A 50g tub of Young's Malic Acid.
Malic acid is the active ingredient in many sour fruits, such as green apples. It helps to speed up fermentation. Add malic acid to to must that is low in acidity, as per your home brewed wine recipe.
Usage: 10g per 4.5 litres. Dissolve it in a small amount of warm water and then add it to your wine.