Wednesday, 24 April 2013

Mailing Yeast

A yeast mailer loaded & read-to-roll
One problem faced by many yeast banks is the sharing of yeasts over long distances. Yeast plates, frozen cultures, slants, etc, can be expensive to send and are highly temperature/time sensitive. There is a better way!

Since the mid-1990's I've been using a simple & cheap method to mail yeasts. I cannot take credit for this - I think I first came across this method in rec.crafts.brewing (for the young whippersnappers out there, this is a usenet brewing group), or perhaps on the ol' home brew digest. It works very well, requires no special tools, is relatively resistant to swings in temperature, and relies on regular letter mail instead of expensive package mail or couriers.

The concept is simple - a foil envelope is built around a piece of absorbent paper. This packet is then sanitized, then liquid yeast is sterilely dropped onto the paper & allowed to dry. The foil envelope is then sealed & mailed. The recipient recovers the yeast (ideally) by placing it on a wort-agar plate and selecting the colonies that grow, or (less ideally) by placing the paper in a tube of media and letting the yeast grow out of that.

Details below the fold...


Preparing the mailers

Step 1: Cut a small piece of absorbent paper (1cm x 1cm is more then enough). Place it in a piece of tin foil large enough to form an envelope around the paper.

Here I have used laboratory filter paper, but any thick absorbent paper will do.

Step 2: Fold an envelope around the paper, making sure the paper is freely moving within the envelope. Use masking tape (or autoclave tape, if you got it) to seal the edges. Do not use a plastic tape like scotch tape.

Also make a tape tab (bottom of image) to hold the top shut. Make sure this tape tab will stick onto tape, not foil. Foil is too weak, and will tear when you try to open the envelope.

Step 3: Close and tape shut the envelope. We now need to sterilize the envelope. This can be done using an autoclave on the dry cycle, or by steaming 20 minutes in a pressure cooker. If using a pressure cooker make sure the envelope is in the steam, not in the liquid!

If you use a pressure cooker, dry the envelopes by placing them in your over at its lowest setting. Depending on the absorbency of the paper you use, they need to bake for 30-60 minutes.

Preparing & mailing the yeast

For best results some yeast preparation is required. The best place to get yeast from is a starter that has completed fermentation, although washed yeast can be used as well. In either case, take a small amount of the yeast (minimum is ~0.2ml) and place it in a tube in your fridge for one to two days (if using washed yeast that has been refrigerated, this step can be skipped). This will allow a sugar called trehalose to accumulate in the cells, which helps protect yeast from drying and temperature changes.

Today we're mailing Brettanomyces
Step 1: Get everything setup; you'll need a flame to create an updraft, your sterile mailers, yeasts (preferably after 1-2 days in the fridge) and one sterile pipette per yeast.

Step 2: carefully open the foil envelope Do not tear the foil. Squeeze the envelope lightly to tent it; if needed, use sterile tweezers to pull the paper into a position where it is accessible.

Step 3: Using the best sterile methods you can achieve, transfer a small amount of yeast to the paper. The correct amount is enough to wet half of the paper. Make sure to use a clean pipette for each transfer as to not cross-contaminate samples.

The paper will draw the moisture away form the yeast, rapidly dehydrating them - this protects them from the rigours of mailing.

Step 4: Keep the envelope open, near your flame, until the paper is dry. The wet region is visible in the middle of the paper.

DO NOT heat the paper to dry, as this will kill the yeast.
Step 5: Re-seal the envelope, place them (multiples can be sent at once) in a normal letter envelope, and mail it. If sending across national boarders you may want to include a statement (see example below) for the customs office declaring the contents of the envelope.

Note: Some places irradiate mail. The foil will not protect the yeast from this treatment. Be sure to avoid mailing yeasts to sites where mail is irradiated.

Sample Customs Letter:
This envilope contains brewers yeast sterilely deposited into protective foil envelopes. These yeasts are being shared between hobbyist home brewers and have no commercial value. These organisms are not hazardous to humans, livestock or plants. Shipment of these non-hazardous organisms is not regulated under the UN's Recommendations on the Transport of Dangerous Goods, nor under the transportation acts of the sending or recipient countries.
Note: letters such as these are not generally required for shipments within a country. If your shipping from/to countries other than Canada or the USA you should check the veracity of last statement.

Recovering the yeast using an agar plate

The best way to recover the yeast is to use an YPD or wort agar plate. These have the advantage of producing single colonies which can be picked, thereby allowing any contaminating bacteria to be excluded.
5 days later the yeast
arrived by mail
Step 1: Sanitize the surface of the packet using starsan or 70% ethanol. Don't soak the packet, as the sanitizing agent may enter the envelope and kill the yeast. Excess sanitizing agent should be dabbed off using a lint-free cloth. Next, working near a flame, open the foil envelope.
Wetted Brett paper on YPD-agar
Step 2: Working near a flame, remove the paper and place it on a prepared YPD or wort agar plate. Add a few drops of sterile water to the paper. Let sit 30-40 minutes, then remove the paper. The paper in the image (left) is about as big as you would want it to be (1.5cm x 1.5cm) - larger pieces should be cut smaller using sanitized scissors.

EDIT: we have found that placing a few drops of sterile water onto the paper at this stage, and leaving the whetted paper on the agar plate for at least 24 hours, greatly improves the success of the recovery.
Brett colonies after 2 days. A steady hand should be able to pick single colonies from the edges.

Step 3: Allow the yeast to grow overnight to a few days at room temp. You should get individual colonies of yeast. Pick a colony and grow it up using the same methods described for a starter. Keep the plate in the fridge, just in case there is something wrong with the colony you have chosen.
Streaking Method

Step 4 (optional): If the density of the yeast is too high, streak out the colonies on the plate using a sterile wire loop, sterilizing the loop between each streak (video):
  • Streak through the colonies (red line)
  • Streak through first streak (green line)
  • Streak through second streak (blue line)
  • Streak through third streak (pink line)
Each streak dilutes the yeast; eventually leading to single colonies
Single colonies can be seen at the bottom of the plate (click for larger image)
Plate after streaking (left). Individual colonies are visible at the bottom of the plate. Pick a colony and grow it up using the same methods described for a starter. Keep the plate in the fridge, just in case there is something wrong with the colony you have chosen.

Recovering the yeast using wort

This is a less ideal method of recovering the yeast, as it doesn't allow you to screen for potentially infecting bacteria. However, with proper attention to sterile technique on both the shippers and receives part, this method works very well. If your planning on doing this method, use a very small piece of paper - I would recommend the punched-out part of the holes created by a paper punch.  Use wort with a gravity of 1.020 fr recovery, as this provides a nutrient-rich environment which does not osmotically shock the yeasts.

The risk of infection can be reduced by adding penicillin (100U/ml) and streptomycin (100ug/ml) to the wort - if you have them.

Brett-soaked paper square

Step 1: Open the packet as described in step 1 of the agar plate method

Step 2: Using tweezers which have been sterilized by heating to red-hot in a flame and then cooled by shaking in the air (obviously, do this in a clean, low-dust room), remove the paper and quickly transfer it to a prepared flask/tube of wort. For a piece of paper the size of a hole-punch, 10ml of wort is sufficient.
Before (left) and after (right)
growth in wort
No Infection!
Step 3: Let grow overnight, or until media is cloudy with yeast.

Step 3.5 (Optional): If you have access to a microscope, it is a good idea to check the culture for signs of bacterial infection.

Step 4: Pour off wort into a larger starter, leaving the paper behind in the culture tube.

That's it! Using nothing more than a few cents worth of materials plus a stamp, you can send yeast nearly anywhere in the world. And let me point out at this time that I am very interested in sharing my yeasts with other yeast bankers around the world. So lets get mailing!


  1. nice write up. I have traded a few strains but it was always slants. Luckly I have access to small coolers and cold packs but this seems like a nice way to do it. I have never made the plates but could you use small strips on slants and pick colonies off that?

  2. I see no reason why that shouldn't work - so long as you've got a sterile source of wort - tube, slant or plate - it should work. The only limitation with the slants is that you cannot streak out the resulting colonies, meaning there is no way to separate the yeast from any infecting bacteria that may be present.

    That said, so long as you use sterile techniques, infections should not be an issue.


  3. In the absence of a pressure cooker or an autoclave, is there anything that I might have in my kitchen that I could use to sterilize these foil packets? Could I just put them in the oven at a low temperature setting for an extended period?

    1. Nevermind - I just went out and bought a pressure cooker! :)