Application of Kjeldahl Method in Food Inspection

Kjeldahl method is a way to determine the total nitrogen in a compound or mixture. As the saying goes, food is the first necessity of the people, and protein is one of the six major nutrients. Protein is considered to be an important component of organism cell tissue. Protein in food is the only source of nitrogen for the human body and plays a role that sugar and fat can never replace. Nitrogen content is an important sign that distinguishes protein from other organic compounds. Then, if you want to know the protein content of food, you must first use Kjeldahl method to determine its nitrogen content. Kjeldahl method is to digest the sample with concentrated sulfuric acid under the condition of a catalyst to convert all organic nitrogen into inorganic ammonium salt, and then convert the ammonium salt into ammonia under alkaline conditions, which is distilled out with steam and absorbed by excess boric acid liquid, and then titrated with standard hydrochloric acid, eventually, the amount of nitrogen in the sample can be calculated. As the nitrogen content of protein is relatively constant, the protein content can be calculated from its nitrogen content, so Kjeldahl method is considered to be the most classic protein quantitative method. Kjeldahl method was first proposed by Kieldahl in 1883, and it is still used as a standard inspection method.


1. The principle of Kjeldahl method

Add concentrated sulfuric acid and catalyst to the sample, mix well, and then heat to digest and decompose. The carbon and hydrogen in the sample are oxidized into carbon dioxide and water, and the organic nitrogen is converted into ammonium sulfate. Alkalization distillation frees ammonia, absorbs with boric acid and titrates with sulfuric acid or hydrochloric acid standard titration solution, and multiplies the consumption of acid by the conversion factor to obtain the protein content.


2. Application of Kjeldahl method

(1) Determination of protein

According to national standards, be used to determine protein in food, crude protein in feed, crude protein in vegetable oils, and gluten protein powder.

(2) Determination of nitrogen

According to national standards, be used to determine non-protein nitrogen content in milk and dairy products, nitrogen content in starch and its derivatives, nitrogen content in soil, total nitrogen content in compound fertilizers, and ammonium nitrogen in fertilizers and urea etc. There are also cases of using Kjeldahl method to determine the content of ammonia nitrogen in water, natural rubber nitrogen and ammonium salt in soy sauce.

(3) Determination of volatile basic nitrogen

Some people use Kjeldahl nitrogen analyzer to determine the content of volatile base nitrogen in tuna meat, in meat and in fish meal. That is, Kjeldahl nitrogen analyzer is used to determine the content of volatile volatile base nitrogen in aquatic products, meat and its products and the uncertainty evaluation. In addition, there are also applications of Kjeldahl method to determine the content of formaldehyde in water products, the content of sulfur dioxide in daylily, and the volatile alkali in caprolactam.


This shows that the application range of Kjeldahl method is very wide. It is precisely because of the need to determine protein content and nitrogen content in various industries that Kjeldahl method has developed rapidly in China in recent years. Currently ,Kjeldahl method is widely used in grain and oil testing, feed analysis, plant nutrient testing, soil fertilizer testing, environmental protection, medicine and other industries.