Friday, December 21, 2012

How Fruit Effects Yeast Viability

There are a number of factors that can reduce the viability of a yeast cake that you may have heard.  These include, high gravity, highly hoped, and dark color, but one I have not heard of is the effect of fruit on viability.  In the past few months I have made several fruit beers, and three of them I collected viability data on the yeast slurries taken from these brews.

I have been surprised to find that yeast slurries from beer made with fruit have a drastically lower viability!

On one of these fruit beers I took a cell sample during fermentation just before adding the fruit and the viability was in the high nineties as you would probably expect during active fermentation.  When the beer was bottled it was down to 1% viability.

The viability of the yeast that was taken from beers that did not have fruit has always been substantial higher. It looks like from this point forward, in order to save the yeast, I will be racking off of the yeast cake before adding fruit. 

The EC-1118 Champagne yeast that I recently harvested off of my Strawberry Champagne contained 19 billion suspended viable cells per litter during fermentation, but after adding the strawberries, the thick slurry only contained 6 billion viable cells per liter.  There were three times as many viable cells suspend in the wine during fermentation than there were in the thick slurry after fermentation completed.  This indicates that there was massive cell death after adding the strawberries.

Although this evidence seems fairly conclusive, I am hesitant to say this is always the case.  But independent of what the causes is, there are significantly less viable yeast cells per liter of slurry.  If a microscope was not used to asses viability before pitching, and a standard cell density was assumed, the wort would end up drastically under pitched, even if using a starter. 

The fruit in these beers has pasteurized and has the cellulose removed with a strainer before being adding to the secondary fermentor.  You can see exactly how it was done in the blog post called "Fruit The Easy Way." In the juice that is transferred to the fermentor there are likely still a plethora of fruit cells.  It is possible that this large cellular mass is being counted as dead yeast cells and thus the viability show as being low. 

Perhaps there is some merit to a secondary fermentation vessel.

Raspberry Cream
Strawberry Champagne
Sasion Terr
Raspberry Wheat


  1. That's pretty interesting. These data you provide… could you be more specific as to how they were obtained? I.e. did you compare the cells from slurries in every case for your table? Were they the same age? Same gravities? Comparing active fermentation suspension and slurry after 3 weeks after fruit isn't a very good comparison. There is a reason why slurries are slurries - spent yeast fall down. Perhaps comparing cells in suspension in a split batch at the same age in same conditions, but one part had fruit added to it and another is a control would be more conclusive?

  2. The time of measurement is approximately the same in all of these. They were measured in slurries after being in the fridge for a few days. It's not a great set of data, so I don't see this as conclusive, but it does seem to indicate that fruit lowers viability.

  3. Yeah, it's kinda interesting. You use methylene blue, right? I wonder why this happens...

    1. Have you seen fruit effect viability? Yes, I use MB for viability counts. When I have time I'm going to double check the viability with a CFU count.

  4. I've never seen it, but I've never looked. Actually I'll add fruits to a blond in a week or two so I'll test that...
    Oh also can you make it easier to comment? It's very tiring to try and type in those weird characters and numbers and getting them wrong 3/4 of the time.

    1. If you could measure the viability that would be great! The "word verification" is now disabled.