Brewers must take special care to prevent air from entering fermentation tanks during the production of beer. Because the fermentation process is such an exact science, most ales and lagers are produced in sealed, sterilized vats, under strictly controlled conditions. Only the desired type of yeast is introduced into each vat along with sterile air, keeping out unwanted yeasts and molds that could spoil the flavor and appearance of the finished product.
There are certain lagers in Europe that are made in open fermentation vats, making the risk of outside infection greater. To compensate for this greater risk, extreme care must be exercised in the sterilizing, handling, and use of the equipment and in the atmosphere of the fermentation chamber. The air entering the chamber is conditioned and filtered, and persons attending the vats wearing sterilized clothing and masks. By this means, a superior product is usually obtained.
Having made homemade wine and beer before, I'm personally familiar with the pleasure an excellent bottle of beverage provides-one that turned out due to scrupulous cleanliness and diligent care. I've also experienced the bitter disappointment that came from a batch of spoiled brew, due no doubt to a "wild" yeast, mold, or fungus that got into it. The lesson learned is that one can never be too clean!