What is isolated whey protein？
Whey protein is a general term for a variety of protein components retained in the supernatant when the pH of milk is reduced to 4.6 and casein is removed by precipitation. It includes β-lactoglobulin (2~4g/L), α-lactalbumin (0.6~1.7g/L), bovine milk serum albumin (0.2~0.4g/L), immunoglobulin (0.5~1.8g/L), lactoferrin, lactoperoxidase, glycopeptide, and bioactive factors and enzymes [1].
β-lactoglobulin (β-Lg)
It is relatively high in whey and plays a major role in whey protein, accounting for about 50% of whey protein, and the content in milk is about 3.2g/L. The isoelectric point of β-Lg is pH5.2, the relative molecular mass is about 18200, and β-Lg has five variants: A, B, C, D, and Dr. β-Lg is composed of 162 amino acids, and its secondary structure is 7-10% α-helix, 43-51% antiparallel β-folding, and the rest is in arbitrary conformation. β-Lg has only two disulfide bonds and one sulfhydryl group. β-Lg exists in the form of dimer (relative molecular weight 36566) in milk, and reversibly dissociates β-Lg at pH8.5, and irreversibly denatures β-Lg dimer into monomer when pH becomes 10.
Alpha lactalbumin
The content of α-lactalbumin (α-La) in milk is about 1.2g/L, accounting for about 3.5% of the total protein of milk and about 20% of the whey protein. The content in colostrum is more, as much as 10%~12%. Alpha-la has two variants: alpha-LAA and alpha-lab. Alpha-la is tightly folded and is basically a globular molecule. α-La is dispersed in milk with 1.5~5μm diameter particles, and plays a colloidal role in protecting casein. The isoelectric point of α-La is pH4.7~5.1, the relative molecular weight is 14175, contains 8 cysteine residues, forms 4 intramolecular disulfide bonds, and the structure is very stable. The secondary structure of α-La is very ordered, with a tight spatial tertiary structure, and α-La is relatively stable in resistance to thermal denaturation.
Bovine serum albumin
Bovine Serum Albumin (BSA) accounts for about 5% of whey protein and has an isoelectric pH of 4.9. The relative molecular weight of BSA is 66,000, and there are two genetic variants of serum albumin isolated from milk. BSA is not synthesized in the mammary gland, but flows into the blood, and is almost indistinguishable from BSA in the blood, consisting of 607 amino acids, with one free thiothiol group (cystine 34) and 17 disulfide bonds, and BSA in milk is transferred from bovine serum.
immunoglobulin
Immunoglobulin (Immunoglobulin) refers to a globulin with antibody activity or chemical structure similar to antibodies, and can have a specific binding reaction with the corresponding antigen, is the main substance of humoral immunity. The immunoglobulins in milk include IgG, IgA and IgM, which have a low concentration in normal milk, the content is about 0.05%~0.11%, accounting for 5%~10% of whey protein, and the concentration is very high in bovine colostrum, the content is as high as 15%, accounting for 50%~60% of colostrum protein.