Zsuzsi said:

How come the substrate doesn't need to be sterilized ? is the well mixed mycelium enough to fight off potential contam ?

And more importantly: what the hell is the mycelium eating when you do it like this ? can mycelium consume coir ?

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The bucket tek version of coir (bod's tek) technically is somewhere between pasteurization and sterilization. Really though, the reason most of us use coir (or verm) is because it's highly contaminate resistance. If we used sterilized substrate it kills all the good microscopic shit so it leaves a lot of space for bad contaminants to take hold. If you use sterile substrate you must use aseptic methodology for spawning your substrate. It's more challenging to do that than pasteurized substrates because of this. (see my tek in my signature for shoebox assembly to see that you can just touch your stuff in open air safely, that's why we use coir)

As for what it's eating... it's not really "EATING" so to say, it's colonizing it. Most of the nutrients come from the grain (in your case BRF), not the coir. To what extent it's eating it isn't very important I've heard others talk about it being like getting offered a huge feast as dessert for having eaten a huge feast. The myc already "feasted" on the highly nutritious grain and can do so for a long time, it doesn't much care about the coir.

Zsuzsi said:

As for the second part of your answer, does that mean that the myc, once "charged" on rice flour, is able to colonize the whole substrate based on that alone ? and provide a greater yield in the end ? what's the limit in terms of tray size from, say, a classic 1/2 pint jar ? when using rice flour, does trying to spread the myc in a tray like this really help with yield or is it useless unless you use grain ? and in any case, is there a way to provide more nutrients to the developing mycelium in the tray without risking contam too much ?

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Kinda, see below for more. Charged wouldn't be the term I'd use, but it probably just means something different for you. But yeah, the brf is the battery that powers the myc to fill up it's new home (see below).

If you shred to bulk you should expect to get anywhere from nothing to approx. 1oz dry per quart of spawn (i.e. 4 1/2pt cakes).

As for developing myc and more nutrients, it's 110% unnecessary after you've got the spawn (in your case brf cakes). Any attempt to add more nutrients will be like trying to add another battery to your remotes or something, there's really no need and it just makes things more likely to explode in your face.

EDIT: If you really want to add more nutes, just use more spawn. This is the safest way to get more nutrients to your mycelium without going into sterilizing your substrate, which requires very specific sterile technique to work with and is much more challenging to do (I say this having just failed at some of my first attempts using sterile substrate, and I feel extremely confident in my ability to grow cubes)

I'm not sure what you mean when you ask "when using rice flour, does trying to spread the myc in a tray like this really help with yield or is it useless unless you use grain ?" I think you're greatly over thinking this though. Your fully colonized brf cakes shredded to straight coir in a bulk container (I recommend shoeboxes to start) will give you as many mushrooms as attempting to do anything else to get mushrooms, but without the highly increased chances of contamination.

This thread has great tips on how to do a brf shred to bulk grow.

LogicaL Chaos said:

It would seem from my perception that mycelium grabs hold of coir sort of like its a new home, as a snail takes on a shell for example.

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