color from blue to white when the samples are placed in tubes
and kept at 98 to 100 deg. F., is dependent upon the number of bacteria
present. By allowing the tubes to stand until curdling occurs, and
noting the nature of the curd, whether the solid curd of the
desirable acid-forming bacteria or the gassy curd of the harmful
types is produced, knowledge is gained of the kinds of bacteria
present.

According to Barthel, milks that reduce the methylene blue within
fifteen minutes contain hundreds of thousands of bacteria per cubic
centimeter. Those that require from fifteen minutes to one hour for
the disappearance of the color are also high in bacteria, and are to
be classed as a poor grade of market milk. If one to three hours is
required, the milk is comparatively low in bacteria, and is to be
classed as a good grade of market milk. When more than three hours
elapse before the disappearance of the blue color, the bacterial
content is low and the milk is to be placed in the highest grade.

The time of reduction is only a rough index of the number of
bacteria present, but it gives a good idea of the keeping quality of
the milk, and of the conditions of production and handling. Of the
above tests the sediment and acid tests are more frequently used.

=Examination of milk sediments.= In the modern municipal laboratory,
efforts are made to determine, as far as possible, the conditions of
production on the farms, by an examination of the milk in the
laboratory. The samples of milk are sedimented in a small
centrifuge, and an examination of the sediment made with the
microscope. The types of bacteria and the number of body cells found
is an indication as to whether any of the animals of the herd are
suffering from inflammation of the udder. The test also gives
information similar to the dirt test since the insoluble dirt will
be thrown down and will impart a color to the sediment.

=Pasteurization of market milk.=.The spread of the pasteurizing
process as applied to market milk has been rapid. This has been due
to the recognition of the fact that only by this process can a safe
milk _i.e._, one free from pathogenic bacteria, be obtained. As
previously mentioned a small proportion of all human beings that
have suffered from typhoid fever become bacillus carriers. It is
impossible to examine all persons who may be concerned in the
handling of milk in order to ascertain whether they belong to this
dangerous and unfortunate