Category Archives: Wine faults

The magic of yeasts

Yeast

Yeast

Yeasts are fungi, which play a vital role in wine production. The number of different species runs into tens or even hundreds of thousands, no one is quite sure. More than one may be used in fermenting the grapes, and this has a big influence on the wine produced. Yeasts produce aromatic compounds as well as alcohol and carbon dioxide.

What yeasts need to make them work
Yeasts need the right conditions to work their magic during wine fermentation:-

  • Temperature. They operate in a temperature range. Too low, and yeasts don’t function, too high and they are killed off. The grape juice may be actively heated or cooled to achieve this.
  • Nutrients. Sugar is the main one, but you need a source of nitrogen, which is provided by amino acids. Winemakers may add selected nutrients.
  • Alcohol level. Different yeasts work in different ranges of alcohol content.

Winemakers split yeasts into two main categories

  • ‘Natural’ or ‘wild’ yeasts. These are the yeasts which are present in the vineyard, and stick to the grape skins. They are also on the wine equipment. These indigenous yeasts give the wine a local character, a sense of place. The term ‘terroir’ is often used to express this.
  • Cultured yeasts. These are developed to introduce particular characteristics to the wine, or to make fermentation more reliable. Usually, the indigenous bugs will be killed off with sulphur dioxide, and then the grape must/juice will be inoculated with the cultured yeast.

What can go wrong with a ‘wild yeast fermentation’

  • Problems with starting the fermentation. The grape must/juice will contain a range of bacteria as well as yeasts, and these compete for the nutrients. You may end up with acetic acid (vinegar) rather than alcohol.
  • Inconsistency. A number of different yeasts may be involved during a fermentation, with some dying off at quite low alcohol levels, and others taking over. If a different yeast takes over, you can end up with a different wine than you anticipated.

Conclusion
There is no right or wrong answer about the use of wild yeasts, it rather depends on the market you are aiming at. Cultured yeasts will help to produce a consistent product, which is going to appeal to the big brands. Small producers may well enjoy the variability.

Sulphur Dioxide – the Magic Molecule

Sulphur Dioxide molecule

Sulphur Dioxide molecule

Sulphite additions to food and wine have become a big issue. However, the sulphites, and the sulphur dioxide that they produce have real benefits.

Why is the term sulphite used?
Sulphur dioxide is sometimes injected as a gas, but solid sulphite granules are usually added instead. The acid in wine rapidly splits this up to produce sulphur dioxide which actually does the work.

Sulphur dioxide occurs naturally in wine
This may seem unlikely, but fermentation of grapes produces sulphur dioxide without any help from humans. The amount depends on the the yeasts that are used, but it is a ‘natural’ product.

Why are sulphites added to wine?
There are two main benefits:-

  • Antioxidantif wine is exposed to air, the alcohol is quickly oxidised to acetic acid (vinegar). So, nature’s natural tendency needs to be moderated, if we are to drink the wine before it goes off. Sulphur dioxide attacks the chemicals which would otherwise produce vinegar. This is less of an issue for red wines which have their own antioxidants.
  • Antimicrobial – wine contains yeasts and bacteria. The magic molecule prevents these from multiplying and spoiling the wine.

Which wines have the most sulphur dioxide?
SO2 binds with many compounds in wine, including sugar. Only the unbound sulphur dioxide is able to fend off oxygen and microbes. So, sweet wines need plenty to protect them.

The importance of quality grapes
Rotten grapes make rotten wine, and need more sulphur dioxide to keep the bugs under control. Grapes with plenty of acidity also need fewer additions.

What are the adverse effects?

  • Sensoryan excess of sulphur dioxide produces a smell of burnt matches, but an awful lot needs to be added to have this effect. This fault is rare.
  • Healthis a difficult issue because there is little proof of adverse effects. It may induce asthmatic attacks in some people. It is worth noting that normal food metabolism in the body generates much more sulphites than you would absorb from wine consumption.

Conclusion
In my opinion, the addition of sulphites at the right time and in the right quantity is beneficial, if not essential, for almost all wine.

Is there a perfect temperature for storing wine?

temperature and humidity recorder

temperature and humidity recorder

Wine is an organic living thing. The temperature a wine experiences can have a big influence on its quality.

So, is there a perfect temperature?
Time has shown that deep cellars in temperature parts of Europe are perfect for wine storage. A constant temperature of around 12°C (54°F) is ideal. A high humidity prevents the cork from drying out.

Increased temperature leads to increased reaction rates
Wine is a complicated mixture of chemicals, which react with each other. As the temperature rises, all of the reaction rates will increase, but some will increase more rapidly than others.

Increased temperature leads to new reaction products
As the temperature rises, new reactions will occur, which simply wouldn’t happen at 12°C. An example is the breakdown of aromatic chemicals in dry white wines. Unpleasant compounds are also produced.

Wines with low sulphites/sulphur dioxide
Natural wines with low levels of sulphites (sulphur dioxide) are increasingly popular. Sulphur dioxide serves two very useful functions:-

  • to limit oxidation of the wine.
  • prevent bacterial growth.

Both of these become much more of an issue as a wine’s temperature rises, and natural wines are particularly vulnerable.

Unfiltered wines
If the temperature gets too high, it is perfectly possible to restart fermentation . This causes plenty of problems.

What if the temperature is lower than 12°C (54°F)?
As the temperature decreases, the reaction rates slow down. So, a wine may not mature in the normal way, but at least it won’t go off. If a wine gets cold enough it may deposit white tartrate crystals, but these do no harm. The alcohol in wine depresses its freezing point, so it needs to be well below 0°C (32°F) before it even starts to freeze.

Conclusion
If you live in a climate where the temperature is above 25°C (77°F), and you don’t have dedicated wine storage, then please keep ALL your wine in a domestic fridge. You will have to remember to take the red out in good time before drinking it, but at least the wine will be in good condition.

Screwcaps versus cork for wine bottles

branded natural cork

natural cork

Natural cork provides a very good seal for a wine bottle, but a screwcap is potentially better still. There are pros and cons for each of them.

Sealing liquids

  • Cork has a unique cellular structure which allows it to be compressed, and then spring back to its original shape. This provides a very good seal to prevent wine from seeping out. It can still seep out if the bottle is overfilled, the cork damaged, or because of poor storage.
  • Screwcaps prevent any wine from seeping out.

    screwcap

    screwcap

Sealing gases

  • Cork. Oxygen can slowly diffuse through the cork into the wine. Some corks are more porous than others, and some have defects; this allows more rapid diffusion of oxygen into the wine, and more rapid ageing.
  • Screwcaps. The original screwcaps allowed almost no oxygen into the wine, and this may cause unpleasant odours to develop. Different seals are now available which allow controlled amounts of oxygen into the bottle.

Contamination

  • Corks can be contaminated with TCA, a very small amount of which makes the wine undrinkable. Refer to this earlier post for details. This is cork’s biggest disadvantage.
  • Screwcaps do not contaminate the wine.

Storage

  • Cork. Bottles have to be stored with the wine in contact with the cork. If not, the cork dries out, and becomes a less effective seal. Rapid temperature fluctuations can cause a cork to move within the bottle neck, which can also cause sealing problems.
  • Bottles sealed with screwcaps do not suffer from these problems.

Long-term ageing

  • Cork has been used as a wine closure for centuries, and there is plenty of evidence that a good wine, with a good cork, stored in a good cellar, ages gracefully.
  • Screwcaps have been around for decades, and there is evidence that some wines age well. However, plenty more trials will be needed over many years, before everyone is convinced.

Conclusion

The scientist in me says that a screwcap with the right seal, is the perfect closure. No bottles will be contaminated, and every bottle in a batch will mature the same way. My heart however, would rather see a cork pulled from a fine bottle of wine, rather than someone crack open a screwcap.

What is meant by a corked bottle of wine?

selection of natural corks

Bleached and unbleached natural corks

A ‘corked’ wine has a mouldy taste and smell. This is caused by chemicals leached from the cork, and is nothing to do with bits of cork floating in the wine.

Where does natural cork come from?

Natural cork is made from the bark of the Quercus suber oak tree, and Portugal is by far the largest producer. The bark is stripped from the tree, and this particular species is able to regrow its bark. Individual corks are then punched out from the bark. Various treatments are carried out along the way.

Origin of cork taint

The main culprit is a chemical called TCA, 2,4,6-trichloroanisole to be precise, although other similar compounds are also responsible. There are several possible sources of these compounds, and they involve fungi, microbes and chlorine treatments, either of the cork oaks, or the punched corks themselves.

How bad is the problem?

There was a time when the percentage of ‘corked’ bottles was estimated at between 2 and 6%. I would say that the incidence has decreased, but the problem has not been eradicated.

What are the cork producers doing about it?

There are two general approaches:-

  • Change the sterilising treatment – in the past hypochlorite bleaches were used to kill off microbes in the cork. Now they use peroxides, or microwave treatments to remove unwanted life in the corks.
  • Use of high tech analytical equipment, such as mass spectrometers and gas chromatographs. These can detect extremely low levels of contamination.

What are winemakers doing about it?

Apart from changing away from natural cork, the best a winemaker can do is to buy top quality corks. Samples from a batch can be tested by immersing them in wine, to see if there is any contamination.

Threshold levels of taste and smell

The threshold level is the minimum concentration at which a person can detect a smell or taste. TCA can be detected at incredibly low levels of a few parts per trillion (yes, a million-millionth). There are two interesting observations:-

  • Some people are far more sensitive to cork taint than others – they have a much lower threshold level. So that one person thinks a wine is just fine, and another that it is undrinkable.
  • The threshold level increases rapidly as the alcohol content increases. Which is why cork taint is very rare with bottles of spirit.

Can cork taint be completely removed by changing to screwcaps or plastic corks?

The answer is no, but the incidence will be much lower. TCA can contaminate other wooden objects in a winery, particularly oak barrels. So some contamination is always possible.