Transcript Introduction Aroma Compounds

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
• Grape-derived – provide varietal
distinction
– Methoxypyrazines (vegetative, herbacious,
bell pepper or earthy aroma)
– 2- isobutylmethoxypyrazine, 3-butylmethoxypyrazine, 3-isopropylmethoxypyraxine
– Threshold 2 ng/L, in wine 9-42 ng/L
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Sauv. blanc, Semillon, Sauv. Cab.
Recognized 4-8 ng/L white wine
Recognized 7-15 ng/L red wine
Undesirable  25 ng/L
IBMP
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
Grape derived thiols
4MMP
Threshold 3 ng/L
Boxtree
3MHA
Threshold 4 ng/L
Passionfruit, grapefruit,
gooseberry guava
3MH
Threshold 60 ng/L)
Grape-derived Aroma
Compounds
• Isoprenoids
– Monoterpenes (fruity, floral)
• Muscat, Gewürtztraminer
– C-13 Norisoprenoids
• -Damascenone (apple, rose, honey)
• Vitispirane (green odor of chrysanthemum, flowery-fruity note)
• Present in many wines
• In Riesling
– Riesling acetal
(fruity, ionone-like)
– TDN (kerosene-like)
citronellol
Introduction Aroma
Compounds
• Yeast and fermentation – volatile
metabolites:
– Esters
– Higher alcohols
– Carbonyls
– Volatile acids
– Sulfur compounds
Yeast and Fermentation Produced
Aroma Compounds
• Esters (fruity flavors)
Compound
Wine (mg/L)
Threshold
(mg/L)
Aroma descriptor
Ethyl acetate
22.5-63.5
7.5*
Fruity, VA, nail polish
Isoamyl acetate
0.1-3.4
0.03*
Banana, pear
Isobutyl acetate
0.01-1.6
1.6***
Banana, fruity
2-Phenylethyl
acetate
0-18.5
0.25*
Flowery, rose, fruity,
Hexyl acetate
0-4.8
0.07**
Sweet, perfume
Ethyl butanoate
0.01-1.8
0.02*
Floral, fruity
Ethyl hexanoate
0.03-3.4
0.05*
Green apple
Ethyl octanoate
0.05-3.8
0.02*
Sweet soap
Ethyl decanoate
0-2.1
0.2****
Floral, soap
*10% ethanol, **wine, ***beer, ****synthetic wine
Swiegers et al., 2005 Austr. J. Grape Wine Res. 11: 139-173
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
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
ethyl hexanoate
ethyl 2-hydroxypropanoate
ethyl octanoate
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
Yeast and Fermentation
Produced Aroma Compounds
• Higher alcohols (fusel alcohols)
– Secondary yeast metabolites and can have
both positive and negative impacts on aroma
Compound
Wine (mg/L)
Threshold
(mg/L)
Aroma descriptor
Propanol
9-68
500**
Fruity, sweet,
pungent, harsh
2-methylpropanol
25.8-110
4
Fruity, wine-like
Butanol
.5-8.5
150*
Fusel, spiritous
Isobutanol
9-174
40*
Fusel, spiritous
Isoamyl alcohol
6-490
30*
Harsh, nail polish
Hexanol
0.3-12
4*
Green, grass
10*
Floral, rose
2-Phenylethyl alcohol 4-197
*10% ethanol, **wine
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
Fusel alcohols
• Conc fusel alcohols produced:
– Amount of precursor - amino acids
Ehrlich Pathway
– EtOH conc, fermentation temp, pH, must
composition, amount of solids, skin contact
time etc. influence conc of higher alcohols
From Linda Bisson: The Fusel Family
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
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
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
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 cerevisiae strains
varies widely 0.1-2 mg/L
• However, commercially used strains
produce less than native strains
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
Yeast and Fermentation Derived
Volatile Compounds
• Volatile phenols (produced from
hydroxycinnamic acid precursors in the
grape must)
-CO2
p-coumaric acid
ferulic acid
4-vinylphenol
4-vinyl-guaiacol
Reduced
4-ethylphenol
4-ethyl-guaiacol
Volatile Phenols
• Trace amounts present in grapes
• Mostly produced during fermentation
from precursors during fermentation
– Saccharomyces cerevisiae
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4-ethylphenol (medicinal, barnyard)
4-ethylguaiacol (phenolic, sweet)
4-vinyl phenol (phamaceutical)
4-vinylguaiacol (clove-like phenolic)
Main
contributor
Present
below
threshold
values
Volatile Phenols
– Brettanomyces/Dekkera spp.
• Produce high conc of 4EP, 4EG, 4EC, regarded as
spoilage organisms
• Band-aid, medicinal, pharmaceutical, barnyardlike, 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
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
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
Hydrogen sulfide
– Amount produced varies with:
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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
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 Oacetyl serine and O-acetyl homoserine,
derived from N2 metabolism, to form org
sulfur compounds
– Otherwise build-up of H2S in cells
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
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
methanethiol
ethanethiol
Sulfur compounds
Swiegers et al., (2005) Austr. J. Grape Wine Res. 11: 139-173
Mousy off-flavor
• 3 known compounds causes mousy aroma
• Lactic acid bacteria (LAB) can produce all 3
compounds
• Dekkera/Brettanomyces can produce 2
ETPY
2-ethyltetrahydropyridine
ACTPY
ACPY
2-acetylpyrroline
2-acetyltetrahydropyridine
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
2-acetylpyrroline 2-ethyltetrahydropyridine 2-acetyltetrahydropyridine
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
2-acetylpyrroline 2-ethyltetrahydropyridine 2-acetyltetrahydropyridine
Snowdown et al. (2006) J. Agric. Food Chem. 54: 6465-6474
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
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
Contact details
• Anita Oberholster
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RMI North, room 3146
[email protected]
Tel: (530) 754-4866
Mobile: (530) 400-0137
http://wineserver.ucdavis.edu
http://enologyaccess.org
http://www.facebook.com/aoberholster