Condensation forming on agar petri dishes is a pain in the backside, the main issue I’ve found is that it obstructs my view of the plate. Condensation forms from temperature differences, try breathing onto a mirror and you’ll notice the warm water vapour from your breath condenses on the cold surface.
The same theory is true when pouring agar. The warm agar contacts the not-warm petri dish, a lid is put on the dish and the warm air condenses against the colder lid. Agar sets below 40c and melts around 85c. There is only one real trick to keeping the condensation down when pouring petri dishes with agar and for only 3 easy payments of $59.99… I’m kidding.
The trick is as simple as you think, reduce the temperature differential between the agar and the petri dish. I always try to pour my agar at around 40c, checking the temperature with the use of an infrared thermometer. You can get the same one I use, here.
I also use a small heater in my lab. I put this on about half an hour before I do any agar work to bring up the temperature of the room and petri dishes. I include a stirrer bar or marble in the bottom of my agar bottle before I sterilise it, since heat rises, the agar at the top cools slower than the agar at the bottom.
I periodically check the temperature and mix the agar using the bar or marble until it reaches the golden 40c. You could use a Sous Vide or a magnetic stirrer with a heating element in it. I then pour, from no great height, wait a few seconds for the initial evaporation and then put the lid on. Clean and clear petri dishes ready for me to contaminate.
Another commonly practiced technique is to take a jar, something wider than your petri dish and fill it with hot water and once you’ve poured your stack of dishes, put the jar of hot water on the top dish and leave it all to cool. This method does prevent condensation on the top petri dishes, but I find it doesn’t travel down very far so if you’re pouring a lot, you need several smaller piles and several jars. Alternatively, you could pour the agar while hot and leave the lids off, allowing the agar to set without the lid, a higher risk of contamination as it’s ‘exposed’ longer, so be extra vigilant if your aseptic technique isn’t great. However, this is a guaranteed way of a clear lid.
If you have an ultra-excessive amount of moisture then under sterile conditions, take the dish lid off, pour off the excess and wipe away any residue with a sterile absorbent like this.
Whichever method you choose, you’ll still get a little bit of condensation forming around the rim. Once sealed, invert your petri dish stack and store inverted until use. The agar will re-sorb some of the moisture from the lids and sides and should remain clear. To avoid further condensation during colonisation and to easily control the climate of my dishes, I recently bought a reptile egg incubator, this is a great way to keep your cultures at a consistent temperature year round, since they heat and cool, and prevents condensation forming from differences between the inside of the dish and outside.
However, is condensation really that big of a deal? In my experience not really, I’ve had dishes with a pool of water colonise just fine and I’ve had dry pristine dishes contaminate, contamination is all to do with the aseptic techniques and culture quality. Condensation does make the culture harder to see and it does look awful when you want to share that beautiful culture!
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