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.

Quercetin is a flavonoid that has been shown to have health-promoting capacities due to its potent antioxidant activity. However, the effect of chronic intake of quercetin on the gut microbiome and diabetes-related biomarkers remains unclear. Male C57BL/6J mice were fed HF or HF supplemented with 0.05% quercetin (HFQ) for 6 weeks. Diabetes-related biomarkers in blood were determined in mice fed high-fat (HF) diets supplemented with quercetin. Mice fed the HFQ diet gained less body, liver, and adipose weight, while liver lipid and blood glucose levels were also lowered. Diabetes-related plasma biomarkers insulin, leptin, resistin, and glucagon were significantly reduced by quercetin supplementation. In feces, quercetin supplementation significantly increased the relative abundance of Akkermansia and decreased the Firmicutes/Bacteroidetes ratio. The expression of genes Srebf1, Ppara, Cyp51, Scd1, and Fasn was downregulated by quercetin supplementation. These results indicated that diabetes biomarkers are associated with early metabolic changes accompanying obesity, and quercetin may ameliorate insulin resistance.