Introduction
Aroma components that are common to the grape cultivar are vital in defining the wine aroma aspect of wine quality. Sauvign blanc, Cabernet Sauvignon and Semillon grape are cultivars of which their typical aroma may be described as constituting vegetative, grassy, herbaceous, gooseberry, asparagus and green pepper nuances. The nuances are caused by a particular group of chemical components referred to as methoxypyrazines, though other chemical components also contribute to the complexity of the aroma. There are three kinds of Methoxypyrazines (grape-derived flavour compounds), namely 2-methoxy-3-isobutylpyrazine (IBMP), 2-methoxy-3-sec-butylpyrazine (SBMP) and 2-methoxy-3-isopropylpyrazine (IPMP). IBMP, SBMP and IPMP have astonishingly low sensory thresholds in wine, which are generally reported at low ng/l levels (Murray and Whitfield, 1978).
Concentration of wine
Research on the occurrence of isobutylmethoxypyrazine has indicated that it has a methodical and regular association with the grape variety and vine growing conditions. The occurrence of isobutylmethoxypyrazine-like aroma in the making of wine is regularly associated with the grape variety, which proves that the methoxypyrazines are produced under genetic control in the grape berry. In spite of the production of noticeable isobutyl-methoxypyrazin levels by Sauvign blanc, Cabernet Sauvignon and Semillon, other cultivars are observed to lack the ability to produce this compound. In another study of different grape varieties using vines with the same vineyard, high levels of isobutylmethoxypyrazine were observed in Cabernet Sauvignon and Sauvignon blanc grapes, but no traces were found in Pinot noir (Heymann, Noble and Boulton, 1986).
Research has indicated that there is a direct relationship between the maturity of grape berries and concentration of isobutylmethoxypyrazine, such that increasing maturity leads to a decrease in concentration. Another study showed that the ripening temperature of the grape berries also affects the concentration. A study comparing fruits at various stages of ripeness, higher levels of methoxypyrazine were observed to occur in cool regions by comparison with warm regions. The level of isobutylmethoxypyrazine was observed to drop below its sensory detection threshold, in warm areas, by the time it got to harvesting time. In cool areas, the level ranged from 20-30 ng/L (Heymann, Noble and Boulton, 1986).
A regular increase in the concentration of isobutylmethoxypyrazine has been observed during fermentation with grape skin contact. This implies that methoxypyrazines can either be extracted from the solid parts of the grape, or produced by yeast-mediated effects. An increase is initiated by ensuring contact of the juice with the grape skins, and it decreases as fermentation progresses. Another important factor affecting the concentration is the vine canopy and the pruning and training system, since the exposure of the fruit to light affects the levels of methoxypyrazine. Within the vine canopy, a consistent lower level of methoxypyrazines is observed in the fruit that is more exposed than in the more shaded fruit, usually half or less of the level of the latter (Heymann, Noble and Boulton, 1986).
The IBMP concentrations in Sauvignon wines from Australia, New Zealand and France were observed to be about eight times higher, on average, than those of IPMP, while those of SBMP were lower. This implies that IPMP and SBMP do not contribute to the overall aroma of grapes and wine of this cultivar. To confirm this, the sniffing technique was used. It showed that IBMP was the most significant contributor to the vegetative aroma of regular wine. Other studies conducted on the typical Sauvignon blanc aroma showed that the addition of IBMP at levels greater than 2 ng/L resulted in musty and green pepper-like aromas in a model and natural wine respectively, implying that the threshold value for wine is 2 ng/L. Addition of the same amount of IPMP to the same wines resulted in musty aromas in the model wine and earthy and leafy nuances in the natural wine. The concentration of the compound added was observed to affect the aroma expression, indicating that other volatiles and individual compound concentration affect the perceived aroma. “Aroma threshold values are only guidelines for aroma intensity and could change through synergistic actions with other components in the medium” (Heymann, Noble and Boulton, 1986).
Biosynthesis
It is proposed that the biosynthesis of alkylmethoxypyrazines involves amidation of the appropriate amino acid. This step is followed by condensation with glyoxal, before methylation. The co-occurrence of the three methoxyyrazines is consistent with the bio—synthetic way suggested a few decades ago. The amino acid leucine is observed to be the source of the C4 side chain of methoxypyrazine, through condensation of its amino amide with an unspecified C2 component, and methylation of the initial pyrazinone condensation product. This suggested biosynthetic pathway complements the three methoxypyrazines through incorporation of leucine, isoleucine or valine. These three are regularly available amino acids in plants. Little is known about the validity of this pathway in vines or other plant material, though the main aspects of the proposed pathway have been shown to apply to the biosynthesis of isopropylmethoxypyrazine by certain bacteria (Allen and Lacey, 1993). The chemical reaction for the biosynthesis process of the three methoxypyrazines is shown below.
Comparison of methoxypyrazine to Ethylmethoxypyrazine
Ethylmethoxypyrazine is a component of grapes and wines, though it is unknown, where it came from. There is no clear association between it and vine variety, berry development and canopy light penetration that is found with isobutyl-methoxypyrazine (Allen and Lacey, 1993).
Quantification of methoxypyrazines
It has been observed that the growing conditions of the grape vine, in terms of soil, climate, variety, and maturity of the berry, have little effect on the concentration of methoxypyrazine. To start with, methoxypyrazines have been observed to be less vital for some varieties of grapes than others, with high levels in Sauvignon blanc, Cabernet Sauvignon, and Semillon. Methoxypyrazines is useful in contributing a wine aroma that is both unique and vital in identifying the particular wine from other varieties. In the second place, the rapid loss of methoxypyrazines level as grape ripening occurs produces results such as 2% of the levels two months previously. This implies that the harvesting date has serious effect on the intensity of the aroma of these components in the wine. The advantage is that grapes harvested early can be blended with those harvested much later, to obtain the desired methoxypyrazine aroma intensity (Allen and Lacey, 1993).
The third aspect is higher level of methoxypyrazine in cool climatic conditions at various stages of berry ripening. Some of the cool regions including New Zealand and Western Australia are capable of developing a powerful methoxypyrazine aroma. the degree of exposure of the fruit to light is dependent on the pruning methods, resulting in fruit with varying concentrations of up to eight times. The effect of growing conditions on berry methoxypyrazine levels raises difficulties in the production of grapes with the appropriate or desired flavour quality. In addition to this, it necessitates research investigation, both new and on reports of previously done research (Murray and Whitfield, 1978).
References
Allen, M. S. and Lacey M. J. (1993). Methoxypyrazine grape flavor: influence of climate, cultivar and viticulture. Die Wein-wisenschaft, 48, 31-37.
Heymann, H., Noble A. C. and Boulton, R. B. (1986). Methoxypyrazines in Sauvignon blanc grapes and wines. Am. J. Enol. Vitic, 42, 103-108.
Murray, K. E. and Whitfield F. B. (1978). The occurence of 3-alkyl-2-methoxypyrazines in raw vegetables. J. Sci. Food Agric., 26, 973-986.