Fuel values of foods

Fuel Values of Foods and Some Common Fuels... 

The bomb calorimeter described in Section 6.4 is ideally suited for measuring the energy content, or "fuel value," of foods. Fuel values are just the enthalpies of combustion (see table). In order to be analyzed in a bomb calorimeter, food must be dried first because most foods contain a considerable amount of water. Since the composition of particular foods is often not known, fuel values are expressed in terms of kj/g rather than kj/mol. 

A bomb calorimeter that may be used to measure heat released upon combustion of a sample. This device is commonly used to determine the fuel value of foods. The bomb calorimeter Is similar to the "coffee cup" calorimeter. However, note the electrical component necessary to Initiate the combustion reaction. 

The fuel value of food is an important concept in nutrition science. The fuel value is generally reported in units of nutritional Calories. One nutritional Calorie is equivalent to one kilocalorie (1000 calories). It is also known as the large Calorie (uppercase C). 

The amount of energy per gram of food is referred to as its fuel value. Fuel values are commonly reported in units of nutritional Calories (1 nutritional Calorie = 1 kcal). A bomb calorimeter is useful for measurement of the fuel value of foods. 

Caloric values of foods (physiological fuel values) are enthalpies of combustion but with an opposite sign, (-AH ), and corrected for energy lost in urine (e.g., as urea) and feces. While enthalpies of combustion of foods are all negative, the caloric values are given as positive numbers. Caloric values for proteins are... 

Fats and carbohydrates are the principal sources of energy in foods. Pure fat has a caloric value (heat of combustion) of 4080 kcal per pound, and pure carbohydrate (sugar) a caloric value of about 1860 kcal per pound. The caloric values of foods are obtained by use of a bomb calorimeter, just as was described above for fuels. The third main constituent of food, protein, is needed primarily for growth and for the repair of tissues. About 50 g of protein is the daily requirement for an adult of average size. Usually about twice this amount of protein is ingested. Tliis amount, 100 g, has a caloric value of only about 400 kcal, the heat of combustion of protein being about 2000 kcal per pound. Accordingly fat and carbohydrate must provide about 2600 kcal of the 3000 kcal required daily. 

Most ctiemical reactions used for the production of heat are combustion reactions. The energy released when one gram of any substance is combusted is the fuel value of the substance. The fuel value of any food or fuel can be measured by calorimetry. 

TABLE 5.4 Compositions and Fuel Values of Some Common Foods ... 

Estimating the Fuel Value of a Food from Its Composition... 

FOODS AND FUELS (SECTION 5.8) The fuel value of a substance is the heat released when one gram of the substance is combusted. Different types of foods have different fuel values and differing abilities to be stored in the body. The most common fuels are hydrocarbons that are found as fossil fuels, such as natural gas, petroleum, and coal. Renewable energy sources include solar energy, wind energy, biomass, and hydroelectric energy. Nuclear power does not utilize fossil fuels but does create controversial waste-disposal problems. 

Why Do We Need to Know This Material The laws of thermodynamics govern chemistry and life. They explain why reactions take place and let us predict how much heat reactions release and how much work they can do. Thermodynamics plays a role in every part of our lives. For example, the energy released as heat can be used to compare fuels, and the energy resources of food lets us assess its nutritional value. The material in this chapter provides a foundation for the following chapters, in particular Chapter 7, which deals with the driving force of chemical reactions. 

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