Contribution of grape and oak wood barrels to pyrrole content in wines – Influence of several cooperage parameters

Abstract

Chardonnay is the world’s most planted white grape variety and has met a great commercial success for decades. The finest Chardonnay wines impart a unique and complex bouquet. Multi-dimensional GC-O recently evidenced 5 pyrroles reminiscent of hazelnut. A quantitative method was developed, highlighting their significantly higher abundance in Chardonnay. However, they proved to be irrelevant in sensory terms, given the low amounts measured in wines compared to their detection threshold. Yet, these aromatic compounds could represent interesting chemical markers of Chardonnay wines. They could also prove to be precursors of thiols newly identified, called thiopyrroles. Thus, it seemed interesting to study the influence of some enological parameters on their concentration in wines. First, the quantitative method of pyrroles in wine was optimized. The validated method was applied to determine pyrroles content of 27 Chardonnay wines elaborated in different containers: stainless steel tank, new and old barrels. The concentration of 1-methylpyrrole-2-carboxaldehyde (MPC), 1-ethylpyrrole-2-carboxaldehyde (EPC), 2-acetyl-1H-pyrrole (AP), and pyrrole-2-carboxaldehyde (PC) were significantly higher in wines made in new barrels than in older barrels or in stainless steel tank. These results showed that these pyrroles can partly be brought by oak wood during the winemaking process. However, pyrroles were also observed in the stainless steel tank modality, which indicated that these compounds have also a varietal origin. Only the 1H-pyrrole content did not seem to be influenced by the type of container, suggesting a purely varietal origin of this compound.

Then, a quantitative method of pyrroles in oak wood extracts was developed in order to study the influence of several cooperage parameters on their content. The influence of three types of traditional toasting on pyrroles concentration in oak wood was studied. Significantly higher concentrations were found in toasted than in untoasted wood extracts for all four pyrroles, regardless of the toasting process. The temperature and the time of toasting were also studied. Results showed that MPC and EPC content increased according to the two parameters studied, whereas AP and PC content tend to decrease in oak wood with a long toasting process. This finding brings new insights on the understanding of the molecular origin of chemical markers of Chardonnay wines.