Stuck fermentation when using two kits and two yeast sachets

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CiderTropica

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I have made around 12 brews and the only one which didn't taste great was a Czech pilsner where I followed the instructions to the letter by adding a load of sugar. The result was a thin, dry, cidery beer which was almost undrinkable. Lesson learnt – I never added sugar again to any kit. Now, I add water up to the desired ABV, and add some dried malt extract if I want to raise ABV. All my beers have been fantastic since then.

The result of this approach is that a beer kit will now only create about half or two-thirds of the volume printed on the label. I'm fine with that, since lower volumes are easier to manage.

On two occasions, I bought two identical kits and emptied the entire lot into the FV to make a larger volume. After allowing to cool, I added both sachets of yeast simultaneously. In both cases, this resulted in a stuck fermentation with SG only falling around half of what it should have done despite the brew sitting in the FV for two weeks. On the first batch, I disturbed the yeast sediment to put it back in suspension. When this didn't help, I raised the temperature of the heating mat slightly. This worked okay and "unstuck" the fermentation.

On the second batch, these steps did not work, so I had to add another sachet of yeast (luckily I had a spare sachet of yeast appropriate for this style). This restarted the fermentation immediately and I ended up with a great product. However, this did increase the fermentation time to over three weeks.

My working theory is that dumping in two sachets of yeast into the top of the FV creates too much competition at the surface and the yeast goes dormant. My approach next time will be to add the first sachet then add in the second one a week later if fermentation appears stuck again.

Any thoughts on this? I realise two batches is not enough data points to draw conclusions.

I will add that I am adding one teaspoon of yeast nutrient per gallon to all my brews after getting the OG right.
 
I think this just shows that kit yeasts can be a bit crap. With enough viable yeast added at the start you should be able to ferment a kit out without the use of nutrient. It's not about yeasts competing with each other.

Bit of context would be useful. What kits were they, and what were the volumes and OGs of the beers?
 

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