[hopfenundmalz]

No malt either.
I have beer in the keg that's getting old.
Can I boil off the alcohol and use that?

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When properly fermented beer in a keg has little to no residual sugar for normal beer yeast to feed off of and grow.  So, no you won't be able to use beer.

But you may be interested in this:

http://brulosophy.com/2015/06/05/real-wort-vs-dextrose-yeast-starter-exbeeriment-results/

The dry yeast manufacturers use raw sugar (or is it molasses?) and yeast nutrients to grow their yeast. Some advertise that the yeast is gluten free, and that means they don’t use barley extract in the process.

What sugar form and how much nutrient in what form?

I believe the dry yeast manufacturers you refer to using molasses are primarily making baker's yeast.  Same species,  but the requirements for life cycle and performance of the yeast are very different.

A local yeast expert that was around here before you said they use molasses. Mark is no longer active here.

How do they make gluten free yeast?

You might have better info than I. I just know that baker's yeast is grown on media like you describe and in that case growth is ALL they care about, really.  If beer yeast is really grown that way it could contribute to the adaptation problem. I use the Weihenstephan lager strain and it generally takes about six generations to come into its own  under actual brewing conditions. But for just that reason, I wouldn't make my initial starter, when I buy a new culture, with a non-malt medium.  Might as well start getting it used to malt right there.  FWIW I generally start from liquid but have tried the dry version.  Only difference I can recall was that it was even slower in the starter and first generation, so I took the opportunity to get a fresh liquid culture at that time.
As to gluten free yeast, well, if it's going into a gluten free wort, it might have an advantage in being pre-adapted!

The yeast are continuously feed sugar and nutrient at 1 P or less, which avoids the adaption problem.

[Robert]

^^^^^
Hmm.   There's a discussion of "fed batch" vs. conventional batch propagation in Chris White and Jamil Zainasheff' s book on yeast and their conclusion is:

"Many brewers have tried adopting the fed-batch process, but few employ it.  David Quain, coauthor of Brewing Yeast and Fermentation and longtime Bass/Coors yeast guru, was once asked if they ever used a fed-batch process.  His answer, 'Fed batch has no place in brewing.'"

[narcout]

http://www.fermentis.com/brewing/industrial-brewing/faq/

"Fermentis dry yeast are propagated in fed batch systems where nutrients and Oxygen are continuously brought to the fermentation media thus enhancing the fermentability of the yeast."

[Robert]

http://www.fermentis.com/brewing/industrial-brewing/faq/

"Fermentis dry yeast are propagated in fed batch systems where nutrients and Oxygen are continuously brought to the fermentation media thus enhancing the fermentability of the yeast."

Well that's as definitive an answer as we'll get, as I doubt they'll give out the composition of their media!   But I still have to wonder, "increase the fermentability" relative to what? As I mentioned, I've tried dry yeast, theirs in fact, and found it very sluggish compared to liquid cultures of the same strain. Thanks for finding that link!

[narcout]

But I still have to wonder, "increase the fermentability" relative to what?

Yeah, I don't know.  The response was to a question about whether dry yeast are suitable for high gravity brewing.

There's some interesting info in the link below that is possibly relevant from Dr. Clayton Clone who I believe works (or worked) at or with Lallemand.

https://koehlerbeer.wordpress.com/2008/06/07/rehydrating-dry-yeast-with-dr-clayton-cone/

"We recommend that the rehydrated yeast be added to the wort within 30 minutes. We have built into each cell a large amount of glycogen and trehalose that give the yeast a burst of energy to kick off the growth cycle when it is in the wort. It is quickly used up if the yeast is rehydrated for more than 30 minutes. There is no damage done here if it is not immediatly add to the wort. You just do not get the added benefit of that sudden burst of energy."

He stated somewhere else (and this info may be in Yeast as well) that because of the way in which it is propagated Lallemand dry yeast has sufficient lipids for 3 to 4 growth cycles right out of the package.  Unfortunately the link for his response no longer works.

[curtdogg]

These things seem to get derailed easily.
I will use 16 oz of wort from tomorrows brew (2nd runnings) basically make a mini beer in a growler,  Cold crash, decant and save for a starter in the future.

Thanks for all the extra info.

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[Robert]

But I still have to wonder, "increase the fermentability" relative to what?

Yeah, I don't know.  The response was to a question about whether dry yeast are suitable for high gravity brewing.

There's some interesting info in the link below that is possibly relevant from Dr. Clayton Clone who I believe works (or worked) at or with Lallemand.

https://koehlerbeer.wordpress.com/2008/06/07/rehydrating-dry-yeast-with-dr-clayton-cone/

"We recommend that the rehydrated yeast be added to the wort within 30 minutes. We have built into each cell a large amount of glycogen and trehalose that give the yeast a burst of energy to kick off the growth cycle when it is in the wort. It is quickly used up if the yeast is rehydrated for more than 30 minutes. There is no damage done here if it is not immediatly add to the wort. You just do not get the added benefit of that sudden burst of energy."

He stated somewhere else (and this info may be in Yeast as well) that because of the way in which it is propagated Lallemand dry yeast has sufficient lipids for 3 to 4 growth cycles right out of the package.  Unfortunately the link for his response no longer works.

Interesting.  I know I was careful to rehydrate only 15 minutes.   I only tried the dry yeast twice.  Once I made a starter as usual, and it was noticeably slower than expected as was the beer.  Another time I followed the direction for directly inoculating the wort.  At the time when I should have been at high kräusen there was still no sign of activity but a weird solventy or medicinal smell (don't remember it precisely.)  Dumped.  I really wanted to find dry yeast to be a convenient, stable alternative source but that wasn't my experience.  To really know what was going on I'd have to do more trials.  But as I buy a new culture so infrequently I'll just stick with liquid. I don't imagine this is strain-specific, I just figured it had to do with propagation effects on dry yeast.
Oh, and Curtdogg -- sorry for the derailment!  Your plan sounds good. But just store it under the beer if you can, without decanting.