Analytical MethodsApplication of gas chromatography–olfactometry (GC–O) in analysis and quality assessment of alcoholic beverages – A review
Introduction
The composition and content of odour compounds determine the quality of alcoholic products. The smell of an alcoholic beverage is the effect of a large number of chemical compounds with different properties (such as polarity or volatility) occurring at widely differing concentrations. The chemical composition of the odours depends on the quality and type of the raw materials, as well as the conditions of the fermentation process.
Alcoholic fermentation conducted under industrial conditions leads to a series of byproducts in addition to ethanol. They include carbonyl compounds, alcohols, esters, acids and acetals, all of them influencing the quality of the finished product. The composition and concentration levels of the byproducts can vary widely. Some compounds appear in high concentrations (hundreds of mg/l); however, a large part appear at significantly lower levels (even as low as ng/l). The influence of the individual compounds on the odour profile can be very different. Quite frequently, compounds appearing in trace quantities have a greater influence on the sensory properties of alcoholic products than those which appear in high concentrations.
In recent years, intensive studies have been carried out regarding the sensory activity of the individual components of food and alcoholic beverage odours, and the dependence between the odour and the chemical composition of the volatile fraction of these products. The majority of the accomplishments within this area can be attributed to the combination of gas chromatography with olfactometric detection.
Section snippets
Overall characteristics of the GC–O technique and instrumentation
Gas chromatography with olfactometric detection is based on sensory evaluation of the eluate from the chromatographic column aimed at discovering the active odour compounds. The role of the detector is played by a properly educated person or a team of evaluating personnel. Qualitative and quantitative evaluation of the odour is carried out for each analyte leaving the chromatographic column. This allows establishing whether a given compound is sensory active at a given concentration (i.e.
Sample preparation methods
Determination of odour substances using instrumental techniques consists of two stages. The first stage of the analysis, isolation of the analytes from the matrix, is particularly important. The appearance of the olfactograms depends to a large extent on the isolation procedure, as numerous comparative studies revealed that the use of different sample preparation techniques (even using different solvents in the case of liquid–liquid extraction) might affect the composition and contents of the
Factors influencing the quality of results
A number of factors affect the quality of results. The volatile compound extraction technique is particularly important, as it affects the representativeness of the isolate and the composition of the eluate subject to sensory evaluation. Sample storage is also critical from the point of view of sample representativeness. To prevent auto-oxidation, wine extracts should be stored in a carbon dioxide atmosphere at low temperature (Bernet et al., 2002). Dividing the extract into small, individual
Quantitative methods
Several quantitative methods exist for the evaluation of the intensity of odours and their relative influence on the odour of the sample (Ruth, 2001). These methods can be categorized into three groups based on the method of determination:
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detection frequency methods (Ferrari et al., 2004, Pollien et al., 1999, Pollien et al., 1997),
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dilution to threshold methods,
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direct intensity methods.
Examples of applications of gas chromatography with olfactometric detection
Gas chromatography–olfactometry studies on odour of alcoholic beverages usually have the goal of determining the relationship between the composition and the content of volatile compounds and the organoleptic properties of products such as beer (Lermusieau and Collin, 2003, Soares da Costa et al., 2004), wines (Campo et al., 2005, Lee and Noble, 2003) and whiskeys (Wanikawa et al., 2002), as well as identification and comparison of the compounds entering the aroma of different alcoholic
Conclusions
Despite the fact that odour detectors have already been in use for over 40 years, literature indicates that in recent years they have been used more frequently, finding applications especially in the analysis of food and beverages (including alcoholic beverages) (Ruth, 2001). The further investigations are still conducted in order to improve GC–O technique, i.e. to achieve a higher sensitivity and better reliability and repeatability of the results.
It can be clearly concluded from this review
Acknowledgement
This research was financially supported by the Department of Scientific Research of Polish Ministry of Scientific Research and Information Technology (grant no. N312 056 31/3446).
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