The best way to maximize the profit per keg is to ensure that the draft beer system is properly balanced, installed and maintained system.
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An out of balance system will create excessive foam which is waste and lost profit, or it may create too little foam, and too much beer is poured into the glass. An incorrectly installed system can create foaming and over pouring resulting in lost profit.
A poorly maintained system will cost the retailer lost profit, not only from foaming and over pouring, but also lost profit when customers stop buying draft beer.
No one wants to drink something that smells like rotten eggs, taste flat or has impurities floating in it. When customers see and taste those things, they will stop frequenting that place of business and that will result in lost sales and profits, not just from draft beer, but for the entire establishment.