Secondary Fermentation: Malolactic Fermentation

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Transcript Secondary Fermentation: Malolactic Fermentation

Introduction to Winemaking:
Part 4. Secondary Fermentation
Dr. James Harbertson
Extension Enologist
Washington State University
Secondary Fermentation
• Secondary fermentation is conversion
of malic acid to lactic acid and CO2
• C4H6O5  C3H6O3 + CO2
• Lactic Acid metabolizing bacteria are
responsible for fermentation
• Deacidification: decrease in titratable
acidity and increase in pH
• Wine stabilization and flavor change
Deacidification I
• Acidity due to malic acid is reduced by 1/2
• Lactic acid is less acidic than malic by loss
of one functional acid group
• This relationship is not always the case:
 RS converted to lactic acid by yeast
 Loss of potassium bitartrate by ppt.
 Malic acid catabolized by yeast
Deacidification II
• pH increase is not easy to predict
• It depends on buffering capacity of wine
• IE the [organic acid] and starting pH
• Lactic acid is weaker acid than malic acid
• The greatest pH change you should see is about 0.2 units.
• In wines with a pH<3.4 the increase will only be about 0.1
Although this is still a desirable change in sourness
generally speaking!
Color of wine will decrease slightly because of anthocyanin
coloration is greater at low pH.
• Sometimes it is necessary to add acid back to
adjust TA to acceptable value
• Don’t add citric acid because ML bugs will convert
it into diacetyl (butter aroma) (Movie time!)
• Tartaric acid is acid of choice for all acid additions
• Although it is expensive and some losses due to
potassium salt precipitates occur
Wine Stabilization
• Wine is stable to further infections by other ML
bacteria and more stable to other infections
• ML strains will use up most of the resources left
after yeast is finished
• Best to inoculate with ML strain and not depend on
“native ML” for control
• Best to do primary and secondary ferments
separate because they can inhibit each other
Flavor Change
• Less Sour!!
• Diacetyl is formed during secondary ferment
• Aroma is described mostly in terms related to
butter aroma
 I.E. Rancid butter, butterscotch, cream
• Aroma change more evident in white wines than
red wines
• Diacetyl is formed from citrate and pyruvate
Wine Style and ML
• Red Wines:
 Happens in most but usually does not change wine style
that much
 Done primarily to stabilize wine
White Wines:
 More infrequent because whites are dominated by grape
aromas, and flavors
 In barrel fermented wines where other aromas are present
it is more acceptable
 Strains available that don’t make as much butter aroma
so it’s use for deacidification is more prominent
Wine Style and ML II
• Vinhos verdes “Green wine”
• Minho region of Portugal
• Lack of ripeness in grapes, not color of wine
• High Malic acid from viticultural practices coupled with no
ML in winery led to bottled wine that was fizzy
Consumers loved it
However it was cloudy and sold in opaque ceramic bottles
Now the wines undergo ML and you can’t get a true vinho
Controlling ML
• Yeast Bacteria Interaction
• Stimulation for wines pH3.3
• Inhibition for wines pH3.3
• Detection
• Cultivation
Yeast Bacteria Interaction
• Yeast and bacteria are competing for nutrients
• Yeast can both inhibit and stimulate bacterial
• EtOH, bisulfite and fatty acid formation by yeast
inhibits bacterial growth
• SO2 binding and the products of yeast autolysis
after sur lies aging stimulate bacterial growth
• Bacteria can stimulate death phase in yeast
• Easiest to inoculate wine with ML strain after
primary fermentation has occurred for simplicity
Stimulating ML fermentation
• Don’t add SO2
• Temperature
 Maintain temperature above 18C
• Acidity
 When pH needs to be raised chemical
deacidifications can stimulate growth
• Ethanol
 Bacteria are inhibited at around 14% EtOH
Stimulating ML II
• Macronutrients and Micronutrients
 Amino acids and five carbon sugars
 Some inoculums have many of the necessary
components present
 Wine is to be stabilized by ML fermentation so
does not make sense to add nutrients which may
encourage growth of spoilage organisms
• Oxygen
 Bacteria like small amounts of oxygen but in
practice anaerobic wine conditions work fine
• Add SO2
 0.8 mg/L molecular pH adjusted
• Temperature
 Below 18C and as low as 13C
• High EtOH does not always inhibit because often
coupled with high pH
• Acidity
 Adding tartaric acid will help but may not stop it
 Wines pH 3.3 and below need to be stimulated
while wines pH’s above this it will be difficult to
Inhibition II
• Elimination of viable bacteria
 Don’t blend wines with differing ML status
• Sterile filtration and sterile bottling
 Like yeast will be filtered out with pore sizes no
larger than 0.45 microns
 Sterilize bottling equipment with heat
• Chemical inhibitors
 Velcorin (dimethyl decarbonate) will work with
SO2 and low pH
 Fumaric acid final concentration (0.5 mg/L) is
needed but low solubility limits its use
Detecting ML
• Measuring malic acid disappearance is best
• pH and TA increases can arise from other phenomenon
• Increase in turbidity and effervescence (not measurable)
• Determination of malic acid can be achieved with paper
chromatography or enzymatic analysis
Enzyme kits are fast but expensive and require a
Paper chromatography requires a fume hood and is time
• Specific strains with desirable characteristics can
be purchased
• They are generally grown in complex media
containing growth factors and a yeast inhibitor
• Companies that sell yeast also sell ML strains
• Wines & Vines guide is directory contains info
• WSU Dr. Edwards works in this area and is an
excellent contact for technical difficulties