Building a population from yeast slants

I think I am following your plan and will rephrase so that what follows is clear. You want to buy a single liquid pack, “borrow” a bit to prepare 7–8 slants, and use the balance for your first brew. When you go to brew again, you will take a slant, make a propagation, and then pitch. This is the part that is a bit unclear. Slants are typically used multiple times where an inoculation loop is used to transfer a bit of yeast into a flask with wort or onto a Petri dish. You could use fewer slants, but I will roll with one slant for each brew and offer a hack on saving a step when using the slant.

The short answer is yes; you have a solid plan to reduce yeast costs per brew. For what it’s worth, you can also make slants by starting with a slurry made from dried yeast and treating the same as a liquid culture. This answer may be all the information you need to confirm your plan, but I will take the opportunity to provide more information about what this method looks like to brewers who have not made slants and who may want to give this a try.

Let’s start by defining a slant. In micro jargon, a slant refers to some sort of solid growth media prepared in a test tube that is allowed to solidify at an angle. The growth media is usually made by purchasing powdered media that include nutrients, agar (a carbohydrate that forms a gel after boiling and cooling), vitamins, and minerals. Specialty growth media may contain selective compounds that select for certain organisms, inhibitors that prevent growth of certain microbes, and/or indicators, such as pH indicators and stains, that help microbiologists understand more about what is growing on the media.

In the brewing world, slants are primarily used to store yeast for intermediate durations by applying cells to the surface of the slant with a loop, allowing a “lawn” to grow on the surface of the slant, then transferring to a refrigerator for storage. Slants are often covered with sterile mineral oil to prevent water loss from the media during prolonged storage; this method allows slants to be stored for many years without issue. Because the yeast applied to a slant is typically taken from a pure culture, the most common growth media is wort agar, a general-purpose media commonly used to grow yeast and mold.

A good rule used to determine propagation volume is for the final wort volume to contain 10% yeast slurry. Discussions about microbiology are always done using metric terms, so I will stick to milliliters and liters for clarity. In your case, you will want a prop volume of about 750 mL to pitch a 2-gallon (8-L) batch. I brew 5-gallon (19-L) batches and would need a prop volume of about 2000 mL. I round to convenient volumes because it’s much simpler to use increments of 100 or 250 mL when assembling basic lab supplies and these are indeed rules of thumb where close enough works well enough.

I like starting with the pitch volume because that helps plan the propagation schedule. The general method followed for yeast propagation is to use 1:10 dilution steps. In your case, that final 750-mL propagation volume is made by adding 75 mL of growing yeast slurry to 675-mL wort. A quick start to yeast activity is important when propagating yeast, just like it is during beer fermentation, because maintaining sterile conditions is impossible without specialized microbiological equipment and methods. This is true at home and in most yeast labs because of the transfer steps involved. The bottom line: Don’t make the first step too dilute because it risks the chance of growing unwanted microbes.

Adding a single colony from a Petri dish to 10–25 mL wort is a typical first step. However, slants have a lawn of yeast, not single colonies. This works to your favor because you can make your first step from slant to wort into 75 mL by picking up the equivalent of about one peppercorn-sized “scoop” of yeast from the lawn. At this point, you can save your slant for another prop or throw it away. You should see yeast activity in your flask within 1–2 days and will want to transfer the entire 75 mL into your 675-mL flask in two to three days. At the end of day five, it’s time to pitch into your 2 gallons (8 L) of wort.

For 5-gallon (19-L) brewers like me, our prop volumes are 200 mL and 2000 mL. Going from a slant to 200 mL is too big of a jump, so we need to start out with 20 mL. One way to do this is to make up a 20-mL starter in a flask. Another way is simply pouring 20 mL of wort on top of the slant. For this to work, properly sized test tubes (~50 mL) containing ~15 mL of media are required. This is the time savings hack mentioned earlier.

That’s about all the specifics I think are needed to answer your question, though key techniques omitted that readers should be aware of are media preparation, wort/media sterilization, proper use of inoculation loops, and proper transfer techniques. These are all critical for successful micro work. The good news is that none of these methods require much specialized equipment, and they are all relatively easy to perform at the proficiency required for success in the homebrewery.

Response by Ashton Lewis.