Are your yeasts on sterols?
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.
Adding water to wine: time to take a sober look part 2
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.
Adding water to wine part 1: Time to take a sober look
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.
Water to wine
Most observers would agree that the style of wine which now dominates the high end of the wine rankings and price scale has evolved over the last couple of decades, toward higher alcohol, higher extract, higher pH and higher levels of new oak. One of the consequences of chasing these high scores is that many winemakers have insisted on leaving the fruit on the vine longer before harvest than before, often to the point that the grapes become partially dehydrated.
In many vineyards, grapes are not truly ripe when the sugar level is just above the minimum contract specification. This is because sugar accumulation is only one component of grape maturation. The skins, pulp and seeds, as well as the acids and salts in the juice, each have maturation curves that are not strictly interdependent or coincident. The flavor of the fruit itself changes as the grapes mature. Fruit can be sweet but not taste ripe, and the evolution of the high-scoring style has put a premium on a flavor profile that tends to the over-ripe. Winemakers may be waiting for a physiological indication of ripeness other than sugar, or they may be waiting for a particular flavor, but they are not waiting simply in order to pay the grower a few percent less.
Bringing in fruit with high sugar content creates potential problems for the finished wine. The first is high alcohol: more sugar means more alcohol produced by fermentation. High alcohol in and of itself is not always a problem, but there is the elusive question of balance to be considered — some wines just taste wrong if the alcohol is too high. The second is the effect on yeast. Dan Berger acknowledges that high sugars can lead to stuck fermentations. I have written before on how high sugar and high alcohol can stress yeast to the point of distress, causing them not only to stop fermenting sugar but also causing them to produce toxins that can make the wine taste hot or that lead to allergic reactions or migraines in sensitive individuals. An uncorrected stuck ferment also means the wine is left with some residual sugar — not a bad thing in itself, as many consumers actually prefer their wines slightly off-dry, but a wine with residual sugar must be sterile-filtered or treated with Velcorin™ if the winery wants to avoid the economic disaster of having it re-ferment in the bottle.
Every vintage, Nature and man conspire to deliver us less than perfect grapes. In waiting for seed ripeness — which is my number one criterion for determining when to pick — the tradeoff may be a higher sugar level. It seems to me that adding some water to the tank is a very minor correction, by which one can avoid too-high alcohols, stuck ferments and their attendant negative effects on wine composition, and filtration or other sterilization. I utterly reject the notion that there is anything deceptive, underhanded or unnatural in this practice.
I expect that winemakers in hotter climates have always been adding a little water to their too-ripe tanks before fermentation. That water addition (along with irrigation) is outlawed in most Continental appellations suggests it has been practiced there as well. However, European vineyards receive more rainfall late in the season than we do here in California. Consequently most European appellation regulations allow addition of sugar before and during fermentation (which is prohibited in California) to bring the potential alcohol of the finished wine back into balance if Nature gives washed-out fruit.
This is an edited version of the original blog posted on John Kelly’s blog: notes from the winemaker.
Search our Blog
Social Media Links
Our Authors
- Karien O'Kennedy
(28) - Bernard Mocke (15)
- Louis Nel (13)
- Bertus Fourie (10)
- John Kelly (10)
- Bruce Zoecklein (5)
- Edo Heyns (5)
- Boela Gerber (4)
- Erika Szymanski (4)
- Travis Braithwaite (2)
- Patrice Pellerin (2)
- Guy Webber (1)
- Sakkie Pretorius (1)
- Jonathan Snashall (1)