has two meanings in the brewery. One meaning refers to the practice of pumping beer from one tank to another via a hose, as, for instance, from a secondary fermentation tank to a lagering tank to leave sedimented yeast and particulate behind. See lagering
and secondary fermentation. The second, more specific meaning refers to the rousing of the wort by recirculating it externally from the bottom to the top of the tank. See rousing. Although the objective implied in the first meaning is obvious and requires no further explanation, the objective of the second meaning is to keep the yeast evenly suspended in the wort during fermentation to ensure complete and prolonged contact of the yeast with all sugars and other nutrients in the wort. Such pumping over must be done carefully, without oxygen pickup. This is why the inlet for the recirculating wort at the top of the tank must be below the beer level. Pumping over should never be done through the spray ball at the top of the tank. Oxygenating fermenting beer would lead to a quick conversion of the precursors acetohydroxy acids into vicinal diketones, including diacetyl. See diacetyl
and vicinal diketones. Pumping over is usually a last-ditch effort used when the fermentation has slowed significantly or has become stuck altogether because the yeast has prematurely settled at the bottom of the fermenter. Whereas the term “pumping over” is rarely used for other circumstances, some breweries use a similar technique to blend beer or disperse hop material during dry hopping operations. See also dry hopping, fermentation, and flocculation.
