I have a hunch (but this may not be factual) that the middle white roots haven't had an opportunity to undergo photosynthesis, so they've gone white rather than maintaining a green color.
All my center phal roots tend to be light yellow or whitish rather than green, and this is my idea as to why. Whether or not it is correct is another thing!
Silken is right, though. Don't cut off roots that are mushy, or feel hollow and papery, or pull off the central "string" when wet. These are dead, and members of this board like to remind me "where there is death there is rotting" - meaning cut off anything dead!