With our crude protein calculator, it is possible for you to determine the percentage of protein contained within a sample. We start with introducing the total Kjeldahl nitrogen method, but with a simple click, you can input the nitrogen content and mass of your sample to use the crude protein calculation formula for any method you want to use.
In the following article, we will explain the Kjeldahl method, which is important for determining crude protein amount. Read on to learn more about what crude protein is, crude protein vs. protein difference, and how to find crude protein by hand. Be sure to also check our protein solubility calculator for more information about this topic.
1. What is crude protein?
Before diving into what crude protein is, it is important first to explain that there are two types of measurements that can be performed to determine the amount of protein in food: these are called crude protein and true protein. So, let's explain this crude protein vs. protein difference.
Do proteins contain nitrogen?
Yes! In fact, the crude protein is something that can be calculated from the nitrogen content of food. An important reminder would be to explain that each of the building blocks used to create proteins, the amino acids, are full of nitrogen. It can sometimes be difficult to use crude protein as a determinant for the level of protein contained within your food since not all the nitrogen you might measure is found in protein. This type of measurement gives you information about the protein content.
The true protein content is the “direct” determination of the amount of protein in your food, and oftentimes, this value is more accurate when you want to give an indication of the protein level you have per food serving.
If you want to understand more about proteins, we have a protein calculator, which you can use to find the optimal protein intake for your weight and activity level and learn more about the significance of different types of proteins for our bodies.
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2. Why do we use crude protein equation if true protein is more accurate?
Crude protein is typically expressed as a percentage of the total weight of a substance. The word “crude” indicates that the measurement is only an approximation of the true protein content.
Measuring the crude protein content of a sample is widely used in the fields of nutrition, food science, or even agriculture to get a percentage estimation of the protein content of a substance in food. We use crude protein calculation formula when there is no particular need to know the “exact” true protein content.
Let's now make an introduction to how to find crude protein with our tool.
3. How to use the crude protein calculator
To use our tool, follow the below steps — you can skip the first four steps if you already know the nitrogen content in your sample:
Input the titrant volume, usually standardized sulfuric acid, that you used to titrate the ammonia.
Specify the titrant molarity you used.
Enter the sample mass you're analyzing.
Report the acid factor of your sample.
Our crude protein calculator will display the intermediate result, i.e., nitrogen content.
You can also enter the nitrogen content by hand if you don't use the crude protein equation based on the total Kjeldahl nitrogen method.
Select the protein conversion factor (Jones' factor) or use the standard factor of 6.25 if the factor is not known.
Don't forget to include the dilution factor of your final volume!
Enjoy your crude protein percentage!
4. How to calculate crude protein
To be able to determine the crude protein percentage, you need to use the Kjeldahl method, which is a laboratory analytical technique that consists of quantifying the nitrogen content of a digested sample. The nitrogen acts as an “understudy” for us to be able to estimate the protein concentration from the following crude protein calculation formula:
- — Crude protein expressed in percentages of the sample's total mass;
- — Nitrogen content expressed in percentages;
- — Conversion factor that usually equal ; and
- — Dilution factor of the used digested volume.
🙋 Jones' factors represent the average nitrogen-to-protein conversion factor. Since proteins typically contain around 16% nitrogen, dividing the measured nitrogen content by 0.16 (or multiplying by 6.25) provides an estimate of the protein content. If you want to obtain more precise results, though, our crude protein calculator offers a list of different substances for which you can select the Jones' factor.
If you are struggling with determining the molecular weight of your sample, or if you want to determine the different types of protein your sample contains, you can easily use the protein molecular weight calculator.
5. Nitrogen content formula from Kjeldahl method
The basic principle behind the reactions in the Kjeldahl procedure is the principle that the nitrogen content in a sample can be determined through a series of chemical reactions.
The first step in the Kjeldahl method involves the digestion of the sample with concentrated acid, e.g., sulfuric acid , which breaks down the protein into its constituent amino acids and other nitrogen-containing compounds. This converts the nitrogen into ammonium sulfate along with other byproducts. The ammonium ions, are then quantitatively determined by titration, allowing for the calculation of the sample's nitrogen content. Below, you can see the chemical reaction:
There is also an important factor to consider, and this is the solubility of the protein in this reaction. The solubility of a protein can affect its nitrogen content because the Kjeldahl method relies on the complete digestion of the protein into soluble forms.
🙋 If a protein is insoluble or only partially soluble in the acid, it may not be fully digested during the Kjeldahl procedure. As a result, some of the nitrogen in the protein may remain trapped in the insoluble residue, and the analysis would underestimate the true nitrogen content of the sample.
To obtain an accurate measurement of the protein's nitrogen content, it is crucial that the protein is completely soluble in the acid, ensuring that all nitrogen-containing compounds are converted into soluble forms that can be quantitatively analyzed.
6. What is the total Kjeldahl nitrogen method?
The Kjeldahl method entails the digestion of food through the use of a potent acid (boric acid), which liberates nitrogen. It is one of several available methods explaining how to calculate crude protein.
Subsequently, this liberated nitrogen is quantified employing titration techniques. The protein content is then determined by converting the nitrogen concentration using a standard factor, typically 6.25, equivalent to 0.16 grams of nitrogen per gram of protein. You must have observed this in the crude protein equation above.
Nevertheless, this method has its limitations, as it doesn't accurately measure true protein content. Indeed, the fixed conversion factor of 6.25 may not be appropriate for all types of proteins, necessitating adjustments based on the specific amino acid composition of the protein in question. Several studies have identified species-specific correction factors for nitrogen conversion: for instance, shrimp and fish are recommended to use a conversion factor of 5.6, cereal products (5.4), and red seaweed (4.59).
Here is the crude protein example for the procedure of digestion:
7. How to calculate nitrogen content in protein
We use the following equation to determine the nitrogen content from the Kjeldahl method:
- and — Volumes of titrant used in the blank test and titration, respectively;
- — Titrant concentration expressed in ;
- — Acid factor (it's one for hydrochloric acid);
- — The constant resulting from the nitrogen molecular mass and adjusting the crude protein equation to milliliters and grams; and
- — Represents the mass of the sample you are analyzing; i.e., the mass of the food, feed, or other material you want to determine the protein content for.
8. Normality of NaOH in the crude protein calculation formula
You might be wondering what the normality of NaOH means or how it is relevant for this crude protein calculator. The normality of a molecule means the concentration of solution, and in a broader sense, normality, is expressed in grams equivalents per liter of solution. You can use the following formulation to determine the normality of NaOH:
- — Mass of solute;
- — Volume of solution; and
- — Equivalent weight of solute.
If you want to explore in depth the usage of other chemical equations, you can, for instance, use the titration calculator to determine the molarity of a molecule with a known solution volume. You may also be interested in the normality calculator to learn more about this topic!
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