chemical elements, namely, nitrogen, sulphur, and phosphorus; in fact, proteins are the most complex of all chemical substances known. Following is the composition of the various nutrients studied thus far: Starch, composed of C, H, O (in the proportion of C6H1005). Protein, composed of C, H, O, N, S, P (and sometimes of other elements). Having demon 22. Tests for the food substances. strated that the various food substances are chemical compounds, each composed of several chemical elements, we are now to carry on experiments by which it will be possible to test for each of these food substances. By this means we shall be able to prove the presence or absence of starch, grape sugar, protein, fat, mineral matters, and water in the foods used by plants, animals, and man. 23. To test foods for starch. Laboratory Study No. 8. Materials: Corn starch, grape sugar, white of egg, fat or oil, salt, water; various foods in the home kitchen; iodine solution (see below); test tubes; gas burner or alcohol lamp. Preparation of iodine solution: A quart (1000 cc.) of iodine solution is made by dissolving in 5 teaspoonfuls (40 cc.) of water, onehalf teaspoonful (4 grams) of potassium iodide, and one-fourth this amount of iodine (1 gram). This solution, when thoroughly mixed, should be diluted to make one quart (1000 cc.). In a clean bottle this mixture will keep indefinitely.1 1. Put a small amount (size of a pinhead) of corn starch in a test tube, add water, shake the mixture, and boil it over a gas flame. Pour into the starch mixture 1 From Peabody's "Laboratory Exercises." Henry Holt & Co. thus formed a few drops of iodine. Describe the experiment, and state what color is produced. 2. Try the effect of iodine on each of the other food substances as follows: Put a small amount of grape sugar into a test tube; into a second tube put some white of egg (protein); into a third some fat or oil; into a fourth some mineral matter (salt); and into a fifth some water. Add a little water to each and boil as in 1 above to cook each nutrient. Add a drop or two of iodine solution to each test tube. Tell what happens. Do any of the colors thus produced resemble at all the color resulting from the addition of iodine to starch? 3. From the preceding, state how you can determine whether or not a substance contains starch. 4. (Optional home work.) Test as many foods as you can (e.g. oatmeal, flour, raw meat, milk, parsnip, potato, onions, apples, beans, rice, pepper) in the following way: Put a small amount of a given food into a test tube or in a sauce pan, add a little water, and boil to cook each food, then add a few drops of iodine. Before making each test make sure that the test tube or saucer is clean. Prepare in your note-book a table like the following, and fill in under each head the names of the foods you have proved to contain or to be without starch. STARCH PRESENT STARCH ABSENT 24. To test foods for grape sugar. Laboratory Study No. 9. Materials: Grape sugar, corn starch, white of egg, fat or oil, salt, water; various food substances common in home kitchen; Fehling's solution (see below); test tubes, gas burner or alcohol lamp. Preparation of Fehling's Solution:- To make Fehling's solution dissolve 3 teaspoonfuls (34.64 grams) of pure pulverized copper sulphate (blue vitriol) in a little less than a half-pint of water (200 cc.). Make a second solution by dissolving in a pint (500 cc.) of water twelve heaping teaspoonfuls (150 grams) of Rochelle salt and 3 (5inch) sticks of caustic soda (50 grams). Fehling's solution does not keep for any great length of time, and hence must be made up fresh a short time before it is needed. To do this, thoroughly mix two volumes of the copper sulphate solution and five volumes of the solution of Rochelle salt and caustic soda, and dilute the mixture with an equal volume of water. It is more convenient to prepare it in small quantities from the tablets that may be obtained of druggists. Before making any tests boil a small quantity of the Fehling's solution in a clean test tube. If it retains its transparent blue color, it is ready for use; otherwise a fresh supply must be prepared.1 1. Dissolve a small amount of grape sugar (glucose) in water in a test tube. Add some Fehling's solution and boil. Describe the experiment and the change in color. 2. Try the effect of Fehling's solution on each of the other food substances as follows: Put a small amount of starch into a test tube; into a second tube some white of egg; into a third tube some fat or oil; into a fourth tube some mineral matter (salt); and into a fifth tube some water. Add a little water to each tube, then pour in a small amount of Fehling's solution and boil as in 1 above. Do any of the colors produced resemble at all the color of the Fehling's solution when it was boiled with grape sugar? 3. From the preceding experiments state how you can determine whether or not a substance contains grape sugar. 4. (Optional.) Test as many foods as you can (e.g. onions, grapes, pears, granulated sugar, honey, molasses, parsnip, raw meat, milk, egg) in the following manner: Put a small amount of a given food into a test tube, add a little water, and a 1 From Peabody's "Laboratory Exercises." Henry Holt & Co., New York. C small spoonful of Fehling's solution, and boil. Before making each test make sure that the test tube is clean. Prepare in your note-book a table like the following, and fill in under each head the names of the foods you have proved to contain or to be without grape sugar. GRAPE SUGAR PRESENT GRAPE SUGAR ABSENT 25. To test foods for proteins (albuminous or nitrogenous substances). Laboratory Study No. 10. Materials: White of egg, corn starch, grape sugar, mutton tallow or other fat, salt, water; piece of meat, milk; concentrated nitric acid, ammonia; test tubes; gas or alcohol lamp. 1. Pour a little concentrated nitric acid on a piece of hard boiled egg in a test tube. a. Describe the experiment and the change in color. b. (Optional.) Wash the egg with water, add a little concentrated ammonia, and note result. 2. Try the effect of nitric acid on each of the other food substances as follows: Put a small amount of starch into a test tube; into a second tube some grape sugar; into a third some mutton tallow or other fat; into a fourth some mineral matter (salt); and into a fifth tube some water. Add a little concentrated nitric acid to each of these foods. Is any color produced like that resulting from adding nitric acid to protein? (In case a liquid is to be tested with nitric acid, the mixture should be boiled before deciding whether protein is present or absent.) 3. From the preceding experiments state how you can determine whether or not a food contains protein. 4. (Optional.) Prepare in your note-book a table like the following, and place in the proper columns the names of the foods tested in class. PROTEIN PRESENT PROTEIN ABSENT 26. To test foods for fats. Laboratory Study No. 11. Suggested as home work. Materials: Butter or olive oil, corn starch, grape sugar, piece of boiled white of egg, salt, water; various foods in the home kitchen, including nuts. 1. Put on a piece of paper a piece of butter half the size of a pea (or a drop of olive oil). Put the paper in a warm place (e.g. in a hot oven or over a heated radiator) for a few moments, then hold the paper between yourself and the light. Describe what you have done, and state the effect of the fat on the paper. 2. Try the effect of each of the other food substances (starch, grape sugar, piece of boiled egg, i.e. protein, salt, i.e. mineral matter, water) on paper, by adding an equal quantity (half the size of a pea) of each to separate pieces of paper. Put the pieces of paper in a warm place as in 1 above. Hold each piece between yourself and the light. State what you have done. Do any of these food substances affect the paper as did the fat? 3. From the preceding experiments state how you can determine whether or not a food contains fat. 4. (Optional.) Prepare in your note-book a table like the following, and place in the proper columns the names of the foods tested at home or in class. |