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Pyrazines

Methoxypyrazines are powerful aroma compounds that have bell pepper and related vegetal aromas. David Raffaele, 2015.

Alkyl-methoxypyrazines (MPS) are nitrogen-containing heterocyclic compounds that are important to for and beverage industries due to their powerful odorous properties and low detection thresholds. The specific MPs responsible for the vegetative, green pepper aromas in vitis vinifera grapes/wines includes 3-isobutyl-2-methoxypyrazine (IBMP), 3-isopropyl-2-methoxypyrazine (IPMP) and to a lesser extent, 3-sec-isobutyl 2-methoxypyrazine (SBMP). The perception thresholds for IPMP are in the range of 0.32–1 ng/L in water and in white wine, while thresholds are about 2 ng/L for IBMP. Detection thresholds in red wines are noticeably higher for IPMP and IBMP, being 1–6 ng/L and 10–16 ng/L respectively.

Because MP levels and detection thresholds are low in wine, analysis requires purification and concentration steps of the compounds prior to actual measurement, which require skill and time. Modern detection instruments that are sensitive to MPs concentrations on the ng/L scale include combinations of gas chromatography-mass spectroscopy (GC-MS), liquid chromatography-mass spectroscopy (LC-MS), high-performance liquid chromatography (HPLC) for purification, gas chromatography combined with time-of-flight mass spectrometry (TOF-MS), or gas chromatography combined with nitrogen phosphorous detectors (NPDs), along with other types of analyses

Empirical studies have shown ways to control MP accumulation and degradation in the grape berry during the growing cycle as well as methods to control their extraction during winemaking. Lesser progress has been made, however, regarding the specific biosynthetic pathways of MP synthesis that are related to these physical parameters. Therefore, future studies will likely focus on the methylation enzymes influencing MP concentrations, S-adenosylL-Met (SAM)-dependent O-methyltransferase (OMT). This would include analysis of the genes responsible for OMT synthesis, hormone signaling, repressor activity as well as the transcriptions factors involved in protein synthesis.

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