The natural colour of green vegetables is due to a mixture of chlorophylls. Under conditions of moist heat, the chlorophyll is converted to an olive green compound, phaeophytin, as a result of the replacement of an atom of magnesium in the centre of the chlorophyll molecule by two hydrogen atoms. The magnesium attaches itself to one of the natural acids in the vegetable. Hence the colour change is favoured by acid conditions and the addition of sodium bicarbonate can reduce it. This is not altogether desirable since raising the pH accelerates the destruction of vitamin C. However it has recently been shown that if magnesium carbonate and calcium acetate are added to green beans sufficiently to raise the pH by only 0–3 of a unit, the canning process results in twenty per cent less conversion of chlorophyll to phaeophytin than the untreated vegetable. Losses of the green colour are increased with time and temperature, and the conditions necessary to sterilize canned peas produce this colour change. For this reason it is normal in Britain to add artificial green colour to canned peas to give them an attractive appearance.
Spencer, M. (1973), "Teach‐in No. 4: Chemical changes during cooking, processing and storage of food Part 2 Colours, vitamins and minerals", Nutrition & Food Science, Vol. 73 No. 3, pp. 11-14. https://doi.org/10.1108/eb058568
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