[James Lorden]

If a wort has 50 gravity points from base malt and 10 additional gravity points are added from crystal malt is the assumtion that the real attenuation (not apparent attenuation) going to be 10 points higher then it would have been whithout the crystal?  In other words none of the crystal will ferment?  Or is a certain portion of crystal fermentable?

[bluesman]

Some of the sugars caramelize during kilning and become unfermentable leaving a residual sweetness in the final beer but there is some residual fermentation acheived from use crystal malt. The lighter the crystal...the more fermentable it will be. However, the enzymes are denatured and therefore it can't self convert.

[James Lorden]

I wonder if malt producers have an idea of what that % fermentable is?  I've noticed that some extract producers now include the approximate % fermentability on their cans.  This would be useful information for recipe formulation.

[weithman5]

or the opposite. how much sweetness can you expect from the crystal.

[narvin]

The lighter the crystal...the more fermentable it will be. However, the enzymes are denatured and therefore it can't self convert.

I was under the impression that starches in crystal malt are already converted.  Although, maybe not all of them are... I can't speak to efficiency using crystal malt in an extract beer.

http://howtobrew.com/section2/chapter13-1.html

As for the original question, some of the sugars are unfermentable, but I haven't seen any spec sheets regarding fermentability  Maybe half?  You should do an experiment 

[a10t2]

Just so no one tries to reinvent this particular wheel: http://www.homebrewtalk.com/f128/testing-fermentability-crystal-malt-208361

[James Lorden]

Awesome, here are the results of an experiment performed by a HBT member...

OK, I think I can make my final conclusions from what I learned with this test. I wish I had better equipment and time to do many more batches and get more data points, but hey, I gotta brew my beers guys

I'll comment first than show all images.

1)Table with all that was tested and results
2)Fermentation/Gravity chart of all that was tested. I excluded this last test I just did (test 16/17/18) to make it easier to understand the graph.
3)PPG or sugar extraction chart
4)Attenuation chart

This experiment bring simple conclusions, may not indicate accurate values due to the reduced data points and accuracy of tools used, but I hope it give us some light to what crystal malts do to our recipes:

A)Crystal malt have sugars but still hold starches that can be converted
B)The amount of sugars that one can extract from crystal malts would increase if mashed with a base malt since the starches will be converted. PPG showed to increase by about 20%, regardless of the kilning level of the crystal malt.
B)The sugars from crystal malts are VERY fermentable, contrary to what we knew. Fermentability will depend on multiple factors like:
-Steeping crystal malt alone will yield sugars that can be attenuated by 50% for crystal 10 and 40% for darker malts.
-Mashing crystal malts with base malts will yield sugars that are almost as fermentable as base malt itself. For the 50-50% rate used, sugars from crystal-10 malts were attenuated by 70% while crystal 40 and 120 by 52% minimums. For a 10% crystal to grist rate, I guess it could be treated just as a base malt, which means very fermentable.

The basic recipe guidelines would be:
1)If steeping crystal malts, expect lower PPG than when mashing. About 50% of the poits you get from the malt will be left to FG for light malts and 60% for darker malts
2)If mashing with a base malt, treat crystal just like a base malt, specially if using lowe amounts like 10 to 20%. So don't blame the crystal malt for a higher FG since most of its sugars will be fermented.
3)Regardless, crystal malts doesn't seem to be the best thing to use to add residual sugars to the final beer. Perhaps mashing at higher temp is the way to go, along with Lactose or Dextrin (that we believe is not fermentable. I may have to test that also)

Note.: All tests were done with mashing/steeping temp at 155F and fermentation with S04 at constant (really constant) 70F.

THANKS TO NILO ON HBT FOR PERFORMING THIS EXPERIMENT!