
Sour apple liqueur is a sweet fruit liqueur flavored to taste of tart green apple.
Sour green apple with sugar at around 20% ABV — three parts, and most stand-ins bring two. Whatever you use, check the acid before the sugar.
Syrup with lemon plus neutral spirit
Sweetness, acidity and strength built separately in roughly equal parts, and the apple note is what this misses; a splash of apple juice puts it back.
Dry and twice the strength, with no sugar and no acid. It gives the apple and rewrites the drink around it: the recipe’s syrup comes back and the measure comes down.
Bright green from dye and sharp from added acid, and the dye is the part daylight destroys. It will not spoil at that strength, but a sun-faded bottle makes the drink look tired. Keep it in a cupboard.