I washed some apples and served them on a hand-carved soapstone platter. When we began to eat them, we noticed the apples had left blue stains on the plate. What could have caused this?
The stain is copper sulphate pentahydrate. It also goes by the name 鈥渂luestone鈥� thanks to its gorgeous turquoise-blue colour.
When dissolved and diluted, copper sulphate makes a highly effective multipurpose treatment for apples and other fruit. It corrects copper deficiency 鈥� a common problem in fruit trees 鈥� and is the most popular spray used by apple growers for killing lichen.
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It is used as a fungicide, algicide, pesticide, antiseptic and germicide in agriculture, animal care and human public health. Mixed with lime it makes , famed for saving vineyards from downy mildew.
Copper sulphate stains blue if it is not diluted enough. Perhaps the apples were dipped in a solution of copper sulphate after picking, to kill fungi or pests.
Do be aware, however, that despite its use in agriculture, .
Toshi Knell
Nowra, New South Wales, Australia
Also on the subject of blue food, we have had two questions that seem to share the same answer 鈥� Ed