According to the Institute of Medicine (US) Committee on Military Nutrition Research, “The available data clearly show that temperature can be an important variable in flavor perception…”. The bottom line seems to be that “…as temperature rises, perceptions of sweetness and bitterness tend to intensify, and perceptions of sourness and saltiness tend to remain the same…” The report details the relationship of heat and ‘oral perception.’
Draft beer is typically chilled with a food-grade propylene glycol system. These systems need to be maintained just like all refrigeration or HVAC systems – but by someone with a special knowledge, like BlueHat Mechanical.
More information at BarNinja.com – How to Clean a Draft Beer and Glycol System