How reliable is the alcohol tolerance of yeast?

I have another question for the experienced mead makers.

I will be starting a few different meads in the next week, and wanted to have some be from off-dry to sweet, but was wondering how much I can rely on the yeast stopping fermentation around it's alcohol tolerance level?

Am I better adding less honey at the beginning, to the level I want the alcohol percentage and ferment it dry, then stabilizing it and adding extra honey to back sweeten, or can I add it all at the beginning and have it end pretty close to it's alcohol tolerance, leaving residual sugar?

By the way, for those interested, I want one basic mead to be dry, two fruit ones to be sweet, one basic mead to be sweet, a cyser to be semi-sweet and JOAM to be semi-sweet.

Yeast is a living thing and so it's alcohol tolerance should always be considered a 'guideline' rather than a rule. If a yeast packet says it's tolerance is 12%, sometimes it can be pushed to 14 or 15%, or sometimes it can just crap out at 8% and refuse to go any more. Yeast health (and alcohol tolerance) depends on must temperatures, must aeration, nutrient additions, and even just overall health of the yeast.

Adding all the honey at the beginning is not recommended unless it's going to be a dry mead (3 lbs per gallon or less) because the osmotic shock can render the yeast just not up to the task. What yeast are you going to use?

For my sweet meads I keep doing honey additions every two weeks until that sumbitch don't be fermentin no more!

I was thinking of D-47, which has an alcohol tolerance of 14%. I was going to be using between 2.3lbs (cyser) and 4.3lbs (raspberry melomel), although all the rest will have about 3.3lbs to be semi-sweet but one which will use EC-1118 to be dry.

I hadn't though of the osmotic pressure, especially for the raspberry melomel. The O.G. with all the sugars would be about 1.162

Just a little question, does anyone know, in BeerSmith, how to make a sugar unfermentable, so that the F.G. increases?

1.162 is very very high for an SG. Drop it down to between 1.090 and 1.100. Then when it ferments down to around 1.020 go ahead and add more honey. Just break your intended amount of honey down into two or three additions and your yeast should be able to to ferment it with not problem. Having said that, if you use the staggered nutrient additions along with this stepped honey additions, this has been known to surpass many yeast's alcohol tolerance by 1-3%. So keep an eye on the SG, when you have added enough honey to surpass the yeast's supposed tolerance, add just small amounts until it ferments no further.

I have seen different conflicting information about adding honey/sugars during the ferment. On one hand it does appear to allow some yeast to push the alcohol tolerance even higher. Adding the sugars in stages during the ferment is method suggested by WhiteLabs to push their High Gravity beer yeast to the 22+% tolerance limit.
However I have also read that adding significant sugars after the yeast are active can lead to stress on the yeast as the osmotic pressure change shocks the system. This can cause off flavors in the mead and the suggestion was to add all the sugars up front and all the yeast to adapt during the growth phase when there is less stress due to alcohol.

I either manner you can start with a lower gravity must that you allow to ferment dry then stabilize and back sweeten once the ferment is complete. This is a more reliable method to get the desired FG but the flavor may be different than one that stops naturally.

craig