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Anita Oberholster

Off-Character Formation during Fermentation. Anita Oberholster. Introduction Aroma Compounds. Grape-derived – provide varietal distinction Yeast and fermentation-derived If known WHAT HOW WHY Prevention and treatment – next talk . Introduction Aroma Compounds.

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Anita Oberholster

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  1. Off-Character Formation during Fermentation Anita Oberholster

  2. Introduction Aroma Compounds • Grape-derived – provide varietal distinction • Yeast and fermentation-derived • If known • WHAT • HOW • WHY • Prevention and treatment – next talk

  3. Introduction Aroma Compounds • Grape-derived – provide varietal distinction • Methoxypyrazines(vegetative, herbacious, bell pepper or earthy aroma) • 2- isobutylmethoxypyrazine, 3-butylmethoxy-pyrazine, 3-isopropylmethoxypyraxine • Threshold 2 ng/L, in wine 9-42 ng/L • Sauv. blanc, Semillon, Sauv. Cab. • Recognized 4-8 ng/L white wine • Recognized 7-15 ng/L red wine • Undesirable  25 ng/L IBMP

  4. Introduction Aroma Compounds • Grape-derived aroma compounds – provide varietal distinction • Thiols very low thresholds (box tree, broom, passion fruit, grapefruit) • Formed during fermentation from odorless precursor (S-cysteine conjugate) • Sauv. Blanc ,Sauv. Cab., Merlot • Seen as positive aroma contributor • More important in white then red wine

  5. Grape derived thiols 4MMP Threshold 3 ng/L Boxtree 3MHA Threshold 4 ng/L Passionfruit, grapefruit, gooseberry guava 3MH Threshold 60 ng/L)

  6. Grape-derived Aroma Compounds • Isoprenoids • Monoterpenes(fruity, floral) • Muscat, Gewürtztraminer • C-13 Norisoprenoids • -Damascenone (apple, rose, honey) • Vitispirane (green odor of chrysan- themum, flowery-fruity note) • Present in many wines • In Riesling • Riesling acetal (fruity, ionone-like) • TDN (kerosene-like)

  7. Introduction Aroma Compounds • Yeast and fermentation – volatile metabolites: • Esters • Higher alcohols • Carbonyls • Volatile acids • Sulfur compounds

  8. Yeast and Fermentation Produced Aroma Compounds • Esters (fruity flavors) *10% ethanol, **wine, ***beer, ****synthetic wine Swiegers et al., 2005 Austr. J. Grape Wine Res. 11: 139-173

  9. Esters • Produced mainly by yeast (through lipid and acetyl-CoA metabolism) • Variable amounts, mixed strains higher levels of esters compared to fermentations with Saccharomyces cerevisiae • Also variety depended • Some esters produced by yeast from specific grape precursors

  10. Esters • Lactic acid bacteria show esterase activity • Esters such as ethyl acetate (nail polish), ethyl hexanoate (apple) , ethyl lactate (creamy, fruity, coconut) and ethyl octanoate (sweet soap) increase with MLF and some others decrease • Suggest that esterases is both involved in the synthesis and hydrolysis of esters • This may increase or decrease wine quality

  11. Esters • Ethyl acetate (nail polish, solvent, glue) • Aroma threshold 7.5 mg/L • Wine normal 22.5-63.5 mg/L, spoiled 150 mg/L • Fermentation temp, SO2 levels, duration of MLF • Biggest influence is air, increased production under aerobic conditions

  12. Yeast and Fermentation Produced Aroma Compounds • Higher alcohols (fusel alcohols) • Secondary yeast metabolites and can have both positive and negative impacts on aroma *10% ethanol, **wine

  13. Fusel alcohols •  300 mg/L add complexity (fruity characteristics) • 400 mg/L (strong, pungent smell and taste) • Different yeast strains contribute variable amount of fusel alcohols • Non-Saccharomyces yeast – higher levels of fusel alcohols

  14. Fusel alcohols • Conc fusel alcohols produced: • Amount of precursor - amino acids • EtOHconc, fermentation temp, pH, must composition, amount of solids, skin contact time etc. influence conc of higher alcohols Ehrlich Pathway From Linda Bisson: The Fusel Family

  15. Yeast and Fermentation Produced Aroma Compounds • Carbonyl compounds • Acetaldehyde (bruised apple, nutty) • Sensory threshold of 100 mg/L, typical conc. in wine 10-75 mg/L • Major intermediate in yeast fermentation • Increase over time due to oxidation of EtOH - due to aeration • Use of high conc of SO2 can cause accumulation of acetaldehyde • Acetaldehyde in white wine is indication of oxidation

  16. Carbonyl compounds • Diacetyl (butter or butterscotch, low conc nutty or toasty) • Aroma thresholds 0.2 mg/L in white, 2.8 mg/L in red wine •  1-4 mg/L buttery or butterscotch •  5 mg/L undesirable – rancid butter • Significant production during MLF by lactic acid bacteria (LAB) • Intermediate in reductive decarboxylation of pyruvic acid to 2,3-butanediol

  17. Carbonyl compounds • Diacetyl (butter or butterscotch, low conc nutty or toasty) • Variety of factors influence production • Fermentation temp, SO2 levels, duration of MLF • Biggest influence is air, increase production under aerobic conditions

  18. Yeast and Fermentation Produced Volatile Compounds • Volatile acids (500-1000 mg/L) • Volatile fatty acids (propionic and hexanoic acid) • Produced by fatty acid metabolism of yeast and bacteria • Acetic acid (90%) • High conc. vinegar-like aroma • Fault  0.7-1.1 mg/L depending on wine style • Production by Saccharomyces cerevisiaestrains varies widely 0.1-2 mg/L • However, commercially used strains produce less than native strains

  19. Volatile acids (VA) • Acetic acid • Excess conc. largely the result of metabolism of EtOH by aerobic acetic acid bacteria • Small increase in VA with MLF • 2 possible pathways • Produced from res. sugar through heterolactic metabolism • First step in citric acid metabolism

  20. Yeast and Fermentation Derived Volatile Compounds • Volatile phenols (produced from hydroxycinnamic acid precursors in the grape must)

  21. Volatile Phenols • Trace amounts present in grapes • Mostly produced during fermentation from precursors during fermentation • Saccharomyces cerevisiae • 4-ethylphenol (medicinal, barnyard) • 4-ethylguaiacol (phenolic, sweet) • 4-vinyl phenol (phamaceutical) • 4-vinylguaiacol (clove-like phenolic) Main contributor Present below threshold values

  22. Volatile Phenols • Brettanomyces/Dekkera spp. • Produce high conc of 4EP, 4EG, 4EC, regarded as spoilage organisms • Band-aid, medicinal, pharmaceutical, barnyard-like, horsey, sweaty, leathery, mouse urine, wet dog, smoky, spicy, cheesy, rancid, metallic • Brett is not an fermentation problem but sanitation problem in cellar/air and barrel

  23. Yeast and Fermentation Derived Volatile Compounds • Sulfur compounds • Sulfides, polysulfides, heterocyclic compounds • Thiols, thioesters • Produced by yeast • Degradation of sulfur-containing amino acids • Degradation of sulfur-containing pesticides • Release and/or metabolism of grape-derived sulfur-containing precursors

  24. Sulfur compounds • Sulfides • Hydrogen sulfide (H2S) – rotten egg • Aroma threshold (10-80 g/L) • Produced by yeast from: • Inorganic sulfur compounds, sulfate (SO42-) and sulfite (SO32-) • Organic sulfur compounds, cysteine and glutathione Glutathione

  25. Hydrogen sulfide • Amount produced varies with: • Amount of sulfur compounds available • Yeast strain • Fermentation conditions • Nutrient status of environment • H2S produced during early – middle stages of fermentation • Associated with yeast growth and respond to nutrient addition • Mechanism not well known • In white wine inversely correlated with initial amount of N2 and glutathione present after fermentation

  26. Hydrogen sulfide • Grape must typically deficient in organic sulfur • Yeast synthesize org sulfur from inorganic sources • H2S is metabolic intermediate in reduction of sulfate or sulfite needed for synthesis • If enough N2 present, formed H2S used by O-acetyl serine and O-acetyl homoserine, derived from N2 metabolism, to form org sulfur compounds • Otherwise build-up of H2S in cells

  27. Thiols (mercaptans) • Formation of sulfides such as DMS (dimethylsulfide, asparagus, corn, molasses) not clear • Mercaptans such as ethanethiol can be formed by reaction of H2S with EtOH or CH3CHO • Yeast can reduce disulfides to thiols such as ethane- and methanethiol

  28. Thiols (mercaptans) • Low aroma thresholds  1.1 g/L • Ethanethiol (onion, rubber, natural gas) • Methanethiol (cooked cabbage, onion, putrefaction (rot), rubber) • Their presence during fermentation suggest that they are by-product of yeast metabolism

  29. Sulfur compounds Swiegers et al., (2005) Austr. J. Grape Wine Res. 11: 139-173

  30. Mousy off-flavor • 3 known compounds causes mousy aroma • Lactic acid bacteria (LAB) can produce all 3 compounds • Dekkera/Brettanomyces can produce 2 ETPY ACTPY ACPY

  31. Mousy off-flavor • 2-ethyltetrahydropyridine (ETPY) • Threshold 150 g/L, up to 162 g/L can be produced by LAB • 2-acetyltetrahydropyridine (ACTPY) • Threshold 1.6 g/L, isolated in wine at levels of 4.8-106 g/L • 2-acetylpyrroline (ACPY) • Threshold 0.1 g/L, detected in wine in trace – 7.8 g/L amounts

  32. Mousy off-flavor • Following needed for mousy-flavor production • L-Lysine, L-ornithine • Responsible for ring formations of 3 mousy heterocycles • EtOH and acetaldehyde • Responsible for the acetyl side chain Snowdown et al. (2006) J. Agric. Food Chem. 54: 6465-6474

  33. Mousy off-flavor • Formation restricted to heterofementative bacteria, general order of magnitued for LAB • Lactobacillus (heterofermentative)  Oenococcus  Pediococcus and Lactobacillus (homofermentative) • Oxygen, high redox potential, high pH, Fe2+- pos environment for mousy off-flavor production Snowdown et al. (2006) J. Agric. Food Chem. 54: 6465-6474

  34. Concluding remarks • Main off-flavors • VA, ethyl acetate, H2S and ethanethiol, acetaldehyde,volatile phenols, mousy • Most off-flavors can be minimized or prevented by • Using clean fruit • Sufficient nutrient and temperature control during fermentation • Good winery sanitation and adequate SO2 use

  35. Contact details • Anita Oberholster • RMI North, room 3146 • aoberholster@ucdavis.edu • Tel: (530) 754-4866 • Mobile: (530) 400-0137 • http://wineserver.ucdavis.edu • http://enologyaccess.org • http://www.facebook.com/aoberholster

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