P-31 - Investigating the Effects of Oil-based Dry Hopping on Yeast Viability

One of the biggest challenges a brewer faces is creating a cost-effective, efficient beer.  A particularly common strategy to minimize cost is to harvest yeast from one batch, and repitch it into a new batch.  If done right, a single pitch of yeast can be used for many fermenters.  It has generally been understood that yeast should be harvested for repitching before dry hopping due to the contamination of the yeast at the bottom of the fermenter with spent hop material.  This stresses the yeast and lowers cell viability; it can also impart undesired aromas into the yeast that could carry over into a new batch of beer if the yeast is repitched.  Historically, dry hopping has been performed with pelletized hop products, however new oil-based hop products, such as HyperBoost™, have been released and are intended to be used in the dry hop.  This raises the question: do these products affect yeast viability and flavor upon repitching?  In this project, we set out to determine if yeast recycled during active fermentation remains viable after the beer has been dry hopped with HyperBoost on day zero of fermentation and if significant aroma and flavor carry over into beer fermented with the recovered yeast.  Benchtop brewing trials were conducted in duplicate to simulate standard brewing procedures. Dry malt extract was used to make 1 L of wort, which was boiled using a hot plate, and dosed with CO₂ hop extract to produce approximately 35 IBUs. The wort was cooled, filtered, and analyzed for hop acid and density data. The wort was dosed with ale yeast and 0.15g/L HyperBoost before the flasks were stored in a dark, temperature-controlled room.  The HyperBoost dose represents a 15g/L T-90 equivalent dose or ~3.9lb/bbl. This dose was intended to represent an above-average use rate for this product.  After 4 days of fermentation, the wort was placed into a column apparatus and cooled to encourage yeast settling. The yeast was harvested out of the bottom of the column and analyzed via a hemocytometer for cell viability. The recovered yeast was then dosed into a new batch of wort made using the same method as the first iteration of wort. This process was repeated for a total of 5 batches.  The final yeast harvest was pitched into a batch without any HyperBoost, to determine the level of aromatic compounds carried from the HyperBoost by the yeast. The standard beer as well as the wort and finished beer with no HyperBoost were analyzed via GCMS.  The data collected indicates that dosing with HyperBoost on day zero of fermentation does not significantly impact yeast viability and does not impart significant hop characteristics when repitched.  Across all yeast generations, the viability maintained an average of ~93% and never dropped below 88%.  All hop-derived aromatics were either not detected or detected below quantification thresholds in repitched beer.