Counting Calories

Physical versus physiological energy value of food

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Every food label contains the ingredients, manufacturer’s information and expiration date along with energy/caloric content information. Regulations previously used the classic calorie unit (cal) and kilocalorie (kcal), but according to the current European regulation, the joule (J) and kilojoule (kJ) per unit of weight is now provided as well. But what exactly does this value tell us?

About Calories and Calorimeters

Figure 1: In an energy calorimeter (left), the sample is burned in a combustion chamber (right, flame) and the released heat is transferred to the surrounding water.A calorie represents the amount of energy needed to heat 1 g of water by one degree Celsius. Combustion calorimeters (see Fig. 1) are used to measure the physical energy content in food.

The sample is burned in a combustion chamber (flame) and the released heat is transferred to the surrounding water (Fig. 1). The water must be mixed enough to ensure uniform heat distribution. Using a temperature sensor, the temperature can be determined to a precision level prior to and after the test.

In the compact static jacket calorimeter from IKA shown here, the calorimeter’s work steps are completed automatically by the device. Since there is always a small flow of heat, this must be determined so that the temperature can be corrected, accomplished using one of the classic correction calculation methods in calorimeter standards for an isoperibolic calorimeter (Regnault-Pfaundler).

Physical Energy Value

Sample preparation is critical in determining energy value. Food should generally be placed in the calorimeter already freeze-dried and homogenized. The calorimeter provides the physical energy value, meaning that the sample was fully combusted. In our bodies, however, these processes do not work in the same way as in a combustion calorimeter. In humans, energy is used to synthesize substances needed by the body and for maintaining body temperature. Special energy-rich molecules are built up that can be used later for the biosynthesis of compounds. This means the organism never fully breaks down the material it takes in. The energy values measured in the calorimeter are thus generally higher than those listed on the food identification label, because these figures describe the value that is actually released from the organism—the physiological energy value.

Physiological Energy Value

Table 1: Determined Energy Value in Comparison to the Labelled Energy Values
  Physiological energy value/J/g Physical energy value/J/g Differences / Label-Calorimeter/J/g
Noodles

Label

15010

Calorimeter

16238

-1228
Yellow gummi bears 14590 14173 417
Sugar 17000 16475 525
Zwieback toast 16930 18435 -1505
Sugar beet syrup 12670 12527 143
Almonds 24660 29134 -4474

The degree of completeness with which the body breaks down the substances absorbed with food depends on the situation and on the particular individual. In addition, some physically oxidizable components such as dietary fiber are generally not broken down. Thus, to determine the physiological energy value, the energy content of the food must first be determined in a combustion calorimeter and then in the stool and urine. The energy in the food minus the energy in the waste is then the energy of the food actually released in the body of the person investigated.

For food labelling, however, an average person is assumed, and this may also be defined differently depending on the country. As a result, anyone who does not correspond to the average acts based on an incorrect energy value or caloric content.

Listing of Caloric Contents

Figure 2: The C 1 static jacket oxygen bomb calorimeter from IKA; the smallest calorimeter footprint available.Above, various foods were evaluated for their physical energy value. The samples were ground to a fine powder, contamination- free, in an IKA Tube Mill control (see Fig. 2). Table 1 shows the measured energy values compared to the specified energy value on the food label.

Since food can be utilized very differently from person to person, the physiological energy value is not equally applicable to all people. The physical energy value allows for better comparability of energy in different foods, as this can be determined directly, without detours and adjustments, by simple combustion in the calorimeter under pressurized oxygen.

Note: This is a shortened version of a longer application note.


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Labs Less Ordinary

Published: December 8, 2015

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