HOW ENERGY OR FUEL VALUE IS MEASURED.—It was stated that the human body could be compared to an automobile, i.e. the "burning" of the fuel foods in the body produced the ability to do work. The quantity of energy that fuel food is capable of giving off is termed the fuel value of that food. Energy has been defined as the ability to do work. Since heat is energy, the fuel value of foods shows, in part, [Footnote 85: Although ash, water, and vitamines nourish the body, it is impossible to measure their nutritive value in terms of fuel value. Fuel value expresses the nutritive value only of the combustible foodstuffs,—carbohydrates, fats, and protein. However, according to Sherman, "the most conspicuous nutritive requirement is that of energy for the work of the body." Hence, the fuel value of a food is often spoken of as its nutritive value (see "Chemistry of Food and Nutrition," Second Edition, by Henry C. Sherman, Ph.D., p. 138).] their nutritive value. If the quantity of heat that is produced by burning a food is measured, the measurement indicates the quantity of energy that the food is capable of giving to the body.
Heat cannot be measured by weight or length, but by the change in temperature which it produces in a given weight of a certain material. The heat unit is not a pound or yard, but a Calorie, or a definite quantity of heat, which, when applied to materials, will produce change of temperature in those materials. If the temperature of one pound [Footnote 86: NOTE TO THE TEACHER—The avoirdupois system of measurement and the Fahrenheit scale of temperature are used in this text. It is believed by the author that less than ten per cent of all pupils taking this course will enter college. Hence, the use of the measurements that are more in keeping with the pupils' practical needs. For the small minority who will enter college, a thorough drill in the metric system is urged. The following formula gives the necessary information for changing from the Fahreheit to the Centigrade scale: Subtract 32 and multiply by 5/9.] of water is 70 degrees Fahrenheit, and it is desired to increase the temperature of that water to 74 degrees Fahrenheit, a certain quantity of heat will have to be applied. It has been found that the quantity of heat required to raise the temperature of one pound of water through any four degrees of the Fahrenheit scale is practically the same, i.e. the quantity of heat required to raise the temperature of one pound of water from 32 degrees to 36 degrees F. is about the same as the quantity of heat that must be applied to raise the temperature of one pound of water from 60 degrees to 64 degrees F. The unit of measurement of heat is taken as the quantity of heat required to raise the temperature of one pound of water through about 4 degrees F.
The Calorie, [Footnote 87: I.e. greater Calorie, distinguished from the lesser calorie by the capital C.] used for food calculation, is approximately the quantity of heat required to raise the temperature of one pound (pint) of water through 4 degrees F. If one pint of water were placed over a lighted burner and heated until it increased four degrees in temperature, approximately one Calorie of heat would have been applied to the water (see Figure 82).
HOW THE FUEL VALUE OF A FOOD MATERIAL IS MEASURED.—Scientists have worked with care to obtain accurate data for the measurement of the heat produced by foods burning in the body. The data accepted to-day differ from those given by Rubner some years ago. [Footnote 88: See "Chemistry of Foods and Nutrition," Second Edition, by Henry C. Sherman, Ph.D., p. 143, "Physiological Fuel Values.">[
1 gram protein yields 4 Calories 1 gram fat yields 9 Calories 1 gram carbohydrate yields 4 Calories Expressing grams approximately in ounces, these data become: 1 ounce of protein yields 113 Calories 1 ounce of fat yields 255 Calories 1 ounce of carbohydrate yields 113 Calories
[Illustration: FIGURE 82.—ILLUSTRATING THE AMOUNT OF HEAT REPRESENTED BY
ONE CALORIE.]
In order to find the fuel value of foods, it is necessary to know their composition. For such data United States Department of Agriculture Bulletin No. 28 is a valuable source.
Flour.—The fuel content of flour is (see United States Department of Agriculture, Bulletin No. 28, p. 58, All Analyses Average):
10.6 per cent protein; 1.1 per cent fat; 76.3 per cent carbohydrates. Then, 1 ounce of flour contains, 0.106 ounce of protein; 0.011 ounce of fat; 0.763 ounce carbohydrates.
The protein in one ounce of flour yields (113 x 0106 =) 11.97 Calories.