Stored purple and red flesh potato tubers were washed, cut into
small thin pieces and freeze-dried for about 48 h using a Alpha 1-4
LSC freeze-dryer (Martin Christ, Osterode am Harz, Germany) after
frozen in liquid nitrogen. The dried potato samples were ground to
a fine powder. All powder samples were vacuum sealed in plastic
bags and stored in a large plastic airtight container with silica gels
at room temperature until use. Potato chips were defatted using
diethyl ether and stored in plastic boxes at room temperature until
use.