Introduction to Winemaking: Part 4. Secondary Fermentation

1. Introduction to Winemaking: Part 4. Secondary Fermentation Dr. James Harbertson Extension Enologist Washington State University

2. 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

3. 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

4. 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.

5. 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

6. 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

7. 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

8. 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

9. 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

10. Controlling ML • Yeast Bacteria Interaction • Stimulation for wines pH3.3 • Inhibition for wines pH3.3 • Detection • Cultivation

11. 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

12. 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

13. 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

14. 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 pH’s above this it will be difficult to stop

15. 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

16. 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

17. 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