Verjus, the tart juice pressed from unripe grapes, is available at specialty food stores.
Pineapple Vinegar: In a medium saucepan, simmer 4 ounces unsweetened pineapple juice over high heat until reduced by half, about 7 minutes. Remove from the heat and stir in 6 ounce white vinegar and 1/4 cup sugar. Let stand until the sugar completely dissolves, about 10 minutes. Pour the infused vinegar into a jar and refrigerate for up to 3 weeks. Makes about 9 ounces.