What happens to the foam stability when using well-modified malt and extending the protein rest?

The correct answer indicates that foam stability becomes unstable when using well-modified malt and extending the protein rest. This is because well-modified malts already contain lower levels of protein, which are essential for the formation and stabilization of foam. The protein rest, a step during the mashing process, is intended to break down proteins into smaller peptides and amino acids that contribute to foam stability.

When the protein rest is extended, the additional breakdown of proteins can lead to an even greater reduction in the protein content available for foam stabilization. Therefore, although a protein rest can enhance certain aspects of beer quality, extending it with well-modified malts can lead to overly fragmented proteins, which do not properly form the stable structure required for maintaining good foam. This results in a foam that is less stable and can collapse more quickly.

Other options suggest improvements or unchanged conditions, which do not consider the impact of both the malt's modification state and the length of the protein rest on the structure and stability of the foam. Thus, option B accurately reflects the consequences of these specific brewing practices.