Abstract Four apple wine fermentation processes have been observed by means of direct-inlet gas-phase FTIR spectroscopy. The apple juice concentrates were each fer- mented by two species of Saccharomyces cerevisiae starters, and the experiment was repeated. The develop- ment of the concentrations of 1-propanol, 4-methylpyridine, acetaldehyde, acetic acid, and ethyl acetate was moni- tored. Two different sampling methods were used – static headspace and direct injection of the must. The perfor- mance of the FTIR method is limited by the high ethanol concentration. It can be mathematically proven that the amount of sample can be selected so that any distortion due to ethanol is minimized. Headspace GC–MS was used for preliminary compound identification. Introduction Formation of volatile compounds is a widely investigated part of wine biochemistry. In addition to their contribution to final aroma and bouquet, the sensory less active com- ponents are also related to the microbiological cascade of the fermentation process. The common knowledge is that the contribution of grape varieties and quality to the for- mation of various volatile compounds is more significant than that of Saccharomyces cerevisiae strain [1]. Thus, variations in many low molecular weight metabolites may be used to follow the cascade and to indicate the success of the fermentation process. Various gas-chromatographic methods have mainly been applied to their identification [2, 3, 4]. Electronic nose-based analyses represent the mod- ern approach [5, 6, 7]. Fourier transform infrared spec- troscopy (FTIR) has also been used for wine process con- trol with promising results [8], but in the liquid phase. In this study, the volatile compounds of apple wine were monitored during the fermentation process by low- resolution gas-phase Fourier-transform spectroscopy. The sampling of the FTIR analysis was a direct-inlet method, i.e. no chromatographic devices were coupled to the FTIR spectrometer. According to our best knowledge, the direct- inlet gas-phase FTIR has not previously been used for wine or fruit wine investigations. The aim of the study was to investigate the possibility of applying gas-phase FTIR spectrometry to the analysis of the wine fermentation process and to monitor changes in the formation of volatile compounds during the process. It should be noted that the principal aim of this work was not to present new infor- mation about the volatile composition of apple wine, but to investigate the possibility of using gas-phase FTIR to monitor wine fermentation. Headspace–gas chromatogra- phy–mass spectrometry was used for compound identifi- cation prior to the FTIR analysis. Experimental Reagents 3-Methylbutanal, 4-methylpyridine, ethyl octanoate, and 3-methyl- butyl acetate (Sigma–Aldrich Chemie, Steinheim, Germany), 1-pro- panol (J.T. Baker Chemicals, Mallinkrod, Holland), 2-methylpro- panol (May and Baker, Dagenham, UK), toluene (Rathburn Chem- icals, Walkerburn, Scotland), 3-methyl-1-butanol, 2-methyl-1-bu- tanol, acetaldehyde, and ethyl hexanoate (Fluka Chemie, Buchs, Switzerland), ethyl acetate (Lab-Scan, Dublin, Ireland), acetic acid (Riedel–de Häen, Seelze, Germany), and 1-butanol (Merck, Darm- stadt, Germany) were used as reference compounds. Purity of the compounds ranged from 95% to 99%. Apple wines The wine fermentation was performed by Scanfrentz (Turku, Fin- land) in 200 L pilot scale fermentation tanks, applying the proce- dure as close to the industrial production as possible. Two yeast strains, Y1 and Y2, of Saccharomyces cerevisiae were used to fer- ment four batches of apple wine. Fermentation of the first two batches (B1 and B2) started simultaneously with yeast Y1 using natural apple juice concentrate. The latter two processes (B3 and B4) started with yeast strain Y2 and with the same concentrate, after Mikko Ahro · Mari Hakala · Jyrki Kauppinen · Heikki Kallio Process control of apple winemaking by low-resolution gas-phase Fourier-transform infrared spectroscopy Fresenius J Anal Chem (2001) 371 : 541–549 DOI 10.1007/s002160101033 Received: 19 February 2001 / Revised: 28 June 2001 / Accepted: 2 July 2001 / Published online: 27 September 2001 TECHNICAL NOTE M. Ahro · J. Kauppinen University of Turku, Department of Applied Physics, 20014 Turku, Finland M. Hakala · H. Kallio (✉) Department of Biochemistry and Food Chemistry, University of Turku, 20014 Turku, Finland e-mail: heikki.kallio@utu.fi © Springer-Verlag 2001