In Alcoholic Fermentation and Yeast
Every year after harvest I get verbally abused by some winemaker or winemakers regarding the alcohol concentrations in their wines.  Apparently they did everything exactly the same as previous years, used the same yeast and all of a sudden the alcohol is 15% instead of 14%. “What did you do to the yeast,” is usually the question. Doing this job for over ten years I am afraid my tact has become less – not that its ever been great – but my stock standard answer is usually that we decided to put some jet fuel into the growth media, whip out some old ACDC and Kiss records (had to dig for those), play them to the yeasts at full volume during production in order to put some hair on their little chests, thinking “this year we are going to nail the winemakers!”
OK so you get my sarcasm. We don’t “do” anything to the yeasts to increase its ability to convert sugar to alcohol. I will spare you the scientific detail but sugar to alcohol conversion is what it is. You cannot get more molecules of alcohol out of a particular amount of sugar molecules. The mystical figure of 0.55 as conversion factor between degrees Brix and alcohol is exactly that – mystical. I do see it making a guest appearance occasionally when people ferment red wine with a low sugar in an open top fermenter, in a well ventilated room, punching down every two hours. In this case the conversion “appears” to be low because a significant amount of alcohol evaporates. Using the same yeast on a higher sugar must in a rotor tank can result in a conversion rate of 0.62. So what does influence your final alcohol concentration?
  • Initial grape sugar – the real value, not the one your faulty hydrometer tells you, or your way of sampling. Actual g/l of sugar is also not directly proportionate to degrees Brix. ( see the Robert Paul article in PDF: Concentrate, this is serious)
  • The amount of whole berries and raisins in the must
  • Residual sugar at the end of fermentation
  • Type of fermentation vessel – how much evaporation can take place
  • Fermentation temperature – more alcohol evaporates from higher temperature juices
In my experience “conversion rates” fall between 0.58 – 0.63. I have seen 0.55, but rarely lately since red grapes are usually harvested at 24°Brix or higher where I come from. White wine fermentations are mostly cool to cold in closed stainless steel tanks allowing for very little evaporation so a conversion rate of 0.63 is more common than 0.55. In fact I have never seen 0.55 in white. I am sure it exists somewhere in countries where people still ferment whites without cooling. Maybe in barrel fermented wines? Let me know.
There can be slight differences between yeast strains in terms of conversion rates depending on how much of the sugar gets converted to by-products such as glycerol, acetic acid and esters, to name a few. However these differences in conversion rates are so minuscule that it is not worth while getting out of bed for, or base your choice of yeast on. Lallemand did a study a few years ago comparing the alcohol conversions of yeast strains under the same conditions. The biggest difference seen between the yeast with the lowest and the one with the highest conversion was 0.51. I suppose a 14.5% wine is better than a 15% wine but labelling laws allow you to print a lower than actual alcohol. Choosing a yeast with a lower conversion might not be very suitable for your wine style and you could end up having a “lower conversion” simply because your wine is semi-sweet.
So until researchers can figure out how to create a non-GMO yeast that naturally turns a percentage of the sugar into white light, you will just have to accept that the higher the sugar – the higher the alcohol, and find other ways to lower the alcohol concentration of wines.
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