A few years go I was working on potato flavour at the University of Reading and came upon a class of potent odorants called methoxypyrazines. They were identified in tubers as early as 1973 and have been associated with the characteristic musty, earthy note of potato (Buttery and Ling, 1973; Nursten and Sheen, 1974). The 2-isopropyl-3-methoxypyrazine (IPMP) has previously been identified in raw potato (Buttery and Ling, 1973) and tentatively identified in boiled potato peelings (Meigh et al., 1973). It is of particular interest due to its very low odour threshold value of 2 ng/L (Seifert et al., 1970) and has a characteristic earthy or raw potato/potato peelings odour. Following baking, different potato varieties have been found to contain different concentrations of methoxypyrazines in the skin, where the highest levels were located (Duckham et al., 2001; Oruna-Concha et al., 2001). Unlike many other flavour volatiles the methoxypyrazine levels demonstrate no clear correlation to tuber storage time (Duckham et al. 2002). IPMP has been identified in cultures of Pseudomonas taetrolens (aka. P. perolens) and Buttery and Ling (1973) have suggested that it may be formed in the soil or on the tuber surface from where it could be absorbed into the tuber. The potato is related to the bell pepper, which can produce relatively large amounts of the related compound 2-isobutyl-3-methoxypyrazine (Buttery and Ling, 1973) which has a characteristic capsicum aroma.
Methoxypyrazines may be seen as a fault in some products but in wine they play a more positive role. Interestingly, the methoxypyrazines have been associated with Sauvignon Blanc grape and wine character (Harris et al., 1987; Lacey et al., 1991; Allen et al., 1991) and Campo et al. (2005) found varietal differences when comparing wines.
The origin of the methoxypyrazines remains unclear and their importance as key odorants may be grossly underrated. They have very low odour thresholds and are typically present below the limit of detection of most analytical techniques without pre-concentration and the use of mass spectrometry. A total of six methoxypyrazines were tentatively identified during an earlier study of baked potatoes but, unlike alkyl pyrazines, did not appear to be generated during the cooking process (Duckham et al. unpublished data). Gallois and Grimont (1985) identified several methoxypyrazines apparently responsible for the characteristic potato odour produced by some members of the Enterobacteriaceae family, namely Serratia and Cedecea strains. Improvements in the sensitivity of mass spectrometers over the past 10 years will ensure more reliable detection and identification of these key compounds. Current studies are likely to more reliably identify them in foodstuffs and from environmental sources hopefully leading to a clearer understanding of their biochemical origins. Watch this space.
Several methoxypyrazines have been associated with off-flavours and taints in foodstuffs and this will form the subject of a future posting.
Allen, M.S., Lacey, M.J., Harris, R.L.N. and Vance Brown, W. (1991). "Contribution of Methoxypyrazines to Sauvignon blanc Wine Aroma". Am.J.Enol.Vitic. 42(2):109-112.
Buttery, R.G. and Ling, L.C. (1973). Earthy aroma of potatoes J. Agric. Food Chem., 21: 745-746.
Campo, E., Ferreira, V., Escudero A. and Cacho J. (2005). “Prediction of the Wine Sensory Properties Related to Grape Variety from Dynamic-Headspace Gas Chromatography−Olfactometry Data” J. Agric. Food Chem. 53: 5682−5690.
Duckham, S.C., Dodson, A.T., Bakker, J. and Ames, J.M. (2001). Volatile Flavour Components of Baked Potato Flesh: A Comparison of Eleven Potato Cultivars. Nahrung, 45, 317-323.
Duckham, S.C., Dodson, A.T., Bakker, J. and Ames J.M. (2002) Effect of Cultivar and Storage Time on the Volatile Flavor Components of Baked Potato. J. Agric. Food Chem., 50: 5640-5648.
Gallois, A. and Grimont, P.A.D. (1985). Pyrazines Responsible for the Potatolike Odor Produced by Some Serratia and Cedecea Strains. Applied and Environmental Microbiology 50(4):1048-1051.
Harris, R.L.N., Lacey, M.J., Brown, W.V. and Allen, M.S. (1987). "Determination of 2-Methoxy-3-alkylpyrazines in Wine by Gas Chromatography/Mass Spectrometry". Vitis 26(4):201-207.
Lacey, M.J., Allen, M.S., Harris, R.L.N. and Vance Brown, W. (1991). "Methoxypyrazines in Sauvignon blanc Grapes and Wines". Am.J.Enol.Vitic., 42(2):103-108.
Nursten, H.E. and Sheen, M.R. (1974). Volatile Components of Cooked Potato. J. Sci. Fd Agric., 25: 643-663.
Oruna-Concha, M.J., Duckham, S.C., and Ames, J.M. (2001) Comparison of the Volatile Compounds Isolated from the Skin and Flesh of Four Cultivars of Potato After Baking. J. Agric. Food Chem., 49: 2414-2421.
Seifert, R. M.; Buttery, R.G.; Guadagni, D.G., Black, D.R. and Harris, J.G. (1970). Synthesis of some 2-methoxy-3-alkylpyrazines with strong bell pepper-like odors. J. Agric. Food Chem., 18: 246-249.
Craig is a consultant, a flavour enthusiast and an unapologetic analytical chemistry geek.