Question: How is the amount of crude protein measured?
About the article "Cereal", section "Nutritional facts".

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Some grains are deficient in the essential amino acid lysine.
That is why many vegetarian cultures, in order to get a balanced diet, combine their diet of grains with legumes.
Many legumes, on the other hand, are deficient in the essential amino acid methionine, which grains contain.
Thus, a combination of legumes with grains forms a well-balanced diet for vegetarians.
Common examples of such combinations are dal (lentils) with rice by South Indians and Bengalis, dal with wheat in Pakistan and North India, and beans with corn tortillas, tofu with rice, and peanut butter with wheat bread (as sandwiches) in several other cultures, including Americans.
The amount of crude protein found in grain is measured as the grain crude protein concentration.
The classic assays for protein concentration in food are the Kjeldahl method and the Dumas method.
These tests determine the total nitrogen in a sample.
The only major component of most food which contains nitrogen is protein (fat, carbohydrate and dietary fibre do not contain nitrogen).
If the amount of nitrogen is multiplied by a factor depending on the kinds of protein expected in the food the total protein can be determined.
This value is known as the "crude protein" content.
On food labels the protein is given by the nitrogen multiplied by 6.25, because the average nitrogen content of proteins is about 16%.
The Kjeldahl test is typically used because it is the method the AOAC International has adopted and is therefore used by many food standards agencies around the world, though the Dumas method is also approved by some standards organizations.
Accidental contamination and intentional adulteration of protein meals with non-protein nitrogen sources that inflate crude protein content measurements have been known to occur in the food industry for decades.
To ensure food quality, purchasers of protein meals routinely conduct quality control tests designed to detect the most common non-protein nitrogen contaminants, such as urea and ammonium nitrate.
In at least one segment of the food industry, the dairy industry, some countries (at least the U.S., Australia, France and Hungary), have adopted "true protein" measurement, as opposed to crude protein measurement, as the standard for payment and testing: "True protein is a measure of only the proteins in milk, whereas crude protein is a measure of all sources of nitrogen and includes nonprotein nitrogen, such as urea, which has no food value to humans.
... Current milk-testing equipment measures peptide bonds, a direct measure of true protein."
Measuring peptide bonds in grains has also been put into practice in several countries including Canada, the UK, Australia, Russia and Argentina where near-infrared reflectance (NIR) technology, a type of infrared spectroscopy is used.
The Food and Agriculture Organization of the United Nations (FAO) recommends that only amino acid analysis be used to determine protein in, "inter alia", foods used as the sole source of nourishment, such as infant formula, but also provides: "When data on amino acids analyses are not available, determination of protein based on total N content by Kjeldahl (AOAC, 2000) or similar method ... is considered acceptable.
"The limitations of the Kjeldahl method were at the heart of the Chinese protein export contamination in 2007 and the 2008 China milk scandal in which the industrial chemical melamine was added to the milk or glutens to increase the measured "protein". 
Scyllatoxin is one of the components of the venom of the Israeli scorpion "‘Leiurus quinquestriatus hebraeus’".
It consists of only 0.02% of the total protein in crude venom. 
The method consists of combusting a sample of known mass in a high temperature (about 900°C) chamber in the presence of oxygen.
This leads to the release of carbon dioxide, water and nitrogen.
The gases are then passed over special columns that absorb the carbon dioxide and water.
A column containing a thermal conductivity detector at the end is then used to separate the nitrogen from any residual carbon dioxide and water and the remaining nitrogen content is measured.
The instrument must first be calibrated by analyzing a material that is pure and has a known nitrogen concentration.
The measured signal from the thermal conductivity detector for the unknown sample can then be converted into a nitrogen content.
As with the Kjeldahl method, conversion of the concentration of nitrogen in a sample to the crude protein content is performed using conversion factors which depend on the particular amino acid sequence of the measured protein. 
Poractant alfa is measured into vials of 80 mg of surfactant (extract) that includes 76 mg of phospholipids and 1 mg of protein of which 0.2 mg is surfactant protein B (SP-B).
The amount of phospholipids is calculated from the content of phosphorus and contains 55 mg of phosphotidylcholine of which 30 mg is dipalmitoylphosphatidylcholine.