Beta-casein milk protein

Beta-casein milk protein

Casein and whey are the two main types of protein in milk. Casein makes up around 80% of the protein in cows’ milk, while around 20% is whey. Beta-casein is one of the major casein proteins, comprising around 30% of the total protein in cows’ milk (Figure: 1). Other proteins present at low levels in milk include antibodies and iron carrying proteins.

Casein Protein in Milk

Figure 1: Beta-casein is one of the major casein proteins in cows’ milk

 

Beta-casein is a 209 amino acid protein that comes in 2 main forms: A2 and A1 beta-casein. These differ from each other by a single amino acid (Figure: 2). Small differences in the amino acid composition of proteins can result in various protein forms exhibiting different properties following digestion.

A2 & A1 Beta Caseins

Figure 2: Sections of the A2 and A1 types of beta-casein proteins showing the single amino acid sequence difference at position 67

 

The beta-casein type of protein produced in cows’ milk will depend on the genes inherited by the cow. As one gene for beta-casein production is inherited from each parent, each cow carries 2 copies of the beta-casein gene.

Of these 2 beta-casein genes, the following combinations are possible;

  • both genes can be for the A2 type of beta-casein, so cows with this gene combination will produce milk which contains only the A2 type of beta-casein, rather than A1;
  • both genes can be for the A1 type of beta casein, so cows with this gene combination will produce milk which contains only the A1 type of beta-casein, rather than A2;
  • one gene can be A1 and the other A2 type beta-casein, so cows with this gene combination will produce milk which contains equal amounts of A1 and A2 type beta-casein.

A simple DNA test which examines a cows’ beta-casein genes allows easy identification of which beta-casein form it carries.

Reference:

  • Woodford K, (2007). Devil in the Milk: Illness, Health and Politics: A1 and A2 Milk. Wellington New Zealand: Craig Potton Publishing. No external link available.
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