Prevalence and Management of Alkyl-Methoxypyrazines in a Changing Climate: Viticultural and Oenological Considerations

Development of Tannins and Methoxypyrazines in Grape Skins, Seeds, and Stems of Two Pinot Noir Clones during Ripening

Two Pinot noir clones (AM10/5 and UCD5) were analyzed for tannin and methoxypyrazines (MPs) in different grape tissues during berry development using liquid chromatography and two-dimensional gas chromatography-mass spectrometry. On a per berry basis, skin tannins reached the maximum level about 2-3 weeks after véraison, seed tannins at around véraison, and stem tannins 4 weeks before véraison. Clone AM10/5 showed significantly higher levels of seed and stem tannins on a per berry basis at harvest. Tannin concentration and composition varied among the different tissues. On a per berry basis, stem tannin levels were comparable to skin tannins but were 3 to 4 times lower than seed tannins, while stem tannins had an intermediate galloylation (5-7%) between seed tannins (12-18%) and skin tannins (2%) and lower prodelphinidin (4-7%) than skin tannins (31-36%). The mean degree of polymerization of stem tannins was similar to seed tannins but lower than skin tannins. MPs, including 3-isopropyl-2-methoxypyrazine (IPMP), 3-s-isobutyl-2-methoxypyrazine (SBMP), and 3-isobutyl-2-methoxypyrazine (IBMP), showed significantly higher concentrations than their sensory thresholds in grape stems but not in skins. The MPs development in stems showed an increasing trend toward véraison and then a decreasing trend toward harvest. Compared to AM 10/5, UCD5 stems showed a higher level of MPs, especially significantly higher concentrations of IPMP and IBMP at harvest. The extraction of MPs from grape stems could contribute negative green and vegetative characters to Pinot noir wines.

Methoxypyrazine concentrations in the grape bunch rachis of Vitis vinifera L. Cv Shiraz: Influence of rootstock, region and light

Vitis vinifera L. cv Shiraz appears unable to synthesise 3-alkyl-2-methoxypyrazines (MPs) in the berry, but can still produce significant concentrations in rachis. MPs are readily extracted from rachis during fermentation, producing Shiraz wines with uncharacteristic "green" flavours. Recently, rootstocks were shown to significantly alter MP concentrations in Cabernet Sauvignon rachis compared to own-rooted varieties, but whether Shiraz followed a similar trend required investigation. This study considered the effect of thirteen rootstocks on the concentrations of 3-isobutyl-2-methoxypyrazine (IBMP), 3-isopropyl-2-methoxypyrazine (IPMP), and 3-sec-butyl-2-methoxypyrazine (SBMP) in the rachis of Shiraz bunches sampled during multiple vintages across several Australian growing regions. Although IBMP was the most abundant, all measured MP concentrations were significantly affected by vintage, rootstock, and region. In addition, vine vigour showed positive correlations with IBMP, which were attributed to changes in canopy coverage impacting rachis light exposure. This hypothesis was explored with light exclusion trials, which significantly increased rachis IBMP concentrations.

Sites of biosynthesis, distribution and phloem transport of 3-isobutyl-2-Methoxypyrazine in Capsicum annuum (bell pepper) plants

The distribution and translocation as well as the biosynthetic sites of 3-isobutyl-2-methoxypyrazine (IBMP) in bell pepper plants (Capsicum annuum) were examined. For IBMP-quantification, a stable-isotope dilution assay (SIDA) was developed, using headspace solid-phase microextraction (HS-SPME) combined with gas chromatography and time-of-flight mass spectrometry (ToF-MS). While IBMP was the most abundant MP in all aerial plant parts, IPMP was dominating in root tissues. The highest IBMP-levels were found in unripe bell pepper fruits. Moreover, feeding experiments with [²H₁₀]-l-leucine revealed that IBMP is biosynthesized in all plant parts, while in roots and ripe bell pepper fruit tissues no incorporation of the labelled precursor was detectable. A potential phloem translocation of IBMP could not be detected.