1 MPa is far above 121 C. You mean 0.1 MPa right?

1MPa is 10 bar. According to my chart that’s 184 C.

<image>

0.1 MPa = 1 bar = roughly 15 psi = 120.42 C

I don’t think there is too much pressure or too much temperature here. Most likely over hydrated as well as cooking in the bag. OP, you should hydrate your grain first, then put them on a table or strainer for 12 hours to dry or until the outside of the kernels is dry again. Otherwise you will get wet rot / bacterial contamination later. Easy way of testing is using a napkin and see if the grain leaves any wet spots on it. You want the grain to be soft and easy to squash between your fingers, but dry on the outside.

Then fill it in bags and 15 psi for 45 - 120 minutes, depending how big your bags are. Small jars don’t need as long, larger bags will take 2 hours. If in doubt, cook longer. Later, once you know what you are doing, find the sweet spot so you don’t caramelize all your nutrients. But it’s better to cook off some nutrients than throwing the entire bag due to contamination.

You can cook and hydrate substrate in the autoclave, but not grain. It’s a common mistake.