Protein – characteristics and methods of determination


The role of protein in the body 

The body of an adult human contains about 10 kg of protein. Ok. 3% of this protein is exchanged in the body. The basic role of the protein consumed is to provide the body with the right amounts and proportions of amino acids for the exchange of tissue proteins in adult organisms and the replacement and construction of new tissues in growing organisms. Proteins are responsible for the proper activity of enzymes, they act as a carrier of nutrients (vitamins, minerals) affecting their better absorption and use. They also act as buffer substances, maintaining the appropriate reaction in various body fluids (plasma, cerebrospinal fluid, digestive tract content). Protein deficiencies are particularly dangerous for children and pregnant women and during lactation. The protein requirement for an adult is about 1g / kg body weight depending on physical activity. 


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Transformation of proteins in the process of food storage and processing 

– Factors affecting thermal changes 

Heating to a certain temperature causes protein denaturation. Thus, cooking, baking, frying, pasteurizing or sterilizing changes the sensory and nutritional properties of food. These changes may be beneficial or undesirable. 

– Enzymatic changes 

In the meat and fish industry, heating is very often aimed at inactivating endogenous enzymes that can cause unfavorable schedule changes. 

– Color changes 

As a result of heating protein products, brown compounds are formed (Millard’s reaction – amine groups). 

– Formation of volatile aromas 

Heat treatment leads to the desired and undesirable odor characteristics.

Volatile aromas form in the skeleton of thermal decomposition of proteins and oxidation of amino acid residues. 

– Impact on nutritional value 

As a result of heat treatment, optimal sensory characteristics (color, smell, taste and rheological properties) develop in food under optimal conditions. Denaturation increases the digestibility of the protein. Protein anti-nutritional ingredients (protease inhibitors) are inactivated. The treatment also leads to the inactivation of undesired enzymes (myrosinases, lipases, proteases) and the destruction of endogenous and bacterial toxins.

Factors influencing the use of protein by the human system 

The digestibility of the protein is the susceptibility to the action of digestive enzymes in the digestive tract. Animal proteins have higher digestibility than vegetable proteins due to fiber. 

Proteolytic enzymes that hydrolyse peptide bonds are involved in protein digestion. The digestion of proteins begins in the stomach and continues in the duodenum. In the stomach, a pepsinogen is secreted by special glands. This inactive proenzyme is activated under the influence of the acidic environment that prevails in the stomach and transforms into active pepsin. Endopeptidases participate in the hydrolysis of peptide bonds within protein molecules. In the duodenum, however, peptidases (a component of the pancreatic juice) are synthesized by pancreatic cells and proteolytic proenzymes such as trypsinogen and chymotrypsinogen (activated respectively to trypsin and chymotrypsin) and exopeptidase. Exopeptidases disrupt extreme peptide bonds, as a result of which free amino acids are cleaved. These enzymes include, among others carboxypeptidases and aminopeptidases that hydrolyze peptide bonds on the side of the free carboxyl group or amino groups. Triplepeptidases and dipeptidases act on the tripeptides and dipeptides. Free amino acids are absorbed by the intestinal epithelium and through the intestinal villi go to the blood with which they are transported to the liver. 

Another enzyme is also secreted in the stomach, the so-called renin. This enzyme is found in young mammals. He is responsible for digesting the protein found in milk. 

Methods of quantitative protein determination 

Direct methods 

– Immunoenzymatic Elisa 

– UV spectroscopy 

– Biuret 

– Lovry 

– Formolowa method 

– Pope Stevens’ method 

– With tannin 

Indirect methods 

– Kjeldahla (dismissal) 

– Dumas 

Determination of the type of protein 

– Electrophoresis 

– Chromatography 

– Extraction 

Determination of the type of amino acids 

– Column and ion exchange chromatography 

– Lombard and de Lange method (determination of tryptophan) 

Determination of denatured protein 

– Sensory 

– Rheological 

– Protein solubility 

– Enzyme activity 

– Ability to bind water 

Determination of the nutritional value of the protein 

– CS (chemical score, a chemical measure of protein quality) 

– EAA (integrated index of essential amino acids) 

– DT (protein digestibility coefficient) 

– BV (biological value of the protein) 

– NPU (net protein utilization rate) 

– PER (protein growth performance index) 

– NPR (net increase protein performance index) 

– South Africa (relative protein value) 


You can read also: Protein, protein, how much should we consume per day?

Posted on: February 25, 2019

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