Here's a recipe I use in the Sludge Press, just to showcase the sintax of the new recipe system:
Note that, as Fumihiko pointed out, the field is named "materials" with an S. Using it without the S is still supported, but doesn't accept arrays.

{
"Id": "SludgePress_SedimentarytoMercury",
"Building": "SludgePress",
"Inputs": [
{
"materials": ["SedimentaryRock", "IgneousRock"],
"amount": 150.0
}
],
"Outputs": [
{
"material": "Mercury",
"amount": 5.0
},
{
"material": "Sand",
"amount": 145.0
}
],
"Time": 20.0,
"HEP": 0,
"HEPout": 0,
"Description": "Extract mercury from rock."
}