During a recent overseas trip, a colleague of mine once again lamented the joys of travelling. This trip kicked off with a baggage issue that ruined her dinner. Other jolly events included fishing her phone out of a toilet, missing a train, almost being run over by an expressionless Parisian, being kicked by a drunken teenager in Lille and finally a screaming taxi driver in Montpellier. Friendly people… the French. But the inside of a fermenting tank can also be a chaotic and even deadly place for yeast.
Modern winemaking can be very stressful and winemakers are putting increasing pressure on their minuscule friends. No, I’m serous! Pressure can indeed be a limiting factor; especially where low pH and high ethanol is concerned. Trapped carbon dioxide gas not only creates turbulence in a tank, but also contributes to a gradual increase in pressure. Pressures upward of 600 kPa (6 atm) typically stop yeast growth (think secondary fermentation of sparkling wine), but not necessarily alcoholic fermentation. In the book “Wine Science: Principles and Applications” by R.S. Jackson, it is stated that a pressure of 3000 kPa (30 atm) and upwards will completely inhibit alcoholic fermentation! What hardy fellows they are, these yeasts!
The use of pressure to control or stop yeast growth is not uncommon in German wineries, but high pressures can cause other problems. Spoilage organisms such as Lactobacillus, Torulopsis and Kloeckera are less sensitive to pressure and can cause a myriad of problems. The latter micro-organism is particularly pesky, as it is quite sulphur dioxide resistant, ferments at temperatures as low as 10°C and can produce high levels of ethyl acetate and amyl acetate.
Some beer brewers postulate that higher pressures have a positive aromatic effect on their ferments, but clear guidelines during vinification have not been established. At least there are many other ways to boost aroma, so don’t be depressed.
Bernard Mocke is a technical consultant for Oenobrands
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 other day I came across this fact sheet: “Reducing Alcohol Levels In Wine” published by the Australian Wine Research Institute (AWRI). Directed at the professional winegrower, this is the best agenda-free piece on wine alcohol levels I have read, period. It’s worth the interested reader’s time.
All of my own efforts to manage alcohol levels in our wines are mentioned here. In the vineyard every year we reduce canopy leaf area to balance crop load, and I have found that irrigating to 85% of evapotranspiration demand right up to harvest prevents runaway sugar accumulation. I have always disdained wines with over-ripe flavors, and so have always picked at the earliest date that I find the various components of the grape to be quote-unquote “ripe” – a personal definition, but one that I am happy with.
I found it amusing that the AWRI paper discusses water adds under the heading of “blending.” Adding a “reasonable” amount of water, for one reason or another, is a common practice in winemaking. We just don’t talk much about it.
I was left scratching my head over the mention of glucose oxidase to decrease the level of fermentable sugar in juice or must. I recall reading a few research papers in the 1990s about this, but didn’t think the technology ever made it out of the lab. I honestly don’t know of any winery that uses this enzyme. Nor have I ever come across a commercial preparation for use in wine. So, pace, “natural” wine aficionados.
Fermenter design does make a difference. I prefer to use fermenters with a must depth of 38″ during peak fermentation, regardless of diameter, and seek to achieve peak fermentation temperatures of around 90° F for my red wines. I have empirical evidence that this approach reduces our so-called “conversion ratio” (the percent alcohol immediately after fermentation divided by the Brix before fermentation) by up to 5%.
By contrast, I have found no consistent evidence that yeast selection has any effect on alcohol level. Whether I conduct a ferment without inoculation, or by inoculation with a selected commercial strain, the final alcohol is the same within measurement error. Incidentally, these days I start every fermentation without inoculation. If the initial Brix is high or if the ferment shows evidence of stress, I inoculate with a commercial strain I feel most suited to the variety. In effect, all our ferments are conducted by mixed strains of yeast.
The AWRI paper discusses the most obvious, the most used, and the most discussed (and often reviled) method of alcohol level management: physical removal of alcohol from finished wine by reverse osmosis or vacuum distillation. I have experimented with these methods on a limited basis with mixed—mostly negative—results. My biggest concern with large-scale alcohol removal is that the wine is nearly always rendered “hotter” by the treatment. I speculate that this is due to removal of ethanol at a faster rate than alcohols of three carbons or more by the processes.
The article mentions de-alcoholizing small parcels of wine and blending back. I have had some good results with this approach and I am experimenting with this method on an ongoing basis, because of the next topic discussed in the article: loss of alcohol by evaporation during barrel aging.
In fact, during barrel aging in our cellar the alcohol level of the wine increases by up to 1.2%-1.5% over two years. During barrel aging, the wood of the barrel acts as a semi-permeable membrane. Wine components inside the barrel migrate through the wood at various rates and evaporate from the outside surface. My a priori assumption is that the rates of migration of water and alcohol are dependent on the differences in concentrations between the inside and outside of the barrel.
Let’s say I put a wine to barrel at 13% ABV; this wine is approximately 87% water. In our barrel cellar, the concentration of alcohol in the air is essentially 0%, while the relative humidity averages about 35%. Water leaves the barrel faster than alcohol because 87%-35%=52% is four times greater than 13%-0%=13% (52/13=4); therefore, the thermodynamic drive for water to leave the barrels is 4x the impetus for alcohol to escape.
The AWRI paper discusses how alcohol levels decrease over time when the average relative humidity of the barrel cellar is 70%-90%, but also discusses the negative issue of mold growth in the cellar in this wet environment. Our barrel aging area was not designed to be wet, and we also store cased goods in proximity to our barrels. Humidification of our cellar is not an option.
My intent is to experiment with vacuum distillation of the wine I use to top our barrels. If we decrease the alcohol level of the topping wine, I believe we can slow the rate of alcohol increase in our barrels over time in our dry cellar environment.
There’s little need for me to rehash the back-and-forth in the wine media regarding alcohol levels: in short, the wheel has turned and we are back in the 1980s when it was fashionable to criticize California wine for having high alcohol.
Here we are again. The difference this time around is that there is a hard number on the lips of the critical: 14%. The narrative being pedaled suggests that wines over this level generally are problematic, inferior, out-of-balance, not true-to-type, lacking: terroir, focus, complexity precision, nuance, etc.
And I’ve commented here and elsewhere that I have noted zero interest in the topic among the visitors to our Tasting Salon. But the “over 14% sucks” meme has a life of its own, it’s out there, it won’t die; sort of like “the President is a foreign-born Muslim.”
Because of this persistent media attention, I figured that it was bound to happen—sooner or later—that one of my guests was going to comment on the “high” alcohol levels on the labels of my wines.
It happened like this. Three nice people came in and tasted through the five wines I had on offer: three Pinots, a Châteauneuf-du-Pape-style blend and last, a varietal Syrah. They seemed to be enjoying them. After the Syrah one of the guests asked “What’s the alcohol on these wines?” I answered “between 14.5% and 14.9%” and a couple of them started muttering “oh, that’s high—so-and-so won’t drink it.”
I politely asked them if they could have guessed that the wines had alcohols approaching 15% without being told, and each of them admitted “no” they couldn’t have. One commented that “…these wines don’t taste hot.” I explained that ethanol doesn’t really taste hot, but that other alcohols do—propanols, butanols, pentanols, etc. and their esters and oxidation products, collectively called congeners in the distillation biz.
These fermentation products are more likely to be produced by yeast under stress, and high initial sugar as well as high final ethanol concentrations are potent stressors, as are nutrient and co-factor deficiencies. In my winemaking I go out of my way to minimize the stresses on yeast (though not so far as to throw diammonium phosphate—DAP, a source of ammonia—at every ferment) and so the levels of these congeners are low in my finished wines. No “heat” on the palate.
I further explained that in fact few of my wines finish fermentation much over 13.5%-14% but they pick up as much as 1%-1.5% during barrel aging. This is because we have a dry barrel cellar. Inside the barrel there is 86% water and 14% alcohol, while outside there is an average of 30% water and 0% alcohol. To a first approximation, the thermodynamic drive for water to leave the barrel is over 3x what it is for alcohol, and so over the course of 2+ years aging in barrel the alcohol level of the wine inside actually goes up.
A wine made from grapes harvested at “optimal” ripeness and put to barrel at 13.5%, in our cellar may well end up near 15% when it is ready to go to bottle. This is not the same as harvesting the grapes over-ripe. Not only do these wines not taste hot, they don’t taste raisined.
Anyway, the offshoot was that these folks bought a case of wine, and intended to put some of in front of their “I won’t drink any wine over 14% because wine over 14% all tastes the same” friends and see what they think. Awesome.
John Kelly is the owner and winemaker of Westwood Wines, Sonoma California. This blog was originally published on his blog: “notes from the winemaker” on the 3rd of January 2012 at 14h52 to be precise.