Man is not merely a physiological organism, but is also a rational and social being. Therefore, problems of food choice and of other behaviour related to food have to be considered from a physiological, as well as from a social, cultural or economic point of view.
In the first chapter of this thesis a review is given of the principal cues of food behaviour and of the factors determining food choice.
Hunger appears to be the main physiological motive for food intake, while appetite is more related to its psychological regulation. The mechanisms reponsible for these cues are not totally understood. Evidence, however, is available that hormonal signals from the bloodstream and the fat tissues play an important role. Further is known that ventromedial and -lateral nuclei of the hypothalamus are involved in the regulation of food intake and that satiety feelings are evoked by the sense of taste in the mouth. Certain elasticity senses in the gastrointestinal tract are suggested to play a role as well.
As to the regulation of the quality and the quantity of the foods to ingest, some (older) authors suggested the existence in man of an inborn quality by which he is able to select adequately the right foods in the right amounts.
Food acceptance, however, is so strongly controlled by psychosocial, cultural and other factors that this selfselection quality is practically totally overshadowed by them. Consequently no practical meaning can be ascribed to it, at least in our society.
Since in food behaviour studies sensory experiments are commonly used tools, in chapter 2 a number of general and specific motives are given for further taste research. They are:
1. The important role of taste and odour in food selection and other behavioural phenomena.
2. Apart from its physiological function food has aesthetic functions, as well.
3. The consumer exhibits preferences and avoidances towards specific sensory food properties and is willing to pay considerable amounts of money for palatability and flavour.
4. The term 'quality' is gradually getting the meaning of 'sensory quality' in our countries.
The applicability of the sensory research on different fields forms another category of motives for taste research:
1. For the food technologist it is of great importance to have at his disposal possibilities for testing the sensory quality of his products and for evaluating the chances of acceptability a new product is likely to meet when it appears on the market.
2. For those concerned with planning and execution of nutritional and health projects, insight in and knowledge of the patterns of tastes and preferences of the target groups and categories are of great use.
3. Taste research has shown itself to be a tool for the tracing of ethnic relations between a variety of races and tribes.
4. It can be demonstrated that clear relations exist between taste sensitivity and some physiological and pathological variables. Although in these fields sensory experiments are rather common, frequently one is struck bij the lack of fundamental knowledge about taste sensitivity. Among others this is caused by the fact that between the different disciplines an exchange of thoughts is very difficult to obtain. The purpose of this thesis is therefore to integrate taste testing experiences from different application fields as much as possible and to make them accessible for others.
As usual in taste sensitivity research a restriction was made as regards the threshold value determination of the four primary taste qualities.
In chapter 3 a review is given of the main types of thresholds. The great variety of threshold data in the literature is discussed. The differences are due to variations in definitions, methodology, quality and purity of the taste substances used, solvent media, and so on. It is needless to say that differences in experimental subjects play an important role, too.
In this chapter attention is also drawn to the discussion about the possible existence of either one receptor system, in which all receptors respond to all taste stimuli or of a receptor system in which at least 4 types of receptors are present to respond selectively to any one of the primary tastes.
In a series of identification experiments (chap. 4) was shown that the mean threshold values resulting out of an arbitrarily increasing series of 15 concentrations of taste substances, were totally identical with those obtained from a series in which 12 concentrations were offered in a mathematically increasing order. Both series were caried out 4 times with sucrose, lactic acid and sodium chloride, with 10 and 15 tasters respectively in the 2 experimental groups. The mean threshold values for the 3 taste substances were 0,016 ± 0,012; 0,0072 ± 0,0038 and 0,016 ± 0,008 Mols/l, respectively. A positive, but weak correlation between the several sensitivities could be demonstrated so that lineair regression equations could be calculated.
Since the methods of determination of the threshold values as described appeared to be very laborious and time consuming, in the next experiment a design was developed in which at least 6 panel members could be examined for their taste sensitivity in the same time, This design actually consisted of a randomized presentation scheme in which were assembled all concentrations, repetitions, placebo's for the 4 taste substances under consideration. As taste substances were chosen 7 concentrations of sucrose, sodium chloride, citric acid an quinine sulphate. The experimental -design was planned in such a way that also the possible influence of the colour of a few kinds of artificial light could be studied (chap. 5). The total project consisted of presenting 7 concentrations of every taste substance in twelve fold, including 96 samples of demineralized water. Sixteen tasters completed the project (= 72 sessions). The mean threshold values of these tasters were in Mots per liter: 0,0091 ± 0,0047 (sucrose), 0,0078 ± 0,0054 (NaCl), 0,00023 ± 0,00022 (citric acid) and 0,000 00167 (quinine sulphate). All judges showed the following order of decreasing sensitivity: quinine sulphate>citric acid>NaCl>sucrose.
Analysis of the responses to the given placebo's made it possible to discriminate between relatively 'good' and relatively 'bad' tasters, according toe their capacity to describe and recognize the samples as water. Both categories showed a different pattern of incorrect responses:
1. All tasters are inclined to describe the demineralized water erroneously as 'bitter'.
2. The good tasters, moreover, seem to describe it frequently as 'sweet'.
3. A decrease in capacity to describe and to recognize the water correctly, is accompanied by a decrease in inclination to call it 'sweet', and an increase in mistakingly calling it 'bitter', 'sour' or 'salty'.
4. Consequently the ratios (incorrectly) sweet: bitter, sweet: sour, and/or sweet: salty, decrease. Probably this is also valid for the ratio salty: sour.
In chapter 4 the effects of training and experience are also studied. An active training period in which the taster gets instructions and advice is usually rather short (4 or 5 sessions) but its effects are very clear. Frequently it is followed by a period during which no further instructions are given, but in which a taster builds up his own experience. Although the effects are usually rather small, they last much longer. Training periods are necessary whenever one starts a new method of research, but experiences appeared to persist for more than half a year.
In the experiment described, the process of building up experience was manifest during the first 50 sessions. The number of correct responses (y) appeared to increase with the number of sessions (x) according to the regression equation y = 57,27 + 0,176x. The correlation coefficient r = 0,489 is significantly different from 0, with p < 0,01. Differences were observed between the tasters as well as in the taster themselves (inter- and intra taster differences). From taster to taster the number of correct responses varied from 53,2 % to 75,2 %.
For an estimation of the intra-individual differences a mathematical treatment of the results obtained was needed. With help of the method of maximum likelyhood it could be shown that the relation between the concentration and the fraction correct responses can be described by the equation for the cumulative normal frequency distribution.
With help of this relation and the observed number of correct responses estimations could be made of the individual parameters p o
, μan σfor the different taste substances (table 28).
Finally an analysis is made of the frequencies with which taste qualities the panel members are inclined to confuse the presented samples. In general, tasters appear to confuse relatively seldom the given taste substances with salt or sour, but relatively often with demineralized water or bitter. Probably the degree of familiarity with primary tastes plays a role. From the incorrect responses an order of decreasing familiarity could be deducted: salt>sour>sweet>bitter>demineralized water. Deviations of this order are connected with the research design used, the circumstances during the experiments, and the 'quality' of the tasters. In tasting subthreshold concentrations of quinine sulphate or of sodium chloride, good tasters do describe them much more frequently as sweet than as sour, while the less good judges confuse them as often with sour as wich sweet.
The effects of colours on taste perception are discussed in chapter 5. These effects are partly associative, partly physiological in character. While the associative character of colours in relation to foods is well known, the physiological aspects in this context need further experimentation. A critical discussion of the scarce data in the literature shows that knowledge about this is mainly empirical. From the experiments of chapter 4 it appears that the physiological action of the different colours in relation to the individual results of the panel members is so weak, that as to the inspected results on an individual basis the influence is negligible, but for the panel members as a group, one ought to take it into account.
Analysis of the group data shows that colour effects are significantly present. Application of identical intensities of sodium light (yellow) and mercury light (blueish) resulted for the panel as a group in a significantly higher amount of correct responses compared with artificial daylight of the same intensity. This was valid at least for the taste substances sucrose, citric acid and sodium chloride. For the bitter substance quinine sulphate this phenomenon was not observed.
It could be demonstrated that the colour effects manifest themselves especially in the steep parts of the log dose response curves. As a consequence of the chosen experimental design this meant for the salt and sour taste substances an influence in the lower range of concentrations used and for the sweet substance an influence in the higher range. The steepness of the curve (a) plays an important role in this. As a consequence of the applied colours the curves showed a shift towards the lower concentrations; in other words: the panel members seemed to be a bit more sensitive. Finally attention is drawn to the fact that the observed phenomena do not form an isolated case, but that in the physiological and psychological literature analogous heteromodal interactions are discussed. According to this literature weak interacting stimuli can result into an enhancement of the cortical alertness in general, and as a consequence into a lowering of the threshold values. Strong interacting stimuli should have the opposite effects. It is concluded that the observed fall in threshold values and its accompanying features can result from the interacting effect of the applied light and that in comparison with the intensity of the taste stimuli, the light stimuli probably are perceived as very weak. The results seem to support YOUNG and HELMHOLTZ'S conception about the existence of several systems of coulour vision.