Prague powder is another name for curing salt. It's basically table salt (sodium chloride, NaCl) to which a little sodium nitrite (NaNO2) and, in the case of No. 2, a little sodium nitrate (NaNO3) has been added.
Prague powder No. 1 is 93.75% table salt and 6.25% sodium nitrite. It's recommended for meats that will be cooked after curing.
Prague powder No. 2 is 89.75% table salt, 6.25% sodium nitrite, and 4% sodium nitrate. It's recommended for meats that require long cures (weeks or months) and will be eaten raw, such as salami or prosciutto.
The sodium nitrate found in Prague powder No. 2 gradually breaks down over time into sodium nitrite, and by the time a dry cured sausage is ready to be eaten there should be no sodium nitrate left.
Sodium nitrite inhibits the growth of bacteria, specifically Clostridium botulinum in an effort to prevent botulism. It helps to preserve the colour of cured meat, and also gives it its characteristic flavour.
© Haydn Thompson 2017