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Just Fine: Improving Wine Quality Through Fining
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A common theme in many of this series of articles is that making wine is a business, and as such winemakers are obliged to maximise the return on investment. One approach to achieving this, is making sure that all the precious grape juice is turned into a wine of some type with an appropriate market value. Let us look at one way in which this is done. Consider the first stages of how white wine is made. The grapes are crushed and the juice is quickly drained off the skins. This juice is called 'free run' and when fermented, produces the highest quality finished wine. The quality is high because the free run juice contains low levels of phenolic substances. Phenolics are found in the skins and seeds of grapes and are responsible for the browning of white wines, and for the production of 'hard' bitter aftertastes. Free run juice typically contains very low levels of phenolics as it has not been in contact with skins or seeds for any length of time, and as such has not had the ability to pick up any of the undesirable phenolic substances.
Most winemakers would dearly love to work exclusively with free run juices when making white wines. However, after draining the juice, the winemaker is left with a sloppy bunch of skins, still rich in juice. Commercial reality dictates that this juice (which can make up 20% of the total volume) is extracted and made into wine. And so it is! The juice is pressed from the skins using a variety of mechanical means. However, this 'press fraction' of juice is phenolic rich and will typically make a lower quality wine. When compared to the wine made from free run juice, wines made from the press fraction are less appealing as they are typically harder, sometimes more bitter, and will age far more quickly. In fact, these wines are pretty unpalatable, and even the most unsophisticated wine drinker would feel dudded if they purchased one.
So what is done to make them acceptable wines? In some cases, the press wines are simply combined with the free run wine (or the two juices are mixed and fermented together). This is done when the press fraction consists of only lightly pressed material, which is low in phenolics anyway. More typically though, the press juices or finished wines made from them, are subjected to processes that strip the phenolics out of them. This process is called fining, and consists of adding a 'fining agent' to the wine and mixing them together. Fining agents are typically naturally occurring proteins or substances that have been synthesised to mimic the action of proteins. Why proteins? Phenolic substances have a strong natural chemical affinity for proteins. So when they come in contact with each other, they react, and precipitate out of the wine or juice. So in effect, protein fining agents are used to strip out the phenolics from wine and juices.
A number of different fining agents are used depending on the type of wine to which they are being applied. Brace yourself because some are downright weird. A common fining agent is gelatin. It is most commonly used to reduce the level of astringency and bitterness in the press fraction of red wines. Another agent used for this purpose is egg whites. Yes, egg whites which contain the protein albumen can be directly added to the wine through the inlet of a circulating pump, but more typically the egg whites come as commercial preparations in a powdered form. Skim milk is a fining agent that is used to remove the bitterness and hardness of white wines and sherries. The active protein in skim milk is called casein. Other casein products are also used to fine the bitterness out of white wines. Two other agents are worth mentioning. Isinglass which is prepared from the swim bladders of certain fish is perhaps the 'Rolls Royce' of fining agents. The primary protein found in this agent is collagen. It is used to fine white wines, but unlike casein does not strip the wine of colour. It also produces small amounts of lees (the bits that fall to the bottom when the fining agent and phenolic precipitate) than other fining agents which means that there is more finished wine and less waste. Lastly, a commonly used agent used to fine white juices and wines is polyvinyl-polypyrrolidone or PVPP. This agent is a synthesised long chain polymer that acts like a natural protein. It is insoluble in wine and as such is mixed in, 'does its job', after which it falls to the bottom of the wine tank. In bigger wineries and breweries (where it is also used), it is often recovered and used again.
Other non proteinaceous fining agents exist, and are used for very specific purposes. For example activated carbon is sometimes used to strip the colour out of Pinot noir juices that are used for sparkling wine production. In an ideal world, the Pinot grapes should be so carefully pressed that the resulting juice contains so little colour that the juice can be fermented directly into base wine. However, in cases where the production of an inexpensive sparkling wine is the winemaking objective, one strategy that can be used is to press for quantity, which typically results in an overly coloured juice. Carbon is then used to decolorise the juice so that the resulting sparkling wine does not have a pink hue. Sure, wines made in this way are of lower quality, but then, there is a market demand for reasonable quality budget priced sparklers.
However no fining agent is 100% specific in what it does. As well as fining out phenolics they also typically take out some aroma and flavour. Some fining agents are more flavour hungry than others, and factors such as this will determine a winemakers choice of agent. As a general principle though, regardless of agent selected, winemakers will use as little as they need to produce the quality of wine that is required. And in case you are wondering if that wine you are drinking contains bits of fish swim bladder or the like, well I suppose technically it does. But the quantities are minute, and the use of the natural clay bentonite (see the previous issue of the Wine Tutor) further assists in the removal of any traces of these fining agents.