It happens, maybe you found a pack of yeast in the back of your fridge from last summer you forgot about, or you took a break for a while and still had yeast on hand before you stopped brewing, maybe you got your ingredients from a homebrew shop that doesn't check dates. However it happened you have found yourself staring at an expired pack of yeast thinking, "can I use this?".On a recent trip to the LHBS I was presented with the opportunity to dig through the expired yeast & see if there was anything I would like to try & revive. I picked out several different tubes of varying age & took them home to see what I could do with them. I researched as best I could on using expired yeast and came up with the following procedures that have now worked for me on several batches using yeast well past it's best by date.For this article I will be attempting to revive one of the older vials I chose, a vial of WLP566 Belgian Saison II that expired November of 2013(a bit more than a year ago).To begin you will want to make a small, weak starter wort with a gravity of about 1.020 this can be achieved using 500ml of water & about 27g of DME. You want to make the wort weak to make things easy on the few remaining yeast cells you have to work with, too much alcohol in the fermented starter is not good for the yeast especially in its weakened state. It is important to make sure that the yeast has all the nutrients it needs to grow additional healthy cells to prepare for fermentation, so I would suggest adding a pinch of yeast nutrient to the starter to provide it with the nutrition it needs.Allow the starter to ferment for a few days, don't be surprised if it takes longer than normal for signs of fermentation to appear, or if signs aren't very noticeable this is normal. Mine took off within the first 2 days, signs of fermentation were weak, however you can see co2 escaping, and some bubbles forming around the edge of the flask.After the initial fermentation of your starter has subsided, you can chill it for about 24 hours to allow the yeast to flocculate to the bottom of the flask. Decant the liquid saving the yeast cake at the bottom. Allow your yeast to warm up to fermenting temps while you make a new starter wort.In a separate pot prepare another small, weak starter wort in the same way as the first this time, but lets make a 1000ml starter 54 grams of DME, and a pinch of yeast nutrient for a gravity of about 1.020. Chill your starter wort in an ice bath & add it to your flask with the yeast slurry in the bottom, recover the flask & allow it to ferment. Signs of fermentation may still be weak, but should start a bit sooner than they did the first time. After signs of fermentation have subsided, (probably 2 days or so) you can once again cold crash your starter to flocculate all of your yeast to the bottom of the flask.Once more after 24hrs or so, you can decant the liquid from the starter & allow the yeast to warm up while you make a new starter wort. This time our yeast should be a bit more viable so I am going to do a normal 1000ml starter wort with 100 grams of DME, for a gravity of about 1.037, I am also going to add a pinch of yeast nutrient again. Fermentation should be a bit more vigorous this time around, mine showed a krausen for the first time.At this point your yeast slurry should be quite viable, you can go ahead and use a standard pitching calculator to figure the amount of slurry you are going to need to ferment your batch of beer, and the amount of subsequent starters you may need to make to reach your cell count. All starters from this point forward should be of standard starter wort strength (between 1.030-1.040 SG)I stepped this one up enough to pitch into a 1.072 OG beer and my "expired" yeast got it all the way down to 1.007 FG that's roughly 90% apparent attenuation!So, if you come across some expired yeast in your fridge at home, or on discount at the LHBS , don't be afraid to use it, It can still make great beer.After this experiment & writing this article I was curious if the same process would work for expired dry yeast, though I have not conducted this experiment with dry yeast I did email two of the largest producers of dry brewing yeast Lallemand, & Fermentis asking if this would work. I was Informed that, yes you can, rebuild cell count using starters with dry yeast as well. Although, with the time & DME involved in making all these starters, & the price of dry yeast I would still probably say it is best to just replace expired dry yeast.Cheers