Campden Tablets x 50
Potassium Metabisulphite in a pressed form. Has an array of uses.
Vintners can use these to sterilise must or crush, particularly on small batches where measuring 1 or 2 grams is tedious. They can also be used to prevent oxidation of beer or wine.
All grain brewers can use these to remove chlorine from their brewing water also, one half a tablet in 23 litres is sufficient to remove the chlorine.
Campden Tablets can also be used to stop a ferment from finishing, leaving you with a sweeter cider or wine.