Secondary Fermentation: Malolactic Fermentation

download report

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
•
•
units.
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.
Re-acidulation
• 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
verdes
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
growth
• 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
Inhibition
 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
stop
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
spectrophotometer
Paper chromatography requires a fume hood and is time
consuming
Cultivation
• 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