To understand why this dessert tastes so good - and why a late-harvest sweet white wine tastes so good with it - we need to understand the role of acidity in the raw materials. Without the malic acid in the apples, for example, they'd be all sugary softness. The same's true of the tongue-tingling oxalic acid in the rhubarb, the citric acid in the orange and the lactic acid in the sour cream: it's the acidity that gives all these ingredients the tang that enlivens the tastebuds, stops the dish from cloying, and urges us to take another bite. Acidity is also crucial in wine, especially if that wine has been made from late-harvest, partially shrivelled grapes. Without acid, very ripe, sweet grapes produce wine that's all flabby, honeyed softness, with no life on the tongue. That's why naturally high-acid white grapes like sauvignon blanc and riesling are chosen to make late-harvest wines: they're quite tart even when overripe.