The conditioning process for bottom-fermented beer is somewhat different. The original Bavarian lager was packed in ice and allowed to rest in contact with its yeast dregs for several months. The yeast slowly produced carbon dioxide, which helped purge the beer of sulfury off-odors. Today, some traditional lagers are still aged for several months; but because storage has the economic disadvantage of tying up money and materials, the tendency is to lager the green beer at temperatures just above freezing for two to three weeks. Carbon dioxide may be pumped in to purge undesired aromas; and centrifuges, filters, and additives help clarify the beer. As a replacement for wooden casks, some beech or hazelwood chips may be thrown into the tank for flavor.