The conversion factor issue
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!”
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.
Tags: fermentation temperature, sugar to alcohol conversion, wine yeast
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7 Responses to “The conversion factor issue”Leave a Reply |
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March 4th, 2010 at 8:27 am
I wonder if you have seen changes in conversion factor induced by sparging the active ferment with oxygen? I have not had the opportunity to conduct a controlled trial recently, but I have reason to believe that introducing one volume saturation of oxygen as few as five times during the active ferment – between 22° and 5°-8° Brix can decrease the final alcohol by 0.5% or more for a starting Brix of 24°-25°. Some of this work I can’t talk about because it was done when I worked for a winery we both know well, but I have also done trials with my own Syrah production where this appears to be the case.
Note that the original reason for adding oxygen to my Syrah ferments was to increase color stability and improve tannin structure on the palate in cases where a significant fraction of the seeds were underripe in the source fruit. This was successful.
- John
March 4th, 2010 at 12:27 pm
Hi John,
No I am not aware that sparging with oxygen during fermentation can lower conversion factor. Respirating yeasts off course form no alcohol as in the case of commercial wine yeast production where biomass formation is the objective. However this is what is referred to as a fed batch culture where small amounts of sugar (molasses) is given to the yeast as it needs it. In the case of grape juice fermentation, you have a batch culture where you start of with all the sugar you plan for the yeast to consume. The moment the sugar concentration of a medium is higher than a certain percentage, yeast will ferment, regardless of the amount of oxygen added. Maybe in your experiments you just replaced some of the volatile alcohol fumes by sparging with a gas? I don’t know. Maybe you did actually stumble upon a loop hole in yeast metabolism, in which case winemakers around the world will reward you with the noble prize for discovering a way to lower alcohol conversion. Keep up the trials!
March 4th, 2010 at 7:15 pm
Was not sparging off alcohol – the rate of gas introduction wa on the order of 1 lpm. There was a difference in rate of fermentation and temperature profile that could possibly have resulted in the difference. I think winery ferments are not the place to resolve the question.
April 23rd, 2010 at 8:42 am
John, I think wineries are the ideal place to do trails…. Maybe we’ll find the answers to our own questions if we experiment more under cellar conditions.
July 19th, 2010 at 6:00 pm
It looks like Wine Network has moved the “Concentrate – This Is Serious” article to:
http://www.winenet.com.au/uploads/pdf/WineNetwork_Concentrate-this-is-serious_RPaul03.pdf
Thank you for writing this article. It never occurred to me that evaporation significantly affected final alcohol.
Erroll
July 20th, 2010 at 1:07 pm
Thank you for the correct link. I updated the article.
October 11th, 2010 at 3:36 pm
The reason for the drop in alcohol when you introduce oxygen is that some of the sugar is converted into yeast like in the yeast processing plant