Yeah! Look, here I have two jars of popcorn kernel grain spawn. On each one of the jars I put in a small piece of a King Oyster mushroom that I bought in the supermarket. I rub 70% isopropanol around the mushroom and then I tear the mushroom open and take out a tiny piece of issue from the inside. These jars were inoculated on May 16, so this is 23 days of growth:
The left one I already shook and the right one is still un-shaken.
What I mean about the hydration is that with rye grain I would often end up with grains a bit too wet and sticky, making it impossible to shake the spawn properly. The colonized grain would turn into a hard block that I had to take out with a spoon! With properly hydrated and surface-dried grain, you can shake the grains and they separate very well, making it much easier to work with them.
What I do is I buy one of these bags from the supermarket (900 g, cost is about $2.5 per kilo or $1.13 per pound):
No need to pre-soak them. I cook them in the pressure cooker for 25 - 30 minutes, starting the timer once pressure is reached without venting (this is not a sterilization step, just a cooking step). Without a pressure cooker they can be boiled for about 1 hour. Then I place them in a colander to dry - at least one hour, over night is even better. The grains should not be wet outside and they should still be firm, but it should be possible to crush them between your fingers if a moderate amount of force is applied.
Since each person has a different setup, the best way to get the right hydration for your own setup is to measure the amount of water absorbed a few times until you are happy with your procedure. You weight the grains before cooking them, and you weight them again after cooking and drying them. The increase in weight for popcorn of 40% - 70% would be a good amount of hydration.
$17 for 4 pounds of rye is quite expensive... That comes out to 4.25 / lb! Current prices I can find for my region are closer to $0.5 / lb for buying bulk. So for a mushroom farmer that buys rye in bulk from as close to the source as possible it makes a lot of sense to use rye because it is a really good grain and cheap, but when it is not sold locally and you have to order it from a specialty shop you are paying a high premium for it! Hope that makes sense!
I do add some gypsum some times, and sometimes I don't add it. I think the main benefit is nutritional in that it adds some calcium. It can help dry out the surface of the grains a bit if you add it after cooking the grain, but it only helps to a degree - if your grains are over-hydrated or too wet, gypsum will not be enough to stop them from becoming a hardened block.
As for pressure cooking the grain - this is done with the grains directly in water with more than enough water to cover them. I use the pressure cooker in this step just because it is faster and more efficient. The rate at which grains absorb water is proportional to the temperature, and the liquid water gets hotter in the pressure cooker due to the increase in boiling point at the higher pressure. This rate depends on the pressure achieved by your pressure cooker (usually 12 PSI or 15 PSI), and a bit in the altitude. That's why the best is to test the hydration with your setup instead of relying on a set recipe.
There is no need to protect the kernels while they dry. The sterilization step comes after, once the grain jars are prepared. In theory you can get unlucky and an heat-resistant endospore could land into the drying grain, so you might want to be a bit clean. I personally I just let it dry by the sink. Get them out as soon as possible while they are hot because this helps the water on the surface evaporate quicker. You can even use a towel to dry them but I find that rubbing grains with a towel is a bit awkward.
That's great! Exciting :D Good luck!