Same lesson, with the figures in motion.
Agar & Slants — Full Protocol
The complete bench recipe for making malt-extract agar and casting both Petri plates and test-tube slants. The quantities come from one tested home setup and work as a proven starting point to adapt to your own equipment.
Materials
- Glass media bottle, 250 ml (e.g. DURAN, with volume marks)
- Precision scale (0.1 g resolution)
- Pressure cooker
- Funnel + coffee filters (used double)
- Light dry malt extract (DME / spray malt)
- Agar-agar powder
- Glass Petri dishes
- Glass test tubes with caps
- Metal test-tube rack
- Aluminium foil
- Spirit lamp / burner
- Microwave (for re-melting the agar)
1. Brew the medium (150 ml malt agar)
- Weigh 15 g light DME into the bottle and top up with water to the 150 ml mark. (= 100 g/L — the same wort strength as a yeast starter.)
- Pressure-cook the bottle for 15 min to dissolve and sterilise the wort.
- Cool the bottle in a cold water bath.
- Filter through double coffee filters and pour the clear liquid back into the bottle. (Removes protein haze so the plates set clear.)
- Add 3.6 g agar-agar (≈ 2.4%). Return the bottle to the pressure cooker.
- Pressure-cook for 20 min until the agar is fully dissolved.
The pressure cooker must reach true sterilisation temperature (~121 °C / 15 psi). Verify with autoclave tape that it fully changes colour — don't rely on time alone.
2. Sterilise the glassware
- Seat the Petri dishes in the metal rack and pressure-cook them ~15 min (low heat, ~5 dl water in the cooker).
- Fill test tubes with medium leaving at least 1 cm to the rim (so nothing spills once tilted ~20–35°). Screw the caps on loosely.
Caps must be only loosely screwed on — a sealed tube builds pressure and the glass can crack in the cooker.
3. Pour plates & set slants
- Re-melt the agar bottle in the microwave if it has set, swirling gently now and then.
- Let the bottle cool until you can hold it against your cheek without burning (~50–55 °C). Hotter burns you; cooler sets before you've poured.
- Work beside a spirit flame (the rising sterile air keeps dust off). Pour ~10 ml into each Petri dish. Let them set with the lid ajar until the condensation clears.
- For slants: after cooking, tighten the caps and lay the tubes at a 20–35° angle until the agar sets as a sloped surface.
- Store finished plates and slants upside-down in the fridge.
The agar boils over very easily in the microwave — heat in short bursts and watch it.
The sloped surface of a slant gives more growth area for streaking inside the narrow tube.
Re-culture the slant every three months. The yeast keeps living slowly in the fridge, and beyond ~3 months genetic drift can't be ruled out. Write the date on the tube — and freeze the strain in glycerol if you want to keep it for years.