I would like to know how long dried yeast stored in the refrigerator (under 40 degrees F) can be expected to be useful?
I'm sure that studies have been done like those with liquid yeast discussed recently, and my own experience has shown that we can expect at least some dried yeasts to last two years or more.
Also, is there a significant difference between different brands of dried yeasts in how long they can be stored and still be usable?
The length of the shelf life is very much strain dependent but there are also slight differences within the same strain from production to production; yeast is a living organism after all. In general we give a shelf life of 2 years and that is basically the minimum for which we guarantee that the yeast still performs to specs.
We test our yeast every 3 months to control the shelf life and find that the yeast often shows good viability and activity even beyond the guaranteed shelf life. We had for instance a Windsor ale yeast that performed still very well after more than 3 years.
So you might be ale to use the yeast after the shelf life is expired but we can guarantee you only 2 years shelf life. This of course requires that the yeast is still in its unopened original pack. Once dry yeast gets in contact with oxygen and moisture activity and viability will drop.