And the winner of "Homebrew Clue" from my last blog post is … Todd Cobb, with the correct answer of “Autolysis – yeast was bad.” Many of you correctly identified this issue in posts on the BYO Facebook page, but none did it faster than Todd, who submitted the answer just 10 minutes after my last blog went live Monday. Below is a detailed answer as to how I came to this conclusion.
Have you ever heard the saying “Your greatest strength can also be your greatest weakness”? This was absolutely the case during the days leading up to brewing my undrinkable rubber band IPA. Simply put, the underlying issue with this beer was that I used a vial of San Diego Super yeast that was expired by almost a year leaving almost no live cells in the vial. When I first saw the date I thought that I could use my powers of yeast cell biology to revive this strain, but my confidence got the best of me. Below I will describe how I properly diagnosed the problem.
Even though my vial of yeast was old I assumed I could overcome the expected low viability with a starter. During the first few days after pitching my starter everything seemed to be going as planned since I noticed signs of fermentation in my airlock. After two weeks of primary fermentation I took a sample and noticed the specific gravity had only dropped from 1.062 to 1.028 indicating that I had some fermentation issues. As always, I tasted the beer after checking the gravity and this is when I really knew I had a major problem because it tasted like an old rubber band (see previous post for more details). After searching around the Internet I found a valuable resource called “The Complete Beer Fault Guide” (http://www.carolinabrewmasters.com/PDF/Complete_Beer_Fault_Guide.pdf) that helped me determine that this off-flavor indicates poor yeast health or autolysis (yeast cell death). This seemed plausible since I started with an old, mostly dead vial of yeast, which could result in severely underpitching the yeast, which can stress them, produce off-flavors, and cause autolysis.
Since I wanted to be sure about my diagnosis I used a sample from the starter to run yeast viability tests using a stain called Trypan Blue. This viability stain cannot enter a live yeast cell because the yeast’s cell membrane is fully intact but when cells die their cell membrane becomes permeable, allowing this blue stain to enter. So by simply counting the number of blue cells compared to non-blue cells using a microscope I could determine yeast viability. I saw that my yeast cell viability was 25% (75% were stained blue and therefore dead). A general rule of thumb on the acceptable viability for using yeast is around 95% viability (5% should be stained). My diagnosis was complete....