[tschmidlin]

What if I were to rack wort over beer that had basically finished fermenting? Or would the yeast not be interested?

I'd make all wort additions before or at peak krausen. Adding new sugars to yeast that are going dormant is just asking for off flavors.

What specific off flavors would be involved?
 Adding new sugar over a period of a week or so is common in raising the alcohol content of wine without any effect.

Just asking....

I would recommend adding any sugars to the wort either before or after fermentation, but not during.  The main reason is that the first thing the yeast will ferment is the simple sugars (glucose and fructose), then it will switch to more complex sugars (maltose, maltotriose, etc).  The presence of glucose inhibits the fermentation of the more complex sugars.  If you add additional glucose while it is fermenting maltose, the cells will stop fermenting maltose and switch to glucose.  Wen the glucose level drops again, they will switch back to fermenting maltose.  This may have an affect on the overall fermentation performance, and might not give repeatable results.

Adding sugar at the end though, seems fine to me.  I do it when I (rarely) add fruit to beers, the yeast don't seem to mind.  I don't have a lot of experience with it though.

[bluesman]

It also depends on what you are trying to achieve. If you want to dry the beer out somewhat, then adding some simple sugar during high krausen will help do that. I agree with Tom in the order with which the yeast will ferment sugar and I also think the yeast prefer to keep things going as opposed to stopping and starting again (not that they won't). It involves more energy for them and ultimately can affect the flavor profile of the final beer.

More often than not, I prefer to let the yeast do their job in one shot. In Phil's case he may opt to keep feeding the yeast continuosly, allowing the yeast to consume all of the simple sugars first and letting them finish by consuming some of the maltose and maltotriose. As long as they are healthy to begin with, and have enough oxygen to sustain the level of active fermentation required to finish, they will consume most if not all of the sugar available. Temperature and yeast strain should also be considered.

[phillamb168]

It also depends on what you are trying to achieve. If you want to dry the beer out somewhat, then adding some simple sugar during high krausen will help do that. I agree with Tom in the order with which the yeast will ferment sugar and I also think the yeast prefer to keep things going as opposed to stopping and starting again (not that they won't). It involves more energy for them and ultimately can affect the flavor profile of the final beer.

More often than not, I prefer to let the yeast do their job in one shot. In Phil's case he may opt to keep feeding the yeast continuosly, allowing the yeast to consume all of the simple sugars first and letting them finish by consuming some of the maltose and maltotriose. As long as they are healthy to begin with, and have enough oxygen to sustain the level of active fermentation required to finish, they will consume most if not all of the sugar available. Temperature and yeast strain should also be considered.

What I don't want to do, though, is go down the road to continuous fermentation in the sense that a lot of the breweries in the 70s tried. I'm concerned about diacetyl with this technique, so what's the difference between continuous fermentation and for example stepping up a yeast culture? Or should I just RDWHAHB as far as this is concerned?