@AmeG
Basically the biggest risk of contaminants is in the early stages. When you're starting from spores, they aren't robust enough to compete against anything. So, you do need to sterilise the medium that you use for germination.
An important thing to remember about this is that, you put the medium into the container and seal it, then you sterilise that. This means you have a sterilised container with nothing living inside it. No bacteria, no fungal spores of any kind.
If you were to take this container (jar or otherwise) and leave it somewhere for 2 weeks, it would not rot. It should not rot. Rotting only happens when there is bacteria present. So, if you've done your job properly, it won't rot. The same bacteria that causes rotting is the same bacteria that would outcompete your mushroom spores.
That's a bit of the why you do what you have do. The way I avoid contamination is by sterilising a sealed container container a growing medium, and then injecting spores in. If you move your syringe from one container to another, use a lighter to burn away anything on the tip of the needle. I do that, and I've had very few problems. No flow box needed.
Maybe I'm just incredibly lucky, but I think the risk of contamination is overstated. I just don't think it is that common. And, to add to that, sometimes I have colonised jars, and I open them, and get a sour smell. A sour smell means that there are bacteria in there. But the thing is, it doesn't matter. It has never been enough to keep me from eventually getting fruits. And none of those fruits have been toxic in anyway.
The reason for this is that, when mushroom mycelium gets mature enough, it has enough of an immune system that it can fight off bacteria and to some degree moulds. So, a little sour smell won't kill it. Just don't eat that part, and don't put it into your grow box.
This becomes even more true when you mix everything together into a monobox to fruit. It would be absolutely impossible to prevent contamination once you put everything into fruiting conditions. There are bacteria everywhere in an ordinary house. So, the reason why this isn't a problem, is again that mature mycelium can fight it off.
But there is one other secret - coir. Coir is anti bacterial. I have never had coir get infected by bacteria. You can keep it moist and in a dark place exposed to bacteria for years, and it will never smell. There just isn't enough food for bacteria in it. This means that, when you make a monotub containing coir, it severely limits bacterial contamination. Maybe the odd grain of rye will catch some bacteria, but it can't spread. Its surrounded by coir. Nothing for it to eat on the way to the next rye grain, and if it runs into mycelium, it can't invade that either. Too well protected.
