Is yeast past it's "best by date" or over 6 months old any good?

Occasionally we end up with yeast that doesn't sell within it's best by date or within the recommended dates set by the manufacturer. We decided to try brewing with some old packages to find out if it would create beer or just do nothing . For this experiment we brewed a Belgian Dark Strong, referred to as a Quad or Belgian Quad.

Brew date was 12/20/20. Two days prior to brewing we took 3 different Belgian yeast strains (2 packages each of Wyeast 3522, Wyeast 1388, Wyeast 1214) and made a 3 liter starter with a gravity of 1.035. All of the packages were dated prior to 5/15/2020.

Typically our starters have a small krausen; This starter showed no signs of activity other than a change in color and take on a milky appearance. After completing our brew session and chilling our wort we pitched our yeast with little expectations.

Our backup plan was to pitch dry yeast should the liquid yeast starter not show any sign of reducing gravity. However we didn't need it. Only after 2 hours of pitching our liquid yeast starter did it begin to show major signs of activity. The fermentation started so quickly and vigorously that is sounded like someone was knocking from the inside of our fermentation chamber.

The resulting beer has all the Belgian characteristics one would expect from a Belgian style beer.

Should we try this experiment again we will start with one package and do a multi-step starter to increase the cell count prior to pitching in our wort.

In conclusion, is using older, out dated yeast a great idea. Probably not so much. But if there are any viable cells in the package and you preform a starter, especially a multi-step starter you can save on that package of yeast that may have been forgotten about in the back of you fridge.

Happy Brewing