Casein extraction: A precipitation reaction

By Dr. Robert D. Craig, Ph.D.• .

A precipitation reaction

• http://nutrafoodingredients.com/products/protein/acid-casein

For CHM 111 and CHM 141

• Metal salts can be used at low concentrations to precipitate enzymes and nucleic acids from solutions. Polyvalent metal ions frequently used are Ca2+, Mg2+, Mn2+ or Fe2+.

Casein extraction

• Casein extraction

The 21 amino acids

A zwitterion-

What is a zwitterion?

• The chemistry of amino acids is complicated by the fact that the -NH2 group is a base and the -CO2H group is an acid.

• In aqueous solution, an H+ ion is therefore transferred from one end of the molecule to the other to form a zwitterion (from the German meaning hybrid ion).

The isoelectric point

• The isoelectric point (pI) is the pH of a solution at which the net primary charge of a protein becomes zero.

• At a solution pH that is above the pI the surface of the protein is predominantly negatively charged and therefore like-charged molecules will exhibit repulsive forces.

Is used to focus DNA in analysisfragments have a charge and respond

to pH changes

The isoelectric point

• Likewise, at a solution pH that is below the pI, the surface of the protein is predominantly positively charged and repulsion between proteins occurs. However, at the pI the negative and positive charges cancel

The isoelectric point

• Isoelectric point the negative and positive charges cancel, repulsive electrostatic forces are reduced and the attraction forces predominate. The attraction forces will cause aggregation and precipitation. The Isoelectric point of most proteins is in the pH range of 4-6.

•

• . The precipitation of casein during cheesemaking, or during production of sodium caseinate, is an isoelectric precipitation

Precipitation

• Precipitation by means of acidification can be considered in terms of simple chemistry as follows, R being the casein protein:

• H2N-R-COO- + H+ → +H3N-R-COO

• casein

• micelle acid casein

• (pH = 6.6) (pH = 4.6)

• Colloidal dispersion Insoluble particles

How to extract from milk

• Can adjust pH

• Make then precipitate as they exist in a specific buffer range

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.Will precipitate proteins as ions go to hydration layer

Protein precipitates

• Protein precipitates left in the salt solution can remain stable for years-protected from proteolysis and bacterial contamination by the high salt concentrations. Ammonium sulfatesalt cannot be used in solutions that have pH > 8 because the ammonium ion has a buffering effect on the solution. Sodium citrate[is a good alternative for solutions above pH 8.