Condensation in Agar Dishes

Reducing condensation in agar petri dishes

Different levels of condensation

If you make your own agar for use in petri dishes or other containers, or if you buy pre-poured agar from online suppliers, perhaps the most common issue you’ll face will be condensation. Most often it won’t be a problem, as you’ll just have a little around the dish edges or perhaps on the lid, but sometimes it can be excessive – to the point of seeing puddles of water inside the dishes. However, it usually isn’t the end of the world.

Why condensation?

If you think about it, the agar in your dishes is almost all water, with a small amount of material added to provide nutrients for mycelium and to turn it into a solid jelly. Anything such as agar which is composed mostly of water can always be subject to condensation and this is triggered by temperature differences – or temperature shock if you prefer.

Think of the windows in your house – they’re not so different from agar dishes. When it’s cold outside and warm and humid inside you will get  condensation on your windows. In the right circumstances you may even have rivers of water running down the inside panes. Double glazing prevents most of it these days but on single-paned windows it’s simply a part of life you get used to. The same process can affect agar in petri dishes.

What causes condensation in petri dishes?

The culprit is temperature differential. As your agar dishes are sealed, the agar changes temperature very slowly. When warm or even room-temperature agar is placed in an environment where the outside temperature is different, condensation is inevitable.

Pouring agar too hot
If agar is too warm when poured into a petri dish, condensation will develop almost immediately after the lid is placed on the dish, because you have warm liquid in a closed space with cooler air outside. The best way to avoid this is to wait as late as you can before pouring, ideally when the agar temperature drops to 50c or slightly less – agar sets at 40c and will start to get lumpy by about 45c, but there are other things you can do too.

Firstly, use a heater to warm up the room you will use to pour your agar. It doesn’t need to be tropical heat, but the warmer the better. Next, you can try placing your (still sterile and sealed) petri dishes on or near a radiator, to warm them up a little, and so reduce the difference in temperature between agar and petri dish. Just be sure the heater can’t damage or melt the dishes! Finally, have a large container of warm fluid (water or even excess agar) at about 45c which you can stand on top of the petri dishes after they have been poured, closed and stacked up. That keeps your stack of petri dishes (and the air inside them) at a fairly constant temperature while everything is cooling.

Temperature shock
Assuming you poured your agar petri dishes and allowed them to cool to room temperature, you might think that immediately putting them into a fridge will help protect them or prolong their life. However, the sudden change of temperature can trigger condensation very quickly, through what can be called temperature shock.

The best way to store agar petri dishes in the short term is to stack them upside down and keep them in a dark place at room temperature or a little lower. Keep them away from draughts, well away from any type of heater and definitely out of the sun – all these things can cause the petri dish to heat or cool quickly and unevenly and will cause condensation. To store them longer-term, put them inside a sealed bag (we use 2 bags, to be sure) inside a fridge and if possible adjust the temperature so that it isn’t too cold. We use a large domestic fridge for agar storage and it is set to level 2 of 6, which seems to work best.

Purchased agar

Perfectly normal and will evaporate with time

Condensation can also be found in agar dishes purchased online – your dishes were probably poured and kept at a little below room temperature prior to dispatch but whilst in transit can be subject to wildly varying temperatures in vans, storage areas or even when sat on the ground waiting to be loaded into delivery vehicles. There’s nothing much you can do about condensation occurring during transit, but if you leave the dishes to settle down at room temperature for a day or two, most of that condensation will fade away.

Puddles of water in your petri dishes?

This can happen. If the dishes are colonised and they’re stored upside down, the water won’t bother the agar or mycelium. If you have a flow hood or still air box, you could unseal the dishes (still upside down) and let the excess water drain out, dry the lid with an absorbent sterile wipe and re-seal them. As long as you’re careful it shouldn’t cause a problem. Use fresh parafilm to seal them, don’t try to reuse the original tape. Alternatively, drain the water and clone your mycelium in a fresh agar dish.

Other remedies..

Leaving dishes ajar after pouring
You’ll often hear about users explaining how they pour their agar into dishes and then leave them ajar for an hour or two to let them cool wait for the condensation to vanish. This generally means leaving the lids slightly offset so as to leave a small gap, and stacking them in front of a (running) laminar flow hood. With a good flow hood this can be effective, but in most cases you’re exposing your agar to every molecule of bacteria in the room. On top of that, you can’t safely stack them very high as they’ll fall over unless you stack them up like a small pyramid – so it only works for smallish numbers of dishes at best. On balance, without a flow hood, trying this only increases the risk of contamination.

Is condensation harmful to agar?

Generally, not hugely. If your agar is wet with water before you try to colonise it with spores or liquid culture, it’s quite likely to cause problems, but if it occurs once your mycelium has begun growing, keep the dishes upside down and try to ignore it. In most cases it won’t do any harm.