Exploring Characters of Odors

There are several parameters of odors we use to document and report our olfactory experiences.  Odor thresholds (ASTM E679 & EN13725) and perceived odor intensity (ASTM E544) receive the majority of attention as key objective methods to quantify odors.1, 2, 3  The character of the odor, often referred to as odor quality, can also be evaluated with standard protocols that greatly reduce the subjectivity of assessments.

Descriptive analysis is a sensory science term used to describe methodologies utilizing a panel of trained assessors to identify and describe specific sensory attributes about a test sample (qualitative) and scaling the intensity of these attributes (quantitative).4  The food, beverage, and consumer product industries have formally used descriptive analysis to obtain detailed information about the appearance, aroma, flavor, and texture of products for well over 70 years.5

Odor Profiles

Long before standard methods were in place, the earliest perfumers and alchemists used their senses to characterize chemicals in their compound library as well as in their innovative concoctions.  Experts in wine, tea, coffee, and many other industries have used their senses to characterize their products for trade and commerce.  The first formal, systematic descriptive procedure was the Flavor Profile Method developed at A.D. Little Corp. in the late 1940’s.6

Odor character is a nominal scale of measurement where odors are characterized using reference vocabulary. 

Odor character is a nominal scale of measurement where odors are characterized using reference vocabulary.  Standard practice has been to develop a standard list of descriptor terms, which are organized with like terms in groups.  Terms with negative connotation (unpleasant) would be grouped with other negative terms and positive (pleasant) terms with other positive terms.7 In 1985, ASTM published a document titled, “DS-61: Atlas of Odor Character Profiles,” which published a standard odor descriptor list of 146 terms.8

The ASTM International E18 Sensory Evaluation Committee originally compiled this master list of 800 terms from published literature and industrial organizations.8  The Committee organized a group of 100 professionals from several different industries to rate the usefulness of the terms and create the more manageable standard list of 146 terms.9  This list and others form the basis for creating profiles of observed odors.

Descriptor Wheels

In the 1970’s American and British brewing and sensory scientists developed a “Beer Flavor Wheel” as a tiered system for describing the flavor (taste and odor) of beers.10 In the 1980’s, the California wine industry developed a wine aroma wheel for the characterization of wines.11

A descriptor wheel is organized with general descriptors at the center of the wheel and more specific characters listed towards the wheel rim.  For example, an assessor may identify a flavor as fruity and move out on the wheel through berry to raspberry or through Citrus and then to orange.  In early assessor training activities, the wheel helps gain familiarity to the lexicon and groupings of different terms.  While the activity of stating what something smells like seems to be a simple task, the wheel tool provides both a standardized descriptor list as well as a visual representation for training and on-going reference.

Similarly, descriptor wheels have been developed in the environmental odor evaluation industry.  In 1986, the International Association on Water Pollution Research and Control (IAWPRC) proposed eight major odor descriptor categories for describing odors from natural waters and illustrated eight categories in a wheel with vegetable, fruity, floral, medicinal, chemical, fishy, offensive, and earthy.12,13,14  This character wheel formed the basis of the St. Croix Sensory descriptor wheel published in 2002 for use in ambient odor assessments.15

descriptors 5.4.20

In 2004, A group of researchers developed an odor wheel specific for categorizing wastewater odors and odorant compounds.16,17 This wheel took an additional step to place the chemical names of odorants along the rim of the wheel to identify compounds potentially responsible for the various descriptors.  For example, hydrogen sulfide listed with the connection to rotten egg character.

More recently, St. Croix Sensory has developed an odor wheel to encompass a broad range of environmental and product/materials odors.  Like the IAWPRCC wheel from 1986, our wheel begins with eight broad odor categories as the inner ring of the wheel; however, there are some differences in the descriptor organization.  Our eight main odor categories include:  Naturally Offensive, Marine, Animal, Chemical, Earth, Vegetable, Naturally Pleasant, and Culinary.  Additionally, we include trigeminal sensations as well as the basic tastes occasionally observed in the mouth while sniffing odors.

Character wheel 5.4.20

These eight main odor categories breakdown further to 22 second tier odor descriptor categories that form the basis for our descriptor laboratory work.  For example, naturally offensive breaks down further to sulfur and decay groupings, while chemical breaks down into petroleum, chemical, plastics, and medicinal.  For each of these twenty-two categories, our odor wheel also includes 2-4 example descriptors in the outer ring to further clarify the types of detailed descriptors intended for each category (e.g. vinyl, rubber, and Styrofoam for the plastics grouping).  During training, exemplars for the outer tier example terms can be provided as training tools for assessors.

The descriptor categories presented on the St. Croix Sensory Odor Wheel are also coded by color to further assist assessors in training as well as in our presentation of sensory testing results.


Descriptive Analysis

Along with the standard list of character attributes, assessors are trained to utilize a scale to quantify the intensity level of each descriptor observed.  In some methods this is a basic word scale, e.g. faint to strong, while other methods use standard references to anchor the intensity scale. At St. Croix Sensory, trained assessors utilize a 0-10 scale of intensity for this rating (ASTM E544).

The odor testing descriptor data can then be plotted individually with an odor profile infographic or multiple samples can be presented on a spider plot (radar plot) format with the distance along each axis representing the intensity scale.  The plots create a pattern that can be readily compared across samples.

A more advanced look at odor characterization involves evaluating the odor profiles of odor samples at multiple dilution ratios in order to investigate how odor character changes as environmental odors dilute downwind or product odors dilute with room ventilation or distance.18  Additionally, these results can also be compared to odor profile analysis of pure compounds.


Characterization Examples

Below, an illustrative example compares the odor characters from inlet and outlet samples from an odor control system.  The odor inlet has considerably higher odor threshold values as well as a higher perceived intensity.  While the odor control system shows a 97% reduction in perceived odor, there is still a residual odor present.  Understanding more about the characterization of the odor control outlet during proper operation can help for future troubleshooting during complaint episodes.

Each assessor considers the second-tier subcategories of the descriptors from our standard odor wheel and rates the intensity of identified descriptors on the 10-point Odor Intensity Referencing Scale.  The average results of the panel are plotted on an Odor Rose™ diagram for each sample.

An example Odor Rose for the odor control system inlet shows highest intensity of the sulfur character attribute, followed by low intensity of decay, fish, animal, earth, and sulfidic vegetable, as well as reporting of trigeminal sensations.

Odor Control Inlet – Odor Rose™ diagram



The Odor Rose for the odor control system outlet shows a reduction in the intensity of all odor characters.  Sulfur, fish, sulfidic vegetable characters and trigeminal sensations are no longer detected.  Decay, animal, and earth show a reduced intensity.  Herbal character is now also present at low intensity in the outlet sample.

Odor Control Outlet – Odor Rose™



A spider graph (i.e. radar or polar plot) can be utilized to directly compare samples by overlaying the character profiles of multiple samples.  Here the inlet and outlet samples are overlaid to show the reduction in intensity of the odor descriptors.




Alternatively, a histogram presents the percentage of assessors in the panel that assigned specific descriptors to the odor sample.  In this example, fewer assessors reported trigeminal sensation, sulfur, fish, animal, and sulfidic vegetable characters in the outlet sample.  The outlet sample had an increase in the number of assessors reporting medicinal and herbal characters.  The same number of assessors reported decay and earth character in both the inlet and outlet samples, even though the intensity of these characters had decreased.

Odor Control System Inlet                                           Odor Control System Outlet



Odor characterization evaluations provide depth to assessments of odorous air samples or product & material samples.  Standard character lists, often arranged as odor character wheels, offer a common lexicon for testing and reporting sensory data.

There is a long history of character lists and character wheel tools used to train assessors for specific product categories or for a broad scope of odor assessments.  Presentation of common exemplar odor references ensure more reliable and specific data.

Using these tools in conjunction with standard methodologies for descriptive analysis provide an objective approach to describing odors.  Results can include a histogram listing of observed odors as well as intensity ratings of each descriptor.  Graphical representation of results allows for comparison of samples for well informed decision making.



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