What purpose does a check valve serve in casks?

A check valve is a crucial component in the operation of casks, particularly in systems where beer is delivered through a beer engine. It ensures that beer can flow out of the cask when the engine is activated but prevents backflow into the cask when the engine is not in use. This mechanism is vital for maintaining the integrity of the beer's quality, as it protects against oxidation and contamination that could occur if air were allowed to flow back into the cask from the beer engine.

In summary, the primary function of a check valve in this context is to manage the flow of beer, ensuring safe dispensing while protecting the cask's contents. This is why this option aligns best with the function described for a check valve in cask systems.