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Introduction to Oxidation in Wine Andrew L. Waterhouse Department of Viticulture & Enology University of California, Davis Oxidation Avoidance Traditional limit to wine preservation Current technology can exclude oxygen Wine oxidation - a fault? Manage Wine Oxidation At crush / press – Anaerobic press to hyperoxidation Fermentation – Add oxygen for yeast, reductive aromas Post fermentation/s – Micro-ox, barrels, racking In bottle – Affects flavor development Oxygen Measurement Standard meter – Limit of Detection ~0.5 ppm Orbisphere or other Clark electrode – LOD ~2 ppb Nomasense, Mocon, others – Oxydots, fiber optic measure-contact not needed! 10 ppb – Temperature sensitive! Antioxidants SO2 Ascorbic acid Glutathione Wine Oxidation Chemistry O OH O2 + EtOH O OH Acetaldehyde + H2O2 Step 1 Step 2 Oxygen reacts with phenols to yield quinone and hydrogen peroxide Hydrogen peroxide oxidizes ethanol to acetaldehyde Known Oxidation Products Quinones – Thiol adducts – Coupling Products Aldehydes – Acetaldehyde – Glyoxylic Acid • (Tartaric Acid product) – Flavonoid coupling OH O O Oxidative Changes in Wine Formation of quinones from catechols – React with thiols, SO2, ascorbate, phenolics Fenton oxidation of alcohols – Formation of aldehydes – Reactions of all other substances Oxygen Pathway in Wine RC=O O2 RC=O Fe+3 Fe+2 RCOH 5 1 (Hydroperoxyl radical) O2 + H Fe+3 RCHOH (Hydroxyl radical) Fe+3 4 2 Fe+2 (Semiquinone radical) 3 (Quinone) (Hydrogen peroxide) Sulfur Dioxide and Oxygen Sulfite is the sink for oxidation O2 + 2 SO2 2 SO3 1 mg of O2 will consume 4 mg SO2 SO3 + H2O H2SO4 Polyphenols are Pro-oxidants Generation of quinone and hydrogen peroxide from dioxygen OH Fe+2 HO O O2 H+ O O + HO OH + OH Catechin + 3-Mercaptohexanol OH O OH O HO HO O OH OH + O S SH OH HO OH Nikolantonaki , ACA 660: 102 (10) Blanchard, AJEV 55:115 (04) OH Catechin dimer in model juice Poupard, J Chrom A, 1179: 161 (08) Quinone Options OH OH ? OH Ascorbate OH O OH HO OH Phloroglucinol O RSH OH S R OH (Quinone) AA's, Strecker Degradation Phenolic Coupling (polymerization) SO2 OH Aldehydes ? OH Mercaptan Trapping Oxygen Pathway in Wine RC=O O2 RC=O Fe+3 Fe+2 RCOH 5 1 (Hydroperoxyl radical) O2 + H Fe+3 RCHOH (Hydroxyl radical) Fe+3 4 2 Fe+2 (Semiquinone radical) 3 (Quinone) (Hydrogen peroxide) Peroxide Competition Fe+3 H2O2 Fe+2 SO2 H2O ·OH EtOH CH3CHO Wine Minor Components Red Wine Composition, Minor Components Acetaldehyde Volatile Acidity Glycerol Sugar Higher Alcohols Sorbitol & Mannitol Phenols Sulfites Minerals * Esters Amino acids Acid Oxidation of Wine Acids (Alcohols) to Carbonyls Pyruvic – Observed in wine – Reacts with anthocyanins to make wine pigments Glyoxylic – Observed in wine – Condenses with flavan-3-ols O OH O H2O2 HO O Fe+2 CH3 CH3 Lactic Acid (or Malic) O H2O2 HO OH O Pyruvic Acid O OH OH Fe+2 OH O HO O OH O OH CH OH Tartaric Acid Hydroxymalonic Glyoxylic Acid Aldehyde Pigment Reactions “D-ring” formation by acetaldehyde and pyruvate R1 O R1 OH OH H HO O R2 OH + HO + O Glu O R2 O or OH O Glu O O R R = H, O OH Alcohol Oxidations with ∙OH Aldehydes known oxidation products Wine Oxidation Aroma and color changes Can be managed – A few key steps need better understanding Many current investigations State of the art and practice today