Crash course: Determining nutritional value of food

Ever read the labels on your food products? If yes, ever wondered how they come to the nutritional values stated on these labels? Or what they mean?

Whereas people tend to think these are hard fixed numbers that have been analyzed in full detail, this is often not the case (nor is it necessary or possible). Foods are natural products so there is a lot of variation in their nutritional values. But still, being a good scientist, you’d probably want to know somewhat more about these numbers and how we come to them.

Therefore, today a crash course in determining the nutritional value of foods. We’ll be discussing carbohydrates, calories, proteins, fats, vitamins and minerals and we’ll discuss how we know how much of all of these is in our foods.

Why nutritional values?

When it comes to understanding food, nutrition is one of the most commonly discussed topics. What foods are good for me? What should I eat to live a long and healthy life? Good questions, that I believe no-one can fully answer for everyone (here’s a great article from the New York Times showing  just how we can all disagree).

Therefore, I tend to stay clear from ‘what’s good for you discussions’ and ‘what’s healthy for you’. That said, I do like understanding my food and its analysis. So instead of discussing what’s healthy, we’ll limit ourselves to discussing what’s in our food. Then it’s up to dieticians and nutritionists to say whether that’s healthy or not. The nutritional values on our food do just that!

What is the nutritional value?

Simply said, the nutritional value of foods tells you how much energy the food contains as well as which nutritional molecules are present in the food and how much. In reality, it’s nothing more than food chemistry & chemical analysis: you determine which molecules are present and how many.

By looking into the nutritional value consumers will be able to determine how much energy is in their food but also which nutrients are present. Typical nutritients mentioned in a nutritional value are the proteins, carbohydrates, fats, vitamins and minerals.

Labelling the nutritional value

Within the European Union there is legislation stating how and when the nutritional value should be labelled on food products. The labelling is regulated through the EU 1169/2011 directive. This is a very extensive piece of regulation on how food products should be labelled. The regulation defines things such as the name of products (you’re not allowed to call something ‘strawberry yoghurt’ if there’s no strawberries in there for instance), but also weight, shelf life dates, etc.

It also describes the nutrition labelling. It describes which nutritional values have to be on the label, how they have to be determined, etc.

Once a manufacturer knows which data has to be put on a label it is time to determine the values for the label. So let’s dig into how these numbers are determined, starting with the energy content.

Energy content

The energy content can be determined using a simple calculation. We know which groups of molecules contribute to the energy content of a food. We also know how much energy each of these types of molecules contain and will give to your body.

So which are these molecules that give your body energy? Well, we’ve seen the three most relevant groups when discussing food chemistry basics: carbohydrates, fats and proteins. Besides that some other groups can contribute to the energy content: alcohols, polyols, fibre and organic acids. But in most foods the carbohydrates, proteins, fats and fibre take up most of the energy content.

It has been defined by analysis how much energy one gram of these different groups of molecules will give to your body (the regulation mentioned above also contains this information). Calculating energy content is then done as follows:

  1. Determine how much of each component is in the food (we’ll come back to that later)
  2. Multiply this value with the energy content for each specific component.
  3. Count up all these numbers and voila, you have the energy content of your food! Let’s do an example here:

Table 1: Calculating the energy content of 50g of product A

Mass in product (g) Energy content of molecules (kcal/g) Energy in product (kcal)
Fats  5 9 45
Proteins  8 4 32
Carbohydrates  3 4 12
Total 89 kcal

This simple calculation has now told us that 50g of product A contains 89 kcal. Whether this is a healthy product or not, no idea! That will depend on all the other nutrients present in this product.

Determining fat, carbohydrate & protein content

So now we know how to calculate the energy content of a product. But in order to do that we needed the quantities of energy containing ingredients such as fats, proteins, etc.

There are two ways to determine these quantities:

  1. The analytical route
  2. The literature route

Option 1: Literature analysis

Let’s start with the 2nd option: the literature route. In this case, no analysis of the actual final product is done. Instead, databases which contain a lot of data on nutritional values of all sorts of products are used.

The USDA has a very extensive database. It contains the nutritional value of a lot of different products, both processed and unprocessed foods. In the Netherlands a database exists as well, from the RIVM.

In case of an unprocessed food, chances are pretty high you will find the nutritional value of your product in one of the tables. In you have a processed food though (e.g. a muffin you baked or a salad you made) your product will not be there, especially if you invented a new recipe!

So, you have to start calculating, in a same way as we calculated energy content. Make a list of all the ingredients in your product. Write down how much of each is in there. Look up the nutritional values of each ingredients per gram of ingredient. Now multiply the amount of material with the nutritional values and you’ve got your values!

Option 2: Analytics

Of course, when there is no available literature data, another way has to be used, that of analyzing the food. This can de done using various chemical analysis techniques.

But, before doing any analysis it is important to consider one thing. Foods are very heterogeneous, that is, they are not identical throughout the whole product. Also, every item is just a little different. For instance, milk milked in summer is often very different from milk milked in winter. The same goes for a lot of other products. Therefore, it is impossible to determine the carbohydrate, protein and fat content of every food to the tenth of gram. There is always some variety and therefore inaccuracy of the results.

Protein analysis – Determining nitrogen content

Foods can contain a lot of different proteins. Each of these is slightly different therefore it is virtually impossible to analyze all the different proteins in a food. Luckily, for labelling only the overall protein content has to be given.

For determining this content a work-around is used. Instead of analyzing protein content, the amount of nitrogen in a sample is analyzed. Proteins contain quite a bit of nitrogen. Moreover, most of the molecules in food that contain nitrogen are actually proteins. Nevertheless, the methods aren’t 100% perfect. Not all proteins contain the same fraction of nitrogen, which causes the calculation of protein content from the nitrogen content to be off at times.

There are two main analysis techniques used for this: the Kjeldahl method and the Dumas method. For some further reading on the details of this method, here are here are two nice articles.

Carbohydrate analysis

Carbohydrate analysis is actually even harder than protein analysis. In most cases the carbohydrate concentration is simply determing by taking the total mass of a product and subtracting all the other components (fats, protein, water, ash & alcohol).

Since there are so many carbohydrates (think of the mono-, di-, oligo- and polysaccharides) there is no general overlaying analysis method. There are methods to identify individual (groups of) carbohydrates though. However, in a lot of cases the method mentioned above is used.

Feel like some more chemistry? The FAO and the University of Massachutes Amherst have written about it in more detail.

Fat content analysis

Most of the fats in foods are the so-called triglycerides. These can be analyzed in foods pretty accurately using a technique call gas chromatography. This is however a more expensive and completx technique. There are other methods as well. Near infrared radiation for instance is used, although more for a process control reason, than an analytical method.

Also, since fats do not dissolve in water a commonly used method is that of extraction. This uses the fact that carbohydrates, proteins, etc. prefer to sit in water, whereas fats do not and prefer another type of solvent. This way the fats can be extracted from the product and their quantity can be analyzed.

Reading your label

So next time, when you’re reading a label of your food, just think about all this and the complexity of properly analyzing and studying foods. Their analysis isn’t easy, nor is the determination of the nutritional value! Luckily, foods remain a natural product (most of the time that is), so will always be slightly different and remain interesting that way.

One Comment

  1. I thought it was interesting that you mentioned how the EU has different regulations based on the nutritional value of the product. I lived in Europe before, but never noticed this difference. It’s great that there are regulations in place and that we can make informed decisions on our diets based off of this information.

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