The intend was to enhance the yeast propagation on the stir plate by being able to aerate the starter beer during yeast propagation when the Kraeusen would get in the way of aeration though the vortex of the stirred starter.

and

Since my approach was working for me I had to check what’s going on here. I thus tested water, a 20 Plato sugar solution and a 20 Plato wort with both the hydrometer and my refractometer. All solutions and the refractometer had the same temperature.

hydrometer in water	This is the hydrometer in water. I have a table that I use to correct the hydrometer reading in Plato for temperature and the slight offset that the hydrometer has
water on refractometer	water measured with the refractometer
hydrometer in 20.4 Plato sugar water	The sugar water (~40g table sugar, 160g water) read after correction 20.4 Plato
hydrometer in 20.4 Plato wort	The wort read after correction 20.4 Plato as well
20.4 Plato sugar water on refractometer	In the refractometer the sugar water read 19.4 Brix. Since refractometers are calibrated for sugar water, it should have read 20.4 Brix
20.4 Plato wort on refractometer	In the refractometer the wort reads 19.5-19.6 Brix (remember that water read slightly under the zero-line). To get the hydrometer reading I have to multiply this reading with 1.04

The reason why my correction factor is different is that the scale on my refractometer is off. I.e. there is already a conversion factor for sugar water that is not 1. But since I checked my refractometer against my hydrometer at various wort gravities and found that Plato is Brix * 1.04 I can still use this refractometer in brewing.

BTW, the ATC of this refractometer is also broken. For every brewing session I have to re-calibrate it with water. Since this is done very quickly, it doesn’t bother me too much.

Conclusion:

If you buy cheap gear, check its calibration.

In my brewing it takes a bit longer than a week for the beer to be done with primary fermentation. It also doesn’t reach final gravity as fast as it was shown in those papers. To make maters worse, fermenting the last few fermentable sugars until the attenuation is close enough to the attenuation limit always tends to take longer than I want it to.

To speed things up I started holding the “Diacetyl rest” rest around 20 C. I intentionally put diacetyl rest in quotes since for most of us the primary benefit of this rest is not diacetyl reduction but speeding up yeast’s consumption of those last fermentable sugars.I prefer to call this rest a maturation rest and may hold it for up to a week, if the beer is still too far away from its attenuation target. (If you don’t know how to determine final extract or gravity target of the beer, check out the Fast Ferment Test.)

Warm fermentation and lagers doesn’t seem to go together, but don’t worry. Low fermentation temperatures are only needed during primary fermentation when the yeast is actively growing and its metabolism shows activity along many different pathways that can leak excessive esters and higher alcohols if the temperature is too high. Once the yeast is done growing and all nitrogen sources have been assimilated, the fermentation temperature can be raised without the risk of creating the flavor compounds that are generally associated with high temperature fermentations.

keg with heating pad and temperature sensor

I do these maturation rests in kegs for a number of reasons. First, I can easily heat them with a heating pad controlled by a temperature controller. Second, I can use this fermentation to carbonate the beer. Lastly, the yeast will consume all the oxygen that is introduced during racking.

The image above shows how the heating pad (you’ll need one that doesn’t have an automatic shut-off) is attached to the bottom of the keg. At the top of the keg I attach the temperature sensor, covered with a piece of foam.

When the beer is racked to the keg, I make sure plenty of yeast is transferred as well. The kegs have a shortened dip tube, which allows for transferring the beer and leaving behind the yeast sediment later.

Finally the whole thing is wrapped in a blanket for further insulation:

Keg wrapped in a blanket

A pressure gauge is attached to monitor the pressure build-up and allow controlled CO2 release.

The experiment was a repeat of a standard experiment that evaluates yeast growth in still, intermittently shaken and stirred starters. I like how easy it is to set up and plan to repeat it again.

Setup

919g of 12 Plato wort were aerated to 100% air saturation on a stir plate. Then 7.8 g WLP 830 yeast sediment from a primary fermentation was added and allowed to un-flocculate completely. The cell density was determined as 22.3 Million/ml with a hemacytometer. If you do the math you find that the slurry contained a total of about 20 Billion cells which means its density was 2.63 Billion/g. Previous slurry density assessments for slurries from yeast propagation ranged from 4-5 Billion/g for this yeast. I assume that trub played a big role in the reduced cell count in this slurry.

Once the yeast was evenly distributed in the wort, the wort was evenly divided into 3 500 ml Erlenmeyer flasks. The flasks were labeled with their empty weight and covered tightly with aluminum foil. I didn’t tighten the cover intentionally. I noticed that this happens when I grab the flasks at their neck to shake them. I made sure that all of them had tightened aluminum foil caps, even though only one would be shaken intermittently.

All starters were allowed to complete fermentation. One sat still, one was shaken intermittently (1-2 times in the morning and 3-5 times in the evening) the last one was placed on a stir plate and stirred continuously. A vortex formed on the stir plate, but was eventually covered with foam when CO2 started escaping. The ambient temperature was around 18 C.

Data:

Once fermentation was complete the yeast was allowed to settle into a dense cake until the beer on top cleared. The beer was poured off and the flask’s weight with yeast and stir bar (in one of the flasks) was determined. From that, the empty weight and the stir bar weight the yeast weight was determined:

I only recorded the yeast weight and did not count the yeast with the microscope. Simply, because I did not want to spend too much time on this. To estimate the number of cells grown I assumed that the sediment had a density of 4.7 Billion/g, an average of the numbers I had assessed in earlier experiments.

Discussion and Conclusion:

The wort volumes for the three starters were not exactly the same, hence the difference in the extract weight.

I was surprised to see that the sill and shaken starters showed no difference in the amount of yeast that was gown from the available extract. This is different from the data reported by others (e.g. the great Maltose Falcon article on yeast propagation: Yeast Propagation and Maintenance: Principles and Practices) where a shaken starter grew significantly more than a still starter and a stirred starter outperformed a shaken starter by about 4x. The caption to the chart mentions that the data point for the still starter was taken from a different experiment, though.

Data I recorded on past yeast propagation steps (usually stirred)  and fast ferment tests (usually shaken) show more difference in extract utilization for yeast growth, but there are also a number of other parameters that have been different for those experiments. Most notably the starting gravity and the initial cell density.

I’ll have to repeat this experiment in the future to see what covering loosely with aluminum foil does and what an airlock would do to the extract utilization that can be achieved. And maybe I also find the time to count the final number of cells.

Kai

But this time I got the wort up to 98 C, which is hot enough to pasteurize it and I decided to go a different route – the no chill approach. While the wort was still hot I covered it with aluminum foil:

The wort sat this way over night on the deck and I moved it into the house in the morning. After 24 hrs the temperature dropped to 32C. During that time the pH fell from 5.27 to 5.14. I’m not sure if this is sign of an infection, but it smelled just fine. So I brought it to a boil. A lot of coagulation already happened in the hot wort and an oddly brown layer of trub developed during heating:

Other than that the boil seems to be going as normal.

I was also 9:30. Not enough time to rush and buy a new burner and I was also not in the mood of starting a wood fire and boil the wort that way. So I decided to cool the wort down and put it into my fermentation chest where it will be sitting at ~0-4 C until I can resume brewing it, which will most likely be tomorrow.

Yes, the beer that can be squeezed from the spent dry hops. Something only a true brewing geek knows to enjoy. Besides being murky like the Ganges in the rainy season, the aroma is pronounced but not overpowering. The bitterness, however is much harsher than the actual beer. It particularily lingers in the back of the throat, which I don’t appreciate too much.

Prost