Volatile Acidity and Oxidation - Appalachian State...
Transcript of Volatile Acidity and Oxidation - Appalachian State...
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.