Sour Cream


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Milk is made up of approximately 3. 0-3. 5% protein, the main proteins in cream are caseins and whey proteins, caseins are globular proteins with phosphoserine residue. Of the total fraction of milk proteins, caseins make up 80% while the whey proteins make up 20%. There are four main classes of caseins; β-caseins, α(s1)-caseins, α(s2)-casein and κ-caseins. These casein proteins form a multi molecular colloidal particle known as a casein micelle. The proteins mentioned have an affinity to bind with other casein proteins, or to bind with calcium phosphate, and this binding is what forms the aggregates. The casein micelles are aggregates of β-caseins, α(s1)-caseins, α(s2)-caseins, that are coated with κ-caseins. The proteins are held together by small clusters of calcium phosphate, the micelle also contains lipase, citrate, minor ions, and plasmin enzymes, along with entrapped milk serum. Casein micelles are rather porous structures, ranging in the size of 50-250 nm in diameter and the structures on average are 6-12% of the total volume fraction of milk. The structure is porous in order to be able to hold a sufficient amount of water, its structure also assists in the reactivity of the micelle. The formation of casein molecules into the micelle is very unusual due to β-casein's large amount of propyl residues (the proline residues disrupt the formation of α-helixes and β-sheets ) and because κ-caseins only contain one phosphorylation residue (they are glycoproteins). Due to κ-caseins being glycoproteins, they are stable in the presence of calcium ions so the κ-caseins are on the outer layer of the micelle to partially protect the non glycoproteins β-caseins, α(s1)-caseins, α(s2)-caseins from precipitating out in the presence of excess calcium ions. Casein micelles are not heat sensitive particles, they are pH sensitive. The colloidal particles are stable at the normal pH of milk which is 6. 5-6. 7, the micelles will precipitate at the isoelectric point of milk which is a pH of 4. 6.