I have about a dozen Lallemand Windsor and Munich dry yeast. The expiration date on the packages are either 12/2013 (3 years past) or 01/2015 (about 2 years past). Are these yeast still usable? For your information, I did not plan to use them in the near future. Thank you in advance for your input.
BTW, I did have them stored in a refrigerator.
I read somewhere (not sure where) that dry yeast loses about 4% viability a year. I can’t confirm it of course, but I’d give them a go by pitching plenty and having a fresh pack or two on hand to pitch if the lag time went very long.
For my money, they’re usable for sure. Especially if you have a second one or two to pitch.
I’ve used 3 year old Nottingham with no problems.
I’ve always used 2% decay rate per month.
They last way longer than anyone will tell you. 4% per year might be about right. Any loss “per month” is bull in my experience, unless perhaps measured in tenths of a percent.
I just use it as a benchmark for calculations in my spreadsheet. Even 2% is not much loss.
You’d still be at > 50% viability after 2 years.
I have an un-opened 500 g brick of Maruibrew 514, best before says June 2014. I plan on doing some higher gravity one gallon brews after Christmas. Kind of nervous about using it, I got it for free though. I am thinking on using double the amount I would normally use for a 1.080 triple. Should I rehydrate it first?
I would rehydrate and do a test batch. If the test batch works then the whole brick likely has the same viability.
Yeah I would defiantly rehydrate.
If that’s true, which it probably is, then keeping dry yeast for 12 years is no worse for them than pitching without rehydration. Or so they say… 50% mortality by not rehydrating.
For me to have a yeast pack last 12 years I’d have to be in a coma for 10 1/2 to 11 years. 
Plan on using two packages when 1 would do normally.