Methoxypyrazine in grapes

TABLE 4.2. Headspace-SPME Conditions for Analysis of 3-Alkyl-2-methoxypyrazines in Grape Juice and Wine2... 

Minor Methoxypyrazines. Stable isotope-dilution mass spectrometry has also allowed quantitative analysis of 2-methoxy-3-(l-methylpropyl)pyrazine (2) sec-butylmethoxypyrazine) and 2-methoxy-3-(l-methylethyl)pyrazine (3) (isopropyl-methoxypyrazine) in grapes and wines. The corresponding trideuterated isopropyl-methoxypyrazine (6) is used in addition to the trideuterated isobutylmethoxypyrazine 5 to provide more accurate quantitative analysis of the isopropyl compound (7, 10,... 

In direct contrast to the monoterpenes and norisoprenoid grape flavour components, no evidence has yet been found for the existence of bound or precursor forms of methoxypyrazines in grapes and, in particular, there is no evidence of glycosidically bound forms. 

Allen MS, Lacey MJ, Brown WV, Harris RLN (1990a) Occurrence of methoxypyrazines in grapes of Vitis vinifera cv. Cabernet Sauvignon and Sauvignon blanc. In Ribereau-Gayon, P, Lonvaud A (eds) Actualitds Oenologiques 89, Comptes rendus 4 Symposium International d Oenologie, 15-17 June 1989, Bordeaux. Dunod, Paris, pp 25-30... 

Response to Viticultural Conditions. In grapes, the behavior of ethylmethoxypyrazine (4) is quite different to that of isobutylinethoxypyrazine (1). Comparison of the concentration of these two methoxypyrazines during ripening provides an example. 

In non-monoterpene dependent grape varieties the flavor is often delicate and subtle, and knowledge of the chemical coiposition of the flavor compounds is almost non-existent. A notable exception in this regard is the recent confirmation of the role of alkyl methoxypyrazines in contributing to the varietal character of Sauvignon Blanc wines (16, 17). However, for other grapes in this... 

Adding to the uncertainty over the existence of ethylmethoxypyrazine is that if the biosynthesis of methoxypyrazines in vines follows a previously postulated pathway (i), in which an amino acid is the source of the alkyl side chain, as already identified in bacteria by the incorporation of valine (22) or pyruvate (23) into isopropylmethoxypyrazine, then the biosynthesis of ethylmethoxypyrazine would require 2-aminobutyric acid, an amino acid not normally found in proteins, although it does exist in grapes and vegetables. 

The role of methoxypyrazines in the herbaceous aroma of certain grape varieties, such as Cabernet Sauvignon, is now well-established. These compounds exist in a free state in grapes and no precursor forms have been identified. 

Sauvignon must attenuates the green bell pepper character produced by methoxypyrazines in insufficiently ripe grapes. 

Hashizume and Samuta 1999). 3-Isobutyl-2-methoxypyrazine (IBMP) is the dominant compound and is located in the grape skin, and being water soluble is rapidly extracted into the must. Although there is unproven evidence that yeast can metabolise this compound, sensory studies suggest that its aroma impact is dependent on the presence of other compounds that partially mask its aroma, such as phenyl ethanol and its ester, which are produced in higher amounts by Saccha-romyces bayanus (Treloar and Howell 2006 Eglinton and Henschke, unpublished). 

In studies of methoxypyrazines, a stable isotope labeled internal standard has not always been used. Calo et al. (6) used 5ec-butylmethoxypyrazine as an internal standard to quantify isobutylmethoxypyrazine in a comparison of grape varieties. Recently, Hashizume and Umeda have used 2-methyl-3-Ai-propylpyrazine as an internal standard to quantify methoxypyiazines in Japanese red wine and grape samples (7). However, the increased potential for lack of precision and accuracy needs to be recognized, and the natural occurrence of 5gc-butylmethoxypyrazine is a drawback to its use as an internal standard. 

In the 1990s, studies of the Sauvignon grapes and wines revealed that several sulfur compounds and methoxypyrazines (grassy note) are typical aroma compounds of these varieties (Harris et al., 1987 Lacey et al., 1991 Allen et al., 1994 1995 Tominaga et al., 1996 Bouchilloux et al., 1998). 

Alkyl-2-methoxypyrazines are compounds present in skin, pulp, and bunch stems of grape, and contribute with very characteristic vegetative, herbaceous, bell pepper, or earthy notes to the aroma of Cabernet Sauvignon, Sauvignon blanc, Semilion, and other wines... 

Accurate studies of grape juice and wine matrix effects in the headspace (HS)-SPME analysis of 3-alkyl-2-methoxypyrazines using a triphase fiber divinylbenzene-carboxen-polydimethylsiloxane (DVB/CAR/ PDMS) was reported by Kotseridis et al. (2008). Also, PDMS/DVB and CAR/PDMS were selected as suitable fibers for analysis of wines (Sala et al., 2002 Galvan et al., 2008 Ryan et al., 2005). The optimized analytical conditions found are described in Table 4.2. 

Hashizume, K., Tozawa, K., Endo, M., and Aramaki, I. 2001. 5-Adenosyl-L-methio-nine-dependent 0-methylation of 2-hydroxy-3-alkylpyrazine in wine grapes A putative final step of methoxypyrazine biosynthesis. Biosci. Biotechnol. Biochem. 65 795-801. 

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