WYEAST is a brand of liquid yeasts, wrapped in aluminium foil, which take effect at the time that the inner bag is smacked. Hence they are called "smack packs". The mechanism works as follows; inside the package is a small plastic pouch with a sterile nutrient solution with wort extract. If this is broken, the liquid mixes with the yeast in the aluminium foil, which will be activated and swells.
It may occur that a foil packaging is already slightly swollen, which is almost always due to a small amount of yeast nutrient left behind during packaging. However, this does not mean that the yeast has gone bad. The inner bag is still intact and the yeast can still be used perfectly.
Using them is very easy: take the smack pack out of the refrigerator, locate the inside bag with your fingers and ‘smack’ it with your fist so it bursts open. Shake the package thoroughly and let it swell at room temperature. Now you’ll have a ready-made pure and sterile yeast culture. Finally cut the package open and add the yeast.
Click here here to view a Wyeast instruction film!