I’m wondering if you can help me with your A38 Juice yeast. It’s been taking longer to hit terminal gravity and some of our batches have been stalling. There have been no process changes, we’re doing everything the same. We’re brewing 20 barrel batches at 17.5ºP. Any thoughts on what might be going on?
Are these with fresh pitches of yeast, or with harvested yeast?
This happens with our harvested yeast and gets worse over time.
What is the target pitch rate, and how are you quantifying this? Are you adding oxygen during knockout? If so, what is the target ppm of dissolved oxygen (DO)?
We’re harvesting about 30 pounds for 20 barrels. We don’t do cell counts. I’m not sure what my pitch rate is. This is how our old head brewer set it up. We add oxygen during the first knockout. I’m not sure what the DO is. We go by sight through the sight glass.
Is this a single 20 barrel batch, or multiple knockouts?
Multiple knockouts. We do one 10 barrel turn and then another 10 barrel turn the next day.
Okay, thanks. When we see issues like this it’s usually due to low viable cell counts. When re-pitched, the issues compound from generation to generation. We need to pin-point where this may be coming from. In multi-knockout situations, we typically recommend pitching enough yeast for the volume of wort brewed on the first day. I’m wondering if 30 pounds is achieving a pitch rate that’s in the ballpark for the first 10 barrels? It’s difficult to say for sure without quantifying this under a microscope, but we can estimate it.
We typically recommend a pitch rate of 0.75 million cells/mL/ºP. To achieve this pitch rate for 10 barrels of 17.5ºP wort, our online Pitch Rate Calculator indicates 12L of yeast. This calculation uses Imperial Yeast’s viable cell count. Since you’re using harvested yeast, your viable cell count may be lower. To be safe, let’s err on the low side and assume ~10%.
To double check your pitch size, we know that 1L of yeast slurry is 2.2 pounds. So, a 12L pitch would be 26.4 pounds. If we add 10% slurry to make up the possible difference in viability, then this comes out to 29 pounds of harvested yeast slurry. For 10 barrels, 30 pounds is probably in the ballpark.
Since the pitch size is close, we should think about oxygen. A38 performs best with 20-25 ppm of dissolved oxygen. It’s difficult to say what the DO levels are without measuring it, but adding O2 to the first batch alone probably isn’t enough to achieve proper cell growth once the second batch is added. I’d recommend adding oxygen to both batches. I’d also recommend using this O2 Flowmeter and DO Calculator to help dial in the specific amount of oxygen necessary to achieve the recommended DO level. This is just an estimation, but it should help get you close.
I have one more question. Do you see excessive overflow during blow-off?
Yes, we see quite a bit of material coming out of the blow-off.
This might be part of the issue as well. A38 is a chain forming strain and is capable of climbing out of the FV, literally losing yeast cells to the floor. Rather than issues with viability or growth, there may just not be enough cells left in the FV to attenuate the batch fully or to harvest at TG. I’d recommend adding a bit of antifoam to the fermenter when pitching the yeast. This shouldn’t impact head retention, flavor, or aroma, but it should help keep yeast cells in the fermenter.
Together, making some adjustments to oxygen during KO and adding antifoam during fermentation, should help increase (and maintain) the overall cell count, improving the fermentation times and stalling issues.
Thank you so much for this information! We’ll look into the O2 flowmeter and antifoam. We’ll keep you posted.