The Science and Technology of Winemaking
Vintners usually use one of the forty species of Vitis vinifera such as Pinot Noir, Chardonnay or Nebbiolo to make their final product. North American species are used as rootstocks for grafting as they are resistant to the phylloxera aphid that attack the roots of grape vines. However, it is not just the grape that is important in creating a quality wine. The concept of "terroir" is also very important as quality depends on growing the grape variety that is right for the soil and is also dependent on the weather in the growing season.
Also important is the skill of the growers to time the pruning and harvesting of the grape so that the level of sugar concentration in the grape, or its maturity, is at an ideal level, which varies with the conditions and has to be decided by each vineyard. It is essential to know how to store your wine.
In the northern hemisphere, grapes are usually harvested in the autumn from around March. Harvesting is completed in the early morning to preserve acidity. Grapes must be without disease or defect and enter the winemaking process as soon as possible after harvesting. Whole grapes are used, with grapes damaged in harvesting used to make acetic and lactic acid.
The different aromas and flavours of wine are created through the use of grape variations and wine production techniques that result in the unique characteristic. Science has proven that there are over 680 different compounds that contribute to wine aroma. Muscat has a more flowery aroma whilst Sauvignon varieties are more fruity.
Fermentation causes changes in aroma and flavour, adding new compounds and changing others. There is a traditional wine yeast called Saccharomyces cerevisiae which has several strains that vintners can select to produce the result they need in any given year.
Scientists studying wine and winemaking, otherwise called oenologists, have been using advanced biotechnology to research the DNA of wine yeast to further understand how it can best work for vintners. The wine industry does not use genetically modified organisms (GMOs) but the science behind GMOs has allowed scientists to see how yeast impacts on the flavours present in the fruit through the creation of compounds known as volatile thiols which are responsible for the characteristic aromas in different wine types. This scientific research has lead to several commercial yeast strains that create the specific metabolic activity needed to achieve an above normal rate of conversion, which creates an increased intensity and varietal character.
Chardonnays have a buttery flavor and thanks to research we now know this is caused by the metabolism of citric acid by malolactic bacteria which produces diacetyl. This has led to research to find a way to select strains that are able to produce more, or less, diacetyl. Science has also demonstrated that changes in winemaking techniques, such as the amount of suphur dioxide used or less stirring can manipulate final diacetyl concentration levels.
Science has also been able to pinpoint the compounds in oak that contribute to wine flavour when barrelled. The species, source, seasoning and coopering of oak can all effect the flavour. Coopers are now able to authenticate the source of oak they are working with, giving winemakers a variety of options so they can predict more accurately the effect of the oak on the characteristics of the wine they are producing.
Ongoing scientific research
It is usual for proteins in white wine to form a haze. Traditionally this has been removed with the use of a clay called bentonite. Scientists are still working to find an alternative.
Another difficulty winemakers want to avoid is incomplete fermentation. Science is leading to greater understanding of the part played by nitrogen in this process.
This research has led to further studies into amino acids which regulate yeast cell metabolism which impacts significantly on both the aroma and flavour of the wine produced. Low levels of amino acids can lead to the formation of the gas hydrogen sulphide, which gives off the smell of rotten eggs.
Phenolic compounds such as tannins and pigments in wine have higher levels in red wines than in white wines, adding a depth to taste and colour which science is growing to understand. Research has shown that grape pigments are highly unstable and must react with stable tannins to maintain colour over the long term. Scientists are continuing to research the reactions in the early stages of fermentation to the red colour is maximised.
Managing tannin formation so that the wine feels good to drink is another challenge faced by both science and technology. Tannins are a large and complex group of compounds that are slowly being quantified. Technology is being used to attempt to manipulate the levels of tannins in wines to produce the right effect.