Geosmin

Geosmin [octahydro-4,8a-dimethyl-4a(2H)-naphthalenol] is characterized by a distinctive earthy, musty odor, and a very low sensory threshold (1-10ng/L). In wine, usually it is present as a metabolite from Streptomyces, and Botritis cinerea and Penicillium expansum moulds growth on grapes. 

TABLE 8.1. Experimental Conditions and Performances of Headspace Sampling SPME/GC Negative Chemical Ionization-MS and SPME/GC High-Resolution-MS Methods Used for Analysis of TCA and TBA in Wine  

GC column 5% Diphenyl-95% dimethyl polysiloxane-fused silica capillary (30m x 0.25 mm i.d. 0.25-pm film thickness) 

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The adverse properties of red beet preparations are without doubt, their high nitrate content, their unpleasant smell due to pyrazine derivatives and geosmin with a low odor threshold, and finally the risk of carry-over of earth-bound germs. - " Since nitrate has been associated with nitrite and nitrosamine formation, the latter exhibiting cancer-inducing properties, procedures have been developed to reduce the nitrate content of red beet preparations. This has been achieved by microbial denitrification. - ... 

Acree, T.E. et al., Geosmin, the earthy component of table beet odor, J. Agric. Food Chem., 24, 430, 1976. 

Tyler, L.D., Acree, T.E., and Smith, N.L., Sensory evaluation of geosmin in juice made from cooked beets, J. Food Sci., 44, 79-81, 1979. 

Bentley, R. and Meganathan, R., Geosmin and methylisobomeol biosynthesis in Streptomycetes. FEES Lett., 25, 220, 1981. 

Sklenar KS, Horne AJ (1999) Effect of the cyanobacterial metabolite geosmin on growth of a green alga. Water Sci Tech 40 225-228... 

Geosmin, 2-Methyl isobomeol (MIB) and 2-isopropyl methoxy pyrazine are known to be produced by various types of actinomycete cultures (10-15). Geosmin and MIB are saturated tertiary alcohols and resist oxidation. The steric configuration of the hydroxyl and methyl groups in both compounds are believed to interact with receptors in the nose, imparting their characteristic earthy odour (16). The four compounds itemised as the key osmogenes in this odorous emission have extremely low odour threshold concentrations. Their occasional occurence in drinking water can lead to widespread complaints and are routinely monitored for within this Authority. 

GERBER, N.N., and LECHEVALIER, H.A., Geosmin, an Earthy-Smelling Substance Isolated from Actinomycetes , Appl.Microbiol 13 935(1965). 

GERBER, N.N., Three Highly Odorous Metabolites from an Actinomycete. 2-Isopropyl-3-methoxypyrazine, Methylisobomeol, and Geosmin , J Chem Ecol 3 475 (1977). 

FI GURE 4.3 Structures of allocholic acid (petromyzonol sulfate is identical apart from replacement of the carboxyl group with sulfate), taurine-conjugated bile acid, and geosmin. 

Some biologically relevant smells may have two functional thresholds, the higher one resulting in a qualitatively different odor. For instance, geosmin in high concentrations smells like musty basement or soil, in low concentration like beets. ... 

Humans may be programmed to be more sensitive to natural food contaminants. For instance, (-)geosmin (rrans-l,10-dimethyl-rran5-9-decalol) occurs in earth, natural surface water, and in foods in contact with soil or water, such as beets, clams, or fish. Geosmin is a microbial, fungal, or algal metabolite. It is a water pollutant and off-flavor compound. The naturally occurring (-)enantiomer has a threshold 11 times lower than the (-I-) enantiomer (Polak and Provasi, 1992). 

Polak, E. H. and Provasi, J. (1992). Odor sensitivity to geosmin enantiomers. Chemical Senses 17,23-26. 

Tosi, L. and Sola, C. (1993). Role of geosmin, a typical inland water odour, in guiding glass, eel Anguilla anguilla (L.) migration. Ethology 95,177-185. 

Fig. 1. Structures of geosmin and 2-methylisoborneol, the two most common causes of off-flavor in farm-raised channel catfish.

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