Wort stability test

I've heard of a simple test to check if the wort is contaminated:

- take a small sample of wort from into a sterile container, just before you pitch the yeast.
- close/cover the container and keep it warm (~80F) for three days
- examine the wort: it should not be carbonated in any way, nothing floats on the surface, check if it smells and tastes good. If so, it passed the test.

So far I did this test twice. First sample (weizen) started showing some signs of infection after 5 days, the one from second batch was clear for weeks.

That's very similar to what we used to do in the lab when we made a new batch of cell media. We'd take a sample and put it in the incubator with nothing else and monitor it for a few days. Signs of infection are quite visible once it gets going.

BUT, just because it develops an infection by itself, doesn't mean it will in the fermentor. The yeast can really take over and once they start producing the alcohol, the others bugs will have a hard time getting going.

This is a very useful test for troubleshooting infections. I use it on occasions, especially when I keep part of the wort unpitched for 24hrs b/c I’m waiting for the other half to get to low kraeusen. This is a way to get away with less pitching yeast but proper pitching rate.

While a failing test wouldn’t help much for the affected batch it would give me vital information in case I do notice off flavors. It can also be used to detect trends. Maybe you are starting to get complacent with sanitation and the test starts showing signs of activity earlier than before. That may actually happen before sanitation gets so poor that it becomes noticeable in the beer. But that’s a lot of work for a home brewer.

I have also used this test to test my R/O water. I took a vial with sterile wort, which I prepare for yeast propagation anyway, and added some RO water. After a few days it turned cloudy which means to me that I cannot assume that my RO water is free of contaminants in case I want to use it to dilute post boil chilled wort or dilute starter wort after the boil. But I have done both with no apparent infection.

I don’t look for carbonation. You should see the sample clear when the cold break settles and then it will get hazy again when the contaminants start to grow.

Kai