Lest identify protein in a food

The biuret method
The biuret method is a colorimetric technique specific for proteins and peptides. Copper salts in alkaline solution form a purple complex with substances containing two or more peptide bonds. The absorbance produced is proportional to the number of peptide bonds that are reacting and therefore to the number of protein molecules present in the reaction system. Thus, the biuret reaction with proteins is suitable for the determination of total protein by spectrophotometry (at 540–560 nm). The method is used extensively in clinical laboratories, particularly in automated analyzers in which protein concentration can be measured down to 0.1–0.15 g l−1. The use of bovine or human serum albumin to standardize the biuret method is well established. High-purity albumin contains only amino acids; its nitrogen content is a constant fraction of its molecular mass and the number of peptide bonds per molecule is known. Since the peptide bond is the biuret-reacting unit in all proteins and the number of peptide bonds determines the absorbance of the colored product, albumin is a reasonable peptide bond standard for all proteins in the mixture.