Volatile Acidity and Oxidation Molly Kelly Enology Instructor Surry Community College Sensory 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
SO 2 and CO 2 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 SO 2 can be converted to sulphuric acid (H 2 SO 4 ), a nonvolatile acid, by small additions of hydrogen peroxide H 2 O 2 (add 0.5 ml of.3% hydrogen peroxide to 10 ml wine) Carbon dioxide (CO 2 ) 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 Sensory threshold much lower than for acetic acid alone Main source: Acetobacter, wild yeasts +CH 3 CH 2 OH Acetic acid Ethyl acetate Acetic acid : detection threshold in wine ~0.5 g/l In clean young wine 0.1-0.4 g/l US legal limit: red 1.4 g/l, white 1.2 g/l Ethyl acetate: detection threshold in wine ~0.08 g/l In clean young wine 0.02-0.1
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 SO 2 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
Acetaldehyde Aroma defects 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 (colorless) quinone oxidation (dark colored)
Acetaldehyde Cure: SO 2 Binds tightly to acetaldehyde Add in increments until free SO 2 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 SO 2 Practice good cellar hygiene and sanitation Keep containers topped Use SO 2 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.