The past century has seen the appearance and disappearance of many systems for odor description. Almost a century ago, Zwaardemaker (1895) proposed a system of 39 categories with further subcategories. In the 1920s, at Arthur D. Little, Inc. in Cambridge, the flavorist Crocker and his associates worked out a four-quality system (fragrant, acid, burnt, caprylic) which was meant to provide numerical signatures to a large range of odorants. The system, later called the Crocker/Henderson system (Crocker and Henderson, 1927) was a very simple one with which to work, and was followed two decades later by the Odor Directory (Crocker and Dillon, 1949). The Odor Directory listed the quality “signatures” of several hundred odorants, in terms of how much of each of the four components every odorant possessed. Still later, Harper and his associates (1968a) publisbed a scholarly review on various types of odor classification, and later developed their own system of 44 descriptor words to classify odorants (Harper et al, 1968b). These approaches, and the spin-offs they engendered, will be discussed.
Rationale and philosophy of odor description
It has been claimed by various researchers that the number of possible odor perceptions ranges into the millions. No two odorants are ever exactly alike. This is in contrast to taste, where two acids (especially the mineral, or nonorganic acids) can be equivalently strong in sourness, and sensorially indistinguishable from each other. In odor perception, no such simple sensory equivalence exists. Each odorant provokes its own unique set of perceptions which differentiates that odor from all others.