An Organoleptic Laboratory Experiment - Journal of Chemical

A laboratory experiment has been designed to accompany the lecture topic. Compounds in ten different classes of organic molecules that are used in the...
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In the Laboratory

An Organoleptic Laboratory Experiment John M. Risley* Department of Chemistry, The University of North Carolina at Charlotte, Charlotte, NC 28223 The Department of Chemistry at UNC-Charlotte offers a two-semester chemistry course with laboratory to non–science majors in order to fulfill a general education science requirement of a liberal arts education. We are using Snyder’s text (1). Among the topics discussed are flavorings in foods and fragrances in personal care products. These are interesting topics for the students. However, there was no laboratory experiment to accompany these discussions. So I wrote an “Organoleptic Laboratory Experiment” for these sections. It illustrates some activities of chemists in the foods and cosmetics industries. Experimental Procedure Samples of chemicals were placed in screw-capped vials. Students were given only the name of the compound and were required to draw the structure of the compound, classify the compound, report whether it was a liquid or solid, and describe the organoleptic properties using a list of terms provided. CAUTION: Students were shown how to whiff the vapors while holding an open vial about 3–6 inches in front of their nose. Do not hold the vial directly under the nose. Do not taste any of the compounds.1 The following classes of compounds and compounds in each class were used: alcohols: butyl alcohol, cinnamyl alcohol, furfuryl alcohol, geraniol, isoamyl alcohol, linalool, menthol, 2-octanol, phenethyl alcohol, pinacol, 2-propanol, tetrahydrofurfuryl alcohol aldehydes: p-anise aldehyde, benzaldehyde, butyraldehyde, isobutyraldehyde, vanillin amines: butylamine, piperidine, pyridine carboxylic acids: benzoic acid, butyric acid, cinnamic acid, phenylacetic acid, valeric acid esters: amyl acetate, dimethyl succinate, ethyl acetate, ethyl acetoacetate, ethyl butyrate, ethyl cinnamate, ethyl formate, methyl acetate, methyl anthranilate, methyl p-hydroxybenzoate, propyl acetate, propylene carbonate ethers: m-dimethoxybenzene, p-methylanisole hydrocarbons: R-(+)-limonene, (1S)-(–)-pinene ketones: acetophenone, d-camphor, 3-heptanone, 4-heptanone, β-ionone phenols: creosol, eugenol, guaiacol

pounds. They then utilized the information to “create” their own fragrances by mixing two or three of the compounds provided in different ratios and then reporting the organoleptic properties of their new fragrances; for example, one drop of eugenol might be mixed with three drops of 3-heptanone, etc. Discussion The sense of smell has been an active area of research for many years, and the literature associated with its many aspects is very extensive. The Amoore theory of smell is a structure–activity (odor) relationship theory that is generally easy for most students at this level to understand. The basis of the theory is that there are seven primary odors: camphoraceous, musky, floral, pepperminty, ethereal, putrid, and pungent. There are seven kinds of protein receptors in the nasal cavity, one for each of the primary odors. The chemical composition (empirical or structural formulas) of a substance has no relationship to its odor. It is molecular shape and size that primarily determine with which receptor a compound interacts, and thus the odor that is detected. For each primary odor, the following shape of the molecule and corresponding shape of the receptor site, respectively, are proposed: camphoraceous: spherical; elliptical bowl musky: disc; larger elliptical bowl floral: disc with tail; bowl with trough pepperminty: wedge; V-shaped trough ethereal: rod; trough putrid: negative charge; positive charge pungent: positive charge; negative charge

Other odors, called secondary odors, occur when a molecule has a shape such that it can fit into more than one receptor site and thus stimulate the receptors to varying degrees to elicit an odor that is a combination of the primary odors (analogous to color vision). It is obvious that a whole range of odors can be detected in a similar manner, which is consistent with our everyday experiences encountering complex mixtures of aromas. The general properties of molecules that elicit odors are that they must: be volatile (have a relatively high vapor pressure) be soluble in the mucus on the receptor cell surface (many of the odorant molecules are hydrophobic and are soluble in aqueous mucus) have a molecular mass less than 400

sugars: D-xylose

The students were required to answer a series of questions to help them focus on some general features of the organoleptic properties of the classes and com-

have concentration-dependent organoleptic properties (the odor of some compounds is directly related to its concentration in solution) have a flexible or rigid molecular structure that determines how the molecules interact with receptors

*Telephone (704) 547-4844; Fax (704) 547-3151.

Vol. 73 No. 12 December 1996 • Journal of Chemical Education

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In the Laboratory

have various functional groups present have appropriate stereochemistry, if required

A practical definition of organoleptics is the perception of aromas and flavors by sensory organs. The terms used to describe the organoleptic properties (odors) are from common experience, and examples of these terms are alliaceous (onion, garlic), balsamic (chocolate, cinnamon, vanilla), citrus (lime), floral (carnation, iris, lilac), meaty, nutty (peanut), spicy, and woody. More detailed information is available in the voluminous literature on this topic; as an example, a recent summary was written by Farbman (2). The Aldrich catalog Flavors and Fragrances (3) provides extensive information on the organoleptic properties of compounds. The variety of compounds for this experiment was selected to demonstrate to students the range of organoleptic properties. I chose compounds that the department already had in stock. Therefore, the experiment may be tailored to suit the needs of others who set up an organoleptics laboratory experiment by reducing the number of compounds, substituting other compounds, or expanding the number of compounds available to the students. The experiment goes much faster if the students draw the structures of the compounds and classify each one before the laboratory experiment, at which time they can report whether the compounds are liquids or solids and describe the organoleptic properties. The students find structures for the compounds in their text (1), and I also make available to them a variety of references that includes chemical catalogs, but not the Flavors and Fragrances catalog by Aldrich (3). Students have generally found the experiment quite interesting and are surprised by the range of organoleptic properties. They sometimes describe the organoleptic properties in terms of a commercial product. Some students ascribe organoleptic properties to compounds that are classified as odorless. It is interesting that a few students debate the organoleptic properties of compounds with other students. Conferring between students is not discouraged because they learn that others may perceive odors much differently. It is amusing when a student is found who does not find, for example, that valeric acid has a putrid, fecal, sweaty, rancid smell when the rest of the class grimaces when the vial is opened; this student apparently has a faulty receptor, which can provide an area of interesting class discussion. Among the questions that students are asked are to: draw general conclusions, if any, about the organoleptic properties of each class of compound smelled; put into a table the organoleptic properties of the six butane-based molecules having five different functional groups, and then assess the effects of the functional groups on the odors of the compounds; compare an aromatic carboxylic acid with an aliphatic carboxylic acid; ascertain if it makes a difference whether compounds are solids or liquids.

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Although the organoleptic properties of the functional groups, in general, are discussed in the lecture section of the course, students are still amazed how functional groups affect the smells. They also express surprise at the effect that changing the structures of the alkyl groups within a functional group has on the smells. And, finally, students find it astonishing that the organoleptic properties of compounds are generally independent of whether the compounds are liquids or solids; most tend to believe that solids have no smell. Many students appear to enjoy the opportunity to “create” their own fragrances. Few students use the compounds with disagreeable organoleptic properties in their creations; instead, they try to create more pleasant odors. There are opportunities to elaborate on this experiment. For example, students could study the relationship between the three-dimensional structures of the molecules and the odors detected using the Amoore theory as a basis. At a more advanced level, this may be used as a biochemistry lecture demonstration relating molecular structure to receptor interaction and odor detection. It might also be used in molecular modeling to analyze molecular shapes in relation to structure and properties. While the experiment is very simple to assemble and execute, my experience is that it is extremely educational. Misconceptions of students are rectified and the lecture material receives positive reinforcement with the experience of having actually smelled the different compounds. Some students report back that they read ingredient labels to see what chemicals manufacturers add to give products their organoleptic properties. Note 1. A referee correctly pointed out that the trend in experimental chemistry is to minimize the exposure of students to potentially harmful chemicals, and that this experiment seems to fly in the face of this trend. We emphasize to students the necessity to limit their exposure to potentially harmful chemicals at all times. Thus the CAUTION contained in this experiment. We tell students that this lab experiment is a special case and is generally an exception to the rule when done correctly. However, we advise students that different chemicals can affect individuals differently, some adversely, and that any time they feel uncomfortable about smelling a sample, they need not do so. Unless a student knows for certain that he or she risks an allergic reaction to a compound or group of compounds if inhaled, then the only assurance a student has is that there is a minimal risk for the average person, who has probably been exposed to most of these chemicals at one time or another in consumer products. This experiment may be considered to be analogous to the “oil of wintergreen” synthesis used in many undergraduate labs. This experiment in no way promotes or encourages the dangerous and stupid practice of “huffing.” Thus far, we have had no problems.

Literature Cited 1. Snyder, C. E. The Extraordinary Chemistry of Ordinary Things, 2nd ed.; Wiley: New York, 1995. 2. Farbman, A. I. Cell Biology of Olfaction; Cambridge Univ.: Cambridge, 1992. 3. Aldrich Chemical Company, Inc. Flavors & Fragrances; Aldrich Chemical: Milwaukee, WI.

Journal of Chemical Education • Vol. 73 No. 12 December 1996