Green plum syrup
Korean syrup made from green, unripe plums (maesil) steeped in sugar: sweet, fruity and tart.
Green plum syrup, maesil-cheong in Korean, is made by layering green, unripe plums of the species Prunus mume with an equal weight of sugar and leaving them for weeks or months until the sugar has drawn out the juice and it has lightly fermented. It tastes sweet with a distinct tartness and a fresh, slightly bitter fruit aroma reminiscent of almond, and it is thinner than honey. Korean cooking often uses it in place of sugar in marinades, dressings and sauces because it sweetens and adds acidity at the same time; diluted with water it is also drunk. You can find it in Asian grocery shops, especially those with a Korean range, and from online retailers, often labelled maesil-cheong or plum extract.
Substitute:
There is no true equivalent. Honey or a light sugar syrup with a little lemon juice or rice vinegar matches the balance of sweetness and acidity, but the fruity, slightly almond-like plum aroma is lost; in marinades and dressings this is barely noticeable, whereas diluted as a drink the difference is obvious.