Smack pack

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A smack pack is the name commonly used among homebrewers for what Wyeast now calls their Activator™ yeast packages.

The sealed package contains liquid yeast as well as a separate nutrient pouch. When the homebrewer "smacks" or presses on the package to break the internal nutrient pouch, the yeast begins to metabolize the yeast and generate carbon dioxide, causing the yeast to swell up. This demonstrates the viability of the yeast, as well as allegedly preparing the yeast better for pitching.