DAIRY PRODUCTS 229
evaporate on the steam bath until the alcohol has been removed or until the
dye is taken up by the silk. The dyeing test is sometimes unsatisfactory,
and in all cases a small portion of the alcoholic solution should be tested by
treating with an equal volume of hydrochloric acid or stannous chloride
solution. The common oil-soluble coal-tar dyes are rendered more red or
blue by the acid and are decolorized by the reducing agent. Most of the
natural coloring matters become slightly paler with the acid and are little
changed by the stannous chloride solution.
(2) Annatto Coloring Matter.—Pour on a moistened filter a basic solution
of the color obtained by shaking out the oil or melted and filtered fat with
warm, dilute sodium hydroxide solution. If annatto is present the filter
paper will absorb the color so that when washed with a gentle stream of water
it will remain dyed a straw color. Dry the filter and add a drop of stannous
chloride solution. If the color changes to pink the presence of annatto is
confirmed.