On 15 February 2013, asteroid 2012 DA14 missed earth by a mere 27,700 kilometers. This ancient 130,000 ton asteroid, spanning 45 meters in diameter, would have released the same amount of energy as a detonating 2.5 megaton atomic bomb, had it collided with the earth. It looks like 15 February 2013 was global meteor day, as a 9 ton behemoth caused widespread panic and injuries as it seared through the sky above Chebarkul, a town in central Russia. Not really cataclysmic, these events, but certainly significant enough to put the thought of mass extinction into our minds.
Not only humans (and don’t forget the dinosaurs) are subject to mass extinction. Micro-organisms are intimately sensitive to changes in their environment. Take a tank of fermenting must. The savvy winemaker will manipulate this very complex environment to suit his and the fermenting yeast’s specific needs, but under certain conditions the yeast population can very quickly become extinct.
The eventual fate of the wine yeast is death. After churning out ethanol, flavour compounds and a myriad of other chemical compounds during its usually short life, the yeast unceremoniously dies. But still their job is not done. These dead cells (lees) also have a very important role, but for now the focus will be on some of the causes of death of fermenting wine yeasts.
Temperature, ethanol concentration, osmotic stress, pH, toxins, pressure, sulphur dioxide and volatile acidity can separately or in combination make your little buddies extinct. So best you follow this multi-part blog, as the next installments will focus on the specific factors listed above.
You might not be able to dodge projectiles from outer space, but you can do a lot to keep your little fermenting soldiers happy and alive right until the end.
Bernard Mocke is a technical consultant for Oenobrands
Recent crackdowns on doping in sport have made all of us more aware of the effects of performance enhancing supplements. This got me thinking. What if winemakers could come up with a legal magic potion for yeast? Like the magic potion that enabled Asterix and Obelix to defeat the Romans time after time. Something that would give mere mortal yeasts super human (rather super yeast) qualities?
An increasing worldwide trend is longer “hang-time”. Delaying harvest might increase berry aroma and decrease acidity, but it creates a unique problem for our little athletes. Increased sugar leads to increased alcohol levels in wine made with these grapes. Fermenting yeast thus run the risk of being smothered in the alcohol they produce as a result of them snacking on sugar. Fortuitously, there is a magic potion that you can give your yeast to boost their viability during fermentation.
First, let’s look at the definition of sterols: “Any of various alcohols having the structure of a steroid, usually with a hydroxyl group (OH) attached to the third carbon atom. Sterols are found in the tissues of animals, plants, fungi, and yeasts and include cholesterol and ergosterol.” Here comes the interesting part. Sterols and unsaturated fatty acids (UFA’s) are survival factors during fermentation, but oxygen is needed for the synthesis of said survival factors. With insufficient amounts, the yeast cell membrane functions poorly, especially during highly anaerobic conditions and especially with increasing ethanol levels. Inadequate sterol concentrations around flux controlling proteins in the yeast cell membrane cause damage to the cell membrane and ultimately results in cell death (read: stuck or sluggish ferment!). The key role between oxygen and sterols now becomes evident. Simply put, controlled and timely oxygen addition = more sterol synthesis = better ethanol resistance = happy yeast = happy winemaker.
In my previous life, I’ve found it useful to add oxygen to red ferments anytime from a third of the way through alcoholic fermentation, up to halfway. This roughly corresponds with the end of the cell growth phase and research has shown that an oxygen addition of five to ten mg/L has a very positive effect on cell viability. Another trick is to combine oxygenation and nutrient addition with a pump-over or punchdown. Complex yeast nutrients contain inactivated yeast, which is a good source of sterols. The abovementioned trick also counteracts reductivity, which every winemaker deals with at some stage.
Research is ongoing to gain more insights into how yeast sterol uptake and synthesis affects cell viability. Ergosterol is one of the main compounds being studied, but I’ve also read a paper which outlines the addition of cholesterol to a fermentation! Fermenting yeast are just as happy with cholesterol as they are with ergosterol, but I seriously doubt if winemakers will be chucking cholesterol by the bucketful into their wholesome red wines!
Bernard Mocke is a technical consultant for Oenobrands.
Part one of this blog gave us some background on high Brix grapes, musts and the resulting high alcohol wine. The easiest (but not always legal) way to counter the effects of a potential high alcohol ferment is to add water to the must. This is usually done prior to fermentation. In warmer viticultural countries, winemakers often employ this technique to dilute musts from grape varieties that are harvested at 27 to 30ºB or higher! The arguments against this moist method are quite watered down by now, but the critics have a valid point. For instance, when you add sugar to wine (chaptalization, as it is called in France), you mainly alter the production of alcohol in the finished wine. However, the addition of water dilutes and impacts an innumerable amount of aroma, tannic and other chemical constituents. So there are obviously two sides to this rather soggy debate.
In California, the addition of water may only be done to prevent a stuck fermentation. Section 17010(a) of the California Administrative Code states: “…and no water in excess of the minimum amount necessary to facilitate normal fermentation may be used in the production or cellar treatment of any grape wine…”. In the less liberal South Africa, the addition of water to must is still in breach of EU wine law and in all probability will still be illegal for quite some time. Some producers (notably in California) take watering back a further step. After bleeding off some of the juice (saignée), water is added to the tank. The initial step drastically alters the juice to skin ratio and decreases the amount of sugar in the must. The watering back further dilutes the sugar concentration. I have heard (and don’t have the hard facts) that another method to eventually reduce alcohol is applied in Australia. It is legal to add water that has been removed from juice via reverse osmosis to other juice or wine seeing that it originates from grapes and not the “black snake.”
Winemakers all over the world are probably most comfortable with reverse osmosis to remove access alcohol in finished wine. Portable units are available that can be used to treat your high alcohol wine and thus effect a significant decrease in alcohol concentration. Another technique is the spinning cone technique, which fractionates wine. After alcohol is removed, the desired volatile components are simply added back to the wine. The downside to these techniques is that important aroma compounds and mouthfeel can be lost. As a winemaker once elegantly put it after treating a high alcohol wine with reverse osmosis: “The alcohol level is acceptable, but now we’re stuck with a bland, soulless wine.”
The winemaker will ultimately decide how the alcohol issue should be remedied. To those that believe that no superior winemaker has ever added water to wine, I guess the American Army policy of “Don’t ask, don’t tell” (Google it if you don’t know what I’m talking about) will be interesting and enlightening reading.
Bernard Mocke is a technical consultant for Anchor Wine Yeast.
One of my favourite stories from the Bible is the one where Jesus turned water into wine during the marriage at Cana. Despite this incredible event, asking most winemakers today whether they dose their ferments with water is like asking somebody about his brother in jail… you just don’t talk about it.
The International Organisation of Vine and Wine (OIV, if you parlez français) prohibits the addition of water in all of its 44 member states, which includes countries such as South Africa, Australia, Chile, Brazil, Uruguay, New Zealand, Austria, Croatia, Czech Republic, France, Germany, Hungary, Italy, Israel, Slovenia, Spain and Turkey. But why is the addition of water such a touchy topic? To answer this, we’ll have to hop into the vineyards and consider the interaction between the sun and the water content of ripening grapes.
Research done in Australia on Shiraz (Australia’s most planted grape variety) showed that ripening grape bunches can lose up to 20% of their weight (all of it water) at the end stage of ripening. The effect of the sun is obvious. This exodus of water basically concentrates sugar, which is further increased by the ripening effect of the sun. Wine producers in the southern hemisphere (i.e. Australia and South Africa) and even northern hemisphere (California) are often faced with the imbalance between grape sugars and tannins (and all the other critical flavour compounds). The problem is thus: sugars develop and build up during the harvest. Often, the heady tannins and usual aroma suspects arrive when the party is almost over, or in other words, when the grapes are already overripe. French winemakers critical of watering back wine (this should more or less include all of them) often state that sugars in their grapes develop more slowly and is thus in concert with phenolic ripeness. For an interesting debate, just mention the word ‘chaptalization’ the next time you encounter one of these winemakers.
Winemakers fiddling with overripe grapes can encounter various problems. Simply put, high sugar musts can lead to stuck ferments, even when inoculated with commercial yeasts. If the commercial yeast does however have a sweet tooth and is impervious to high alcohol levels, a high alcohol wine can result, where an alcohol level of 15% or higher is common. The Lower Alcohol Lobby frowns upon high alcohol wines (for various health, safety and other socio-political reasons) and is a powerful driving force behind many health initiatives and various research projects at leading universities. Another factor is tax that is levied on alcohol in some countries.
What is a winemaker to do, apart from having grapes picked earlier and fermenting with yeasts that have a lower alcohol conversion factor? (The latter doesn’t really exist.) There are a few techniques available, of which adding water to wine is paramount in this blog. But you will have to wait for part two of this blog to read all about these controversial techniques.
Bernard Mocke is a technical consultant for Anchor Wine Yeast.