Building a 12-liter stirred yeast starter

12-L stirred yeast starter in use
Photos by Greg Paterson

As my homebrewery has expanded and evolved, I’ve encountered challenges in making yeast starters for my favorite liquid yeast strains large enough to pitch into my wort batches. At $15 to $25 per yeast pack here in Canada ($11-$20 in the U.S.) in 2023, a 60-L (16-gal.) standard strength beer could cost me up to $75 (CAD). Sachets of dry yeast are easier to scale up inexpensively, so this isn’t much of a concern for standard ale and lager yeasts.

My first magnetic yeast stirrer and 2-L Erlenmeyer flask was a good start. The stirrer top was smaller than the glass base of the flask, and that was unstable. I scavenged the electronic components into a larger flat enclosure (Polycase WA-42) and upgraded to a 5-L flask. This was more than adequate for pitching yeast into my 40-L (10.5-gal.) batches of standard strength beers — but what was I to do for my tripels and imperial stouts? On more than one occasion I’d brew a standard strength beer (single, stout, ale, etc.) as the full-batch starter and re-pitch my little yeasties.

With my recently built electric brewery pumping out 60-L (16-gal.) batches (sometimes with double batches in a parti-gyle brew day) replicating the magnetic stirrer option seemed silly, and flasks seem to top out at 5-L (at least easily attainable versions). Brainstorming and engineering design mode: INITIATE!

The plan was for a direct-drive, work-gear DC motor, with an attached paint stirrer, all mounted inside a 12-L Fermonster PET carboy. I built my design in Autodesk Inventor CAD software and links to all the files can be found at the end of this section.

The plan was for a direct-drive, work-gear DC motor, with an attached paint stirrer, all mounted inside a 12-L Fermonster PET carboy.

Having built the thing, here are a few lessons from hindsight. Any holes in the lid need to be sealed with an O-ring, a bung, or silicone. The fermenter needs to have a slight positive pressure to force CO2 out though the airlock. Any leak will make it seem like your yeast is dead (trust me). Additionally, my stirrer tends to wobble when in operation and is noisier than expected. If I restarted this project, I’d consider using a more rigid transmission system that is sealed, like a through-wall gearbox. This would also help prevent any cross-contamination of sanitizer into the motor or motor lubricant into my starter. A metal plate to reinforce the flat of the lid would help as well. Here’s the build process.


Tools and Materials

  • 12-L Fermonster carboy with lid
  • Stainless stir rod (40 x 8 x 300 mm)
  • Shaft coupling (8 x 8 mm)
  • Motor worm gear “C” (24-V 260-RPM)
  • Electric enclosure (minewas 3-D printed)
  • Motor enclosure mount (3-D printed)
  • Power supply (24-V 1A)
  • DC plug
  • Grommets
  • (6) Button-head socket cap screws (M5 x 0.8 x 18 mm)
  • (1) Socket-head cap screw (M4 x 0.7 x 18 mm)
  • (1) Socket-head cap screw (M4 x 0.7 x 8 mm)
  • (1) Socket-head cap screw (M3 x 0.5 x 6 mm)

1. Mechanical Design

I selected my motor based on the lid size and threaded hole layout. Next, a shaft coupling is needed to join the shaft to the motor, as well as to make it easy to disconnect for cleaning. Shaft length must be less than the distance from the coupler to the bottom of the fermenter, remember that it is okay to shorten a longer stirring rod, and that the stirrer must fit into the opening of the carboy.
The motor I selected also had threaded holes on the side opposite the shaft, so I elected to mount the electronics enclosure to the back side.

2. Controls Design

As I was trying various Polycase mini-enclosures in the CAD, I realized I’d be able to 3-D print a custom-sized enclosure and motor mount. I designed the enclosure with spacers to hold the pulse-width modulation (PWM) control board off the bottom surface and left space for the various electrical connections inside the enclosure. With the cylindrical motor adding some complexity, I designed and 3-D printed a motor mount for the enclosure. Figure 2 on the right shows a few important details, highlighting the motor-enclosure mounting piece. You can see the stainless steel button-head cap screws through the Fermonster lid restraining the motor (also seen in Figure 3b below), the four socket head cap screws holding the enclosure to the mount, and the two long socket-head cap screws that mount both to the motor. The balance of the visible fasteners hold the control board inside the enclosure and seal the lid of the enclosure. Links on the previous page provide a more detailed view of the components and their layout in this design.

3. Drilling & Milling

Start by drilling your six holes in the Fermonster solid lid. One hole for the motor shaft (9-mm clearance for my motor). Note that you could also undermount the shaft stack, and seal around that protruding ridge. Four holes for the motor mounting screws. Now you can test fit the motor and drill the final lid hole with room to install the airlock. The PDF drawing package (Figure 3a on the right) has a 1:1 scale drawing you can use to mark your hole locations.

My 3-D printer is a FLSUN QQ-S Delta-type. I bought mine on Amazon for around $500 (CAD), and a 1-kilogram roll of material is roughly $20. My preferred material is polylactic acid (PLA) and my backup is polyester (PETG). The PLA material I have found to be very robust and reliable with clean prints. When using PETG, my prints are stringy and often have rough surfaces. The enclosure, lid, and motor mount were all simple prints as designed with only a bit of hole clearing needed post-print. Note that you don’t need to thread holes for your screws, as they tend to self-tap into the plastics if your hole inner diameter matches the minor pitch of your threads. Two holes need to be drilled for the DC pin connector and motor wiring grommet. For ease of design, I always recommend the metric system.

4. Wiring

Wiring is a snap with this one if you’ve followed me so far. The control panel came with the motor, so I just needed to supply 24 volts and connect the motor. Remember that in DC wiring red is positive while black is negative. Starting at the motor, pass the two lead wires through one of your drilled holes in the enclosure with the grommet installed (Figure 4a on the right), and connect it to the two terminal blocks marked “motor.” The switch and potentiometer come pre-wired (Figure 4b below), so just pass them through the cutouts in the enclosure lid and reconnect them as needed. Two new wires (18-gauge) are needed to connect to the supply terminal blocks, so strip them and you’ll have to solder them to the through-wall DC-pin connector. I selected a standard 5.5-mm diameter, 2.5-mm length power barrel connector, as this is what I use for all my small electronics projects.

5. Finishing & Testing

Based on this design the order of operations for install is:
1. Fasten the motor to the lid
2. Install the enclosure body to the motor mount
3. Fix the enclosure and motor mount to the motor
4. Batten down the control board into the enclosure
5. Connect all your wires
6. Install the switch and potentiometer into the lid
7. Bolt on the enclosure lid
8. Install the airlock
9. Invert the assembly and install your shaft coupling
10. Insert the shaft into the coupling and tighten securely
11. Put some water into the fermenter (don’t run the motor without resistance)
12. Turn the potentiometer all the way counter-clockwise (zero speed to start)
13. Plug in the power supply and the power barrel
14. Turn on the switch
15. Set the speed to keep the yeasties moving
16. Congratulate yourself on a job well done

6. Starting Your Starter

No need to disassemble everything unless maintenance or changes are needed. Just empty any liquid from the airlock, unscrew the Fermonster lid, and the whole stirrer assembly lifts out. You can rinse as-is or remove the shaft coupling for a more thorough cleaning.

Sanitize all the wet surfaces, the shaft, shaft coupling, and your airlock. Pour in your starter wort, yeast nutrient, and yeast. Screw on the lid, plug in the power, and switch it on. An anti-foaming agent, like FermCap-S, is recommended to make sure electronics remain dry and foam-free.

You’re one step closer to flavor country. Cheers!

Issue: September 2023