Impact of sugar additions to primary?

So, me being a brewing rookie, I have a question regarding the addition of sugars to the primary during the later stages of fermentation. And no, I am not doing this right now, but I stumbled across the process online and have some questions, just out of pure curiosity. It seems as if the main purpose of doing this is to boost ABV and/or dry out a beer.

My question is : When one adds sugar to the primary after the bulk of fermentation has completed, how does this impact the duration of fermentation? Obviously, one would think that it would extend it, but is their a general rule of thumb (i.e. 1lb added to 5g takes about a week to ferment out with US-05)? Would you take a gravity reading prior to the addition and then another after a week or so and use that as your basis for determining whether it is done or not?

No rule of thumb that I know of...but, if anything takes a week with US-05 you probably are doing something wrong . You can take gravity before and after, that is really up to you. I personally only take a final gravity reading at kegging. My exceptions are occasionally taking a reading when fermentation slows up when doing a high gravity beer to make sure I don't have any issues as this is the point where you can still make adjustments to save yourself from getting a stuck fermentation or if the fermentation seems to be abnormal I will take one for diagnostic purposes. Otherwise, my process is to let the beer sit a week or two after fermentation appears to have ended so I know I will hit terminal gravity before going to keg...no reason to waste 6 ounces of beer just to see .

In a higher-gravity beer, or a beer that uses a lot of sugar, the best time to add sugar is at high kraeusen, i.e., right when the yeast reaches its peak activity (about 24 hours after you add the yeast, give or take). That is because if you have a lot of simple sugars in the wort, the yeast will preferentially metabolize it, which interferes with its ability to ferment maltose. Adding the sugar after the yeast has already observed what is in the environment tricks it. In normal gravity beers with only a small percentage of sugar, you can just add it late in the boil.

If you add the sugar at high kraeusen, it shouldn't--in my experience--affect overall fermentation time too much. I don't think I have tried adding sugar alone later on. But I have had experience adding some other stuff that contains sugar--e.g. ginger or fruit syrup--in an already-mostly-finished beer. That will set the yeast back to fermenting for a few days, usually. I guess the answer is just "it depends." You can't really be in a rush with the beer.

motorneuron said:

In a higher-gravity beer, or a beer that uses a lot of sugar, the best time to add sugar is at high kraeusen, i.e., right when the yeast reaches its peak activity (about 24 hours after you add the yeast, give or take). That is because if you have a lot of simple sugars in the wort, the yeast will preferentially metabolize it, which interferes with its ability to ferment maltose. Adding the sugar after the yeast has already observed what is in the environment tricks it. In normal gravity beers with only a small percentage of sugar, you can just add it late in the boil.

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I will in part disagree with this due to what moto says about the yeasts preference for simple sugars. Going from memory so I apologize if I am inaccurate...yeast metabolize sugars starting with glucose then fructose, sucrose, and finally maltose. Because maltose is the tricky sugar you want the yeast to start metabolizing that before slamming them with a bunch of easy food or else they might spend the rest of their reserves eating the newly added sugar and fail to take care of the maltose. IIRC the exponential phase where sugar is metabolized runs from 1-4 days or so, so you would want to add the sugar towards the end of that (I add sugar on day 3). Finally, I think sugar additions impact on yeast have more to do with percentages of the different sugars than gravity but obviously higher gravity brews put additional stress on the yeast.