On the composition of food and how it is adulterated : with practical directions for its analysis / by W. Marcet.

  • Marcet, W. (William), 1828-1900.
Date:
1856
Catalogue details

Licence: Public Domain Mark

Credit: On the composition of food and how it is adulterated : with practical directions for its analysis / by W. Marcet. Source: Wellcome Collection.

    29/236 (page 9)
    dissolved in water. Enough sulphate of copper is added to a sample of the clear aqueous extract to give it a decidedly blue colour; the potash solution poured into this mixture vrill first induce a precipitate, disappearing, however, when an excess has been used, and a dark blue fluid is thus prepared, which is now to be heated; as soon as it com- mences to boil (if any sugar be present,) a red precipitate will occur in the liquid, subsiding to the bottom of the test tube, and consisting of suboxide of copper. In order to avoid the trouble of preparing solutions of sulphate of copper and caustic potash as often as a test for grape sugar is wanted, a test fluid can be conveniently made by adding, first, a small quantity of potash to a solution of sulphate of copper, and then adding enough tartaric acid to dissolve the precipitate thus obtained. By mixing, subsequently, a large quantity of potash with this solution, it will acquire a deep blue colour, and may be preserved in this state as an excellent means of detecting the presence of grape sugar. Another sample of the clear fluid extract from the flour deprived of albumen by boihng and subsequent filtration, is now evaporated to dryness, and the residue boiled with alcohol to remove all the sugar; the substance insoluble in alcohol consists of dextrine, which differs but slightly from gum, or mucilage. When dissolved in water, and boiled with a diluted mineral acid, dextrine as well as gum is con- verted into grape sugar; thus, with the tartrate of copper and potash test, the presence of these substances may also be determined. A small quantity of hran is usually found in flour; it is obtained by mixing the flour with a large quantity of water, and then decanting the fluid before the starch has time to subside. The bran will then be observed settled at the bottom of the vessel.