A. Oxidation may cause foods to darken at the tops of jars.
Oxidation is from air in the jars or too little heating or processing to destroy enzymes.
Over processing may cause discoloured foods throughout the containers.
Pink and blue colours sometimes seen in preserved pears, apples, and peaches are caused by chemical changes in the colouring matter of the fruit.
Some fruits such as apples and pears for example, will quickly turn brown in colour once peeled and exposed to the air. Even if soaked in a anti oxidation solution they may still turn a light brown colour.
Iron and copper from cooking utensils (or from water in some localities) may cause brown, black, and grey colours in some foods.
When preserved corn turns brown, the discolouring may be due to the variety of the corn, to stage of ripeness, to over processing, or to type of pan used. Packing liquid may dissolve colouring materials from the foods.