Post-Fermentation Wine Treatments & Techniques

Transcription

1 Post-Fermentation Wine Treatments & Techniques 2006 Stephen Skelton MW Reading material Making Good Wine by Bryce Rankine. Sun/Macmillan 1989 ISBN Understanding Wine Technology by David Bird MW. DBQA Publishing 2005 ISBN The Winemakers Encyclopaedia by Ben Turner & Roy Roycroft. Faber & Faber 1979 ISBN Applied Wine Chemistry & Technology by Anton Massel. Heidelberg Publishers 1969 Some useful websites Tom Cannavan s Wine Pages The Wine Doctor Improved Winemaking Lallemand Yeasts, Enzymes and Bacteria Martin Vialatte Oenology The notes below constitute a brief summary of possible treatments and techniques for still wines after the alcoholic fermentation has ceased. Techniques for sparkling, fortified and VDN wines may differ considerably. Not all techniques may be legal in all countries or under all appellation regulations. 1. Racking after fermentation. i. To remove wine from primary yeast deposits (gross lees), plus grape particles and general debris, to start process of clarification. ii. To remove wine from primary yeast deposits prior to transferring to barrel for ageing. i. Transfer by pumping or gravity from one tank to another. Lees can then be settled and/or filtered and blended with clarified wine or sent for distillation. ii. Transferring from one barrel to another. With red wines, the number of times will depend on wine type/style. Burgundy, Bordeaux, Rioja. 2. Lees-ageing in tank i. To permit the malo-lactic conversion to take place. Also allows for a partial MLF to take place and be controlled more easily than in barrel. ii. To allow for a controlled measure of yeast autolysis for flavour change. iii. Allows for the introduction of staves or oak chips for flavour change. iv. More cost effective than barrel ageing. i. Lees ageing can take place on either the gross lees (the totality of the sediment after fermentation) or on the fine lees those lees that remain after the first racking. The extent of the gross lees will depend on the pressing techniques and how much pre-fermentation settling and racking has taken place. ii. Sur Lie maturation (as in Muscadet region) takes place on gross lees and wine must be left until end of March in year following harvest. iii. Maturation on fine lees in tank is more usual and found with many white wines.

2 3. Malo-lactic conversion (malo-lactic fermentation or MLF). Considered usual for most red wines. Optional for whites. Not usual for wines with residual sugar or fortified wines. i. Reduction of acids. Conversion of harsher malic acid to softer lactic acid. Acid reduction can be between 1 and 4 g/l. ph. ii. Beneficial effect on flavour of wine, esp. whites by production of diacetyl. Between 2 and 4 mg/l is considered ideal. iii. Wines are more stable, esp. if a low SO2 and no-filtration regime is proposed. i. Presence of lactic acid bacteria (LAB) Lactobacillus, Leuconostoc & Pediococcus. (Leuconostoc oenos is now more correctly known as Oenococcus oeni) ii. Wine must have low free and total SO2 levels below 10 mg/l free (15 mg/l free maximum) and mg/l total. (Some strains will accept slightly higher free and total SO2 levels, but cell multiplication may well be adversely affected). iii. Between 3.2 and 3.5 ph. At ph 3.0 or below, most strains of LAB will not grow iv. Minimum temperature of 15 C (20-25 optimum). At 10 or below, LAB will not grow and above 30 they will not survive. c. Dangers of MLF i. Oxidation esp. if time to start is prolonged and temperatures are high. ii. Off flavours. Too much diacetyl gives rise to unwanted rancid butter tones. iii. Most strains of LAB cause slight increase in volatile acidity. iv. If MLF starts when wine still contains sugar, production of acetic acid is increased. v. Loss of fruit aromas. vi. High phs leading to loss of SO2 activity. vii. Instability if only partial malo. Malic acid should be below 0.1 g/l for complete malo-lactic conversion. viii. ph usually raised by MLF which (if above ph 3.5) might aid development of spoilage bacteria. ix. Wines that have not gone through complete MLF may well contain dormant LAB and must be bottles through a 0.45 micron filter for 100% security. d. Other observations. i. 1 gm malic acid is converted to 0.67 gm lactic acid plus 165 ml of CO2. ii. LAB growth is slow when alcohol is higher than 13.5%, although some strains may adapt to local conditions. iii. Production of diacetyl depends on various factors: 1. Some strains of LAB produce higher levels than others. 2. Higher initial levels of citric acid favour diacetyl production. 3. Slow multiplication of LAB cells increases diacetyl production. 4. Low levels of SO2 favour diacetyl production. 5. Higher oxygen content of the wine increases diacetyl production. iv. In higher alcohol wines (esp. reds) inoculation with LAB before the end of alcoholic fermentation may make MLF easier as nutrient levels are higher. v. Bayanus yeasts use more nutrients and therefore are preferable if MLF is not wanted in susceptible wines. vi. Partial MLF is usually achieved by carrying out a complete MLF on a portion of the wine, blending it with the remainder, raising SO2 levels and filtration.

3 4. Retention of residual sugar through partial fermentation i. To make wines with residual sugar ii. Sugars retained from fermentation are mainly glucose and fructose and tend to taste fruitier than wines sweetened with sterile juice, CGM or RCGM. i. Refrigeration ii. Racking off yeast iii. SO2 additions to kill yeasts iv. Combination of 2 or more methods 5. Barrel (cask) ageing i. Controlled oxidation. Helps soften wines esp. high acid whites and fix colour in red wines. ii. Increases complexity in wines. iii. Adds oak flavours iv. Adds yeast (lees) characters. b. Type/size of barrel: Chablis Feuillette 132 lt, Bordeaux Barrique 225 lt, Burgundy Piece 228 lt, Hogsheads 300 lt +, Puncheons 500 lt +. i. 225 litre barrel has about 90 sq.cm. per litre of content. ii. 500 lt barrel has about 60 sq.cm. per litre of content. c. Origin and type of oak. i. French Oak (Quercus robur)- Allier, Limousin, Nevers, Tronçais, Centre of France. ii. American Oak (Quercus alba) iii. Other areas/regions - Germany, Slovenia, Russia. d. Handling of oak staves i. Ageing, length of time and climate. ii. Sawn or hand split staves. e. Barrel treatment i. Level of toasting ii. Toasted heads. f. Cost of barrels. i. Number of times barrel used. ii. Re-coopering and shaving. g. Inner-stave etc h. Length of time in barrel i. Red wines approx months ii. White wines approx. 2-6 months i. Percentage of new oak. i. 100% - cost implications? ii. Less than 100% j. Temperature of storage important. Higher temperatures accelerate maturation process and flavour extraction from wood. k. Lees stirring (bâttonage) increases lees/yeast characters. l. Completion of alcoholic fermentation in barrel, esp. in reds, helps integrate oak characters and makes smoother wines. m. Handling techniques i. Storage at 2pm reduces need for topping. ii. Racks with rollers.

4 iii. Requires wines that have gone through complete alcoholic fermentation and malo-lactic conversion. iv. Types of bung. Wooden. Silicone. 6. Other methods of oaking Cost savings of both labour and materials. i. Chips or shavings g/l. Can be used either pre or post-fermentation. ii. Staves/planks in tank. iii. Oak extract not legal in EU. 7. De-acidification i. If wines are still too acidic after fermentation (de-acidification is best carried out pre-fermentation), then de-acidification of wine is still possible. ii. In EU, post-fermentation of new wine or wine still in ferment (usually taken to mean wine still on gross lees) is not restricted, but postfermentation, only 1 g/l may be removed. i. Calcium carbonate. 1 gram of CACO3 removes 1.7 g/l of tartaric acid. Usually used on must, not wine. Added to proportion of product which is completely de-acidified, then blended with whole. Large amounts of CO2 produced and acid reduction takes time. Only removes tartaric acid. Can lead to tartrate instability and raised ph levels. ii. Double-salt de-acidification. Special preparation of calcium carbonate which contains a small amount of calcium tartrate-malate ( Acidex ). Added to proportion of product which is completely de-acidified, then blended with whole. Removes both tartaric and malic acids. Action is relatively quick. Wine/must needs filtering or centrifuging soon afterwards. iii. Potassium bicarbonate or potassium carbonate. Preferred by some as does not lead to tartrate instability problems. 1 g/l will remove 0.75 g/l of acidity as tartaric acid. ph of wine does not usually rise above 3.6 and action is quite quick. 8. Acidification i. Lowers ph which makes SO2 more effective ii. Inhibits microbial and bacterial spoilage iii. Accentuates fruitiness and gives wines better balance iv. Encourages clarification of wine after fermentation. i. Between 1 and 4 grams per litre of L(+) tartaric acid (as opposed to racemic or DL-tartaric acid) are added, depending on many different factors. Citric and malic acids not normally used. ii. In EU, there are strict legal limits on acidification of wine, depending on which zone (A, B or C) wine comes from. 9. Micro-Oxygenation (Microbullage) i. Stabilises colour in red wines ii. Reduces vegetal characters iii. Builds middle body in wine

5 iv. Corrects slight sulphide problems v. Increases maturity vi. Reduces reliance on barrels for maturation i. Introduction of pure oxygen via controlled dosing machine. 1 ml per litre per month is considered a minimum (which is about the same as a barrique delivers). 10. Sulphur Dioxide, addition of after fermentation. i. Antioxidant combines with oxygen to form sulphuric acid in small amounts. Stops wine taking up oxygen. ii. Antiseptic kills bacteria which might harm wine such as acetobacter which oxidise alcohol to produce acetic acid (VA). Also lactobacillus which cause malo-lactic conversion to take place. iii. Anti-oxidasic. Kills enzymes which promote oxidation known as oxidases. iv. Removes acetaldehyde (created when alcohol oxidises) which causes a sherry-like nose. v. Will have some effect on stopping yeast growth, but some yeast strains are only affected by very high levels of SO2. Brettanomyces yeasts are controlled by normal SO2 levels. i. E220 Sulphur dioxide as a gas or in aqueous solution. ii. E224 Potassium metabisulphite (in EU only for juice) iii. E228 Potassium bisulphite (in EU only for juice) c. SO2 in wine is termed: bound, free and total. i. Bound SO2 is that which has combined with oxygen, sugars, aldehydes and ketones and is no longer effective. ii. Free SO2 is that which is still active. iii. Total is the sum of the two. d. Molecular SO2. SO2 plus water or wine forms sulphurous acid which then turns back into un-ionised SO2 (having neither positive or negative charge). The rate at which this change takes place is ph and acidity dependent. Wines with low ph and high acids require less free SO2 for the same level of protection. e. Post-fermentation levels likely to be very low (below 5mg/lt free) as the protective effect of CO2 is reduced with racking/pumping/aeration. f. Some yeast strains are able to produce SO2, but most strains in commercial use do not. g. Unless malo-lactic conversion required, free SO2 needs bringing up to keep wine secure. An addition of mg/l after 1 st racking would be typical. Wine would then have around 35 mg/l but this will depend on many factors ph and acidity, handling methods, temperature, storage conditions. ph 3.0/ mg/l; ph 3.2/ mg/l; ph 3.4/ mg/l; ph mg/l. h. SO2 can remove colour of red wines i. At bottling total levels of SO2 are limited by law in the EU. Levels of free SO2 are not regulated. i. Dry reds 160 mg/l ii. Dry whites 210 mg/l iii. Reds with 5 g/l sugar or more 210 mg/l iv. Whites with 5 g/l sugar or more 260 mg/l v. Sweeter wines mg/l j. Free SO2 levels at bottling:

6 i. Dry whites 25 mg/l 45 mg/l ii. Dry reds 15 mg/l 25 mg/l iii. Dessert 45 mg/l 60 mg/l 11. Topping with inert gas To reduce contact with oxygen Sparge airspace with inert gas. i. CO2 ii. Nitrogen 12. Clarification of wine Visual quality. Wines with sediment (esp. whites) are usually commercially unacceptable. i. Racking off and settling. Wines transferred from one tank to another. Yeast lees can then be settled, filtered or sent for distillation. Aeration of wine during racking may be beneficial and help reduce H2S. ii. Filtering. Various techniques used to pass wine through a filter medium. iii. Centrifuge. High-throughput machine which spins out sediment. iv. Flotation systems. 13. Filtering i. Stability correct filtration will remove organisms which cause spoilage such as yeasts inc. Brettanomyces and lactobacillus. Will also prevent alcoholic fermentation in wines with residual sugar and malo-lactic conversion in susceptible wines. ii. Marketability wines esp. white wines are not generally acceptable unless clear and free of visible particles. i. Depth filtration relies on depth of filter aid to remove particles. Not 100% effective. Diatomaceous earth (kieselguhr) is filter medium. 1. Rotary drum vacuum (RDV) filter. Can be used for juice and wine, as well as settlings and yeast lees (tank bottoms). Usually used for early stages of clarification. 2. Earth filter. Can be used for all stages of filtration. Very cost effective. Disposal of earth poses problems. ii. Sheet filtration. Plate and frame filter. Can be used for relatively clear products and right up to sterile filtration. Sheets made of cellulose. Asbestos no longer permitted. iii. Surface filters. 1. Membrane or cartridge filters. Particles are screened (sieved) out and remain trapped on the wrong side of the filter. Liable to become blocked micron will stop yeasts micron will remove bacteria. 0.2 micron used for sterilising water. Can be integrity tested for 100% security. 2. Cross-flow or tangential filtration. Arrangement of membrane filters that are self-cleaning. High capital cost, but very effective and can tackle quite dirty product. 3. Ultra filtration. Filter membranes that can be tailored to filter out certain individual elements sugar, acids, tannins, colours. Not yet in commercial use.

7 c. Rules for successful filtration i. Filter as little as possible ii. Clarify before filtration by racking and settling iii. Use coarsest grade of filter (either sheets or earth) that will do the job iv. Use combination sheet filters (two or more grades in one pass) that reduce handling (pumping). v. Avoid pressure surges in both sheet and membrane filters to retain integrity vi. Avoid pressure surges in earth filtration to reduce break-up of cake. d. Difficult to filter wines i. Some wines, especially those made from botrytis affected grapes contain complex long-chain polymers or colloids which block filters. The polymers are usually the polysaccharides of various sugars such as glucose, mannose, galactose, arabinose, fructose and rhamnose. Can also be pectins. Enzymes can be used to break down the polymer molecules esp. of glucan. 14. Alcohol reduction i. Taste. Wines too high in alcohol can taste too warm. ii. Public perceptions about high alcohol wines iii. Taxation implications (wine over 14% pays more tax in US. Wine over 15% pays more tax in UK) i. Blending ii. Watering sometimes illegal, usually negative impact on flavour. iii. Physical methods iv. Vacuum (cold) distillation spinning cone v. Reverse osmosis 15. Volatile Acidity (VA) reduction i. High levels of VA (mainly acetic acid) are detrimental to taste and stability. Levels over 0.75 g/l are often noticeable. ii. Very high levels are illegal (1.2 g/l of acetic acid max. for reds and 1.08 g/l for whites in EU) i. Physical methods 1. Vacuum (cold) distillation spinning cone 2. Reverse osmosis 16. Sweetening i. Marketing requirements ii. To balance acidity b. How i. Stopped fermentation ii. Addition of sweetening agent 1. Sterile grape juice (süss-reserve) 2. CGM (Concentrated Grape Must) 3. RCGM (Rectified Concentrated Grape Juice) 4. Sucrose converted to glucose and fructose via action of acids (inversion). 17. Fining

8 i. To improve stability and clarity. Proteins, tartrates, heavy metals etc can give rise to deposits and hazes. ii. To improve filterability. Colloids can block filter media. Unstable proteins are removed by introducing another substance with a different electric charge (positive/negative) i. Egg white (albumin) removes harsh tannins from red wines. ii. Albumin (extracted from egg whites) removes tannins and phenolics from both red and white wines. iii. Gelatine, made from animal skins/bones removes tannins. Also available from vegetable sources iv. Casein (from milk) removes colour and tannins from white wines. v. Isinglass ichtyocol made from the swim bladders of fish, esp. sturgeon and fish waste very pure form of gelatine removes tannins from white wines. vi. Bentonite also known as montmorillonite an alumino-silicate clay mined in various parts of the world (Wyoming). Acquires negative charge when mixed with wine and used for removing (positive) protein particles. Cannot be overused but also removes flavours. For both reds and whites. vii. Ox blood albumin (not used since 1987) removes colloids. viii. Tannin extracted from oak galls used in conjunction with gelatine (tannin 1 st, gelatine 2 nd ) to remove colloidal proteins. ix. Silica Sol (Kieselsol) silicon dioxide mineral origin and available in either positive or negative versions. Removes other fining agents from white wines. x. PVPP (Polyvinylpolypyrrolidone). Finely milled plastic used for removing phenolics from white wines that are suffering from pinking or browning. xi. Calcium phytate used (rarely) for removing iron from wines. xii. Acacia (gum arabic). Acacia is a naturally occurring polysaccharide and is related to the polysaccharides found in grapes (large sugar molecules). Used to stabilise unstable colloids. Must only be used after cold stabilisation. 18. Blue Fining a. Potassium ferrocyanide used for removing excess iron and copper which can cause hazes (casse) and even deposits in wines. Wines high in copper will age more quickly. Less of a problem with stainless steel tanks, fittings and pump parts. Bronze impellors were a major source of copper. A slight amount of iron should remain in the wine after fining so that cyanide is not formed. Amounts used is controlled by qualified chemist 19. Citric acid, addition of. a. Helps prevent iron casse. b. Added to finished wine as can be converted to acetic acid by fermentation. c. Allowed up to 1 g/l in EU. 20. Copper sulphate, addition of. a. Removes hydrogen sulphide (H 2 S) in wine. H 2 S is caused by reduction of SO2. Also from sulphur sprays still present on grapes. H 2 S smells of rotten eggs. b. CUSO4 is added up to 1 mg/l. Exact amount needs to be determined by analysis. 21. Ascorbic acid (Vitamin C), addition of.

9 Has powerful anti-oxidant properties. Should only be used in conjunction with SO2 otherwise browning will occur. Allows for lower levels of free SO2 at bottling. Additions of up to 150 mg/l allowed in EU. The isomeric form erythorbic acid used in some countries, but not allowed in EU. 22. Sorbic Acid, addition of. Sorbic acid stops yeasts fermenting but does not kill yeast. Needs SO2, alcohol and acidity to be effective. Additions of potassium sorbate are allowed in EU up to 200 mg/l, although most bottlers would add 150 mg/l. Must be used just prior to bottling otherwise a geranium tone can develop. c. Not a substitute for sterile bottling. 23. Tartrate stabilisation (cold stabilisation) a. Tartaric acid, although harmless, is often unacceptable to consumers (and winebuyers). After fermentation, when wine contains alcohol, tartrates become unstable and crystallise. Often though of as sugar or even glass shards. i. Cold stabilisation. Good for removing potassium bitartrate, but not calcium bitartrate. Wine is lowered to just above its freezing point (-4 C for 12% wine, -8 for 20% fortified wine) and held for 5-8 days. Not always 100% effective and considered unreliable. ii. Contact process. Wine lowered in temperature to around 0 C and finely ground potassium bitartrate crystals are then added and stirred for 1-2 hours. Wine then filtered and excess tartrates removed. Also continuous process available. iii. Ion exchange (similar to a water softener). Ion exchange resin swaps calcium and magnesium for sodium in the resin. Resin can be cleaned by flushing with salt solution. Wine contains enough sodium bitartrate to prevent crystallisation. Not legal in the EU. iv. Electrodialysis. Special membranes are able to filter out unwanted elements when electrically charged. b. Metatartaric Acid i. Combines with existing acid crystals (micro-crystals) to stop them getting to a visible size. ii. Can prevent tartrate crystals forming for up to 18 months, although effectiveness is temperature dependent. At 25 C effect only lasts 6 months. iii. 100 mg/l is maximum allowed by EU and is dose usually used. 24. Pasteurisation a. Wine heated to C for a few seconds. b. Kills bacteria and yeasts. c. Very effective and cheap. d. Requires very good sterility post-pasteurisation. e. Mainly used in conjunction with bottling line Stephen Skelton MW