Question

The Kjeldahl method is the most widely used method for determining the protein content of foods. It is based on the fact that protein is the only macronutrient (major nutrient) that contains nitrogen. The method gives the value of g of nitrogen per 100 g of food. That value is then multiplied by a constant, 6.25 g protein per g of nitrogen, to give the result expressed as g of protein per 100 g of food. a. Explain why this constant, 6.25, gives only an approximate value of the proteirn content of food. What other nutrients in foods affect the value of g of nitrogen per 100 g of food, and explain how these will affect the final result? b. In terms of overall amino acid composition, and primary, secondary and tertiary structure, explain why proteins can vary so much in their physicochemical properties, like solubility in water and susceptibility to heat and pH. OR From your general knowledge of food chemistry and your personal experience, explain why cooking of foods increases the nutritional value, and minimises deterioration of food quality with time. Your answer may extend to different methods of cooking.

Answer 1

a. there are total 20 type of amino acids monomer by which protein polymer could be made-up of. Sequence of these monomers are also different in different protein macromolecules. In protein chain each amino acid is bounded with amide bond but depending on amino acid the 'R' group is different for each amino acid so the constant 6.25 gives only an approximate value of protein. Again relative ratio of amino acids could be different for various proteins. On the basis of nitrogen weight, amino acid residue weight comes out around 6.25 but it could not be exact value for a food.

b.Overall structure of protein depends on the type of amino acids which make its skeleton. these overall structures could be primary, secondary or tertiary. It depends on type of H-bonding into the protein molecules. some protein molecules are fibrous proteins where intermolecular H-bonding is preferred instead of intramolecular. Some proteind form intramolecular H-bonding (like enzymes) so these proteins are globular in shape (albumins). Fibrous proteins make intermolecular H-bonding like this the free O-H bonds are not availabe to make H-bonds with water molecule so these proteins are insoluble in water. Instead of this albumins form H-bonding with water molecules and soluble in water. Again globular structure of enzymes provide them specific lock sites where substrate can react.

Due to cooking denaturation of protein takes place where all the specific structures of protein lost and all parts of protein molecules are exposed to digestive environment to digest completely and easily.