Pitching Whole Starter vs Cold Crashing Effects on Taste

Question

Earlier, I made a thread trying to find a reason for a yeast/green apple flavor in my beer.. After a lot of thought and poking around, I have an idea of why this is, but I wanted your opinions on whether this is the case.

I think it is because I added the entire starter to my wort rather than cold crashing.

This did not happen on my first batch, where I used only one smack pack of Wyeast 1056 on 4.5gal @1.052OG. It has happened on the next three, where I used very large starters. For all of these, I went for a very high pitch rate. The second batch, I used a half gallon starter, and the OG was was probably just over 1.070 if memory serves. The next two, I used a full gallon of starter (the local supply store I was going to seemed to never have fresh liquid yeast; I have switched stores because of this). So 20-25% of the volume of wort was starter wort.

Would this cause the yeast-like aroma I am getting in my beers--especially because I am brewing styles that should not have this? Any input is appreciated. Thanks!

Answer 1

It's certainly possible - a starter is only fermented to completion, but not conditioned, so byproducts of fermentation, such as acetaldehyde (green apple) and acetolactate (which becomes diacetyl - butter/butterscotch) are still left in the beer. This have low taste thresholds (50ppb for diacetyl), so it doesn't take much for you to notice then.

In a regular beer the conditioning period allows the yeast to reabsorb these compounds and convert them to ethanol and butanediol, which have much higher flavor thresholds. But since this has not typically happened in the starter, the starter wort will be full of these off flavors.

During the primary ferment, even more of these flavorful compounds will be produced. If the yeast is not given enough time to condition the beer, they may simply remain in the beer. And given that 20-25% of the beer was starter, that's a large proportion of additional by-products to condition.

Although ales don't typically require a diacetyl rest due to the already high temp, it's worth considering raising the temp towards the end of primary towards 21-22C to encourage the yeast to condition the beer faster, and hold it there for 3 days.

Also, as you've hinted in your question, pitching the yeast starter wort is usually not done when it's so large. In a dark, flavorfull beer, I might pitch a 1l starter, but otherwise cold-crash the starter and decant the wort before pitching the yeast. Diluting the beer by 20% means you're getting only 80% of the beer you wanted - less flavor from malt/hops, lower FG, body and abv.