RECENT DEVELOPMENTS IN THE FIELD OF ESSENTIAL OILS By ERNEST GUENTHER, Ph.D. Vice-President and Technical Director, Fritzsche Brothera, Inc., New York 11, N.Y. LIKE EVERY field of human endeavor, the industry of essential oils is subject to the continuous change. Without change there would be no progress. The series "The Essential Oils" (64) covers the essential oil in- dustry up to the years 1948-1952, but since then many developments have taken place--in the laboratory and the producing regions--which, as part of a highly specialized field, have not come to the attention of all users of these complex products. The purpose of this paper is to review these re- cent developments. ANALYTICAL PROCEDURES* I. Improvements in Existing "PFet Methods" Little work has been reported along these lines but further standardi- zation of existing methods has been accomplished by various trade and scientific organizations, among them the Essential Oil Association (56) (the latter has so far published 100 Specifications and Standards covering essential oils, isolates and aromatic chemicals), the manufacturers in Grasse (168), the Essential Oil Sectional Committee of the Indian Standards Institute (80) and the British Standards Institute (26). The International Organization for Standardization (I.S.O.) has a Technical Committee working in this field (81). One important method now being evaluated in various laboratories is a modification of the existing Hydroxylamine Hydrochloride Method for the determination of aldehydes: the use of tertiary butyl alcohol has been suggested as a solvent to prevent acetal formation during the process (83). II. Instrumentation A major development in the field is the extended use of instrumentation, * For bibliographic reviews covering this subject see the reports by E. Guenther and E. Langenau, published in zlnal. Chem., 21,202 (1949) 22, 210 (1950) 23, 217 (1951) 25, 12 (1953) 27, 672 (1955). A further review is slated for publication in the same journal in 1957. 290

RECENT DEVELOPMENTS IN ESSENTIAL OILS :291 particularly the introduction of improved instruments that give readily reproducible results, and of modern automatic recording devices that eliminate tedious plotting by hand. (A) Use of ultraviolet spectra has been given great impetus by the publication of Sale, et al. (143), of his work on lemon oils. (B) Use of infrared spectra has been greatly extended. A number of laboratories, under the direction of experienced researchers in the United States and abroad, are engaged in the isolation and purification of essential oil constituents, and in the measurement of their infrared spectra. Some of this valuable work has been published in the literature, some is available through commercial laboratories specializing in this field, but much is still in the development stage, hence unpublished. Libraries of reference spectra are being compiled, with modern machine-indexing of the spectra. Once a great many of the spectra have been assembled and indexed, it will be relatively easy to identify a compound isolated from an essential oil. One of the advantages of infrared spectroscopy is that minute quanti- ties of a substance, liquid or solid, are sufficient for examination. The uses of infrared spectra include: (a) Identification of components of essential oils. (b) Elucidation of structural formulas of newly detected components. (c) Selection of raw material for the manufacture of isolates. (d) Detection of adulteration. (e) Evaluation of pure materials by determination of the percentage of important constituents (56) e.g., true pipefine content of oleoresin pepper. (f) Control of manufacturing processes. The importance of infrared spectra to our industry is demonstrated by the fact that the Scientific Section of the E.O.A. has established a per- manent subcommittee to study infrared techniques and their application. (C) New distillation equipment. Modern laboratory fractionation columns are so efficient that they permit sharp separation of the various fractions, the separating power sometimes being equal to that of 150 single operations in the older, conventional stills. III. Liquid-Liquid Extraction This technique has been found very helpful in research work on fruit flavors. It involves distribution of a substance between two immiscible solvents. Multi-stage apparatus with counter-current distribution halve been developed (44). IF. Chromatography Chromatographic techniques have become standard methods in every modern laboratory investigating essential oils and their components. Much of the rapid success recently achieved in the isolation of essential