In Processing

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.

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 19 July 2012.

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