Mega-Starter

Building a 12-liter stirred yeast starter

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.

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.

Links:

• GrabCAD Profile https://grabcad.com/greg.paterson-1/models
• Fermonster Stirred Yeast Starter CAD: https://grabcad.com/library/stirred-yeast-starter-1
• PDF Drawings & BOM: https://bit.ly/BYO-Stirrer

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)