The Disaster

I was distracted with several people talking to me and my daughter running around while I was making my wine kit.  I grabbed what I thought was yeast and I tossed it into the wine.  I noticed the yeast looked a bit like potassium sorbate and thought it was unusual.

A day later the wine kit showed no signs of fermentation.  I forgot to clean up earlier in my haste, and noticed an emptied wrapper was lying on the basement floor.  The wrapper said “Potassium Sorbate”.  I could almost feel the camera quickly pan out as I was hit with the sudden epiphany.  I added the wrong packet.

For the readers who do not make their own wine, potassium sorbate is added to the end of the wine fermentation to ensure that the fermentation is complete.  It is a preservative that stops yeast from reproducing, and prevents any renewed fermentations from other yeasts or bacteria.  Potassium sorbate is added to many products, like grape juice to prevent spoiling.  If you add the potassium sorbate at the beginning, you are preventing any fermentation from starting.  No fermentation means no wine.

I quickly signed on to a wine forum.  I sheepishy asked the question to which I already knew the answer.  Was my wine kit doomed?

Can someone call a wine medic?

Some of the answers were hopeful.  They suggested that I try anyway, and see what happens.  Most said what I knew deep inside.  I have a bucket of $150 grape juice that will never be wine.

I talked to the owners at my favorite homebrew store in Denver.  They were not hopeful, but suggested I try making a starter and add that to the wine kit.  If I did nothing, the kit was lost so I might as well try.  I created a starter using 100% Welches grape juice, and tossed it in two days later.

To my shock, and to the disbelief of many on the wine forum, the kit started fermenting.  The fermentation was the most vigorous fermentation I’ve ever seen for a wine kit.  It actually blew out the airlock twice.  The starter had worked, and in the process destroyed my understanding of how potassium sorbate worked.  I had to know why.

How it worked

I searched the internet for articles regarding potassium sorbate.  Richard Roseweir in B.C. Canada directed me to an article at the British Columbia Amateur Winemakers Association (BCAWA) written by Bill Collings.  The article showed that the effectiveness of potassium sorbate is related to the amount of alcohol in solution.  The higher the alcohol content, the less potassium sorbate required to prevent fermentation.  The amount of potassium sorbate in wine kits is enough to prevent fermentation in wine, but not in grape juice.

For example if your wine is 10% alcohol, you need 0.20 grams/liter of potassium sorbate.  When the alcohol content reaches 14%, you need only 0.07 grams/liter.  The effectiveness of the potassium sorbate as a preservative is dependent on the amount of alcohol in your wine.  In my case there was no alcohol so the amount of potassium sorbate added was not enough to prevent fermentation.

The effectiveness of potassium sorbate is dependent on several factors: the wine pH, the concentration of free SO2, the percent of alcohol in solution, the concentration of sorbate, and the viable yeast concentration.  All have to be in the correct amounts to prevent renewed fermentation.

Another reason why my starter worked is potassium sorbate prevents fermentations from starting or renewing.  It does this by interfering with the reproduction cycle of yeast.  It does not kill the yeast.  When I added the starter, the yeast was already fermenting.

Conclusion

The lesson I learned was the potassium sorbate does not kill yeast as most brewers and winemakers believe.  Potassium sorbate only prevents fermentations from starting again, and its effectiveness is dependent on other factors.