At some point in the past, I had purhased a large amount (maybe a pound) of instant yeast at a restaurant supply store. I transfered it to a glass container, and have been keeping it in the fridge. I don't remember when I bought it -- I suspect it was 2 years ago. (I know it was more than a year old).
When I make bread, it's still rising, and if I try proofing it, it works ... but the time to get bread to double in volume is taking longer (sometimes up to twice as long) as many recipes give as estimates.
Is there some good test that I can use to estimate what the decreased activity is, so I can adjust the amount of yeast that I'm using? Or is there some other technique that I can use to compensate (eg, start proofing the yeast with sugar before using?)