Amylase Enzymes
TroubleShooting
Keith Bachman - Ozark, Missouri asks,
I just brewed a doppelbock (my first lager) with a target original gravity (OG) of 1.095. Knowing pitching rates are super important for lagers, I made a robust starter. I used two packs of Wyeast Munich Lager II (Wyeast 2352-PC) in a 2 L (2 qt.) starter of 1.040 OG wort. Once the 2-L (2-Qt.) starter was finished, I cold crashed my starter, then created another 2-L (2 qt.) starter of 1.050 OG wort. By my calculations, after the second starter I should have had about 820 billion cells. After transferring the wort to my carboy for primary fermentation, I oxygenated the wort with pure oxygen. I pitched the yeast and transferred the carboy to my controlled fermentation chamber set at 55 °F (13 °C). After two weeks in primary the gravity was at 1.050 and stayed at 1.050 for the next three days. Fearing a stuck fermentation, I read various ways to fix it and continually came across using amylase enzyme. Would amylase enzyme help? Should I make another starter and re-pitch?
At first glance, wort fermentability could be the problem with this brew. It seems that plenty of healthy yeast was used, the wort was well oxygenated and fermentation conducted at a very comfortable range for this yeast strain. One would expect a healthy fermentation from 1.095 to about 1.025–1.030 in two to three weeks. Having fermentation stop at 1.050 could certainly be an indicator that something is unusual with wort fermentability. And adding amylase enzymes could remedy this sort of issue. But my gut feeling is that this is not a wort fermentability issue.
The enzymatic power of most malts these days, including the Munich and Vienna malt types typically used in doppelbocks, is consistently where brewers have little worries about mashes not converting or mashes not producing fermentable wort. This is especially true when using a two-step mash at the temperatures used in your brew. My gut feeling is you have a yeast issue.
The rule of thumb pitching rate for lagers is 1 billion cells per Plato per liter. Assuming you brewed 5 gallons or 18.9 liters you need 435 billion cells (18.9 liters x 23 Plato x 1 billion), which is half the amount you calculated. OK, so you may have been a bit conservative in your target and aimed to overpitch. That is typically not the cause of a stuck fermentation. I am not sure that you grew this many cells, however.
There is another rule of thumb that I am going to recall here and that is the 10x rule when propagating yeast. The 10x rule means that yeast is moved up in volume by 10-fold increases. Assume that you grow up yeast in a 100 mL starter. At the end of a fairly normal growth cycle the cell density usually grows to about 100 million cells/mL. This assumes the culture is exposed to air through a cotton plug or the growing yeast is intermittently aerated during growth. The 10x rule means that the culture density is diluted to 10 million cells/mL after the transfer. When using this rule the batch size is usually 10 times larger than the last jump in the step when the gravity of the wort is around 1.040 (10 °Plato). In the case of high gravity beers the final pitch volume may change, but the propagation steps are about the same.
So what does this rule have to do with the price of doppelbock in Ozark? When yeast is put into a growth media and the idea is to grow cells, it is key that there is sufficient cellular building blocks for cells to actually reproduce. If yeast is grossly overpitched, reproduction may never occur and if cells do divide the daughter cells may not be healthy. To put this into animal terms, it’s a similar concept to underfeeding a large population of deer. Too many deer on a plot of land may very effectively devour all of the edible food in sight, but the individual health of the population may well be very poor.
Your starting point was a smack pack of yeast. Dilute this yeast slurry in 2 liters (2 qts.) of wort and you have a good starting point for healthy yeast growth. Using the 10x rule your next step should have been 20 liters (5.3 gallons) of wort, or a normal batch size of average strength wort. Instead you fed this slurry with another 2 liters (2 qts.) of wort and likely put your healthy culture into an environment that did more harm than good.
On top of that, you crashed your culture to help separate yeast from beer produced in the first propagation step. Unfortunately most lager yeast strains are really not very flocculent, and this is a dicey method to use. After your first step of propagation you had yeast that was healthy, vital and ready to munch on maltose . . . and then you stuck it in the cooler, putting the yeast in a quiet place. When you pitched this yeast into 2 liters (2 qts.) of wort it probably perked up very quickly because of the high cell density, metabolized all of the nutrients in the wort, increased in cell number to some degree by producing daughter cells that were most likely not very healthy. And when you pitched this culture into your delicious sounding doppelbock wort the stage was set for a stalled fermentation.
Kräusening your brew with about 2 liters/2 qts. (10% of the batch size) of a new culture of yeast in high kräusen may be the best thing to do. High kräusen occurs about 24–36 hours after pitching and is noted by the high crown of foam that grows above the fermenting wort. This is the stage of fermentation when yeast growth is maximal and is the best time to jump-start a stuck fermentation, or simply to carbonate a finished batch by kräusening. If your fermentation is stuck, you should see things pick up and the gravity begin to drive down towards your expected completion. If fermentation does not resume you may be playing around with adding enzymes, but I am betting heavily against that happening.
