As Milk contains protein, it turns violet when reacted with Copper Sulphate and caustic soda solution.
Biuret reagent is an alkaline solution of 1% CuSO4, copper sulfate. The violet color is a positive test for the presence of protein, and the intensity of the color is proportional to the number of peptide bonds in the solution.
We used Biuret's reagent to detect the presence of proteins in solution. The reagent is pale blue when pure, but when mixed with proteins, the resulting reaction produces a pale purple color.
Biuret test is given by an amide linkage. It is present in proteins, polypeptides and urea.
Casein and whey protein are the major proteins of milk. Casein constitutes approximately 80%(29.5 g/L) of the total protein in bovine milk, and whey protein accounts for about 20% (6.3 g/L) (19-21). Casein is chiefly phosphate-conjugated and mainly consists of calcium phosphate- micelle complexes (20).
Caseins are one of the main types of protein in milk which cluster together with calcium and phosphate to form tiny particles called micelles. When light hits these casein micelles it causes the light to refract and scatter resulting in milk appearing white.
Biuret solution is a blue liquid that changes to purple color when it reacts with proteins present in food. Biuret solution is made by mixing copper sulfate with sodium hydroxide. Copper sulfate gives a typical blue color to this solution, which is turned to purple after reacting with the proteins.
The reason behind this colour is the formation of a chelate complex or the copper coordination complex. Cu (II) or cupric ions create a chelate complex of violet colour, using oxygen of water and the unshared electron pairs of peptide nitrogen. Since this complex absorbs light in 540 nm, it appears violet.
The biuret reaction is based on the complex formation of cupric ions with proteins. In this reaction, copper sulfate is added to a protein solution in strong alkaline solution. A purplish-violet color is produced, resulting from complex formation between the cupric ions and the peptide bond.
When the biuret reagent is added to the milk powder in a test tube, it rapidly gives a purple color in the test tube and confirms the presence of protein. .
A positive test is indicated by:
a deep blue/purple color due to the copper ion complex with the amide group of the protein.
The blue colour will change to violet if protein is present. If protein is not present, the blue colour will remain.
What is the Biuret Test? The Biuret test is used to detect the presence of proteins in a sample. The test is founded on the idea that copper ions in an alkaline solution can form a complex with the peptide bonds in proteins, resulting in a change in color, from blue to violet.
Because the bonds in biuret molecules ([H2N-CO]2NH) are similar to peptide bonds, a positive result for this test is also received when the analyte contains them.
Phenolphthalein is an indicator — a chemical which changes colour depending on whether it meets an acid or a base. It turns purple if it meets something basic, such as ammonia; it stays colourless if it meets an acid like vinegar or a neutral substance like water.
Therefore, the biuret test can also be used to gauge the amount of protein present in the analyte. In this test, the presence of peptides results in the formation of pale purple coloured (or mauve coloured) coordination compounds of the copper(II) ion (when the solution is sufficiently alkaline).
The biuret solution can only test for proteins. If the food sample is put into a biuret solution and the solution turns purple/violet, then it is an indication that proteins are present in the food. If the solution remains blue, then there are no proteins present in the food.
Negative Biuret Test: No formation of violet/purple color (or formation of blue color) solution after the addition of Biuret reagent.
The reagent turns violet in the presence of peptide bonds -- the chemical bonds that hold amino acids together.
Proteins are detected using Biuret reagent . This turns a mauve or purple colour when mixed with protein.
The normal color of biuret reagent is blue. The reagent turns violet in the presence of peptide bonds -- the chemical bonds that hold amino acids together. The proteins detected must have at least three amino acids, which means that the protein must have at least two peptide bonds.
Is it bad for them? In a word, yes, cows' milk is bad for cats. Most cats are actually 'lactose intolerant' as they don't have the enzyme (lactase) in their intestines to digest the sugar in milk (lactose), meaning that milk which contains lactose can make them poorly.
The nutritional importance of dairy is engrained in our dietary conscience. And certainly, cow's milk does contain micronutrients that support strong bones and provide other health benefits.
Whole-fat dairy — Whole or 2% milk contains protein, calcium, vitamins A and D and saturated fat. Although typically only recommended for young children, new studies are showing that adults who consume full fat dairy typically eat less at meals and stay satiated longer than those drinking low-fat dairy.