Amorim, the world’s biggest producer of cork closures for wine bottles, last week hosted a workshop on wine problems. For publication in week starting 2 April 2018.

A key factor when discussing problems with wine is the distinction between a fault and a taint. Faults are connected with the fermentation process while taints are the result of external factors like the environment.

Many common taints are caused by a group of chemical compounds known as haloanisoles. The best known are 2,4,6-trichloroanisole and 2,4,6-tribromoanisole, usually abbreviated as TCA and TBA respectively. Both occur when fungi, mould or bacteria in wine come in contact with halogens such as chlorine in pesticides in the vineyard or processes like bleaching. The problem can be transferred from the cork, though seldom through the cork, so TCA and TBA are technically both a taint and a fault.

The result is harsh aromas described by a variety of offensive terms. My favourite is the smell of a wet dog drying by the fire. Another is the dank tang of wet and mouldy cardboard. Our noses can detect them in the tiniest of concentrations. They are not harmful, and over time humans lose sensitivity to haloanisoles if they smell them often.

In late 2016 Amorim developed a new cork called NDtech that guarantees TCA is not present in the cork or the level is so low humans cannot detect it. Each cork is tested and any stopper with more than 0.5 nanograms of TCA per litre is rejected. The figure of 0.5 nanograms means half a part per trillion and is incredibly small – equivalent to one drop of water in 800 Olympic-size swimming pools.

Amorim says it spent five years and 10 million Euros researching and developing its new cork.

Dr Paulo Lopes is a research scientist and oenologist with Amorim based in Porto in Portugal. He presented the wine faults seminar in London. Dr Lopes said all corks processed via NDtech receive a “non-detectable TCA guarantee”.

Ways to prevent haloanisoles in a winery include reduction in the use of chlorine or bromine as cleaning agents, keeping humidity below 70 per cent to reduce the number of microbes in the air, and strict monitoring of cleanliness. “This is a problem that it is always better to prevent than cure,” he said. One way a winemaker can resolve a problem is to blend tainted wine with larger quantities of clean wines so the concentration of haloanisoles is below the level at which humans notice them.

Another wine fault that divides opinion among wine connoisseurs is known as “brett”, short for brettanomyces. It falls into the general category of volatile phenols. The fault occurs when the dekkera or brettanomyces yeast reacts with wine to produce the volatile phenol 4-ethylphenol.

The level of brett can enhance or ruin wine flavours. Thus 110 nanograms per litre of brett will give aromas of smoke, cloves and spice but 430 nanograms per litre will make the wine smell like walking into a pharmacy, or unwrapping a Band-Aid.

The style and age of a wine and the use of oak can mask or accelerate brett aromas. “The sensory perception threshold of volatile phenols depends very much on the style and structure of the wine,” Dr Lopes said. Thus an older oaky red with brett can enhanced by it, while some white wines will be degraded. A classic example is Chateau Musar red from Lebanon, famous for the high levels of brett which make this wine so distinctive and attractive.

Wines that spend a long time on yeast lees or receive extended skin contact are more likely to suffer from brett, Dr Lopes said, as do natural wines that limit the use of sulphur dioxide to reduce oxidation of crushed grapes. “Natural wines need to be careful with SO2. It is difficult to make natural wine without SO2.”

The simplest way to prevent brett contamination is to ensure the winery is clean, and to dispose of barrels that contain the yeast. Infected wines can be saved through filtration with activated carbon.

Another common fault is known as volatile acidity, abbreviated as VA. This causes wine to smell of nail varnish, vinegar or bruised apples. It tastes bitter. It is usually caused by wild yeasts reacting with oxygen before and during fermentation.

Again, the fault is only a problem for some people. Many people like the “rancio” aromas associated with oxidised wines like sherry and madeira. “Some oxidation is beneficial for flavours,” Dr Lopes said. He noted that oxidation can only be prevented, and cannot be reversed.

The best way to manage VA was to grow healthy grapes with natural anti-oxidants, he said. Some winemakers could blend their wines until the VA levels were below the threshold that people would notice.

The final fault Dr Lopes discussed fell into the category of “reduction” associated with the chemical mercaptan. This is where wines have vegetative or reductive aromas like the smell of cooked cabbage and taste metallic or bitter. At very low levels these chemicals can enhance flavours.

The April 2018 edition of Decanter magazine notes that of the 19 billion bottles of wine produced a year, about 12 billion use cork stoppers (Amorim makes about 5 billion of these). Screw-caps account for another 4.7 billion with the rest using plastic closures.

The English scientist Robert Hooke discovered the potential of cork in 1665 when he observed it under a microscope. He verified it was composed of tiny pentagonal or hexagonal prisms that he called cells from the Latin cellula, for small room.

Each cork stopper consists of about 800 million sealed cells. Between them is a gaseous mixture that allows for compression up to about half its width without loss of flexibility. Cork is the only solid that when compressed on one side does not increase in volume in the other.

Cork is removed from cork oak trees every nine years without damaging the tree. The largest areas of forest are in Portugal, Spain, Italy, France, Morocco, Tunisia and Algeria. The world has a total of about 2.2 million hectares of cork oaks.

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