For selection, we need to grow yeast on a solid media in a Petri dish.
This solid media is obtained by dissolving agar into wort (which provides the nutrients).
The basic property of agar is that it melts above 95°C, and remains liquid until cooled to about 45°C.
Thus you can pour the liquefied media into the plates and let if solidify.
Instead of pouring many plates at once, I store the solid sterile media in small 100 ml bottles, sealed with a rubber closure.
When the need arises, I simply heat a bottle until media liquifies, then pour it in 5-6 plates.
The quantity of agar needed varies greatly, depending on the source, grade and form of the agar you use, so you need to experiment a little.
Too much agar and the surface becomes too hard, too little agar and the surface remains too soft and you risk cutting it with the loop when you streak.
I use flaked agar from health food stores, and 15 grams dissolved into 1 liter of wort works nicely for me.
Prepare the media
Take a jar of canned wort and pour it in a clean pot, leaving the trub behind.
Add the agar and mix
Allow the agar to soak for 10 minutes
Apply heat and bring to boil, stirring until agar is completely dissolved. Beware, the mix has an astonishing boil-over potential
Pour the hot liquid into 100 ml bottles and place rubber stoppers. During sterilization, pressure develops inside the bottle and the stopper will pop out. To avoid this, insert a piece of cotton string between the the rubber stopper and the bottle mouth. This creates an airway to release pressure during sterilization
Cover the bottle closure with aluminum foil
Place bottles in pressure cooker and autoclave for 20 minutes at 15 psi (1,05 kg/cm2)
When done, remove from heat and let the pressure cooker cool until you can open it safely
Remove the bottles, pull out the pressure relief strings. The strings are now wet, and should come away with no effort
Allow to cool then store in refrigerator
Pour the plates
The usual precautions apply: scrub the table surface and your hands with alcohol. Wear a mask, keep your work area as dust-free as you can.
Since you need to work as quickly as possible, you should practice with water and empty Petri dishes ahead of time, to gain the necessary dexterity.
Put a bottle of solid media in a water bath in a pot and apply heat until it melts
Allow to cool to about 50-55°C (monitor water bath temperature)
In the meantime, take 6 sterile Petri dishes and place them on the table, cover on top, smaller media half on bottom
When the temperature of the bottle reaches 50°C, quickly proceed as follows
Hold the bottle in your right hand, agitating continuosly to keep the temperature uniform and avoid partial solidification
Remove the rubber stopper and discard it
Flame the bottle mouth
Partially lift the cover of the first dish
Pour about 15-20 ml of liquid to cover at least 2/3 of the plate surface
Gently swirl the plate to spread the liquid media on all the surface. There should be a layer of about 3-4 mm of liquid media
Close the plate cover
Repeat steps 8 through 11 with the other Petri dishes
When done, wait until media solidifies without moving the plates. Having poured the media at a relatively low temperature (50°C), very little condensation will form on the inner side of the cover
Turn the dishes upside down (media on top, cover on bottom). From this moment on, plates should always be kept upside down
Incubate at 30°C for 24 hours in a dry, dark, clean, dust-free. Condensation will disappear
Check media surface for mold or bacteria growth, and discard contaminated plates
Seal with electrical tape and store in refrigerator or use immediately
Petri dishes with solid media keep for several weeks if refrigerated, but even if sealed with tape after a few weeks the agar releases water and becomes unsuitable for healthy yeast growth.
At times I have observed mold contamination, possibly due to spores from the sealing tape.
It might thus be advisable not to seal the plates, and simply stack them and store them in sealed sterile plastic bags.
Of course, the best solution is the one adopted by professionals, that is to seal the plates with Parafilm™ and store in sterile bags.
