Re: why do some potatoes turn black on the inside on cooking ?

The graying and/or blackening of potatoes following cooking is referred to 
as after-cooking darkening. The cause of after-cooking darkening has been 
studied by scientists since the early 1900s. Today, it is generally 
accepted that after-cooking darkening is the result of a dark colored 
complex of ferric iron and an orthodihydric phenol (probably chlorogenic 
acid). The mechanism of after-cooking darkening is influenced by other 
factors as well. Such things as iron content, amount of orthodiphenols, 
presence of organic acids and pH are also probably involved. If you would 
like an indepth discussion of after-cooking darkening the book Potato 
Processing, 3rd Edition, William F. Talburt and Ora Smith, The AVI 
Publishing Company, Inc, Westport, CT is an excellent resource. The book is 
out of print, but many University Libraries should be able to get a copy.

I hope this information provides a brief answer to your question.

Sincerely,

Al Bushway
Professor of Food Science