SNIF method

The description of the off-odor, identified at the beginning of the study in the global sensorial evaluation, which has been separated into several components by the gas chromatographic sniffing method can have seemingly large deviations in the descriptions of the individual components from the initial off-odor description. 

Guaiacol has a powerful smoke-like, somewhat medicinal odor (Arctander, 1967) it is described as phenolic, burnt by Blank et al. (1992b), phenolic, spicy (1992a), and perceived as phenolic, aromatic by Holscher et al. (1990). The flavor is described as smoky, woody, phenolic, meaty (Chemisis, 1996). Wasserman (1966) found an odor threshold in water of 21 ppb with a flavor threshold of 13 ppb. According to Buttery et al. (1971) the odor threshold in water is 3 ppb, 2.5 ppb for Semmelroch et al. (1995). For Pollien et al. (1998), guaiacol has a high aroma impact in a brew and even more in an instant coffee prepared with the same blend (GC-olfactometry, SNIF method, see Section 3.3). 

Bobillot S., Brevard H., Feria-Morales K. and Mermoud A.F. (1998) GC-sniffing method for identification of coffee aroma profiles. 17th Int. Colloq. Chem. Coffee (Nairobi, 20-25.7.1997) (ASIC, 1998), 214—17. 

Chaintreau A. (2001) Quantitative use of gas chromatography-olfactometry the GC- SNIF method. In Techniques for Analyzing Food Aroma R. Marsili Ed. Marcel Dekker, New York, Second Edition, in press. 

The very low activation energy for 2-acetyl-l-pyrroline may help explain why this volatile is found by sniffing methods in nearly every food stndied. It appears that this compound is very readily formed even nnder mild heating conditions. This fact, coupled with its extremely low sensory threshold, make its detection in foods likely. The Strecker aldehydes, isovaleraldehyde and phenylacetaldehyde, followed similar kinetics during heating. This is due to the similar mechanism of formation and the fact that they are both consumed through reaction to form 5-methyl-2-phenyl-2-hexenal [39]. 

The GC- SNIF method has been developed to achieve the following criteria ... 

The GC- SNIF method has also allowed the elucidation of the process-induced modifications of the coffee aroma by comparing impact odorants of an instant coffee with the corresponding brew (29). It was applied as weU to the characterization of three Champagne wines (17), and to determine which were the impact odorants protected by the addition of an antioxidant in the wine (34). 

The possibility of a forced-choice detection method on at least two sniffing ports. 

They should also make it possible to use forced choice methods by providing at least two sniffing ports which each may carry the odourous stimulus or pure air. Panels used to obtain valid odour threshold data should at least have 20 members. For practical work using relative measurements only panels of at least 6 trained subjects are recommended. Finally, it is pointed out that odour units/m3 are no direct indication of the perceived intensity of an odour. 

Percentage that would be obtained by mere guessing. If, in conclusion, forced choice methods give a better impression of the true sensitivity of the subject than yes-no methods, olfactometers should be equipped to make their use possible. This means that the subject should have access to at least two sniffing ports, one of which delivers the odorous air, whereas the other(s) do give pure air only. 

The aroma of foods is caused by volatile compounds which are perceived by the human nose. Many studies (reviews in [1, 2]) have indicated that only a small fraction of the hundreds of volatiles occurring in a food sample contribute to its aroma. To detect these compounds, a method proposed by Fuller et al. [3] is used. In this procedure, which is designated gas chromatography-olfactometry (GC-O), the effluent from a gas chromatography column is sniffed by an expert who marks in the chromatogram each position at which an odour impression is perceived. 

SNIF-NMR and IRMS have been adopted as official methods by... 

Jain (20), in his introduction, notes that the direct injection of the blood sample does not give "ghost" or interfering peaks. The column can be used for a long time. The method is not limited to ethanol, since methanol, isopropanol, and acetone as well as toluene, methylethylketone, and xylene (associated with glue sniffing) can be quickly determined. 

For quantitative evaluation a set of 6 serial dilutions of the original sample that should have an accurately known concentration of -2.0%. The concentration can be adjusted to give the optimum odor data. Each sample is diluted by a factor of 3 from the previous sample (i.e., 1 ml sample diluted with 2 ml of solvent) and then given a random number using a double blind labeling method. The samples are sniffed in random order. 

More recently, enantiomer ratios have been used as evidence of adulteration in natural foods and essential oils. If the enantiomer distribution of achiral component of a natural food does not agree with that of a questionable sample, then adulteration can be suspected. Chiral GC analysis alone may not provide adequate evidence of adulteration, so it is often used in conjunction with other instrumental methods to completely authenticate the source of a natural food. These methods include isotope ratio mass spectrometry (IRMS), which determines an overall 13C/12C ratio (Mosandl, 1995), and site-specific natural isotope fractionation measured by nuclear magnetic resonance spectroscopy (SNIF-NMR), which determines a 2H/ H ratio at different sites in a molecule (Martin et al 1993), which have largely replaced more traditional analytical methods using GC, GC-MS, and HPLC. 

Figure G1.8.3 Sniffing chromatogram of eight volatile compounds in a reference mix obtained by the detection frequency method using eight assessors (see Alternate Protocol 1). Compounds 1, 100 ng 2-butanone 2, 20 ng diacetyl 3, 500 ng ethyl acetate 4, 100 ng 3-methyl-1-butanol 5, 20 ng ethyl butyrate 6,100 ng hexanal 7, 100 ng 2-heptanone 8, 500 ng a-pinene.

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