Uncorking what gives a wine its unique taste

Overtones of fruit. Undertones of flowers and herbs. Underlying hints of soil, stone, sand or dry leaves.

With such colourful descriptors, it is no wonder that wine appreciation is often described as an art.

But there is plenty of science behind the aromas and flavours in each bottle of red or white that is being popped this festive season.

To make wine, you need two main ingredients - yeast and grapes. Through fermentation, the yeast helps convert grape sugars to alcohol for both red and white wines.

"The fundamental difference between the two is that grape skin is removed in white winemaking but the whole grape is used for red winemaking," said Professor William Chen, director of Nanyang Technological University's food science and technology programme.

•Riesling: A grape variety originating from Germany which is acidic, with a strong citrus and stone fruit characteristic, such as lemons and pears. Old Rieslings, or Rieslings grown in hot areas, can sometimes develop a kerosene or burnt rubber aroma, which is considered attractive to some.

•Sauvignon Blanc: Originally from Bordeaux, Sauvignon Blanc is now grown all over the world, especially in the Loire of France and in New Zealand. It is typically fresh with high acidity and has strong citrus aromas, along with those of fresh herbs and cut grass.

•Chardonnay: It was first grown in the Burgundy region of France, but is now the most planted grape variety around the world. A ripe Chardonnay may taste fruity, with hints of apple, melon and grape.

•Cabernet Sauvignon: This variety of red grapes came about from a chance crossing of red Cabernet Franc and white Sauvignon Blanc plants in 17th century France. Today, Cabernet Sauvignon is grown in most wine-producing countries. Sommeliers often describe it as tasting like blackcurrant or green bell pepper. Older wines will also have tones of dry leaves, tobacco and cedar. Of all red wines, Cabernet Sauvignon is the most full-bodied and has the highest amount of tannin - which is what causes the mouth to feel dry when sipping wine.

•Merlot: These wines usually taste more fruity and are easier to drink, compared with Cabernet Sauvignon wines. The fruity aromas of Merlot are due largely to the presence of esters, which are chemical compounds released during the reaction between acid in the grapes and the alcoholic sugars during fermentation.

•Pinot Noir: Originally from the Burgundy region of France, Pinot Noir grapes require plenty of tender loving care to grow. They grow only in specific conditions - which is why a good bottle of Pinot Noir wine is considered a rare find. This is also the wine adored by some aficionados. It has strong hints of red fruits such as strawberry and raspberry.

•Source: Gerald Lu, vinepair.com

"In general, the presence of grape skin (which contains tannin) in the red wine contributes to its distinct taste from white wine," said the wine connoisseur.

When you sip wine, tannin is what causes the dry sensation in your mouth.

Connoisseurs may also describe wines as having other fruity or flowery aromas, or smelling like herbs or earth. But it does not mean that strawberries, soil or grass had been blended into the wine.

The key lies in substances known as aromatic compounds, which are produced in chemical reactions during winemaking.

Compounds such as esters, pyrazines, terpenes and thiols have smells similar to those from fruits or herbs that people are familiar with.

That is why the sense of smell is so important for sommeliers such as Mr Gerald Lu, 31.

To hone it, he literally smells everything he comes across, from the good, like fruits in a supermarket, to the bad - an old carton box - and the downright nasty - like his own sweaty shoes and socks.

He has even smelled quirky objects, such as pencil shavings, to get the scent of wood mingled with lead.

But the results have paid off.

Today, after some 10 years of training, the vice-president of the Sommelier Association of Singapore can distinguish between calamansi and jaffa lime - usually the same thing to the man in the street - just by smelling them.

To him, calamansi is acidic and smells more sour, while jaffa limes are more aromatic, fragrant and sweet.

There are three broad "groups" of wine flavours and aromas - floral, fruity or herbal; spicy; and earthy - which are all caused by different compounds.

Fruity or flowery aromas are usually caused by compounds known as esters - the result of a chemical reaction known as hydrolysis - when acids in the grapes react with alcoholic sugars in the presence of oxygen.

"In general, wines that smell more fruity or flowery (have grapes that) are grown in warmer climates which receive more sunlight," said Mr Lu.

"Plants photosynthesise in the presence of sunlight to form sugars, so the more sunlight there is, the more sugar content there will be in the grapes."

Herbaceous, grassy aromas reminiscent of bell peppers or capsicums are caused by compounds known as pyrazines.

This is most apparent in wines made from the Sauvignon Blanc grape variety.

"Pyrazines are compounds that are present in the berry during the first phase of growth, just before it ripens," said Mr Lu. During ripening, sugars enter the berry and significantly reduce the concentration of pyrazines.

The theory among sommeliers is that wines with such scents contain grapes that are usually grown in cooler climates. They get less sunlight and therefore less sugar from photosynthesis.

But what makes the wines from each vineyard taste unique?

The determining factors are the grapes used and the winemaking processes, said Prof Chen.

He added: "In Europe which is referred to as the Old World for wines, wines are generally differentiated from one another by the region that a particular wine grape is grown in, such as Bordeaux, Bourgogne and Champagne.

"Outside Europe, in North America, Australia and South America, which are referred to as the New World, wines are more commonly known for the type of grapes, like Sauvignon and Shiraz."

But for all the science behind a glass of wine, experts such as Mr Lu think the art of wine appreciation lies in using all the taste and aromatic "clues" to paint a picture of what happened during the year in which the wine was produced.

"You may understand how the smells develop, due to chemical compounds caused by reactions that are in turn affected by factors such as temperature, sunlight availability and water stress.

"But it is understanding all these things, and painting a picture in your mind of what happened during that year, during that vintage, that harvest. We call this terroir."

In essence, it is the subtle art of using your palate to get a sense of place.