Q:

How will reducing and freezing affect the flavor of stock?

I have been working on making beef stock for about a month now.  I am happy enough with the outcome that I would like to make larger batches and preserve it.  It is currently the consistency of jelly after bringing down its temperature and letting it cool in the fridge overnight.
I am considering reducing the stock down to about 25% of its volume, letting it cool and set.  Then I want to cut it into pieces, vacuum seal them and freeze them.  Then, when I want to use it, add water to it until it is back to its original volume.
I am wondering how much this will affect the flavor of the stock.

A:

I've certainly never detected any adverse effects on the flavour from freezing and/or reducing stock. 
However I no longer reduce stock before freezing as it seems like a waste to boil off loads of water only to add it later, when the only benefit is saving surprisingly little freezer space. It comes out a little more concentrated from my new slow cooker anyway, as it's hotter than the old one.
Stock is simmered for hours anyway; getting it very slightly hotter for a few more hours won't do any harm. 

