Packaged Food

Deepshree keeps telling me to become thin. And I just do not care as I do not know how to become thin. In today’s lecture I got some hope.

I learnt that in any food, the energy is given by Carbohydrates, Fats and Proteins. If one buys packaged food, the label generally mentions the number of calories in the food. The number of calories is an indicator of how much energy the food will produce. If I consume fewer calories, the chance of my becoming thin is a higher. For example, if I eat a cucumber, the number of calories consumed is very less; but the stomach will get filled as the volume of food consumed is comparatively much bigger (85 grams of cucumber generates 11 kcal of energy). For example, notice the following label.

Food Label Sample

Food Label Sample

The label says that in every 100g serving, there is 379 kcal of energy. That is if I have 100 grams of the food in the package, then my body can get at the maximum 379 kilo calories of energy. The number of calories that my body will actually get is going to be much lower as human body is not very efficient in converting food into energy. And my body is extremely inefficient as I am diabetic. However, the question is whether this food will actually give a maximum of 379 kcal of energy or not.

The first verification that can be done is to see if the calculations shown on the label is correct. To verify if the energy calculation is correct, one can do the following.

  • Multiply the number of grams of Protein in the Food Label by 4. (8.5 * 4 = 34 cal)
  • Multiply the number of grams of Carbohydrates in the Food Label by 4. (84 * 4 = 336 cal)
  • Multiply the number of grams of Fat in the Food Label by 9. (1 * 9 = 9 cal)
  • So, total energy content in 100g = (34 +336 + 9) cal = 379 cal

Now, the label could have been made by using an Excel Sheet. How do I know that actually every 100 grams of this food actually has 379 cal?

I have learnt a method. It is as follows.

  1. Take 100 grams of the food material.
  2. Take 1 kg of Water in a container. Preferably I would take a vessel with a very thin base and body and preferably an Aluminium vessel and not a Steel vessel. (Thermal conductivity of Aluminium is 235 W/m K; and thermal conductivity of Steel is 47 W/m K). The reason will become clear in Step 6. (Specific Heat of Water is 4180 Joules / gram-Kelvin or 4180 Joules / gram-Celcius).
  3. Place the vessel of water on the 100 grams of food material. A stand will need using for sure.
  4. Record the temperature of Water in the vessel. Let us call this temperature as T1.
  5. Now, burn the food material.
  6. Once, the food material is completely burnt, record the temperature of Water in the vessel. Let us call this temperature as T2. (Using a better conductor would make sure that least energy from the burning the food material would be lost in heating the vessel).
  7. Calculate Q = (1000 g * (4180 J / (g C)) * (T2 – T1)C) = x Joules.
  8. Now, 4180 Joules = 1 Calorie. So, x Joules can be converted to Calories. Let us say that it is y calories.
  9. If y is somewhere close to the 379, then the food label has reported the energy contents correctly.

The industries actually use a device called “Bomb Calorimeter” to find the number of Calories in a Food Material. There is least loss of energy from burning the Food Material in the Bomb Calorimeter which was not possible in the above method.

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