Role of Grape-Extractable Polyphenols in the Generation of Strecker Aldehydes and in the Instability of Polyfunctional Mercaptans during Wine Oxidation

Abstract

Wine longevity is a complex multifactor phenomenon in which the weight of the different factors is not well known. One of the key factors of wine longevity is related to its resistance to oxidation. This property can be defined as the ability of the wine, under an exposure to oxygen, to keep its color, avoid accumulation of acetaldehyde and Strecker aldehydes (SA), and keep as long as possible labile varietal aroma compounds, such as polyfunctional mercaptans (PFM). The goal of the present work is to assess the specific role played by grape polyphenols on these abilities. For that, polyphenolic fractions extracted from Garnacha, Tempranillo, and Moristel grapes were reconstituted to form model wines of identical pH, ethanol, amino acid, metal, and varietal PFMs contents. Models were subjected to a forced oxidation procedure at 35 °C (50 mg O2L−1 for 35 days) and to an equivalent treatment under strict anoxia. Chemical characterization of polyphenolic fractions and oxidized and unoxidized (controls) wine models was carried out. In general, oxidation causes increases in redox potential, tannin activity, and in the levels of SAs. Similarly, oxidation causes decreases of great magnitude in free and total PFMs and of moderate magnitude in total polyphenol index, pigmented tannins, and in TDN. Polyphenolic profiles significantly determined oxygen consumption rates (5.6−13.6 mg L−1 day−1), SAs accumulation (ratios max/min around 2.5), and levels of PFMs remaining (ratio max/min between 1.93 and 4.53). By contrast, acetaldehyde accumulated in small amounts and homogeneously (11−15 mg L−1). The accumulation of SAs is positively and significantly correlated to the content on phenolic acids, monomeric flavanols, and nonpigmented tannins and negatively correlated to the contents in prodelphinidins, anthocyanins, and color. Overall, SAs accumulation may be related to polyphenols, producing stable quinones. Tempranillo samples, with highest delphinidin and prodelphinidins and smallest catechin, consume O2 faster but accumulate less SA and retain smallest amounts of PFMs under anoxic conditions. The ability to protect PFMs as disulfides may be negatively related to the increase in tannin activity, while pigmented tannins could be related to 4-methyl-4-mercaptopentanone decrease. The varietal polyphenolic profile exerted a deep effect on the generation of Strecker aldehydes and on the instability of polyfunctional mercaptans, and hence, on the longevity of wine aroma.