Proteins

Contents 4.1 Introduction ..................................................................................................................... 57 4.2 Major Methods for Measuring Protein Nitrogen .............................................................. 58

4.2.1 Kjeldahl Method .................................................................................................. 58 4.2.2 Dumas Method .................................................................................................... 58

4.3 Muscle Protein Analysis ................................................................................................... 60 4.4 Protein Solubility Classes ..................................................................................................61 4.5 Methods for Analysis of Soluble Proteins ......................................................................... 63

4.5.1 Biuret ................................................................................................................... 64 4.5.2 Lowry ................................................................................................................... 64 4.5.3 Bicinchoninic Acid ............................................................................................... 64 4.5.4 Coomassie Blue Dye Binding (Bradford) ..............................................................65 4.5.5 Problems with the Colorimetric Protein Assays .....................................................65 4.5.6 Ultraviolet Protein Absorbance ..............................................................................67

4.6 Electrophoretic Methods ...................................................................................................67 4.7 Enzyme-Linked Immunosorbent Assay Methods ............................................................. 70 4.8 Special Problems with Collagen ....................................................................................... 70 4.9 Summary ......................................................................................................................... 71 References ................................................................................................................................. 71

4.1 Introduction Proteins are the major nonwater ingredients of muscle foods and constitute almost 20% of the weight of lean muscle tissue. Proteins are polymers of amino acids that perform diverse functions. Th ey make up the enzymes that catalyze biochemical reactions to sustain life. Muscle tissue also

contains large quantities of proteins involved in the process of muscle contraction. Since the amino acid composition of proteins from food animals is similar to human muscle and muscle makes up almost 50% of our body’s weight, muscle foods are an excellent source of the amino acids needed for growth, repair, and maintenance. Protein content determinations are required for measuring muscle food composition to meet human nutritional requirements,1 for product formulation and to meet legal requirements for product identity in meat industry, and for research on the role of specifi c proteins in foods and biological processes.