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Rancid
Identifying Spoilage
Use 'rancid' to describe foods with fats or oils that have gone bad, primarily from smell or taste. 
Not Just Smell
'Rancid' can also describe a stale taste, not just smell, which is important in food quality. 
Safety Cue
Describing food as 'rancid' is a warning that it may be unsafe to eat, as it is likely spoiled. 