Bill Tindall, E.Tn.
I occasionally helped a local walnut steaming operation. It was "common knowledge" there that the steam dissolved the colored stuff and diffused it to the sap wood. The lumber to be steamed was dead packed to get good contact between the white sapwood and dark heartwood.
I provided this common knowledge on some Forum and was promptly buried by the wood expert John Arno with contrary technical information(1) on what really happens. Skeptical, in spite of the technical papers supporting the heat and steam process, I put a piece of sapwood in the pressure cooker and sure enough it began to darken once it reached near the boiling point of water. I planted a walnut tree in memory when he died.
1. L. L. Johnson, Forest Products Journal (1966) pg 525.