1/9/2017 Sulfur Taints Wine Flavor 101 January 2017 Linda Bisson Department of Viticulture and Enology University of California, Davis Why Are Sulfur Taints a Problem? Low thresholds of detection Chemical reactivity Difficulty in removal Difficulty in masking 1 1/9/2017 S-Volatiles: Negative Impacts on Flavor Hydrogen Sulfide: Rotten egg Fermentation and Post-fermentation: S-taints derived from amino acids and other metabolites Sur lie S-taints Sources of Sulfur Compounds Sulfate metabolism/biosynthesis Degradation of sulfur containing amino acids Inorganic sulfur • Non-enzymatic • Requires reducing conditions established by yeast Degradation of S-containing pesticides/fungicides 2 1/9/2017 HYDROGEN SULFIDE Hydrogen Sulfide Formation Due to release of reduced sulfide from the enzyme complex sulfite reductase Reduction of sulfate decoupled from amino acid synthesis Sulfate reduction regulated by nitrogen availability (methionine) and stress See significant strain variation 3 1/9/2017 Hydrogen Sulfide Formation: Strain Variability Hydrogen sulfide plays an important population signaling role – Inhibits respiration: coordinated population fermentation – Inhibits respiration: inactivation of bacteria and other yeasts Hydrogen sulfide formation is protective against stress Strain variation due to exposure to different environmental conditions in combination with the multiplicity of roles of H2S Current Understanding of H2S Formation Nitrogen levels not well-correlated with H2S formation, but generally see increased H2S at lower nitrogen Under complex genetic control Tremendous strain variation in H2S production 4 1/9/2017 Factors Impacting H2S Formation Level of total nitrogen Level of methionine relative to total nitrogen Fermentation rate Use of SO2 Vitamin deficiency Presence of metal ions Inorganic sulfur in vineyard Use of pesticides/fungicides Strain genetic background Timing of Formation of H2S H2S Brix Time 5 1/9/2017 Elimination of Hydrogen Sulfide Rely on volatility and fermentation gas or inert gas sparging to remove – Need to make sure it is gone and not just converted to a non-volatile form Use of volatiles stripping technologies Precipitation via copper – Emerging issue: health and environmental concerns about copper Use of fining agents Use of strains not producing sulfides HIGHER SULFIDES 6 1/9/2017 Higher Sulfides Emerge late in fermentation and during sur lie aging Release of compounds during entry into stationary phase by metabolically active yeast Come from degradation of sulfur containing compounds by viable cells – Biological – Chemical » From reaction of reduced sulfur intermediates with other cellular metabolites? » Formed chemically due to reduced conditions? Degradation of cellular components: autolysis – Enzymatic – Chemical Common Volatile Sulfur Compounds Methanethiol: CH3-SH Ethanethiol: C2H5-SH Dimethyl sulfide: CH3-S-CH3 Dimethyl disulfide: CH3-S-S-CH3 Dimethyl trisulfide: CH3-S-S-S-CH3 Diethyl sulfide: C2H5-S-C2H5 Diethyl disulfide: C2H5-S-S-C2H5 7 1/9/2017 Common Volatile Sulfur Compound Ranges in Wine Hydrogen sulfide: Trace to 80 ug/L Methanethiol: Trace Ethanethiol: 1.9 -18.7 ug/L Dimethyl sulfide: 1.4 - 474 ug/L Dimethyl disulfide: Trace to 1.6 ug/L Dimethyl trisulfide: 0.09 - 0.25 ug/L Diethyl sulfide: 4.1 - 31.8 ug/L Diethyl disulfide: Trace - 85 ug/L Sulfur Compound Aroma Descriptors Dimethyl sulfide: cabbage, cooked corn, asparagus, canned bean/vegetable Dimethyl trisulfide: meaty, fishy, clams, green onion, garlic, cabbage Diethyl sulfide: garlic, onion Diethyl disulfide: overripe onion, greasy, garlic, burnt rubber, manure Ethanethiol: onion, rubber, natural gas Methionol: cauliflower, cabbage, potato Methional: musty, potato, onion, meaty Mercapto-3-methyl butanol: meaty 8 1/9/2017 Ehrlich Pathway S-Compounds Ehrlich Pathway: source of fusel oils Removal of N from amino acid compounds Generates aldehyde Aldehyde reduced to alcohol In fermentation see high concentrations of methionine-derived “fusel” compounds: Methionol (100-6,300 ug/L) and Methional (generally trace, but reaction products are more aromatic) Sources of Higher Sulfides S-Containing Amino Acids S-Containing Vitamins and Co-factors Glutathione (Cysteine-containing tripeptide involved in redox buffering) 9 1/9/2017 Management of S-Taints Diagnosis of Taint Taint Prevention Taint Mitigation DIAGNOSIS OF SULFUR TAINTS 10 1/9/2017 Correct Diagnosis of Fault Is Important Is it an S-containing compound? When did taint first appear? What factors are associated with appearance of the taint? Is It a sulfur-containing compound? Be familiar with the characteristic off-odors of sulfur compounds Other classes of off-odors can be reminiscent of S-compounds Thresholds of detection are so low compound may be difficult to detect chemically 11 1/9/2017 When did taint first appear? Provides important clues as to the reason taint is occurring If know why it is being made can take steps to prevent formation What factors are associated with appearance of the taint? Always found with a specific vineyard? Associated with unsound fruit? Associated with specific processing? – Inert gas blanketing – Type of vessel Associated with specific fermentation conditions? 12 1/9/2017 TAINT PREVENTION Preventing S-Taint Formation Vineyard Wine chemistry Yeast strain selection Fermentation management 13 1/9/2017 Preventing S-Taint Formation Vineyard – Judicious use of elemental sulfur – Eliminate reliance on S-containing pesticides – Address excesses of metal ions – Address vine stress: nutritional or otherwise Wine chemistry Yeast strain selection Fermentation management Preventing S-Taint Formation Vineyard Wine chemistry – Minimize use of ‘enabling’ practices » Temperature » Solids content » “Reductive” aging conditions » Role of the ‘lie’ in ‘sur lie’ Yeast strain selection Fermentation management 14 1/9/2017 Conclusions Sulfur taints can be controlled Need to use correct strain Need to minimize use of compounds in vineyard and winemaking techniques that amplify S- compound formation Many factors leading to S-taint appearance are still not well understood Sulfur Compound Tasting Glass 1: Control wine Glass 2: Hydrogen Sulfide (30 ug/L) Glass 3: Dimethyl sulfide (20 ug/L) Glass 4: Diethyl sulfide (30 ug/L) Glass 5: Diethyl disulfide (70 ug/L) Glass 6: Ethanethiol (20 ug/L) 15 1/9/2017 Sulfur Compound Flight #2 Spiked Compounds Hydrogen sulfide: rotten egg, fecal Dimethyl sulfide: cabbage, cooked corn, asparagus, canned vegetable Diethyl sulfide: garlic, onion Diethyl disulfide: overripe onion, greasy, garlic, burnt rubber, manure Ethanethiol: rubber, strong onion 16.