Package of 30.
Potassium metabisulfite. Use Campden to kill wild yeast on fresh fruit juices and musts, i.e. wine must or fresh cider. This will remove wild yeast and bacteria from the mixture so you can ferment it with the yeast strain of your choice. Use at a rate of 1/2 to 1 tablet per 5 gallons to kill off wild yeast and bacteria. Allow 48 hours for the sulfites to dissipate.
You can also use campden tablets to break up chloramine from your tap water prior to brewing. Crush up and add 1 tablet per 5 gallons and toss it in your water before you add anything else, and go on about your brew!