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
The recent cold weather in large parts of our country (South Africa) made me remember a tasting I had with a winemaker. As far was icewine was concerned, he got the concept of minimum and maximum temperatures all mixed up and insisted that he harvested his frozen grapes at a minimum temperature of -7˚C. After realising that his mind was frozen from all the alcohol he had consumed during our tasting, I accepted the futility of trying to explain to him that he was actually referring to the maximum temperature requirement.
It is believed that frozen grapes were already harvested in the Roman times. Other reports indicate icewine production in Germany as far back as 1794. Another documented case stated that German vintners anticipated a very harsh fall in 1829. Grapes were left hanging on the vines for later use as animal feed. After discovery that these grapes yielded very sweet must, icewine was born! Austria, Canada and certain states in the USA also produce icewine, with China, USA, South Korea, Hong Kong and Singapore being the top markets for icewine.
In Canada, all icewine must have at least a part of it made from grapes that have been frozen naturally on the vine and then pressed whilst still frozen and without any intervention (no artificial freezing allowed after harvesting). Several challenges daunt the winemaker here. The grapes must survive animal, insect and bird activity, whilst combating mould and raisining. Only healthy, frozen grapes are thus harvested, which considerably limits the amount of grapes that can be processed. The cost of icewine it thus high, which is illustrated in an extreme case by Canadian producer, Royal DeMaria. Five cases of Chardonnay icewine was released in 2006, with a price tag of C$ 30,000 per half bottle!
On the technical side (my favourite side), I’ve just read a very interesting article about yeast adaptation concerning Riesling icewine juice fermentation. Juice with concentrations of up to 46 degrees Brix were fermented. This is quite challenging for many yeasts and it should come as no surprise that juice concentrations higher than 42 degrees Brix would not be able to be fermented to 10% v/v ethanol. Also, acetic acid produced as a function of sugar consumed was positively correlated to the glycerol produced. Glycerol and acetic acid are well-known markers for yeast stress, where acetic acid can represent up to 20% of wine TA (in icewine). Those of you familiar with icewine will know what I’m talking about when I say that it has a ‘slight’ bite to it…
A new threat to the chilly tradition that is icewine making is however gaining ground. Charles-Henri de Coussergues, Quebec icewine maker, says: “The danger now is that other wine regions start using the name ‘icewine’ for a product made the artificial way.” He is of course referring to a technique called ‘cryoextraction’. In essence, grapes are artificially frozen (-7˚C or lower) and the rest of the process is similar to traditional icewine making. The benefits here are larger production at less cost and better control over grape quality. Also, one doesn’t have to wait around for winter to do its work. But then the old-schoolers insist that traditional icewine just tastes better and more complex, possibly because of the extended hang time under harsh conditions.
The frosty debate between the ‘naturals’ and the ‘cryo-extractors’ continues. What do you think? Is there room for both these schools of thought in the already crowded wine market?
Bernard Mocke is a technical consultant for Oenobrands.
Karien’s comments on the hype that was made about glycerol, makes me nostalgic. Everybody was caught in the frenzy to find ways to increase glycerol in wines, and people went to extremes to achieve their goal.
Glycerol is one of the base chemicals used to make dynamite, and the commercial production of glycerol involves yeast fermentation, where SO2 is added constantly. To protect itself from the SO2, yeast produces glycerol. Winemakers tried to simulate this effect by stressing the yeast during fermentation. Treatments included adding small amount of SO2 constantly during fermentation, adding unfermented juice to wine (osmotic shock) and stressing the yeast by cooling it suddenly. Many winemakers (and some of the best) practised fermentation practices where the must was cooled to 10°C, allowed to rise to 15°, cooled quickly to 10°C, allowed to warm etc, to increase the amount of glycerol in the wine.
All these techniques probably resulted in an increase in the glycerol concentration, but had no effect on quality resulting from the glycerol.
Most people have heard about the French paradox so I thought I’d inform you about the lesser known German paradox. I started travelling to Germany in 2001 as a fermentation consultant to the wine industry and did so on a yearly basis until starting a family put me out of international travel for a while. I tended to visit the same people every time I went and it was fascinating to monitor their progress year after year as they started implementing what I advised. I must admit I was sometimes quite surprised / relieved at the positive effects of my advice myself. Wines that I thought were beyond hope (some Muller-Thurgaus) were all of a sudden mistaken for a new world Sauvignon blanc! Now whether that is a good thing or not is a topic for another day. The mere fact that it went from unpalatable (in my opinion) to very pleasant indeed is what one should focus on.
In 2001 most of the cellars I visited (take note I am not generalising) fermented their whites at temperatures between 18 – 22°C. This is fine for what I call “forgiving” grapes such as Riesling. Riesling has a lot of varietal character not influenced by yeast and fermentation temperature and therefore top quality white wines can be produced at these fermentation temperatures. However, there were certain other candidates that I won’t mention by name out of fear for my safety and that of my family’s, that quite honestly were not so great. I focussed my advice on these wines, what the winemakers themselves viewed as “neutral” varieties. They were all German varieties that I had no experience in tasting so I had no idea if they were truly “neutral” or not. I decided to investigate by advising colder fermentation and more aromatic yeast strains. These winemakers gradually shifted their fermentation temperatures to between 15 – 17°C and changed to different, more aromatic, yeast strains. Some now even ferment at 13 – 15°C.
And this is exactly where the paradox set in… The wines were much more aromatic. All of a sudden the grape varieties were not so neutral after all. The winemakers loved it, the consumers loved it, their buyers loved it, I liked it, BUT they could not sell it. Why not? Because they could not pass the certification of the wines. They were refused their QBA numbers because the wines “were not typically German.” Some kept on submitting the wines for certification until they struck a “younger” panel that would then pass the wines – only just though. The irony of the situation was that these “new style” German wines would then sell out in three months!
The situation has subsequently become better and I believe it becomes easier every year for winemakers to pass their more modern style (but still identifiable as German) wines, especially in the southern wine producing areas. But it certainly is a very strange and frustrating position to be in – to struggle to get certification for your better quality wine. A paradox indeed.