scope:

Introduction

Sensory problems are increasingly coming to the fore as man attempts to improve his food and stem the tide of water and air pollution. Basic data need to be collated and new knowledge acquired. One purpose of this publication is to tabulate the threshold values of odor and taste substances. The compilation also serves as the first step in determining gaps in knowledge.

Recent analytical advances such as gas chromatography and mass spectrometry have permitted the isolation and identification of compounds at rates and at concentration levels scarcely dreamed of a decade ago, but nonetheless, man often is capable of responding to substances at even greater dilution levels. This, however, is not an irresolvable problem. Through suitable concentration means, the odor or the taste substance can be generally brought within the range of present analytical methods, but one has to be able to relate the chemical determination back to the level detectable by man. Knowledge of threshold values is thus of paramount importance. In the control of water and air pollution, threshold determinations are used to evaluate the effectiveness of different treatments and to establish the level of odor control necessary to make the product acceptable. Therefore, threshold data may be incorporated into regulatory specifications to define permissible odor limits, for example, from dieselpowered equipment or industrial waste discharges.

The flavor of foods is generally a complex mixture of compounds. The first step in evaluating the importance of different components is to obtain information about the threshold value of the pure material.

Data on threshold values are scattered widely in the literature. Probably even more is unpublished. The collecting of available data into one monograph for comparative purposes should serve two purposes, aside from those already mentioned. Hopefully, those possessing unpublished data will make it available either through publication or submission to an agency such as Committee E-18 of the American Society for Testing and Materials so that the data can be added to future editions of the present compilation. The second benefit related to the problem the Committee encountered in assembling the data. There are questions as to the reliability of many of the values listed. Experimental procedures and precautions followed in obtaining the data were seldom described in unequivocal terms. Often critical conditions of testing were not described at all. The collating of information should serve not only to point up where there are gaps but also to make evident that standardization of test procedures and full reporting of test conditions are needed if data are to be fully trustworthy.

A clear consistent definition of threshold terminology is requisite to any effort to classify data. To this end, the appropriate terms contained in a previous Committee publication (ASTM Designation E 253-67T) have been used. They are:

detection threshold, n-the minimum physical intensity detection by a subject where he is not required to identify the stimulus but just detect the existence of the stimulus.

difference threshold, n-the smallest physical difference between two stimuli which can be correctly identified as sensorially different. (The British Standard Institution puts it more simply, the smallest change in concentrations of a substance required to give a perceptible change.)

just noticeable difference or JND, n-See difference threshold.

recognition threshold, n-the minimum physical intensity detected by a subject where he is required to identify the stimulus in some manner. (The British Standards Institution puts it more simply: the lowest concentration at which a substance is correctly identified.)

supra-threshold, n-above the threshold. Also called supraliminal.

threshold, absolute, n-the minimum physical intensity of stimulus that elicits a response a specified percent of the time. Synonym limen.

threshold, terminal, n-(1) the maximum intensity of a stimulus that will produce a given type of sensory experience without change in modality. (2) intensity of stimulation above which increase in intensity cannot be detected. (The British Standards Institution: the concentrations of a substance above which changes in concentration are not perceptible.)

In the foregoing list, the presence of three terms (namely, detection threshold, recognition threshold, and absolute threshold) to refer to absolute sensitivity does not mean that there are three different kinds of absolute sensitivity. The term absolute threshold has a more general meaning than the other two and may be applied both to the detection task and to the recognition task. Consequently, we can measure an absolute threshold for detection and an absolute threshold for recognition. In practice, it has been more common for investigators to measure absolute sensitivity for detection than absolute sensitivity for recognition. This practice has caused many to use the terms detection threshold and absolute threshold interchangeably. Probably little harm has resulted from such usage, since those who have measured the absolute threshold for recognition have usually not reported their values as simply "absolute threshold."

Certain recommendation for publishing useful values by future authors are suggested. The majority of the values reported in this compilation have been obtained under less than desirable conditions. While this study does not purport to tell the analyst specifically how to determine sensory values, certain precautions are recommended.

Detection thresholds are critically dependent on three main things: (1) sensitivity and ability of the panelists; (2) purity and stability of the compound; and (3) contamination of equipment and space with background odors. Unfortunately, it was generally difficult or impossible to evaluate the effect of these factors from the threshold values reported in the literature.

The Committee cannot assure that all the values taken from the literature as they appear in this compilation are free of error, particularly where details in the publication are rather vague, and also in those cases where the original reference could not be checked. Reasonable care was exercised. If errors occurred in coding the purity of the compound or in stating whether the threshold was for detection or recognition, we would appreciate the authors using the form at the back of this book to call the error to our attention so that the entry can be corrected in the next edition. Furthermore, authors should consider standardizing expressions of concentration to weight/weight, milligram/kilogram; weight/volume, milligram/litre; and volume/volume, microlitre/litre so as to reduce the number of ways of expressing concentration. As may be seen from the code at the bottom of each page, 24 different methods presently exist. Furthermore, use of the International System of Units (SI) would be in conformity with Public Laws 90-472 and 94-168, expressing the intent of the United States to move toward the SI units.