Which term best describes a malt that has been excessively modified?

The term that best describes a malt that has been excessively modified is "over-modified." When malt undergoes modification, it refers to the biochemical changes that occur during the malting process, which break down starches into fermentable sugars and alter protein content among other transformations.

Excessive modification indicates that the malting process has continued beyond the ideal point where the malt is considered optimal for brewing. This can result in excessive enzyme activity and the breakdown of too many proteins, which can lead to issues such as over-attenuation in the brewing process. This means that the sugars that are supposed to contribute to body and mouthfeel might be overly degraded, potentially resulting in a thin and less balanced beer.

The other terms do not apply to excessively modified malt. "Under-modified" refers to malt that has not undergone sufficient modification, which could lead to a lack of fermentable sugars and poor extract efficiency. "Optimum" implies that the malt is at an ideal stage of modification, not excessive. "Conditioned" typically refers to malt that has been stored or treated to stabilize its properties, but does not address the extent of modification.