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Sensory Evaluation of Food Flavor AMIHUD KRAMER
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University of Maryland, College Park, Md.
In the absence of adequate physical-chemical methods for measuring quality attributes of foods, it becomes necessary to resort to the human instrument. Such sensory procedures may be classified as descriptive, preference, or difference tests. Descriptive testing is accomplished by one or more trained individuals who provide descriptive evaluation of quality, usually with the aid of reference samples. Preference tests used for consumer acceptance utilize large numbers of tasters. Difference tests employ trained panelists. If a test fails to demonstrate a statistically significant difference, it is assumed that the samples are the same. This last procedure approaches most closely the use of the human taster as a foboratory instrument. evaluation of foods has been practiced ever since food was first Sensory produced. Various attributes of quality were measured and decisions
made on the basis of sensory—human—evaluation. Although isolated i n stances of the development of instrumental methods for measuring esthetic qualities of food, and statistical procedures for interpreting re sults, have appeared from time to time during the past century or even earlier, only recently have such procedures become available i n sufficient numbers so that an attempt could be made to organize them as a distinct discipline ( 9 ) . Since these food attributes are to be measured through the human senses, it is logical to classify this discipline i n broad categories i n accord ance with the human senses of sight, kinesthetics (feel or the muscle sense), and taste and smell. In certain special instances, sound may be considered a fourth category—for example, crackle of breakfast cereals, crispness of some vegetables, or sizzle of carbonated beverages (4). Thus, attributes of appearance are evaluated by the eye; kinesthetics by the muscle sense i n the hand, but primarily i n the mouth; and flavor by the taste buds in the mouth and the odor-sensitive patch i n the nose.
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Hornstein; Flavor Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
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During the last several decades, there has been very substantial suc cess i n the development of instrumental or chemical methods for measur ing appearance properties such as color, gloss, size, shape, and visual de fects. Kinesthetic attributes, which may be defined i n such descriptive terms as hardness, grittiness, and fibrousness, may also be measured satis factorily by objective means. The principles of these measurements are thoroughly understood and all that needs to be done to evaluate the ap pearance, or kinesthetic quality of specific food commodities is to relate scalar values obtained by instrumental methods with human evaluation in order to interpret such scalar values i n terms of preferences, or differ ences, as they may be sensed by a human consumer. T o this day, how ever, flavor factors are particularly difficult, if not impossible, to measure by any but the subjective human instrument. Thus, i n a food quality measuring system where objective methods should be relied upon, the use of a sensory taste testing panel is i n a sense an admission of failure except where an objective method is being tested for conformance to human evaluation. U n t i l there is a thorough physicalchemical understanding of flavor perception, it w i l l be difficult to con struct an instrument capable of measuring flavors, qualitatively and quan titatively, i n a manner similar to the evaluation of flavor by the human senses (3). Thus far there has been only one serious attempt to develop an i n strument that w i l l simulate human response to odor (6). This electro chemical approach is still i n the testing stage with testing of isolated com pounds. W i t h the rapid development of radically new research tech niques, such as gas chromatography accompanied by spectroscopy and nuclear magnetic resonance, there is the hope that a means for measuring flavor quality directly may be discovered i n the not too distant future (3). F o r the present, the four-dimension taste attributes of sweet, sour, salt, and bitter may be measured by determining quantitatively the chemical components contributing to these specific properties. In the odor area of flavor, much of the work with gas chromatography is in a similar direction —that is, identifying the volatile components that may be associated with the specific odor of the food product (3,11). The study of the interrela tionship of all of these volatile and nonvolatile substances to the percep tion, and more specifically to the preference, for particular flavors by the human consumer has only begun.
Statistical Procedures Because of the vast number of individual substances that are isolated on the gas chromatogram that may or may not have some relationship to flavor perception, it is necessary to use rather elaborate statistical proce-
Hornstein; Flavor Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
Downloaded by RUTGERS UNIV on May 17, 2017 | http://pubs.acs.org Publication Date: January 1, 1969 | doi: 10.1021/ba-1966-0056.ch004
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dures to determine which specific components in which specific combina tions have significant effects on flavor perception. This can be accom plished by using a statistical procedure called "stepwise multiple regres sion." In this procedure, the human taste panel is the criterion by which the usefulness of any particular component is measured, as contributing to the flavor sensation. Thus, the taste panel score is considered as the "de pendent" variable, which is correlated with every one of the "independent" variables. These are the individual components, volatile or not, as found by chemical procedures including gas chromatography that may con tribute to the flavor sensation. Data for each component are correlated with the taste panel data separately and all of the components are correlated with each other, to form a correlation coefficient matrix. The highest correlation coefficient between panel results and one of the independent variables is selected and all of the other correlations are recalculated on a partial correlation basis. The next highest correlation is then selected and if the multiple correlation is significantly better than the previous single high correlation, both are retained for use i n a regression equation, and the remaining correlations are recalculated, omitting the two selected. This process is repeated until all of the components which contribute to a significant increase i n the multiple correlation coefficient with the taste panel data have been se lected. In this way, the components having a significant effect on flavor perception are selected while the other components are omitted from the final regression analysis. Synergistic effects are revealed as significant interactions and antagonistic effects as negative correlations. The success of this procedure depends on the presence of an adequate set of samples i n which all variables are represented at all levels of quality. Such studies were conducted with fresh and processed green beans, where 32 components were separated by gas chromatography and corre lated with taste panel scores for flavor preference. In this situation, the taste panelists were instructed to score for intensity and quality of bean flavor. O f the 32 components, six were found to contribute towards a significant correlation with the taste panels. The multiple correlation co efficient, using these six components, was 0.76. Since the coefficient of determination is the square of the coefficient of correlation, this would i n dicate that these six substances explain just a little more than half (0.76 = 0.58 or 58% ) of the variability i n the panel scores. Although this indi cates a highly significant relationship between these six components and flavor evaluations of the taste panel, it still does not explain almost half of the variations in the panel scores for flavor. W h e n measures of fiber content (indicating toughness or stringiness of the beans), size, and color, were also included in the analyses, the multiple correlation coefficient rose to 0.94, indicating that almost 90% (0.94 = 0.88) of the variations i n 2
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Hornstein; Flavor Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
Downloaded by RUTGERS UNIV on May 17, 2017 | http://pubs.acs.org Publication Date: January 1, 1969 | doi: 10.1021/ba-1966-0056.ch004
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panel evaluations for flavor were explained by these six gas chromato graphic peaks, plus measurements of appearance and texture of the prod uct. This then is a demonstration that at least the ordinary consumer (if not the highly trained master taster) is influenced in his flavor perception, not only by the four primary taste stimuli, and an unknown number of odor stimuli, but also by stimuli affecting the sight and muscle senses. Stated i n another way, this is merely repeating what has often been sus pected—that the response to flavor quality is not entirely a function of taste and odor alone. Another possible explanation for the relatively low multiple correlation coefficient between panel scores and gas chromato graphic data alone is that some relatively high boiling compounds, which were not measured by the gas chromatographic procedure used, are also contributing to flavor (11). Taste Panels The use of a taste panel, as described above, is merely to provide scalar equivalence to objective methods. The usual application of the taste panel is for the many situations where a complete objective-instru mental procedure for evaluating quality is not available ( 7 ) . Since a l l types of taste panels may be grossly influenced by human psychological factors, results should be interpreted statistically. Thus, the taste panel conducted on a statistical basis can be said to be a psychophysical test based on psychometries. As with any statistical procedure, the condi tions must favor and one must assume a completely independent response for each individual who does the testing, and each sample must be pre sented under similar conditions. These sensory panels may be classified as descriptive, preference, or different panels. Descriptive testing is accomplished by one or more trained individuals who arrive at a descrip tive evaluation of the product quality, usually with the aid of specific odor, taste, or texture samples used as reference points. Such descriptive panels are useful particularly for product development, where specific odor or taste notes are to be enhanced, or attenuated. There is no room for statistical treatment, and the procedure may be described as being more of a highly skilled art than a science ( 2 ) . Preference tests are used ordinarily merely to evaluate consumer acceptance. Relatively large numbers of individuals are used and each panelist is required to select the sample he prefers or to rank the samples in the order of his preference. Difference tests employ panelists who are selected for their aptitude or trained to detect differences. If a test fails to demonstrate a significant difference, it is assumed that the samples are the same. This difference-type panel approaches the use of the human taster as a laboratory instrument (7).
Hornstein; Flavor Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
Downloaded by RUTGERS UNIV on May 17, 2017 | http://pubs.acs.org Publication Date: January 1, 1969 | doi: 10.1021/ba-1966-0056.ch004
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A member for such a difference-type panel is selected on the basis of two criteria: the individual's ability to distinguish among different samples, and his ability to duplicate results on the same sample. A third criterion may be of value only when an absolute scorer is required, such as when quality grades are provided (9). The environment in which the panelist works is important i n a num ber of respects. If he is to evaluate for flavor only, it is desirable to mask color differences. The panelist should also be relatively isolated so that he cannot be influenced by other panelists. The samples that are sub mitted to him should be as homogeneous as possible from the standpoint of the quality attributes other than the one which he is required to evalu ate. Thus, if flavor is being evaluated, color, size, and texture should be as uniform as possible for all samples. Order of presentation of the samples can be important. Ordinarily if the product is generally liked, the sample presented first w i l l be scored too high; if a product is disliked, it w i l l be scored too low. To avoid such errors i n order of presentation, samples can be presented entirely at ran dom, with sufficient replication, or they may be presented i n a latin-square design i n which each sample is presented in every position (5, δ ) . M u c h has been said regarding the number of samples that can be tasted at one sitting. Some authorities insist that a true value cannot be obtained unless one sample and one alone is tasted at a particular time. Others insist that fatigue, or adaptation, sets in very rapidly after three, four, or five samples have been tasted at one sitting. In general, the opin ion of some workers ( 8 ) is that particularly where samples are not very strongly flavored, or spicy, and do not have a carry-over effect, a time lapse of 1, possibly 2 minutes from one tasting to another is sufficient. The more samples are tasted within one session the more precise are the results likely to be, and more acuity may be achieved in differentiating small differences. This is because flavor memory is good over short periods of time but tends to fade rapidly from one day to the next. Ough et al. (10) present another attitude. Responses are ordinarily recorded on some scalar basis. This can take the form of the usual scoring system on the basis of 1 to 10 or 1 to 100, with the top value indicating the highest intensity i n the case of a dif ference test or the highest desirability in the case of a preference test. A scale balanced about 0 is frequently useful, particularly where two attributes are considered, one being positive and the other negative. Thus, for example, if the panel is required to score for sweetnesstartness, the center of the scale may be neither sweet nor tart and indi cated as 0. Positive values indicate degrees of sweetness while negative values indicate degrees of tartness. In the case of preference tests a simi lar hedonic scale can be used with increasing positive values indicating degrees of liking from "like slightly" to "like e x t r e m e l y w h i l e increasing
Hornstein; Flavor Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
Downloaded by RUTGERS UNIV on May 17, 2017 | http://pubs.acs.org Publication Date: January 1, 1969 | doi: 10.1021/ba-1966-0056.ch004
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negative values would indicate degrees of disliking, from "dislike slightly* to "dislike extremely." Another method of recording responses is to rank the samples i n order of preferences or intensity. Such a ranking procedure has the advantage of handling responses which are not necessarily evenly spaced i n a nonparametric manner. It should, however, be limited to ranking up to seven, possibly nine samples. Beyond this number, the ranking procedure becomes cumbersome ( 7 ) . Recently Schutz (12) has suggested a Food Action ( F A C T ) rating scale whereby the panelist indi cates how frequently he would wish to purchase the product. Finally, results obtained by these procedures, if they are obtained i n dependently and psychological bias is either eliminated or balanced, can be then analyzed statistically, ordinarily b y the analysis of variance. B y this statistical procedure, differences due to method, time of presentation, and quality characteristics measurable b y other means than panel scores, can be isolated and removed separately, and a decision can then be made on the basis of statistical probability regarding the difference i n the par ticular attribute of quality which is being measured. Results of such an analysis would not only indicate a general difference i n intensity or prefer ence as indicated b y all the panelists as a whole but also demonstrate any significant interactions among panelists and samples. Thus, i t is possible that a number of individuals may prefer, or may be particularly sensitive, to a certain flavor while others may prefer another flavor attribute. Such results can be uncovered by the use of panels whose data are analyzed b y these methods. Literature Cited (1) Beidler, L. M., "Facts and Theory on the Mechanism of Taste and Odor Perception," Quartermaster Food and Container Institute, Symposium, Chemistry of Natural Food Flavors, Chicago, Ill., 1957. (2) Caul, J. F., Advan. Food Res. 7, 1-40 (1957). (3) Dimick, K. P., Corse, T., Food Technol. 10, 360-364 (1956). (4) Drake, Β. K . , J. Food Sci. 30, 556-9 (1965). (5) Eindhoven, Jan, Ibid., 29, 520 (1964). (6) Hartman, J. D . , Food Technol. 11, 130 (1957). (7) Institute of Food Technologists, Food Technol. 18 (8) 1135-1141 (1964). (8) Kramer, Α., J. Agr. Food Chem. 9, 224 (1961). (9) Kramer, A . "Fundamentals of Quality Control for the Food Industry," Avi Publication Co., Westport, Conn., 1962. (10) Ough, C . S., J. Food Sci. 29, 506-9 (1964). (11) Pyne, A. W . , Ibid., 30, 192-200 (1965). (12) Schutz, H. G . , Ibid., 30, 365-74 (1965). RECEIVED May 4, 1965. Miscellaneous Publication 542. Contribution 3661, Maryland Agricultural Experiment Station (Department of Horticulture).
Hornstein; Flavor Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
