I baked a white spud potato and when done I cut it open and I noticed crystallized grainy sugary substance. What is that?

What you’re describing is sugar that very occasionally concentrates withing the potato core, usually following a series of extreme warm and below 41 F degrees temperature swings. This will sometimes convert some starch within the potato to convert to sugar. Typically this manifests itself in a discolored core center, but can also occur with little visible evidence of the conversion. In any case, this is harmless and the potato can still be prepared as desired, albeit with a slightly sweet flavor imparted.

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