What do you call a can that contains spoiled food but appears normal on the outside?

The term that is most appropriate for a can that contains spoiled food while appearing normal on the outside is "flat sour." This condition is specifically associated with the spoilage of canned food due to the growth of microorganisms, typically certain bacteria, that produce acids without generating gas. Consequently, the can maintains its normal shape without swelling or bulging, hence the term "flat."

This situation often occurs when the food inside has undergone an anaerobic fermentation process but does not lead to the gas buildup that would cause the can to be visualized as swollen or bulging. Recognizing flat sour is vital for food safety because it indicates spoilage that might not be externally visible, thereby emphasizing the importance of not only relying on the physical appearance of canned goods but also understanding their potential microbiological safety risks.

In contrast, "sharp sour" typically refers to a noticeable sour taste caused by bacterial action, which is distinct from the condition of the can itself. "Swollen can" and "bulging can" both describe physical deformities in the can that indicate gas production, which implies different spoilage mechanisms often associated with pathogenic spoilage and is fundamentally different from the flat sour condition.