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Canning -- Pickled and Fermented Foods
Why do pickles turn a green color?
Acid and heat are responsible for turning the bright green skin of cucumbers to the olive green of pickles. During pickling, the magnesium atom in the center of the chlorophyll molecule is replaced with two hydrogen atoms to form pheophytin, and olive green pigment.
If copper replaces the magnesium, the pigment turns a bright green. Copper kettles used to be used to prepare pickles and this imparted a bright green color. This not a recommended practice because copper can be toxic. Another "old timey" practice was to add a blue stone, copper sulfate, to give the green color or even a copper penny. Again, none of these practices are recommended.