Ingesting foods or drugs can alter rated mood. Moods have been theorised as reinforcers that cause ingestion. This assumption may be incompatible with the current two-system models of affect, where 'moods' are less intense yet more protracted than emotions, and affective states are caused by primary rapid affect processing and secondary cognitive appraisal. In ingestion research, moods may be transient rather than protracted and significant changes on mood rating scales are found without reportable changes in mood. Conclusions: Transient mood is caused cognitively and the temporal dynamics of mood are important. Consequently, when ingestion directly causes changes in affect these may be brief emotions rather than moods. In the absence of emotion, ingestion may provide input to the cognitive processes that cause transient mood, but physiological change cannot easily be inferred backwards from mood ratings. There are a number of unresolved questions about the relationship between rapid affect processing, cognitive appraisal and learning.