Limitations And Future Improvements

A. Sample Preparation

As originally designed, GC-''SNIF'' includes the headspace sampling of the product to be analyzed. Of course, the GC-''SNIF'' data treatment is also applicable to any other injection mode, such as a liquid extract. However, the headspace (HS) composition is the most representative of the odor perceived by the nose, as long as the method used to collect volatiles does not alter their relative amounts. The static-and-trapped headspace technique (S&T-HS) does not distort the HS composition as a function of volatilities, and allows the collection of a greater vapor volume than a gas syringe (14,43). However, usual HS trapping agents still have some limitations (limited breakthrough volumes for very volatile compounds, alteration of very labile compounds upon desorption, etc.). Cold traps are prone to be blocked by the formation of ice. Therefore, extending the performances of HS trapping agents would be a great help for any GC-O technique.

B. Chromatographic Problems

When performing the evaluation of odorants after GC separation, odors are smelt out of their context, i.e., in an isolated form. Up to now, little is known about their possible synergies and/or their relative contributions between odorants perceived simultaneously (21). Attempts to ascertain GC-O results by reformulating a flavor with these impact odorants are rare [e.g., (12,33,44)] and not always conclusive (45). Conversely, peak coelutions may occur with all techniques, giving rise to the overevaluation of the impact of a given odorant.

The injection into a GC column of a series of components in the same amount is known to give different peak heights and peak widths as a function of their retention time. Does it affect their odor intensities perceived at the sniffing port (18)? This has never been investigated, in spite of the fact that it might significantly change their real odor impact and modify their contribution ranking in the overall flavor.

All three phenomena are inherent in all GC-O techniques. Therefore, conclusions drawn from the character of isolated constituents using such a technique should be applied with care to interpret the character of the overall odor.

C. Analysis Duration

Although GC-''SNIF'' seems to perform better than other techniques (17,32), in terms of data acquisition and data treatment duration, obtaining a final olfacto-gram still takes days, versus weeks for dilution methods with several judges (13,15,22). To enable analysts to routinely perform GC-O runs with quantitatively reliable results, the analysis duration should be still decreased.

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