit on the number of compounds that can be at least partially integrated. In principle, integration permits the detection of natural substances whose myriad components could all be far below threshold. The mechanism of taste integration is speculative, but the facts are congenial to the hypothesis of multiple parallel channels for the processing of intensity and quality.

Detection thresholds for sodium chloride were compared in aqueous solution, in mixture with a sucrose masker, in mixture with a citric acid masker, and in mixture with both of these maskers together. Separately the two maskers raised the threshold of sodium chloride by three to four times, and together by over nine times, a result consistent with independence (additivity) of the two masking effects. To achieve comparable masking with either sucrose alone or with citric acid alone would require increasing their masking concentrations by about ten times. Hence multiple masking can be a far more efficient means of concealing a taste, whether an unpleasant one (e.g. the bitter taste of medicine) or a pleasant one (e.g. a salty or sweet condiment). Multiple masking has dietary and culinary significance, especially for middle aged and elderly persons concerned about salt intake, because their thresholds for NaCl, whether with or without maskers, are typically two or three times higher than those of youthful persons.

When one taste (masker) is strong enough, it can completely mask another task (target) of different quality. How strong the masker must be to do this depends on how strong the target is. As the target concentration is increased, the masking concentration must be increased, too, but in ever-increasing proportion. To quantify the conditions for such complete masking, the target's detection threshold was measured as a function of the masker's concentration, from zero to strong. This was done for 12 binary combinations of sucrose, sodium chloride, citric acid and quinine hydrochloride. The 12 functions generated show that some tastants mask each other much more efficiently than others. Masking gives new insight into the role of aging in taste: older (66-90 years) subjects' thresholds, regardless of masking concentration, always measured a constant factor higher than younger (18-29 years) subjects' thresholds (about two to seven times higher, depending on target tastant). Thus, with increasing level of the masker, the thresholds of young and elderly go up in parallel. Thresholds of tastants in water alone are false predictors of elderly persons' ability to perceive ingredients like salt and sugar condiments in foods, where, because of masking, their thresholds can be several times higher than in water. Age manifested itself relatively mildly in sucrose and citric acid, moderately in sodium chloride, and strongly in quinine hydrochloride.

Detection thresholds were measured for sweet (sucrose), salty (sodium chloride), sour (citric acid), and bitter (quinine hydrochloride) and for the 11 possible mixtures of these four substances. These 11 mixtures (6 binary, 4 ternary, and 1 quaternary) all turned out to be stimulus additive, in the sense that a person could reliably detect mixtures whose individual components are weaker than their unmixed thresholds. Tastants too weak to be perceived alone can thus make impact when in mixtures. The threshold concentration for a given compound was reduced in approximate proportion to the number of compounds added to it. This liberal heteroquality additivity contests the widespread belief that heteroquality mixtures (different chemicals evoking different qualities) are non-additive and homoquality mixtures (different chemicals evoking the same quality) are additive. Heteroquality additivity emerges on appropriate definition of the subject's task by forced choice (unavailable to earlier investigators), in order to skirt methodological pitfalls. Operating together, homo- and heteroquality additivity may concomitantly enable a person to sense natural mixtures of hosts of weak constituents, such as drinking water. In this regard, gustatory mixtures may function much as do mixtures of frequencies in audition and mixtures of gaseous compounds in olfaction.

This study aims at understanding the role of mixtures (mutual quality suppression) in the evaluation of impact of the human aging process on the perception of taste. Heretofore, the effect of aging on taste has been directed at threshold and suprathreshold magnitudes of single chemicals (e.g., NaCl, sucrose, citric acid) in aqueous solution. Although absolute thresholds typically rise in advanced age (2 to 9 times, depending on the study), suprathreshold magnitude assessed by magnitude matching seems (except for bitter) to resist change in the way presbycusis spares suprathreshold loudness, fostering the impression that aging may handicap the aged little in the perception of food. Asked, however, to discriminate the presence-absence of the prescribed salt flavoring (nominally suprathreshold) in tomato soup, the young outperformed the middle-aged who, in turn, outperformed the elderly. Moreover, NaCl thresholds in the presence of tomato measured several times higher than in water, but the difference between the young and the elderly continued to hold. Elevation of threshold to much higher levels by mixture suppression leaves the young-elderly difference unchanged, implying that the elderly may fail to detect salt levels that really count in their diet. To examine the relation between age and taste mixtures, we measured detection thresholds: (1) for NaCl in citric acid, from zero to strong; (2) for sucrose in citric acid, from zero to strong; and (3) for citric acid in sucrose, from zero to strong. Whether in water alone or in a weak or strong suppressor, the elderly subjects' threshold was consistently 2 or 3 times higher than that of the young. Moreover, the way in which threshold for one quality rises with concentration of a suppressor is the same, except for constant upward displacement of the elderly peoples' threshold. In general, both young and elderly confuse salty and sour (show large suppression at all concentrations of the suppressor) much more than they confuse sweet and sour (seen mainly at high concentrations of the suppressor). Study of other mixtures is planned.

This study reports on the ability of young, middle-aged, and elderly persons to discriminate the presence-absence of the salt flavoring in a published recipe for a soup. Although all but one young person tested was able to make this discrimination above chance level, more than half of each of the two older groups failed to do so. Both the discrimination score and the absolute threshold for NaCl in water solution correlated significantly with age over a span from 18 to 89 yrs. These results add to previous ones from a similar study of the discrimination of an aromatic spice, marjoram (Cain et al., 1990), in demonstrating that taste and smell weakness revealed in recent psychophysical tests can reveal themselves in the perception of everyday food and beverage preparations. A secondary study compared thresholds for NaCl in water with NaCl thresholds in the presence of tomato, the principal ingredient of the published recipe. These thresholds, presumably because of "mixture suppression" of tastes, were from seven to ten times higher than thresholds in water. Although thresholds for young and old were more alike in the tomato medium than in water, the elderly nevertheless needed over twice as much salt concentration than did the young just barely to appreciate its presence. Computer evaluation of the salt content of a large number of soup recipes revealed that (a) the salt content of the tomato soup we used to study discrimination was reasonably representative of other published recipes for tomato soups and (b) that the salt content of the average of these tomato soup recipes was greater than that of the average of non-tomato soup recipes.

An experiment explored how well young, middle-aged, and elderly subjects could discriminate the presence or absence of the spice marjoram in a soup prepared according to a published recipe. Whereas most young made the discrimination at a level above chance, most middle-aged and elderly failed to. A clinical test that entails odor threshold and identification revealed that olfactory ability also diminished with age among these subjects. Furthermore, the odor thresholds measured in the test correlated significantly with the subjects' ability to discriminate flavor. Subsequent testing wherein subjects sought to discriminate flavor with their noses blocked confirmed that olfactory ability largely underlay the discrimination. The results emphasize that losses in olfaction measured most commonly with environmental odors are serious enough to have an impact on discrimination of flavors in everyday foods, even among persons of middle age. Loss of such discriminative capacity may entail risks of avoiding dangerous and overlooking beneficial ingredients in foods.

Over several years, we have compared olfaction in elderly subjects with that in young subjects. The main tests have been 1) absolute threshold, 2) magnitude matching to assess whether aging impairs suprathreshold odor and trigeminal strength, and 3) odor identification. As have other laboratories, we have found impairment by all three measures. In one study, we devised a composite score of olfactory competence based on all three measures; the outcome suggests that it is uncommon for a person over 70 years to escape a degree of impairment, although the degree varies from person to person. We speak here of persons living at home and regularly attending senior citizens' centers in New Haven. After testing, we often gave a questionnaire having to do with the subjects' health and occupational history. Under the circumstances of our subject selection and testing, the questionnaires have revealed no striking correlations. Among the questions asked have been the following: 1) Do you have a problem tasting? 2) Do you have a problem smelling? 3) Have you noticed changes in tastes of foods? 4) Do you enjoy food? Of 276 elderly subjects, 17% answered yes to one or more of questions 1-3 and/or no to question 4. Only 5.8% of the total complained of weakened food enjoyment all or some of the time. Several were unable to say why. Some blamed a lifelong lack of interest in food; others, restricted diets, expense of foods, denture problems, etc. In contrast, the great majority (about 260 persons) rated the pleasure of eating with enthusiasm. Of 262 young subjects, 10% answered yes to one or more of questions 1-3 and/or no to question 4, and only four (1.5%) complained of food. Considering the losses involved, the rate of elderly complaining seems low, especially compared with much higher rates from institutionalized persons (for example, up to 33%). The reasons may or may not have to do with sensory factors, but the conclusion seems justified that a persons may profoundly enjoy the pleasures of food and drink despite a weakening of the nasal components involved.

The sweetness of sucrose depends on the temperature as well as the concentration of a solution. The main effect is that relatively low concentrations gain sweetness as temperature increases. This effect diminishes with progressively higher concentration and finally becomes negligible at about 0.5 M. At this concentration the various functions that relate perceived sweetness to concentration for various temperatures converge. The mechanism of the taste-temperature interaction is speculative, but the interaction is large enough to be of practical interest in the perception of common foods and beverages as well as a variable to be strictly controlled in taste experiments. An examination of method of tasting showed that swallowing stimuli did not substantially increase perceived sweetness.