Culturing Yeast and Using Slants

By | April 12, 2011

Acknowledgments

Information contained here was retrieved from various internet sources, such as the Home Brew Digest, the Usenet newsgroup rec.crafts.brewing, and the Yeast.FAQ, compiled by Patrick Weix. Particular acknowledgement is due to Jack Schmidling, whose r.c.b. post on this subject motivated me to finally get going on doing this. Note: I use metric units throughout this document–sorry, you’re just going to have to live with it!

Contents

I. Overview
II. Equipment you need
III. Materials you need
IV. Preparing blank slants
V. Inoculating the blank slants with yeast
VI. General comments

I. Overview:

Some of what you gain from doing slants is a) indefinite (practically speaking) storage, b) assured maintenance of the original generation, and c) ease of sharing yeast with brewpals. Also, when it comes time to make up a starter to pitch into a batch of beer, you get 500 ml of active starter within 24-36 hours every time. Finally, if you are doing slants, then when someone sends you a sample of yeast, or you get one from some other means, you can make yourself a renewable lifetime supply from that sample.

In the descriptions below, click on each thumbnail image to bring up a larger-sized version of the photo.

II. Equipment you need:

  1. A bunch of glass vials or test tubes that have caps that can a) withstand temps of 100 deg. C and b) form a tight seal. I started with 30, and I think that is plenty, because as you proceed, using the yeast in them, you just reculture into the spent ones as necessary. I use flat-bottomed glass vials, capacity about 50 ml, because they are easier to handle–you don’t need a test tube rack. I got them in Sydney–it’s a long, odd story–but you should be able to get them from some kind of scientific supply outlet. Check the yellow pages under “laboratory equipment”. My 30 vials + caps cost me A$25, roughly 80 cents per unit.
  2. A dish made of something like pyrex, that can also withstand boiling temperatures without exploding!
  3. Something to use for your starter vessel, like an old-style milk bottle, an Erlenmeyer flask (that’s what I use), or other glass vessel that has a mouth to which you can affix a rubber-stopper + airlock.

The other hardware you will already have if you brew beer: a scale, big pot to boil in, kitchen stove, refrigerator, spoons, etc etc etc.

III. Materials you need:

  1. Either gelatin or agar-agar, available at some Asian food stores. This is the growth medium. I use gelatin and it works just fine–easy, cheap, always available. I’ve used both, and the only difference is that agar has a higher melting point than does gelatin, so that if you live in a hot climate it’s probably the better bet.
  2. A bag of dried malt extract. One bag will last you the rest of your life as far as keeping a full supply of yeast slants on hand is concerned.
  3. A bit of ethyl alcohol. 250 ml will last you for years of culturing use.

That’s it.

IV. Preparing “blank” slants:

Now, the assumption here is that you are culturing from a pure source, like a Wyeast pack, one of your bottled beers, or a slant that someone sends you. If you are culturing, say, from a bottle of commercial bottle-conditioned beer, extra steps are required to isolate pure cultures (bottling strains are rarely pure). This is much more involved, I have never done it, and have no intention to any time soon, so I will skip it here.

The first step is to make up a bunch of slants to use. It is very easy:

  1. Bring 1 cup (about 250 ml) water to a boil. Remove from heat, add 15 grams of dried malt extract, and stir till dissolved. Put back on the heat and boil for 5-10 minutes to ensure sterility. Remove from heat.
  2. Pour a packet of gelatin (15 to 20 grams) or the equivalent weight of agar-agar into this “wort” and stir till COMPLETELY dissolved. Now pour this mixture into as many of your vials/test-tubes as you can; a small funnel is useful for this step. Fill the vials about 1/4 full– do NOT fill them all the way up. Keep at least one vial empty for use in the next stage–see below. I typically fill about 15-20 vials with these amounts.
  3. Now place the pyrex dish mentioned above inside a large pot that has a lid (like your brewkettle). Place your partially-filled slant vials in the dish. Here is one place where having the flat-bottomed vials makes life easy–just stand them up in the dish. (If you have test-tubes, you will need a rack to stand them in, and the rack must sit in the pyrex dish. If your rack is the right shape and size, you may be able to omit the pyrex dish.) Put a couple cm of water in the pot, outside the pyrex dish–be sure it doesn’t come up over the dish’s lip. Crank up the heat so that this water boils (full power will probably not be required), and keep it boiling for about 20 minutes. If you wish, you can put the vial-caps in there too, or just sterilize them with your favorite chemical agent–doesn’t matter. My usual procedure is to just toss the vial caps into the water that is boiling. What is happening at this stage is that the steam from the boiling water is sterilizing the vials, the growth medium, and the caps (if they are in there too). Leave the lid on the pot with a gap to allow the steam to readily escape. If you have an autoclave or pressure cooker, so much the better– use that by all means. But simple steaming seems to have the desired effect by itself, and it’s very easy to do.
  4. Now turn off the heat, and have a couple homebrews while it cools off. What you have at this point is sterilized vials + liquid growth medium. You need to wait for things to cool to at least 40 deg. C before attaching the sterile caps, otherwise the cooling growth-medium will cause the vials to either suck the caps into the vials, or actually implode. Once cool enough, put the caps on the vials firmly. You are now out of the woods as far as sanitation is concerned.
  5. Now you must cool the vials while placing them at an angle of about 40-45 degrees. I do this by standing the vials in a box or box-lid (one with a good tall lip around it, and propping an end up at the appropriate angle. When you do this, the surface of the still-liquid-but-cooling gelatin + malt extract will of course stay horizontal. Let the vials sit like this for 24 hours, after which time the gelatin + malt will be as solid as it gets (which is still a bit soft and yielding–ideal for this purpose). After cooling, the surface of the medium is at the angle you propped the box-top up to–that is, it’s “slanted”, hence the name. These are now ready to be “inoculated” with cells of your favorite yeast. Store them in the fridge until you are ready to inoculate them.The time to do these 5 steps is no more than an hour or two (depending on how many homebrews you have :-}).

V. Inoculating the blank slants with yeast:

OK, so now you have a bunch of slants. At this point, the procedure depends on what the source is for your yeast to be cultured. I’ll describe doing it from a packet of Wyeast, and then comment on variations used for other sources.

Culturing your slants from a Wyeast pack:

Before you start, you should lay out your working area in an organized way to minimize having to get up and down, reach long distances for things, etc. Sort out your worldly affairs before beginning–i.e., take a leak, feed the dog, take out the rubbish. Then wash your hands thoroughly, and begin. Have your slant vials, an unwrapped paper clip or long needle, a cotton ball or folded up paper-towel, your vial of ethyl alcohol, and your starter vessel laid out on clean paper toweling, along with an empty, unused slant vial that has been sterilized, along with its (also sterilized) cap.

  1. As per usual procedure, get your Wyeast ready to make a starter, i.e. by breaking the inner packet and waiting a day or two for it to swell up.
  2. Stop breathing. Shake the Wyeast pack well, and then open it using standard procedures. Pour a tiny bit of the Wyeast solution into your empty and waiting vial, and pour the rest into your starter. Cap your vial, and attach your airlock to your starter. Resume breathing. Your starter will be ready to use the in the batch of beer you will brew tomorrow or the next day; the small quantity of solution will be used to inoculate slants right now.
  3. Now repeat the following sequence of actions for each slant vial you wish to inoculate. First, take a deep breath and hold it. Wipe your needle/paperclip with alcohol-moistened cotton/paper towel to sterilize it. Open the slant vial to be inoculated. Then open the vial with the bit of Wyeast solution in it. Dip your needle into the Wyeast, and then *lightly* poke it into the surface of the gelatin + malt growth medium in the slant vial. Poke it in a bunch of places. Smear it around. Just try hard not to touch the walls of the vial. When done, withdraw the needle, cap the slant vial, and cap the Wyeast vial. Exhale. You now have a fully inoculated slant vial. Keep doing this for as many vials as you wish to inoculate (see comments below).The total amount of time to do these 3 steps is about 20 minutes for 10 slants, maybe a little more the first time you do it, till you develop the knack.
  4. When done, leave the vials out at room temperature (20 deg C) for a week. Within a couple of days you will see a cloudy film on the slant surface, and a few days later it will develop into a milky white layer about a mm thick. Sometimes, depending on the strain of yeast involved, the ambient temperature, and the richness of your growth medium, the CO2 evolved from the yeast growing on the slant surface may begin to push the cap up off the vial. No big deal–just bleed the gas out by cracking open the cap for a moment, and press the cap back down firmly. After the week is over, wrap the tops of the vials (where the caps meet the vial walls) with electrician’s tape, and put the vials into a ziplock bag and pop them in the fridge, where they will keep for at least 3 months in a perfectly viable condition. Obviously, if you are keeping more than one yeast type around, you will want to label the vials somehow–masking tape works perfectly for this.

VI. General comments:

  1. How many vials should you inoculate at a time? I usually do 3 or 4 vials of a given yeast type in a session, because I rarely use the same yeast 4 times in 3 months. When 3 months is nearly over, I simply reculture the strain by doing the above to 3 or 4 new slants, but using an “old” slant as the source instead of a vial with a few ml of Wyeast solution in it. Otherwise the procedure is identical. Reculturing in this way does not increment the strain generation-number because the yeast have not made enough copies of themselves for mutation to occur.
  2. If culturing onto slants from a bottled-beer source, I would make a small-volume starter and wait till that is fully active, then pour off most of the liquid, swirl around what is left, and put several ml of that into the empty vial as the source. WARNING! Many types of bottle-conditioned beer use either a different kind of yeast in the bottle than they fermented the beer with, or it is the same kind but has mutated, or any of several other possible complications. If you culture from a bottle of commercial beer, *taste* the small starter you make from it when inoculating your slants. If it tastes good (or at least, not bad) you are probably OK. But you should still test the yeast on a small, pilot batch of beer before committing your entire batch to it. Even when using a bottle of your own homebrew, things can happen, so I would recommend these cautions in that case as well. Caveat brewor.
  3. Making a starter from a slant: Before starting, be sure to let your slant sit out for an hour or two so it can SLOWLY come up to room T from fridge T. Now make a starter as usual. After boiling up your starter mini-wort (I usually use a 1040 wort, which I get by boiling up 50 gr of dried malt extract in 500 ml water), cool it, and then pour a bit into your slant vial (a sanitized funnel is useful here too). Swirl it around to loosen as much yeast as you can. Usually not all the yeast comes off the slant surface by simple swirling, so have your needle and alcohol ready in this case. You just wipe the needle sterile with the alcohol, and then gently scrape the slant surface till the yeast comes off (don’t worry if some of the gelatin tears loose– doesn’t matter). Then pour that into your starter vessel (along with the rest of the mini-wort of course) and attach the airlock. At 20-22 deg. C, the amount of yeast on a typical slant will produce a vigorously working starter in 24-36 hours; a little more time is required at cooler temperatures. You may wish to step this initial starter up to a larger volume, say 1.5 litres, a couple days later, which is not a bad idea. When pitching the yeast from the starter, pour in most of the liquid part, and then swirl your starter vessel to ensure that all the yeast cells that collect at the bottom of the vessel are suspended in the remaining liquid; then dump that into your fermenter as well. In my brewing, starters produced in this way give good starts–I’ve never seen a lag time longer than 12-15 hours (in cooler weather), with 8 hours being the norm.
  4. Things that can Go Wrong: The most common problem in making your own slants is that mold can take hold on the slant surface. When this happens, it is absolutely unmistakeable–a big greenish fuzzy patch just grows like wildfire. Simply ditch that slant–it is hopeless. This is why you should always make at least three new slants when propagating from an “old” one–so that you will be more likely to have one pure one if things go wrong. It is also possible for bacteria to get a foothold too. In my experience, this has not happened so far. At first, I tasted every starter I made to be sure, but after 50 times with no problems I gave up doing this. As long as the slant surface looks “good”–i.e., a nice milky layer of yeast, no other colors or shapes–you should be fine.

The simple procedures outlined here should allow you to keep essentially limitless supplies of every strain you use into the indefinite future. The amount of time expended to have endless pure yeast on hand is a couple hours per strain, and this brings your cost per batch for yeast very quickly to essentially zero. And hey–I LIKE being a yeast rancher! I look in on ’em all the time, talk to ’em (“Hi boys, I’m home!”)…

Original article is available on the web here: http://home.comcast.net/~david.s.draper/beer/slantuse.html

© 2007 Dave Draper, david at draper dot name
First written in Sydney, Australia, January 1995
Minor revisions: May and July 1997, Dallas, Texas
Major revisions October 2006
Last updated 15 July 2007