Ascorbic Acid, also known as Vitamin C Powder is light in acidity and flavour, and is a fantastic tool to stop the oxidisation of fresh juices and cocktails.
A half of a bar spoon of this magic acid in any bottle of prebatched home made cocktail will help stabalise and give it a bit more longevity in your fridge. Always taste as you to as this may turn your drink too sour.