Foods and the Theory of Digestion : Part 5

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 13 of 27)

Phosphorus. - Examine a small piece of phosphorus, noting that it has to be kept under water. Lay a small piece on the table and observe the tiny stream of white smoke rising from it, formed by slow oxidation. Dissolve a piece as large as a pea in a teaspoonful of carbon disulfide in a test tube, pour this on a piece of porous paper, and lay the paper on an iron support. When the carbon disulfide evaporates the phosphorus takes fire spontaneously. (The heat from the slow oxidation is sufficient to ignite the phosphorus in the finely divided condition.) What is the most striking property of phosphorus? What purpose does it serve in the match?

Sulfur. - Examine some sulfur, noting its color and the absence of odor or taste. (Impure sulfur may have an odor and a taste.) Burn a little sulfur in an iron spoon, noting that the compound which it forms with oxygen by burning has a decided odor.

Other Elements. - Magnesium. Examine and burn a piece of magnesium ribbon, noting the white compound of magnesium oxide which is formed. Iron. Examine pieces of the metal and also some of its compounds, as ferrous sulfate, ferric chloride, and ferric oxide or iron rust. Sodium. Drop a piece of the metal on water and observe results. Sodium decomposes water. It has to be kept under some liquid, such as kerosene, which contains no oxygen. (It should not be touched except with the fingers wet with kerosene.) Chlorine. Pour strong hydrochloric acid on a little manganese dioxide in a test tube, and warm gently over a low flame. The escaping gas is chlorine. Avoid breathing much of it.

Composition of the Nutrients. - The simplest way of determining what elements make up the different nutrients is by heating them and studying the products of decomposition, as follows:

To show that Carbohydrates contain Carbon, Hydrogen, and Oxygen. - Place one half teaspoonful of powdered starch in a test tube and heat strongly. Observe that water condenses on the sides of the tube and that a black, charred mass remains behind. The black mass consists mainly of carbon. The water is composed of hydrogen and oxygen. These three elements are therefore shown to be present in the starch. The experiment may be repeated, using sugar instead of starch.

To show that proteins contain Carbon, Hydrogen, Oxygen, Nitrogen, and Sulfur. - Place in a test tube some finely divided protein which has been thoroughly dried (dried beef or the lean of hard cured bacon). Heat strongly in the hood of a chemical laboratory or some other place where the odors do not get into the room. First hold in the escaping gases a wet strip of red litmus paper. This will be turned blue, showing ammonia (NH3) to be escaping. Next hold in the mouth of the tube a strip of a paper wet with a solution of lead nitrate. This is turned black or brown on account of hydrogen sulfide(H2S) which is being driven off. Observe also that water condenses in the upper part of the tube and that a black, charred mass remains behind. Since the products of decomposition (H2O, NH3, H2S, and the charred mass) contain hydrogen, oxygen, nitrogen, sulfur, and carbon, these elements are of course present in the protein tested.

To show the Presence of Mineral Matter. - Burn a piece of dry bread by holding it in a clear, hot flame, and observe the ash that is left behind. This is the mineral matter present in the bread.

Tests for Nutrients. proteins. - Cover the substance to be tested with strong nitric acid and heat gradually to boiling. If protein is present it turns yellow and partly dissolves in the acid, forming a yellow solution. Let cool and then add ammonia. The yellow solid and the solution are turned a deep orange color. Apply this test to foods containing protein such as white of egg, cheese, lean meat, etc.

Starch. - (a) Place a small lump of starch in one fourth of a pint of water and heat gradually to boiling, stirring well. Then add enough water to form a thin liquid and fill a test tube half full. Add to this a few drops of a solution of iodine. (Prepare by dissolving a crystal of iodine in 25 cubic centimeters (1/20 pint) of a solution of potassium iodide in water and add water to this until it is a light amber color.) The starch solution is turned blue, (b) Cut with a razor a thin slice from a potato. Place this in a weak solution of iodine for a few minutes and then examine with the microscope, using first a low and then a high power. Numerous starch grains enclosed in cellulose walls will be seen.

Dextrose, or Grape Sugar. - Place a solution of the substance supposed to contain grape sugar in a test tube and add a few drops of a dilute solution of copper sulfate. Then add sodium hydroxide solution until the precipitate which first forms is redissolved and a clear blue liquid obtained. Heat the upper portion of the liquid slowly to near the boiling point. A little below the boiling point the blue color disappears and a yellow-red precipitate is formed. If the upper layer of the liquid is now boiled, the color deepens and this may be contrasted with the blue color below. Apply this test to the sugar in raisins and in honey.

Fat. - Fat is recognized by its effect on paper, making a greasy stain which does not disappear on heating and which renders the paper translucent. Try butter, lard, or olive oil. Also show the presence of fat in peanuts by crushing them in a mortar and rubbing the powder on thin paper. If the substance to be tested contains but little fat, this may be dissolved out with ether. If a drop of ether containing the fat is placed on paper, it evaporates, leaving the fat, which then forms the stain.

To show the Effect of Alcohol upon protein. - Place some of the white of a raw egg in a glass vessel and cover it with a small amount of alcohol. As the albumin (protein) hardens, or coagulates, observe that the quantity of clear liquid increases. This is due to the withdrawal of water from the albumin by the alcohol. Since the tissues are made up chiefly of proteins, a piece of muscle or of liver may be used in the experiment, instead of the egg, with similar results.

To illustrate the Digestive Process. - To a tumbler two thirds full of water add a little salt. Stir and observe that the salt is dissolved. Taste the solution to see that the salt has not been changed chemically. Now add a little powdered limestone to the water and stir as before. Observe that the limestone does not dissolve. Then add some hydrochloric acid and observe the result. State the part played by the acid and by the water in dissolving the limestone. Apply to the digestion of the different classes of foods.

D.C. Heath and Co. - Publishers
Original copyright 1909