A lot of people like to shake or mix their grain spawn when it is 30 to 50% colonized, but 10% makes much more sense.
If the grain bag is colonizing from a single location such as from the injection port or an agar wedge and you let it colonize to 30% – there were a significant amount of fully colonized grain touching other fully colonized grain: they basically do not contribute to the colonization rate.
If you could calculate how many pieces of grain on average, touch a single piece of grain, you could then opt to mix at that ratio, and once you did mix it – you could have a single fully colonized piece of grain touching uncolonized grain. All of these would be contributing to colonizing the rest of the remaining uncolonized grain.
So, how many pieces of grain is touching a single piece of grain?
We’ve done a small experiment where we soaked a single piece of grain in super glue, and placed it on top of other grain, then we buried it with more grain and gave it a gentle tap to emulate mixing the grain.
Our results show that, on average, a single piece of grain is touching 10 or more other pieces of grain. This would suggest that optimal mixing would occur at around 10% assuming you can properly break apart the colonized grain and mix it effectively and evenly through out the uncolonized grain.