Parafilm is a sealing film, it’s made from a blend of plastics and paraffin wax; making it stretchable, moldable, waterproof, odorless, semitransparent and self-adhering giving it a wide range of uses in labs. In mushroom growing, we use this as a way of sealing agar dishes to keep them sterile. It’s also used around bottles and master slants. It’s pretty straight forward to use Parafilm. Take a piece, stretch it, work it around your dish and stick it to itself.
For my 90mm dishes I use 2 squares and fold it back on itself doubling the thickness. I do this because I’m ham-fisted and it works out more reliable for me to use more rather than cutting it in half length ways as you’ll see next.
First, peel the paper backing off making sure that side faces inwards on your petri-dish as this is the clean side. If you want, double fold it like me and pin the start of the film on the petri-dish with one finger while working it around. Stick it to itself and make sure it sealed all the way around.
You can also cut it horizontally, doubling the amount you can use, and do the same thing.
You could also use a whole sheet but that’s quite wasteful.
You could just use clingfilm or micropore tape or even regular old tape. However, parafilm is purpose built for this job so if you’re doing 20 or 30 dishes at a time, it’s much easier to use this then messing around with sticky tape or clingfilm, mainly because of it’s stretchiness.
I love parafilm and I’ve never sought the need to look for alternatives. One roll will last me about a year and costs around £30 in the UK for a 76m 50mm wide roll. Another brand is called Nesco Film but it’s not as common, at least in the UK.
A couple of downsides to parafilm are it tends to dry out a bit and become brittle. I guess this is because the petroleum-based wax evaporates off, so if you have slants you want to keep for a while, treat them with care and reseal them every year or so.
My lab is in my garage so when it becomes colder the film is far less stretchy so you may end up snapping or tearing the film as you try to stretch it so keep it at room temperature, not garage temperature.
It’s a bit fiddly to remove paper backing once you have gloves on, I find tearing off a corner of the film and backing gives me a purchase to hold onto then I’m able to separate them.
Parafilm does allow a small amount of gas exchange but mycelium is mostly able to survive with the air that is captured inside the container. Parafilm isn’t something you would use instead of an air filter, for example, on a grain jar or bag.
I sell it in lengths of 5 metres in my shop if you want to try it out yourself before buying 76m of the stuff. Any questions feel free to contact me!