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Computer-based Fourier-transform infrared spectroscopy (FT-IR) was used to identify food-borne, predominantly fermentative yeasts. Dried yeast suspensions provided the films suitable for FT-IR measurement. Informative windows in the spectrum were selected and combined to achieve optimal results. A reference spectrum library was assembled, based on 332 defined yeast strains from international yeast collections and our own isolates. All strains were identified with conventional methods using physiological and morphological characteristics. In order to assess identification quality, another 722 unknown yeast isolates not included in the reference spectrum library were identified both by classical methods and by comparison of their FT-IR spectra with those of the reference spectrum library. Ninety-seven and one-half percent of these isolates were identified correctly by FT-IR. Easy handling, rapid identification within 24 h when starting from a single colony, and a high differentiation capacity thus render FT-IR technology clearly superior to other routine methods for the identification of yeasts.

Fourier-transform infrared (FT-IR) microspectroscopy was used in this study to identify yeasts. Cells were grown to microcolonies of 70 to 250 m in diameter and transferred from the agar plate by replica stamping to an IR-transparent ZnSe carrier. IR spectra of the replicas on the carrier were recorded using an IR microscope coupled to an IR spectrometer, and identification was performed by comparison to reference spectra. The method was tested by using small model libraries comprising reference spectra of 45 strains from 9 genera and 13 species, recorded with both FT-IR microspectroscopy and FT-IR macrospectroscopy. The results show that identification by FT-IR microspectroscopy is equivalent to that achieved by FT-IR macrospectroscopy but the time-consuming isolation of the organisms prior to identification is not necessary. Therefore, this method also provides a rapid tool to analyze mixed populations. Furthermore, identification of 21 Debaryomyces hansenii and 9 Saccharomyces cerevisiae strains resulted in 92% correct identification at the strain level for S. cerevisiae and 91% for D. hansenii, which demonstrates that the resolution power of FT-IR microspectroscopy may also be used for yeast typing at the strain level.Traditional identification of yeasts is achieved by applying physiological and morphological tests, which determine enzyme production profiles and growth characteristics (5, 16). Various rapid and in some cases automated identification systems for routine analyses of yeasts are commercially available and easy to use. However, identification results are questionable to a certain degree because these systems originally were developed for clinical application and databases therefore do not include an adequate number of common environmental yeast species (10,27). In recent years many techniques that identify yeasts according to the fatty acid composition of the cell membrane (3, 6) or genotypic characteristics such as temperature gradient gel electrophoresis (14), electrophoretic karyotyping (28), restriction fragment length polymorphism (12, 25) and randomly amplified polymorphic DNA (1,4,25) restriction enzyme analysis of PCR-amplified rDNA (11) or mitochondrial DNA (24), RNA probes (15,18,23), and rDNA sequencing (7, 29) have been developed. However, their application in the routine analysis of yeasts in the food industry is limited by their high cost and the requirement for highly skilled personnel.A rapid and very inexpensive method to identify microorganisms is Fourier-transform infrared (FT-IR) spectroscopy (13,19). Absorption of infrared light by cellular compounds results in a fingerprint-like spectrum that can be identified by comparison to reference spectra. Once an extensive and well designed database of reference spectra is established, reliable identification results are obtained within 25 h of starting the identification from a single colony (17,21,22).FT-IR microspectroscopy is a novel tool to characterize microorganisms (20). Microcolonies grown on a solid medium are transferred fr...

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