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The principal acids in grapes are malic and tartaric acid. Both give freshness to wine, especially white wine, to give it structure. Malic acid tastes harsher, like sour green apples, and the winemaker may convert it to the more mellow lactic acid (found in milk) via malolactic fermentation. Acid balances the alcohol and sugar (in the case of sweet wine). Wine without adequate acid tastes flabby or cloying. Acidity in wine ranges from flat or soft to fresh, lively, crisp, mouthwatering, sharp or taut.'

Aged Spätlese Riesling tastes less sweet
The sweetness of a Spätlese Riesling derives from both the residual sugar and the ripe fruit sweetness. As the wine ages, the amount of residual sugar remains the same but the ripe fruit sweetness evolves into other bouquet such as honey, nuts, etc. This secondary bouquet also masks the residual sugar sweetness. Therefore, the wine appears to be not sweet.

Alcohol (August newsletter)
Alcohol in wine is converted from sugar in grape by yeast by means of fermentation. About 16.5g sugar is required for 1% of alcohol depending on the efficiency of the yeast. The riper the grape, the higher the sugar level hence the higher the alcohol in wine. Therefore, wine from warmer regions usually has higher alcohol. Winemakers can manipulate the alcohol level by planting vines in different altitudes and facing, picking the grapes earlier or later, blending wine of different alcohol levels or chaptalisation (adding sugar to juice prior to fermentation to increase final alcohol level). Fortification is adding spirit to wine (either during or after fermentation) to make sherry, port, madeira and vin du naturel. The latest technologies are reverse osmosis and spinning cone to reduce alcohol from the final wine. Dilution with water, a logical way to reduce alcohol, is not permitted in the EU.

Barrel fermentation versus barrel ageing
White grapes, after destemming and pressing, can be fermented in small barrels to give extra complexity. The resulting wine is softer and fuller-bodied, with less obvious and more integrated oak. Unlike barrel ageing which gives a deeper colour to the wine, barrel fermentation results in a wine that is paler in colour because the lees forms a coating on the wood surface thus limiting the colour extracted into the wine. Colour pigments also drop out during fermentation.

Brettanomyces (Brett)
Brettanomyces, or brett in short, is a yeast that is present on grapes, vineyards and wineries. It is a controversial subject as both winemakers and consumers can have different and opposite views. Brett growth in wine, usually associated with unhygienic wineries and equipment, releases four aromas. Depending on their ratio, they can contribute to the wine complexity, or cause spoilage. The positive aromas are the appealing cloves, spices and smokiness, while the negative aromas are the unmistakeable barnyards, band-aid and sweaty saddle. Chateau Beaucastel and Chateau Musar are advocates of brett as it adds the unique complexity and terroir to wine, but most New World winemakers who believe winemaking is a science, think even a tiny amount of brett is a fault.

Bubbles in sparkling wine is one of the indications of quality. They are dissolved carbon dioxide formed during the secondary fermentation. Champagne and sparkling wine made in traditional method is left undisturbed on lees in bottle for a period of time after secondary fermentation. The longer the time is, the finer and more persistence the bubbles it developed because of the interaction with mannoproteins in wine. Sparkling wine produced in this method has 5-6 bar pressure and more than 20 million bubbles. By comparison, sparkling wine produced in Charmat (or tank) method, such as Prosecco, usually has lower pressure of about 4 atmosphere, with less and bigger bubbles.

Carbonic maceration
Unlike conventional alcoholic fermentation where oxygen is required, carbonic maceration is an anaerobic fermentation technique whereby whole bunches of  grapes are put in tanks blanketed by carbon dioxide. The gas permeates the grape skins and fermentation occurs inside the berries, uncrushed. Alcohol is generated as a by-product alongside other estery chemicals such as banana and bubble gum aromas. The resulting wine is very low in tannin and lacks the structure for ageing. Beaujolais is made this way and so is some Valpoleceila.

Corked taint (TCA)
Corked taint comes from a chemical called Trichloranisole (TCA), the product of the reaction between the moulds on the cork and the chlorine solution that was used to bleach the cork. Cork taint ranges from mild to severe. Cork taint strips the wine of any fruit aromas. A very corked wine would smell dull, damp and of wet cardboard. Some people may not spot a mild corked wine but think the wine is closed or lack of fruit. If in doubt, taste it. Corked wine has a mute palate and no finish. Better quality control and use of ozone to disinfect cork reduce the incident but unfortunately, corked taint is still around, especially among older vintage where hygiene was slack. Any wine that uses cork as closure could be corked, including champagne and port.

Extraction is a key process of red winemaking. Colour, tannins and flavours are all in the skins and the trick is to extract the right amount into the wine. Too much extraction is like brewing the tea for too long - the colour is dark and bitterness dominates the flavour. Most extraction occurs during fermentation. The factors affecting its rate is temperature (higher temperature, more extraction), contact time (hence punching down or pumping over), medium (colour is more soluble in water therefore easier to extract before fermentation while tannin is more soluble in alcohol). Vintage port uses foot treading, which has the right pressure to extract the maximum favourable components in a short time without breaking the pips - pips contains harsh tannins and are unwanted.

Fermentation temperature
Fermentation temperature has a big effect on the wine style because different aromas are produced in different temperature and the more volatile compounds evaporate quickly if fermentation temperature is too high. White wine is usually fermented in cool temperature (18ºC-23ºC) to retain the varietal flavour. Low temperature (12ºC-18ºC) favours the production of ester and terpenes compounds that give tropical fruit aroma, which is prominent in a lot of New World white wine. 

Grape tannin
Tannin comes from either the skin of the grapes or from new oak. Tannin can’t be tasted but it affects the texture of the wine. Wines from unripe or over-extracted grapes taste bitter and harsh, while ripe tannin gives richness—but too much will mask the aromas. It is important in red wine to give it its shape and definition. Acidity accentuates astringency in tannin whereas alcohol masks it. The astringency in tannin can soften over time, but there must be enough fruit flavours to support it. Tannin or texture can be described as coarse, firm, grainy, chalky, ripe, smooth, silky and velvety.

Lees are the sediments left in wine after fermentation. Some wines are deliberately left on lees (sur lie) for a period of time to increase the complexity and mouthfeel. Wines acquire a toasty aroma and have a better integration with oak. Lees stirring (bâtonage) is the technique to promote the uptake of lees flavour. Because lees absorb oxygen, wines ageing on lees for a longer period are fresher. This is particular obvious when one compares a Dom Perignon Oenothèque and Vintage of the same year. Oenothèque ages on lees for 14 years but it tastes younger and fresher than vintage, which ages on lees for 7 years.

Length is a sign of quality in wine. According to Paul Mason from Martinborough Vineyard, it is linked to vine age: the older the vines, the more concentrated the fruit, giving more fruit weight to the wine on palate. For this reason wine from a high yield crop will lack length because the fruit is not concentrated enough. Another factor impacting length is the extraction of fruit tannin in post fermentation maceration, which is important in giving wine the back palate it needs to carry the fruit weight through.

Grape skins give colour, tannins and phenolics to a wine, so skin contact is crucial in making red wines. Winemakers in the New World favour pre-fermentation maceration at low temperature to extract the maximum colour but not too much tannin, leading to a fruity wine. However, to make a long-lived classic wine such as a Bordeaux or Barolo, the key is to do the maceration after fermentation to extract tannins and phenolics.

Malolactic fermentation
Most red wine undergoes malolactic fermentation (MLF) to convert the sour, green apple-like malic acid to the softer lactic acid. The resulting wine has a softer and rounder texture. Winemakers, however, avoid MLF in fruity and aromatic white and rose wines because these need the acidity to keep their freshness. Chardonnay is one white grape that can benefit from MLF when the aim is produce a wine with more complexity and ageing potential, such as the great Burgundies.

Oak flavour and tannin
Oak contributes both flavour and tannins to wine. However, the flavour is only brought out by the toasting of the wood. High temperature changes the chemical compounds in the wood and releases the aromas, but at the same time reduces the tannin level. Wine aged in lightly toasted barrels has vanilla and clove aromas, while medium toasted barrels make the wine milder and softer (less tannins). Highly toasted barrels release smoke, chocolate and coffee aromas and even less tannins. French oak has silky tannins and aromas of the more subtle cedar, spices and toasted nuts. American oak, on the other hand, is powerful and bold, imparting sweet flavours of vanilla, coconut and butterscotch to the wine.

Residual sugar
Sugar in grapes is converted into alcohol during the fermentation process, at a rate of approximately 16.5g/l of sugar to 1% alcohol. If sugar is not fermented completely, residual sugar will be left behind, contributing to the sweetness. Under normal circumstances, grapes are harvested when the sugar level is between 180g/l and 230g/l. Therefore, a German Kabinett Riesling with 8% alcohol will have approximately 40g/l residual sugar. For sweet wine, sugar in grapes is concentrated by dehydration (eg. noble rot wine, ice wine, straw wine). The sugar level may reach over 300g/l before fermentation, resulting in wine with 12% alcohol and 100g/l residual sugar. Fermentation may stop naturally or be deliberately stopped by the winemaker. Winemakers may add 2-5g/l süssreserve (concentrated grape juice) to dry wine for a more pleasing palate.

Sulphur dioxide
Some sulphur dioxide is naturally produced by yeast during fermentation. Winemakers also deliberately add it to wine to protect it from oxidation and microbial spoilage. The EU and other countries have upper limits on how much sulphur dioxide is permitted, ranging from 150 to 400 ppm (mg/l) depending on the wine style. Natural wine producers try to make wine without sulphur dioxide but quality can be hit and miss, and the wine may not last depending on the quality of grapes and cellar practices. In contrast, dried fruits (raisins, apricots, prunes, etc) contain a much higher sulphites level, from 500 to 2,000 ppm.

Wine grapes
Although both wine grapes and table grapes belong to the same species, Vitis vinifera, they are very different. Unlike table grapes which are large, juicy, thin skinned and (usually) seedless, wine grapes are completely the opposite. The flavours in a grape come from skin, not pulp. Therefore wine grapes have much thicker skin. They are also smaller to keep a high skin to pulp ratio thereby concentrating the flavour. Wine grapes are also much sweeter and less watery than table grapes. They contain at least 50% more sugar. The pips are not used in winemaking but they are good indicators to assess the ripeness of the grapes.

Wood ageing
Most consumers presume wood ageing or aged in wood is wine aged in wood barrel. However barrels are expensive for most mass-market wine. There are a few tricks winemakers can employ: from hanging a bag of wood chips in fermentation tank (like a giant tea bag), to fixing a huge log of wood in the middle of the tank. These techniques can only impart wood flavour into wine but cannot mature wine like barrels do because of the lack of gas exchange. Next time when you see ‘aged in wood’ in back labels, chances are the wine is only subject to wood treatment, not ageing in barrels.

Yeast is essential in winemaking as it converts sugar in grapes to alcohol during the fermentation process, and more importantly, produces various by-products that contribute to the complex aromas of wine. Yeasts are abundant in wineries and some winemakers rely on wild yeasts to start the fermentation spontaneously (or naturally), believing that wine will be more complex and more expressive of terroir. The downside, however, is the possibility of producing off-flavours. Some winemakers (especially big brands) choose to use cultured yeast for fermentation for more efficient and reliable fermentation. Cultured yeast is cultivated from natural yeasts, and is available for different wine style ranging from neutral and specific aroma enhanced strains to high alcohol or sugar tolerance strains particular suitable for secondary fermentation of sparkling wine and dessert wine.

Yield, usually expressed in tons/hectare or hectolitres/hectare, is the quantity of grapes harvested from a given area of vineyard. A vine bearing less fruit produces more concentrated and flavoursome grapes, resulting in higher quality wine. Wine from high yield vines, in contrast, is insipid and dilute. This makes sense because a vine has finite resources to ripen the grapes, so clearly the less bunches it needs to ripen, the more it can put into each bunch. Try a bottle of classed growth Médoc against an AOC Bordeaux and you will immediately notice the difference.