Dairy proteins

Research output: Chapter in Book/Report/Conference proceedingChapterpeer-review

10 Scopus citations

Abstract

Dairy proteins are generally divided into two groups: caseins and whey proteins, which are distinctly different in their structural, physical, chemical, and functional properties. In fresh milk, caseins exist in micelles (130-160 nm in diameter), which are formed by the association of individual submicelles (~10 nm) through amorphous calcium phosphate bridges (McMahon & Brown, 1984). The micelles exhibit hydrocolloidal characteristics and are readily suspended in the aqueous phase of milk. In bovine milk, there are four casein proteins, i.e., αs1-, α s2-, β-and κ-caseins, which represent approximately 38%, 10%, 36%, and 12% of whole casein, respectively (Table 6.1). Caseins are phosphorylated as monoesters of serine, but the degree of esterification differs among caseins. The αs1-, αs2-, and β-caseins contain multiple phosphorous groups, which can form complexes with calcium leading to precipitation. κ-Casein, on the other hand, contains only one phosphorous group and, therefore, is less sensitive to calcium (Fox & Kelly, 2004). κ-Casein is also glycosylated, making it more hydrophilic than other proteins in the casein family. Caseins are hydrophobic proteins. The preponderance of proline residues (an imino acid), which are uniformly distributed throughout the molecules, yields caseins an open structure, and thus, high surface hydrophobicity. Because caseins have low levels of secondary (e.g., α-helix) and tertiary structure, they do not exhibit thermal transitions during heating (Swaisgood, 2003). However, the hydrophobic and polar residues are not uniformly distributed along the sequence. They tend to form clusters and patches, thus giving casein molecules amphiphilic structures with a high surface activity and explaining why caseins are excellent emulsifiers. Their relatively open structure also makes caseins highly susceptible to enzymes, including proteases and transglutaminase (Kuraishi et al., 1997). Whey proteins in fresh milk are present as individual noncomplex proteins and are highly hydrophilic. Together, they represent approximately 20% of total protein content in milk. β-Lactoglobulin is the most abundant protein in whey, followed by α-lactalbumin, immunoglobulins, serum albumin, and the proeoytic products from cheese making, collectively referred to as proteose-peptone (Table 6.1). When the pH of fresh milk (pH 6.7) is lowered to about 4.7-5.0, the calcium phosphate linkages between casein submicelles are destabilized, leading to hydrophobic association and subsequent precipitation of casein. The proteins that remain soluble are whey proteins. The preponderance of nonionizable polar side chain groups in whey proteins renders them soluble even at their isoelectric pH. The high solubility of whey proteins is responsible for their excellent gelling and water-binding properties.

Original languageEnglish
Title of host publicationIngredients in Meat Products
Subtitle of host publicationProperties, Functionality and Applications
Pages131-144
Number of pages14
DOIs
StatePublished - 2009

ASJC Scopus subject areas

  • General Materials Science
  • General Chemistry

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