18. Protein Analysis

Protein content analysis of foods and ingredients is important for a variety of reasons. This chapter covers the instrumentation, principles, procedures, advantages, disadvantages, and applications of various protein analysis methods that are based on the unique characteristics of proteins and amino acids. The Kjeldahl and Dumas methods measure nitrogen. Infrared spectroscopy is based on absorption of a wavelength of infrared radiation specific for the peptide bond. Copper-peptide bond interactions contribute to the analysis by the biuret, Lowry, and bicinchoninic acid (BCA) methods. Specific amino acids are involved in the Lowry, BCA, dye-binding, and UV 280 nm methods. The BCA method also utilizes the reducing power of proteins in an alkaline solution. These and other protein analysis methods differ in their speed and sensitivity. Because of the complex nature of various food systems, problems may be encountered to different degrees in protein analysis by available methods. Certain methods are required as official methods for nutrition labeling, rapid methods may be suitable for quality control purposes, and other very sensitive methods are required for work with a minute amount of protein.