Without yeast there would be no beer. Here are two pictures from our automatic cell counter. The one on the left is a brightfield picture of our house yeast. The one on the right is our house yeast stained with a fluorescent dye called propidium iodide. The dye will bind with non-replicating DNA in yeast cells. Basically, if you see a glowing cell, it’s dead. This is how we determine how viable our yeast is for pitching. Too many glowing yeast cells means the yeast sample isn’t very viable and we’ll toss it rather than re-pitch it into more beer.