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Rhubarb
Rheum rhabarbarum
The vegetable that acts like a fruit — rhubarb's bright red-green stalks are so acidic they cannot be eaten without sugar, but when cooked with sugar they produce a tart, uniquely flavoured ingredient for pies, crumbles, and jam; forced Yorkshire rhubarb, grown in dark sheds, is a protected food with a distinctive pale pink colour and more delicate flavour.