Picture: Living yeast cells.  Some of the yeast cells are elongated and "wild" in appearance, but wildness can seldom be established on the basis of appearance alone.

Domestic draught beers are not pasteurized, and in some instances are not filtered.   Many types of wild yeast present in the air can grow on the faucet or in the keg.   These yeasts are not necessarily true beer spoilers, but they can cause surface growth if the lines are not cleaned and maintained, or if the beer warms above 50 degrees.  

The beer can provide an excellent food source to promote the growth of these organisms, which can exist for long periods of time in the beer lines.  These lines, when used, will introduce any organisms present into the freshly tapped keg. 

Yeast can appear as either a heavy surface film, or as a white colored substance.

Note: The information in this section was provided by and reproduced with the permission of The Coors Brewing of Golden, Colorado.