Detection threshold, sensory, methoxypyrazines

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. 

Compounds of Importance. The three identified methoxypyrazines (1), (2) and (3) are very similar, both in structure and in the factors that influence their occurrence. One of them, isobutylmethoxypyrazine (1), is almost invariably dominant typically, it has an eight-fold or higher concentration than the other methoxypyrazines (Figure 1). Of the other two methoxypyrazines, the most abundant is isopropylmethoxypyrazine (3). Its concentration sometimes exceeds its sensory detection threshold. As they all have a similar sensory detection threshold, of 1-2 ng/L in water. 

The sensory detection threshold of jec-buty I methoxypyrazine and isopropyl methoxypyrazine are very similar to that of isobutylmethoxypyrazine, but the jec-butyl- and isopropyl- components are relatively minor methoxypyrazines, typically 10% or less of the isobutylmethoxypyrazine concentration. This suggests that they are less important to wine flavor than isobutylmethoxypyrazine although isopropylmethoxypyrazine may be more important, in red wine, than its sensory detection threshold suggests (9), and its more earthy aroma (i, 9, 14, 15) may contribute to nuances of wine aroma. The origin of sec-butyl- and... 

Ethylmethoxypyrazine. Unlike the isobutyl-, isopropyl- and sec-butyl methoxypyrazines, the existence of 2-ethyl-3-methoxypyrazine (4) (ethylmethoxypyrazine) in fruits and vegetables has not been consistently confirmed in the past. While evidence for its existence was found, unambiguous identification by mass spectrometry was prevented by the trace concentration of this component. Of all the methoxypyrazines so far mentioned, it has an aroma most similar to raw potatoes 16) and synthetic material adds potato flavor to processed potato products (18). It is also described as earthy 15), and it has a sensory detection threshold of 425 ng/L (17). Given its aroma, it is not surprising that its possible occurrence in potatoes has been the subject of several studies. 

Methoxypyrazines can contribute to wine flavour even at low parts per trillion levels. Sensory detection thresholds in water have been determined as 2 ng/1 (parts... 

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