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The Fuel Value Of Food

( Originally Published Early 1900's )

How Fuel Value is Determined. By fuel value of food is meant the amount of energy (heat and work) which a given quantity of food will produce. As the fuel value of coal is measured, for the sake of convenience, in terms of the heat it will produce, so the fuel value of food. has been determined for all the common foods in terms of heat value, as can be seen from the tables of this book. The diagram shows an apparatus for measuring the fuel value of food. A weighed quantity of the food to be tested is placed in a crucible inside the closed vessel D. Electric wires W are placed in contact with the food so that it may be burned by means of an electric current. The vessel D is supplied with oxygen through the tube 0, and is surrounded by a measured ,quantity of water within vessel C. A and B are larger vessels to prevent loss of heat. The temperature of the water in C is read on the thermometer T. From the weight of the food, the quantity of the water, and the increase of temperature of the water due to the burning of the food, the "fuel value" of the food may be calculated.

Standard of Measurement of Heat Value. The temperature of the water, however, is not the same as the amount of heat in the water. Two liters of water would contain twice as much heat as one liter at the same temperature. The unit of temperature is a degree; the unit of heat is a Calorie (from the Latin, color= heat). A Calorie is the amount of heat necessary to raise one liter of water one degree Centigrade.

If, for example, in the experiment above, an ounce of food was found to raise the temperature of a liter of water forty degrees, we should say that an ounce of that particular food has a fuel value of forty Calories ; or a pound (16 ounces) has 640 Calories. The tables on food composition give the fuel value in Calories for each pound of some of the common foods.

Daily Fuel Value Requirements. The quantity of food required each day varies chiefly with the amount of exercise taken. It has been estimated that a grown person resting in bed needs sufficient daily food to furnish about 2000 Calories of heat. The energy requirement may rise to 5000 or more for a man doing hard manual labor. The average man requires food yielding about 3000 Calories of heat daily. About four-sevenths of this should come from carbohydrates; a little more than two-sevenths from fats, and a little less than one-seventh from proteins. The table on page 278 gives the quantities of common foods necessary to furnish 100 Calories of heat. From this it is easy to calculate a ration for a day or for one of the three daily meals.


The following experiments will prove of interest and should be performed in the laboratory. The tests given are the usual color tests for starch, protein, and reducing sugar. It is particularly important that the tests be applied in Experiments VIII and IX.

EXPERIMENT I. To test for starch. Place a drop of iodine solution on starch paste (a teaspoonful of starch boiled in a cup of water). If the paste is thick, the color will be an intense blue, almost black. Dilute the paste until the color is a bright blue.

There are four principal kinds of sugar* that concern us in the study of the common sugars: cane sugar, which is the common granulated sugar ; grape sugar, or glucose, found in grapes, but commercially manufactured from cornstarch ; milk sugar, in milk ; and malt sugar, in fermenting grains, especially barley, but also found in the mouth as a result of the digestion of starch.

The test here used is known as Trommer's test.' Prepare a 5% solution of caustic soda and a 10% solution of copper sulphate. Keep these in separate bottles until ready for use. To perform the test, take about half an inch of the copper solution in a test tube and add about the same quantity of caustic soda, or add this until a bright blue and almost clear solution results. Add the sugar solution to be tested and heat the mixture. If grape, malt, or milk sugar is present in the solution, the mixture will turn first yellowish, then red. These three sugars are known as "reducing sugars." No such effect is produced by cane sugar or by starch. Test them also and compare with a test made by heating Trommer's solution.

EXPERIMENT II. Test for grape sugar. Take some cheap stick candy or grind up some raisins and make a solution of either. Filter. Perform Trommer 's test on filtrate.

EXPERIMENT III. To change cane sugar to a reducing sugar. Make a solution of cane sugar. Add a few drops of hydrochloric acid to the solution and boil. Neutralize with NaOH and make Trommer's test. Reducing sugar is found to have resulted.

EXPERIMENT IV. To test for cane sugar. If the food to be tested is sweet or a sugar is thought to be present, make a solution and filter. Test the filtrate. If no reduction takes place, the sugar, if any, is cane sugar. Add a few drops of hydrochloric acid and boil. Neutralize with NaOH and perform Trommer's test. The cane sugar, if present; has been changed in art to grape sugar.

EXPERIMENT V. To change starch to grape sugar. Place a little starch paste in a test tube, add a quantity of acid, and boil ten or fifteen minutes. Note how the mixture clears up, which indicates that the starch is changed to some soluble substance. Neutralize with NaOH and make Trommer 's test.

EXPERIMENT VI. Two tests for proteins. (a) The "violet color" test for protein. Shake up the beaten white of an egg (almost pure protein) in 100 c.c. (3% oz.) of water. Place a little of the mixture in a test tube, add ten drops of caustic soda solution; then drop by drop add a little copper sulphate, shaking after each drop and noting the color changes. The violet color after several drops indicates protein.

(b) The "orange color" test for protein. Add several drops of strong nitric acid to a quantity of the egg white solution as prepared in (a). A white precipitate occurs. Boil. The precipitate becomes yellow. Cool the test tube and add ammonia; the orange color resulting indicates protein.

EXPERIMENT VII. To test for fats (roughly). A large quantity of fat or oil in a food, can, of course be recognized easily. A rough test for small quantities is as follows: mash the food to be tested in pure chloroform or ether. (If ether is used, beware of the flame.) Filter the liquid and place a drop on a clean white sheet of paper. If a grease spot is left after the chloroform or ether evaporates, the food contains fat.

EXPERIMENT VIII. Fopd Analysis. With the tests described above, analyze the foods your teacher gives you for sugar, starch, protein, and fats. Test onions, meat, liver, and milk. Test wheat flour in the following manner: make a small quantity of stiff dough. Take two tablespoonfuls and lay one aside for comparison later. Place the other in a sack of cheesecloth, and in a tumbler of water wash out all the starch possible, squeezing the rag vigorously and changing the wash-water occasionally. The gluten, a protein, is left in the rag as a sticky mass. Test this. Compare the quantity of gluten with that of the original spoonful of dough. What is the approximate proportion of protein to starch in wheat flour?

EXPERIMENT IX. To show the change of starch to sugar in the mouth. Test a cracker to be sure that the specimen contains no sugar to start with. Chew a cracker for as long a time as possible without swallowing. Then place the resulting milky mixture of cracker and saliva in a test tube and test for sugar, using Trommer's or Fehling's test. (The saliva may be added to a quantity of starch paste in a test tube kept at body temperature for half an hour and then tested.)

EXPERIMENT X. To show the digestion of protein. (a) First, make an artificial gastric juice in the following manner: to 100 c.c. of distilled water add 3/4% of 1 c.c. of strong hydrochloric acid. This makes a 3/4% solution. To this add a teaspoonful of pepsin.

(b) Prepare several tubes of solid egg-white. To do this place in a test tube several small glass tubes about half an inch long. A pipette or eye-dropper, broken into short lengths, will do. Over the tubes pour the beaten white of an egg. Be sure the glass tubes become filled with egg-white. Now coagulate or harden the egg-white by setting the test tube in hot water. When the egg-white is hardened, remove the glass tubes, for example, by breaking the test tube. Clean off the glass tubes on the outside and place them in a beaker of artificial gastric juice and set them in a warm place until most or all of the solid egg-white has been dissolved (digested) out of the tubes.

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