Chlorophyllin keeps Canned Vegetable looking fresh

Demonstration Experiment on Video

Chlorophyllin keeps Canned Vegetable looking fresh

Objectives: Chlorophyllin-Copper-Complex, Food Dye

Peter Keusch

German version

Supermarket products:

instant softener (ingredients: citric acid, amido sulfonic acid)
canned peas (ingredients: sugar, Aromen), firm: Bonduelle

Chemicals:
CuSO₄ · 5 H₂O

Apparatus and glass wares:
2 beakers 100 mL
2 beakers 50 mL
2 flat porcelain dishes d = 10 cm
2 snap-cap vials 20 mL
Petri dish d = 9 cm
hotplate

Hazards and safety precautions:

Copper(II) sulfate is harmful when swallowed.

Safety glasses, protective gloves, good ventilation!

Experimental procedure:

5 g of peas placed in each of two beakers are mixed with 50 mL of water and 2.5 mL of an instant softener. A spatula full of CuSO₄ · 5 H₂O is dissolved in one of the two samples. The two samples are heated for 5 minutes.

Results:

In the presence of Cu ²⁺ ions the peas are colored bright green. The peas in the other beaker remain unchanged.

Video clip (Download RealPlayer .rm file)

Discussion and background:

A derivative of chlorophyll is formed in which the magnesium in its center is removed and replaced with copper. The replacement yields a bright green pigment (chlorophyllin copper complex) which can be used as a food colourant.

Foods having coloring properties that contain chlorophyll are very unstable. When green vegetables are cooked long enough, the magnesium gets removed from the center of the porphyrin ring and replaced by a hydrogen atom. The chlorophyll a gets turned into a molecule called pheophytin a, and the chlorophyll b gets turned into pheophytin b. With this one simple change, the color of the vegetable changes from bright green to olive-gray. Hence, in order to provide the structure with a little bit more stability, the magnesium ion in the chlorophyll molecule is synthetically replaced with copper.

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