P-34
Solid phase micro-extraction technique for alcoholic beverage
volatiles analysis: advantages and limitations.
JEAN-PIERRE DUFOUR, Russel Keast, Rana Wierda, Patricia Delbecq, and Michelle
Leus, Food Science Department, University of Otago, Dunedin, New Zealand.
Solid-phase microextraction (SPME) has been shown to be a simple and effective tool for detecting trace levels of volatile compounds in foods and beverages. This procedure has been used for profiling the volatiles of beers, fruit-flavored malt beverages, wines and spirits. However, the usefulness of SPME in the quantitative analysis of volatiles of alcoholic beverages is questionable as increasing levels of ethanol have been shown to interfere with their adsorption on the fiber coating. The aim of this study was to evaluate the advantages and limitations of the SPME technique for the analysis of alcoholic beverage volatiles, including the influence of beverage ethanol level on SPME efficiency, the behaviour of the fiber coatings (e.g. ageing of the coating, selectivity of the coating, saturation of the coating and compound displacement) and the potential of SPME-GC-olfactometry (GC-O). The effect of beer ethanol content was investigated by comparing SPME and head-space volatile responses. Increasing the level of ethanol decreases the volatility of beer compounds. By using an internal standard it was possible to compensate for the observed changes in beer compounds volatility using the head-space technique. However, increasing ethanol levels had a variable effect on the adsorption of volatiles on the SPME fiber. Quantification of beer volatiles using an internal standard is thus only possible when adjusting the ethanol content to a reference level. Effect of ethanol on the analysis of spirit volatiles was investigated using tequila samples. Dilution of the spirit resulted in a large increase in the number and amount of volatiles adsorbed (optimum around 16% alcohol). Interestingly, this increase in SPME performance could not be attributed to the sole decrease in ethanol level. The data could be explained by the progressive aggregation of ethanol molecules above a critical ethanol concentration. Such microemulsion favoured the dissolution of hydrophobic molecules and reduced their volatility. Over 180 compounds were detected by SPME-GC-MS using the optimised conditions. Beer volatiles were extracted using ethylacetate. The concentrated extract was analysed by GC-MS and GC-O. Beer samples were also analysed using SPME-GC-MS and SPME-GC-O. SPME compared favourably with the long tedious solvent extraction method. Over fifty odour active compounds (e.g. fruity, flowery, aldehydic, medicinal, solvent, sweet, fatty acid, yeasty aromas) were detected using the SPME-GC-O technique. The evolution of the fiber coating efficiency and selectivity with uses was investigated using a reference mix of standard compounds. Large variations in the adsorption of compounds were observed with increasing number of exposures, the rate of decrease in the adsorption being affected by the experimental conditions. The reference mix has been used as a quality control of the fiber coatings.
Prof. Dr. Ir. Jean-Pierre Dufour. Studies: MSc., 1975- PhD., 1979 (Louvain, Belgium). Research fellow (Johns Hopkins University, School of Medicine, Baltimore,MD) 1979-1981. Appointments: Catholic University of Louvain, Professor 1981-1993 (Head of the Department of Brewery and Food Industries, 1987-1993); Universidade Catolica Portuguesa, Escola superior de Biotechnologia (Porto, Portugal), visiting Professor 1989-1994; University Senghor (Alexandria, Egypt), associated Professor 1992-1995; Expert for EEC and UNIDO (Implementation of sorghum malt for the production of lager beer in Africa) (1994-1996); Otago University, Dunedin, New Zealand, Professor (1995- present), Chairperson and Head of Department of Food Science.