Introduction to Winemaking: Part 4' Secondary Fermentation

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Introduction to Winemaking Part 4. Secondary
Fermentation
Dr. James Harbertson Extension Enologist Washingto
n State University
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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

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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

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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 pHlt3.4 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.

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Re-acidulation

• Sometimes it is necessary to add acid back to
  adjust TA to acceptable value
• Dont 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

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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

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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

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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 dont make as much butter
  aroma so its use for deacidification is more
  prominent

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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 cant get a true
  vinho verdes

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Controlling ML

• Yeast Bacteria Interaction
• Stimulation for wines pH?3.3
• Inhibition for wines pH?3.3
• Detection
• Cultivation

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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

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Stimulating ML fermentation

• Dont 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

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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

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Inhibition

• 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 pHs above this it will be difficult
  to stop

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Inhibition II

• Elimination of viable bacteria
• Dont 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

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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

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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