Origin and Incidence of 2-Methoxy-3,5-dimethylpyrazine, a Compound with a “Fungal” and “Corky” Aroma Found in Cork Stoppers and Oak Chips in Contact with Wines

Abstract

This study identifies a previously isolated bacterium as Rhizobium excellensis, a new species of proteobacteria able to form a large quantity of 2-methoxy-3,5-dimethylpyrazine (MDMP). R. excellensis actively synthesizes MDMP from L-alanine and L-leucine and, to a lesser extent, from L-phenylalanine and L-valine. MDMP is a volatile, strong-smelling substance detected in wines with cork stoppers that have an unpleasant "corky", "herbaceous" (potato, green hazelnut), or "dusty" odor that is very different from the typical "fungal" nose of a "corked" wine that is generally due to 2,4,6-trichloroanisole (TCA). The contamination of cork by MDMP is not correlated with the presence of TCA. It appears possible that R. excellensis is the microorganism mainly responsible for the presence of this molecule in cork bark. However, other observations suggest that MDMP might taint wine through other ways. Oak wood can also be contaminated and affect wines with which it comes into contact. Nevertheless, because 93% of the MDMP content in wood is destroyed after 10 min at 220 °C, sufficiently toasted oak barrels or alternatives probably do not represent a major source of MDMP in most of the cases. Due to MDMP's relatively low detection threshold estimated at 2.1 ng/L, its presence in about 40% of the untreated natural cork stoppers sampled at concentrations above 10 ng/cork suggests that this compound, if extracted from the stoppers, may pose a risk for wine producers.

Quantification of cork off-flavor compounds in natural cork stoppers and wine by multidimensional gas chromatography mass spectrometry

“…It has also been found as a malodorous compound in water supplies [15] and seems to be generated by bacteria [16,17]. It is an extremely potent aroma compound with an odor threshold of about 2 ng/l in white wine [3].…”

“…It is an extremely potent aroma compound with an odor threshold of about 2 ng/l in white wine [3]. In the literature, MDMP is seen as the most important substance affecting cork stoppers and thus wine next to TCA [3,17]. The other alkyl methoxypyrazines, IPMP and IBMP, with a vegetative, green odor are flavor relevant compounds with very low odor thresholds in many vegetables [18] and also in V. vinifera varieties with IBMP as the major methoxypyrazine [19] (odor threshold in white wine 1-2 ng/l [20]).…”

Investigating a persistent odor at an aircraft seat manufacturer

“…[18] These organisms were identified as the source of 3,5-MDMP in tainted wine corks, with Rhizobium excellensis believed to be the main source. [19] Rhizobium bacterial species are generally found in soil. Several bacteria within the phyla Proteobacteria, Bacteroides, Firmicutes, and Actinobacteria are known to produce an array of pyrazines, including 3,5-MDMP.…”

Health hazard evaluation report: evaluation of an unpleasant odor at an aircraft ejection seat manufacturer.

“…Pyrazine production by these organisms can vary, depending on the culture conditions. The Proteobacteria species known to produce 3,5-MDMP, including Rhizobium excellensis (Rhizobiales order), Serratia odorifera (Enterobacteriales order), and Chondromyces crocatus (Myxococcales order), were all identified in tainted wine corks [Chatonnet et al 2010]. We did not identify these specific organisms in the Sanger sequencing analysis.…”

“…Some bacteria have been demonstrated to produce 3,5-MDMP specifically, including Rhizobium excellensis, Serratia odorifera, and Chondromyces crocatus [Chatonnet et al 2010]. These organisms were first identified as the source of 3,5-MDMP in tainted wine corks, with Rhizobium excellensis believed to be the main source [Chatonnet et al 2010]. Rhizobium bacterial species generally are found in soil.…”