s not
appreciably enhance the keeping quality, owing to the fact that the
bacteria adherent to the dirt particles are washed off in straining,
and pass through the pores of the strainer.

=Filtration of milk.= It is possible to remove all bacteria from water
and other fluids and thus render them sterile by passing through
filters of unglazed porcelain. This process can not be used with
milk for the fat globules are larger than the bacteria (see Fig. 6)
and any process that would remove the latter would also remove the
former. The term "filtration" is applied to a process used in some
European cities for the removal of the insoluble dirt that has been
introduced into the milk. Suitable containers are filled with layers
of coarse sand at the bottom and with finer sand at the top. The
milk is introduced at the bottom and is forced upward through the
sand. Such a filtering process is a very efficient means of removing
the dirt; but unless the filters are kept scrupulously clean, the
bacteria are likely to grow in the filtering material, so that the
number of organisms in the milk may actually be increased by the
filtering process. It is necessary to remove the sand daily and
thoroughly wash and sterilize the same. The extra care required in
keeping these sand filters in sanitary condition has been the great
objection to their employment in this country. Filters of other
material such as cellulose have been employed but with no marked
success.

=Clarifying milk.= A much more efficient and less troublesome means of
removing the insoluble foreign particles from milk is to pass it
through a cream separator, allowing the cream and skim milk to mix
in the same container. The slime that collects on the wall of the
separator bowl is made up of dirt, casein, bacteria, and the
cellular debris from the interior of the udder. The bacteria are
heavier than the milk serum, and would, therefore, be deposited on
the wall of the bowl were it not for other factors that in a measure
prevent this. The movement of the fat toward the center of the bowl
carries into the cream a considerable proportion of the bacteria in
the milk. The slime will always contain many more bacteria than the
milk, but the per cent of bacteria thus removed is relatively low,
due to the small amount of slime obtained from the milk, so that the
actual effect of clarification on the keeping quality of milk is
insignificant. The complete removal of all insoluble and the