A. Darkening of foods at the tops of jars may be caused by oxidation due
to air in the jars or by too little heating or processing to destroy
enzymes. Overprocessing may cause discoloration of foods throughout the
containers.
Pink and blue colors sometimes seen in canned pears, apples, and peaches
are caused by chemical changes in the coloring matter of the fruit.
Iron and copper from cooking utensils or from water in some localities
may cause brown, black, and gray colors in some foods.
When canned corn turns brown, the discoloring may be due to the variety
of corn, to stage of ripeness, to overprocessing, or to copper or iron
pans.
Packing liquid may cause fading of highly colored foods. The use of
plain tin cans will cause some foods to lose color (p. 4).