Methoxypyrazine isolation

Minuscule quantities of naturally-occurring pyrazines have been found in some foodstuffs and are largely responsible for their flavor and aroma. For example, 3-isopropyl-2-methoxypyrazine is isolated from green peas and wine and a seasoned wine connoisseur can identify a ppt quantity. In addition, 2-methyl-6-vinylpyrazine exists in coffee. 

The specific compounds that are responsible for the "pea" flavor have not been identified. Bengtsson and Bosund (52) suggested that acetaldehyde, hexanal and ethanol were important, while Murray et al. (53) isolated three methoxypyrazines that have very low taste or recognition thresholds and might, therefore, be of major significance in pea flavor. 

The use of headspace solid-phase microextraction (HS-SPME) combined with GC was effective for isolation and determination of alkylpyrazines in cocoa liquor <2004JBS267, 2004MI291>, and similarly methoxypyrazine in wine was analyzed by HS-SPME/2-D GC <2005MI1075>. 

Methoxypyrazines are compounds with very low detection thresholds which must be determined at very low levels. For these compounds, different selective isolation methods have been proposed (Allen et al. 1994 Sala et al. 2002). Some authors use a simple extraction (Kotseridis et al. 1999 Falcao et al. 2007) or an optimized headspace SPME procedure (Chapman et al. 2004 Prouteau et al. 2004) using in most cases isotopically-labelled internal standards to compensate for matrix effects. In spite of the claims of the authors, all these methods present some difficulties to accurately determine the compounds at the lowest levels at which they can be found. A recent report has presented an advanced method combining the preconcentration ability of headspace SPME with the selectivity of comprehensive GC (Ryan et al. 2005). 

Quantitative Analysis. To study those factors that influence the occurrence of methoxypyrazines, it was necessary to be able to quantify them. Furthermore, it was desirable that this be possible even at the lowest levels that might be relevant. This indicated a need for their quantitative analysis to extend to concentrations below their sensory detection threshold of 1-2 ng/L, preferably by an order of magnitude. Accurate quantitative analysis at such extremely low analyte concentrations is difficult to achieve. If a moderate sample size is to be used, and if the analysis is to cope with some losses in isolation, then the technique needs to have a detection limit of a few picograms. Furthermore, it must provide quantitative data with adequate accuracy and precision for meaningful interpretation. 

Althou the amino aclci/sugar solutions were buffered in an effort to maintoin pH, a drop in pH was encountered with Increasing reaction times. Therefore, after heat treatment, each sample was adjusted to pH 9.0 with O.IN NaCH. One ml of a solution containing 2-methoxypyrazine in distilled water (2 ppm) was added as an internal standard. Firml sample volume was 15 ml. lyrazines were then isolated, s >arated and quantified using an... 

Three extremely odorous pyrazines, 3-isopropyl-2-methoxypyrazine, 2-s-butyl-3-methoxypyrazine, and 2-isobutyl-3-methoxypyrazine have been shown to be present in green peas, and are likely to be of major significance in the flavor of peas (59). The volatile oil of green bell peppers has been found to contain 2-isobutyl-3-methoxypyrazine as a major component (60,61). The alkylpyrazines in potato chips (62,63) and roasted peanuts (63) have been examined. 2-Isopropyl-3-methoxypyrazine has been characterized in the vacuum steam volatile oil of potatoes (64), 2-ethyl-3-methoxypyrazine in cooked potato (65), and 3-ethyl-2,5-dimethylpyrazine and 2-ethyl-3,5-dimethylpyTazine as the components important to the aroma of baked potato (66). A variety of alkylpyrazines have been identified in roasted sesame seeds (67) and 21 pyrazines have been identified in the aroma components isolated from roasted green tea (68). 

A large number of 2-alkyl-3-methoxypyrazines has been isolated from raw vegetables (59, 60, 64, 65, 69, 80, 1099), bell peppers (60,61) and gambanum oil (49). 2-Isobutyl-3-methoxypyrazine is already finding commercial use as a flavoring material (368) and 2-ethoxy-3-methylpyrazine may be used for pineapple flavor (981). The odor characteristics (367, 368, 976, 977) and structure (977) of some alkoxy alkylpyrazines have been examined. 3-Guanidino-6-hydroxymethyl-l-methyl-2-oxo-l,2-dihydropyrazine has been isolated from seeds of Stizolobium hassjoo and on mild alkaline hydrolysis gave 3-hydroxy-6-hydroxymethyl-l -methyl-2-oxo-l, 2-dihydropyrazine (1060). 

Three derivatives of 2-methoxypyrazine (175) were isolated from the volatile constituents of green peas those in which R is isopropyl,... 

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