This second kind of dirt is generally found in
milk when the first kind is present in any quantity. It is more liable
to be harmful than the other, because it enters the milk from the water
used in cleaning the receptacles or from some contaminated source.
[Illustration: FIG. 2]
28. Whenever dirt is present in milk, bacteria are sure to be there; and
the greater the quantity of dirt the greater will be the number of
bacteria. Should the housewife desire to compare the cleanliness of
several lots of milk, she may filter a like quantity from each lot, say
a quart or a pint, through small disks of absorbent cotton. If, after
the milk has passed through the cotton disk, very little dirt remains on
it, as in Fig. 2 (_a_), the milk may be considered as comparatively
clean; if the cotton disk appears as in (_b_), the milk may be said to
be only slightly dirty; if it appears as in (_c_), the milk is dirty;
and if it appears as in (_d_), the milk is very dirty. Milk that leaves
a stain like that in (_d_) contains more bacteria than milk that leaves
a stain like that in (_c_), and so on through all the lots of milk.
Filtering milk in this manner, however, does not indicate whether the
bacteria are disease producing. Such information can be secured only by
microscopic examination, and only then by persons who have a knowledge
of such matters.
29. Since, as has been pointed out, bacteria cling to all dirt, the
dirt that milk contains is one of the causes of souring and putrefaction
of milk, and may be a cause of disease.
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