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How does the fermentability of wort affect the speed of fermentation, if at all?

If I have two batches of wort mashed at different temperatures (say 148 °F and 156 °F - 64/69°C) and I pitch the same amount of the same yeast into both, what will happen in the first few days?

Will the gravity of the more fermentable wort drop faster, or will it go at the same speed as the other and just continue longer?

What I am really trying to get at is whether the drop in gravity during the first 24-48 hours of fermentation can be used to estimate the final gravity. My experience says no, but I haven't done enough brews in nearly identical conditions to be sure.

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I haven't seen that to be the case. I know of no literature that has looked at this specifically. My experience is that the rate of fermentation is strain dependent and not wort dependent. The yeast are not slowed by non-fermentables as they seek out fermentables. I am sure there may be differences at the extremes, but in the frame work of your question it shouldn't effect the rate much.

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You'll need to pitch the correct amount of active yeast for a quick start to fermentation. Yeast will have a lag phase as it wakes up, then an aerobic growth/lag phase as it reproduces, then an anaerobic fermentation phase followed by a cleanup phase. Assuming you are pitching just a normal home brew pitch of yeast, such as one vial or smack pack, but different wort gravities, the one with the most fermentability should theoretically take a little longer to get started, because it should need a longer growth phase to get the yeast cell population high enough. Yeast can sense the amount of fermentability in wort, and will continue reproducing in the growth phase before it has enough brothers and sisters, then begins fermentation.

To the second part of your question, if you have a quick and vigorous start to fermentation in the first 24 hours you are doing something right. Final gravity and apparent attenuation are strain dependent, but you should be able to maximize it to its potential by keeping the yeast happy. That means pitching an active and healthy yeast OR pitching healthy yeast into oxygenated wort where it will reproduce, keep everything at the optimal temperature, controlling your temperatures, and having yeast nutrients in the wort. You could go for the high end of the temperature range for your yeast strain for maximum attenuation but that may introduce off flavors.

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I suppose one might ask if pilsners ferment faster than stouts, as those two beers are examples from both ends of the non-fermentables spectrum. From experience they under go primary fermentation at about the same rate.

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No decent estimates of final gravity can be projected from early readings.

Speaking from personal experience the amount of unfermentable sugars and proteins doesn’t affect th speed of the fermentation process to a great extent. The number of live yeast cells pitched and amount of oxygen locked in the tank will.

Rehydration and common lag time for the chosen yeast strain also play a part. But for instance, using a clearing agent in the boil has little impact of the speed of fermentation or the appearent attenuation. But it does, I imagine, affect the yeast’s ability flocculate.

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It depends on alot of factors temperature- yeast culture and volume of wort. A fast start doesn't always mean final gravity will be achieved. My advice would be to make sure you aerate your wort shacking the carboy isn't enough. l suggest using a aquarium stone

  • You are missing the point. I was asking a theoretical question about a case where all conditions were equal except the wort fermentability. – Gluten-Reduced Brewer Feb 7 '17 at 23:52

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