Energy Value of Foods Energy relationships in chemical reactions is a topic which is now included in most general chemistry courses and texts. A related area in which students have some interest and familiarity is the calorie content of foods. Many of the packaged food products on the shelves of the local supermarket now display an approximate nutritional analysis. A typical analysis includes percent protein, carbohydrate, and fat (lipid), along with the number of calories per ounce serving. Including a discussion of how the calorie content of foods is determined when illustrated by data from labels of food packages promotes student interest in the topic of energy changes. This paper briefly summarizes how the calorie cantent of food is determined and describes several examples using data from packaged foods. Euell Gibbons not withstanding, most students recognize that humans cannot derive nutritional value from eating trees (cellulose). Although cellulose has no caloric value in the body, both indigestible cellulose and digestible starch have the same heats of combustion. Since many foods sueh as leafy vegetables, fruits, and whole grain cereals contain cellulose, i t is apparent that the calorie value of foods cannot be determined by simply comhusting a sample of the desired food item in a calorimeter. Foods derive their caloric value from the protein, carbohydrate, and fat which they contain. Other food components such as water, minerals, and indigestable fiber do not contribute t o the caloric content. An approximate analysis of the major food components is performed as follows.' First, the moisture content is found from the weight loss during air drying. The fat content is then found from an ether extract determination. The protein content is based on the amount of reduced nitrogen present as determined by the Kjeldahl method. To obtain the protein content the average nitrogen content is multiplied by 6.25 (the protein factor). The indigestible fiber content is determined by digesting a sample first in dilute acid, and then in dilute base. The residue consists mainly of fiber and a little mineral matter. This residue is dried and weighed. It is then ignited to constant weight. The difference in the two weights represents the weight of indigestible fiher present in the sample. The percent of carbohydrate is given by subtracting the sum of the percentages of moisture, fat, ash, protein, and fiber from 100 percent. When carbohydrates and fats are metabolized in the body they are completely oxidized to carbon dioxide and water just as they are in the homb calorimeter. Therefore, the experimental heats of combustion are used to provide caloric values for these two nutrients. Although there are slight differences in heats of combustion among individual compounds of the same nutrient type, on the average the combustion of one gram of most carbohydrates liberates shout 4.0 kcal. Fats average about 9.0 kcal g-'. Proteins on combustion liberate about 5.6 keal g-I. However, protein is not oxidized completely in the body since the major end product of protein metabolism is urea. For this reason, the energy value of protein in the body is less than that obtained in the bomb calorimeter. The value 4.0 kcal g-' is used for the caloric value of protein in the body. When applying these values, it should be recognized that the word Calorie when associated with food is actually a kilocalorie. The number of calories corresponding t o an ounce of food can he determined from a knowledge of the weight of protein, carbohydrate, and fat present per ounce serving, and the use of the "4-4-9" kcal per gram heat of combustion relationship. As an example, we can illustrate the calculation of the calorie value of sunflower nuts. One ounce (28.4 g) of sunflower nuts contains 7 g of protein, 5 g of carbohydrate, and 12 g of fat. Using the "4-4-9" heat of combustion relationship, we first calculate the number of kilocalories liberated by each type of nutrient. Upon summing these and rounding off t o two significant figures, we obtain 160 kcal per ounce serving. The caloric content per ounce given on the label was also 160 Calories. The table shows some nutritional data from the labels of various types of packaged foods, and compares the reported calorie content with that estimated by the above procedure. Caloric Value of Some Packaged Food Items Item (Serving size1
Protein'
Carbohydrate
Fat
Calories
kcaib
corn Meal (1 o z i Non-Fat Dry Milk (3.2 or) Evaoorated Milk ( 4 fl. or.) s u n f l o w e r nuts ( 1 oz) Milk Chocolate (1.05 0 2 )
n Nutrientamounts are in grams. b Calculated caloric value. CThe heat of combustion of ethanol is 7 . 1 kcal g? which suggertr that this beer contains 11.49 ethanol.
Describing how the energy content of foods is determined can also provide a stimulus for further discussion of related topics such as: utilization of energy by the body to do work and synthesize new cell components, the necessity of a balanced diet, the logic (or lack of it) of fad diets, and the reason why humans can not metabolize materials sueh as cellulose. 1 (a) Triebold, H. O., and Aursnd. L. W., "Food Composition and Analysis," D. Van Nostrand Company, Inc., Princeton, N.J., 1963. (b) Hart, F. L., and Fischer, H. J., "Modern Food Analysis," Springer-Verlag. New York, N.Y., 1971.
Colorado S t a t e University F o r t Collins, 80523
80 / Journal of Chemical Education
Ned A. Daugherty Kenneth W. Watkins
