Volatile Acidity and Oxidation

Molly KellyEnology Instructor

Surry Community CollegeSensory Evaluation for Winemakers Workshop

April 14, 2011

Volatile Acidity

• Organic acids are more volatile(more easily vaporized) than the non-volatile or fixed acids (malic and tartaric acids)

• Volatile acids are able to be steam distilled

• VA = acetic acid + ethyl acetate

• The main volatile acid in juice and wine is acetic acid (avg 0.5g/L)

• VA is an indicator of spoiled wine

Characteristics of VA

• Ethyl acetate and acetic acid are produced in the ratio:

1 part ethyl acetate to between 5-10 parts of acetic acid

• Acetic acid smells like vinegar

• Ethyl acetate smells of nail polish remover

Acetic acid and ethyl acetate

• Acetic acid: pungent, vinegar

• Ethyl acetate: nail polish remover, fruity

– Dominant component of VA

• Around threshold, VA may increase perception of fruitiness

• Eventually becomes solvent-like

VA• 2 components

– Smells like: vinegar (acetic acid)

– Fingernail polish (ethyl acetate)

• Comes from – Yeast (Brett)

– Normal by-product of Saccharomyces growth

– LAB during primary fermentation

– Metabolism of citrate by O.oeni (LAB)

– Acetic acid bacteria

Other volatile acids

• Minor quantities of other volatile acids (formic, butyric and some fatty acids) are formed during fermentation

• Along with less volatile acids , lactic and succinic acid

• Lactic, succinic, sorbic acids are slightly steam-distillable

SO2 and CO2

• Sulfur dioxide additions to juice and wine can distill across and exist as sulphurous acid

• These are not considered part of VA and need to be neutralized or removed before VA determination

• SO2 can be converted to sulphuric acid (H2SO4), a non-volatile acid, by small additions of hydrogen peroxide H2O2

(add 0.5 ml of .3% hydrogen peroxide to 10 ml wine)

• Carbon dioxide (CO2) if distilled across can exist in solution as carbonic acid

Sources of VA (acetic acid)

• Saccharomyces• Spoilage yeasts• Acetobacter• Hydrolysis of compounds from oak• Oxidation of grape phenolics

• Note: ethyl acetate is formed from acetic acid and ethyl alcohol

• Ethyl acetate is often found accompanying the presence of acetic acid

AAB

• Oxidize ethanol to acetic acid

• Can grow in barreled or bottled wine

• Can grow using small amounts of oxygen absorbed during clarification and maturation

• Only two recognized genera: Acetobacter and Gluconobacter

AAB

• Moldy grapes have a high population of AAB and can lead to spoilage after crushing

• Most serious consequence of spoilage by AAB is the production of high levels of acetic acid (volatile acidity)

• Recognition threshold for acetic acid is 0.7 g/L

Acetic acid

• Formed by yeast at low levels during AF

• Produced by LAB during MLF

• Commercial LAB strains generally produce low levels, but spoilage LAB produce more (main source)

• *LAB don’t produce ethyl acetate

Acetic acid and ethyl acetate

Acetic acid Ethyl acetate

+CH3CH2OH

Acetic acid : detection threshold in wine ~0.5 g/LIn clean young wine 0.1-0.4 g/LUS legal limit: red 1.4 g/L, white 1.2 g/L

Ethyl acetate: detection threshold in wine ~0.08 g/LIn clean young wine 0.02-0.1

Sensory threshold much lower than for acetic acid aloneMain source: Acetobacter, wild yeasts

VA post fermentation sources

• Headspace in barrels

• Oxidation of wine

Legal limits for VA in wines (expressed as acetic acid)

• Red: 1.40 g/L

• White: 1.20

• Dessert: 1.20

• Export (all types): 0.90

• Late harvest: In the US white wines produced from juice of 28 Brix or more-VA can be 1.5 g/L; Red wines produced from must of 28 Brix or more-VA can be up to 1.7 g/L

Prevention of VA

• No headspace

• Gas headspace

• Maintain free SO2 at appropriate levels

• Removal

– Blending

– Reverse osmosis followed by ion exchange of the permeate

AAB control

• Low pH (acid)

• Minimize oxygen incorporation

• Maintain cool temperatures (<50F)

• Free SO2 15-30ppm

• Reverse osmosis

• Vinegar?

VA

• Need to monitor VA: may be increasing but still below sensory threshold

• Reverse osmosis:

– Expensive

– Does not significantly remove ethyl acetate

• Reduce to 0.06-0.07g/100ml-NOT all gone

• Can return

Methods of analysis

• Sensory analysis

• Steam distillation

• Enzymatic analysis

• Gas chromotography

• HPLC

Cash Still

• Switch on cooling water

• Fill boiling chamber with distilled water so that water level is ~1 in above heating coil

• Add 10 ml of sample

• Add 0.5 ml of 0.3% hydrogen peroxide

• Rinse funnel with distilled water

• Switch on heater

• When water boils, let some steam escape for ~15 sec , then close stopcock

Steam distillation

• Collect 100 ml of distillate

• Switch off heater

• Add 2-3 drops phenolphthalein

• Titrate with NaOH to pink endpoint that lasts 10-15 sec

• Va (g/L)= mls NaOH x N NaOH x 60 x 1000

1000 mls of wine

Calculation• For a 10 ml sample volume:

Conc of NaOH(N)

Equation

0.1 mls NaOH x 0.6

0.067 mls NaOH x 0.402

0.0167 mls NaOH

** This is g/L, for g/100 ml move decimal point one place to the LEFT.EX. VA=0.72 g/L or 0.072 g/100 ml

Errors

• Interference from:

– CO2

– SO2

• Loss of distillate from loose seals

• Not using distilled water in boiling chamber

• Forgetting to close stop cock

Errors

• Forgetting to switch on the cooling water for the condenser

• Letting the water in the boiling chamber get too low

• Not transferring the sample quantitatively from the funnel to the sample chamber

• Sample loss due to suction into boiling chamber

Aroma defects• Acetaldehyde

– Over-ripe bruised apples

– Sherry

– Nut-like

• From wine aging (chemical oxidation of ethanol)

• Increased color depth in white wines (golden)

• Brickish tint in red wines

Oxidation

• Changes observed: browning, decreased varietal aroma, nutty or sherry-like aroma

• Oxidation occurs in must as well as in wine

• Phenolic compounds are the main substrates for oxidation

Must oxidation

• Rapid and catalyzed by PPO (polyphenoloxidase) enzyme

• Preferred substrates are phenols (hydroxycinnamates)

• Botrytis-infected grapes: laccase enzyme causes oxidation

– Oxidizes more substrates

– Less sensitive to SO2

Wine oxidation

• Slower than must oxidation

• Not enzymatic, but rather an auto oxidation reaction

Phenolic compound quinone

(colorless) oxidation (dark colored)

Acetaldehyde • Cure: SO2

– Binds tightly to acetaldehyde

– Add in increments until free SO2

begins to increase

– All acetaldehyde bound

– Casein, Polycase (PVPP+Casein)

Final thoughts

• Inoculate with known cultures (vs native)

• Provide adequate nutrition

• Monitor critical parameters

– pH, VA, free SO2

• Practice good cellar hygiene and sanitation

• Keep containers topped

• Use SO2 appropriately

Prevention is always better than a cure

• Control insects

• Sterile filter

• Isolate infected wine

• Smell and taste wine regularly

• Train cellar staff in early detection

Thank you!• Mr. Gill Giese, Viticulture

• Mary Simmons, Asst. winemaker

• SCC Viticulture &Enology Students

Questions?

References

• Ritchie, G., Fundamentals of Wine Chemistry and Microbiology, Napa Valley College, CA 2006.

• Lourens, K., Enzymes in Winemaking, Wynboer, 2006. www.wynboer.co.za/recent articles/0411enzymes.php3

• Margalit, Y., Winery Technology and Operations, The Wine Appreciation Guild, 1996.

• Iland, P., Chemical Analysis of grapes and wine, P. Iland Wine Promotions, Campbelltown, SA 2004.