Methods for gas chromatography-olfactometry: a review

Abstract

Gas chromatography-olfactometry methods are used in flavor research to determine the odor active compounds in foods. In this review, the four major methods for gas chromatography-olfactometry are described and their potentials and limitations discussed. The methods include dilution analysis, detection frequency methods, posterior intensity methods and time-intensity methods. The value of gas chromatography olfactometry data is shown to depend directly on the gas chromatography-olfactometry method, as well as on sample preparation and analytical conditions. Each of the methods has been used frequently and has its advantages and disadvantages. However, on the methodological side, a considerable area is still to be explored, which would contribute to the interpretation of the data and would improve the value of these techniques for both fundamental and applied research.

Introduction

Progress in analysis techniques over the last decades has led to long lists of volatiles determined in foods [1]. Some of these volatile compounds contribute to the odor of a food and some to the aroma of a food. Odor perception can be considered the response to odor active volatiles that enter through the nostrils (orthonasal) and aroma perception is the perception resulting from volatiles that enter from the mouth and respiratory system (retronasal) [2]. The perception of volatile compounds released from foods by the human nose depends on the extent of release from the food matrix and on the odor properties of the compound. There are indications that only a small fraction of the large number of volatiles occurring in food actually contributes to the odor and aroma [3]. Therefore, the distinction between odor active compounds and the whole range of volatiles present in a particular food product is an important task in flavor analysis. An interesting approach is sniffing the gas chromatographic effluent of a representative isolate of volatile compounds of a food, in order to associate odor activity with the eluting compounds. Many of the ‘chemical’ detectors are not as sensitive as the human nose for many odor active compounds [4]. Experience shows that many odor active compounds occur at very low concentrations; their sensory relevance is due to low odor thresholds. Therefore, the peak profile obtained by any ‘chemical’ detector does not necessarily reflect the aroma profile of a food [5]. Gas chromatography-olfactometry (GC-O) was proposed by Fuller et al. as early as 1964 [6] and has shown to be a valuable method for the selection of odor active compounds from a complex mixture [7]. With the early GC-O devices, reproducibility was a serious problem, which was caused by discomfort from sniffing hot dry effluent gases and the lack of sensitivity of the ‘chemical detectors’ to the odor active compounds. The latter problem is still with us today. Dravnieks and O'Donnell [8] published a GC-O design in 1971, which minimized the discomfort of sniffing. The hot GC effluent was combined with humidified air to reduce nasal dehydration. Nowadays, the same principle is still used in most GC-O apparatuses. In general, it is very difficult to judge the sensory relevance of volatiles from a single GC-O run. Initially, volatiles were sniffed individually when eluting from a GC column and a description of the odor was given for each retention time, corresponding to an odor active compound [9]. GC-O is limited to this screening for odor active volatile compounds, unless any quantification of the chemical stimuli and of the assessors’ responses is performed. It should, of course, be kept in mind that in GC-O, single compounds are assessed. This approach does not provide information on their behavior in a mixture, although it indicates the relevance of some compounds for the aroma of a food. Recombination of odor active compounds in the food matrix to match the original aroma of the food and subsequent sensory evaluation can be used to prove the correct selection of odor active compounds as a final step in aroma analysis. In addition, correlations between the odor active compounds present and the sensory data of food products also indicate the relevance of the compounds.

The purpose of this paper is to review recent developments in GC-O analysis techniques, including those methods that have not received much attention in reviews before. Classic and newer GC-O methods will be compared and their potentials and limitations discussed.