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Physical vs. Physiological energy value of food

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Dr Heinz-Dieter Isengard, Professor at the University of Hohenheim, Institute of Food Science and Biotechnology, Germany and Kai-Oliver Linde, Corporate Product Manager, IKA-Werke , Germany give a holistic view of ‘energy value’ of food

Every food label contains the ingredients, the manufacturer’s information and the expiration date along with information on the energy/ caloric content. EU regulation no. 1169/2011 as well as the Food Information Regulation 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? Where does the information come from and how is the caloric content precisely determined?

Calories and calorimeters

20160515ep30The word ‘calorie’ is derived from the Latin word calor for ‘heat’. A calorie represents the amount of energy needed to heat 1 g of water by one degree Celsius. So-called combustion calorimeters (see Fig. 1) are used to measure the physical energy content in food.

A sample under pressurised oxygen is completely burnt here under controlled conditions. All components of the previously homogenised and prepared food are completely oxidised. The organic components are present after combustion in the form of CO2, water and acids in the combustion chamber of the calorimeter.

20160515ep31As shown in Fig. 1, the sample is burned in a combustion chamber (flame) and the released heat is transferred to the surrounding water. The water must be mixed sufficiently enough in order to ensure uniform heat distribution. Using a temperature sensor, the temperature can be determined to a precision level of one ten thousandth of a degree celsius prior to and after the test.

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The compact static jacket calorimeter from IKA shown here has an uncontrolled (static) jacket — as the name suggests — that provides an insulating function. All of the work steps related to the calorimeter, such as water and oxygen handling, are completed, entirely automatically, by the device. Since there is always a small flow of heat, this must be determined so that the temperature can
then be corrected. This is accomplished using one of the classic correction calculation methods in calorimeter standards for an isoperibolic calorimeter (Regnault-Pfaundler) (see Fig. 2).

Physical energy value

20160515ep32The sample preparation is a decisive part in determining the energy value. Food should generally be placed in the calorimeter already freeze-dried and homogenised. The result is influenced mostly by the water content of the sample.

The calorimeter provides the so-called physical energy value.

This means that the sample was fully combusted. In our bodies, however, these processes do not work in the same way as in a combustion calorimeter, but are rather more staged in a great number of individual steps during which a comparably very small amount of energy is released.  This energy is used for the synthesis of substances needed by the body and for maintaining the body temperature. Special energy-rich molecules are built up that can be used later and at other points for the biosynthesis of compounds. In other words, one does not need to constantly eat in order to have energy available and to build up materials. This means that the organism never fully breaks down the material it took in; it eliminates a part thereof, primarily a part that it cannot break down, but which can be physically burned.

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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 so-called physiological energy value.

Physiological energy value

So that a person can use food optimally, one prerequisite is first to crush it well, to chew it, to allow the saliva to act, then it can be better digested and the individual components optimally processed by the healthy body. Fats are in some cases stored or, to cover basic energy needs, are accessed directly and converted into energy. However, the preparation of the food is important here.  Some foods may be poorly digested in the raw state and poorly utilised by the body.

In order to know the optimal personal energy value for food, one would first have to determine one’s own basic energy expenditure. There are studies in which people were evaluated specifically in that regard.  This includes, among other things, breathing, CO2 output (‘combustion efficiency’ of the body), at rest and while working.

As mentioned, the body does not always break down the substances absorbed with food with the same degree of completeness.  This depends on the situation at the moment, but also, of course, on the particular individual.  In addition, some physically oxidisable components such as dietary fibre are generally not broken down because humans lack the necessary enzymes.  Thus, in order to determine the physiological energy value, the energy content of the food must first be determined in a combustion calorimeter and then also the stool and urine examined.  The energy in the food minus the energy in the faeces and urine is then the energy of the food actually released in the body of the person investigated.

Such studies are common practice, for example in the field of agriculture and animal feed research.  Here the standard DIN EN ISO 9831 is frequently applied, such as in carrying out nutritional studies [1].

For food labelling, however, an average person is assumed, and this may also be defined differently depending on the country, which exhibits an assumed average metabolism for processing food.  In humans, the process of food processing, energy use and usability is still affected by a variety of other factors.  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

20160515ep33Various 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. 3).  Table 1 shows the measured energy values compared to the specified energy value on the food label.

Since, however, as already mentioned above, food can be utilized very differently from person to person, and depending on how well cooked and chewed the food is, the physiological energy value is not equally applicable to all people. The physical energy value allows for better comparability of the energy in different foods, as this can be determined directly and without detours and adjustments by simple combustion in the calorimeter under pressurised oxygen.  Under this precondition, would a physical energy value not be better suited for a general comparison of the energy/ caloric content of foods?  Where applicable, the energy content of substances that are indigestible by humans could be subtracted from the total value.

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