The isoelectric point of milk is complex, as milk is a mixture of components. However, the primary component influencing its isoelectric point is casein, which has an isoelectric point (IEP) of approximately 4.6. This is the pH at which casein proteins have no net electrical charge.
Understanding Isoelectric Point (IEP)
The isoelectric point is the pH at which a molecule, like a protein, carries no net electrical charge. Above the IEP, the molecule will carry a net negative charge, and below the IEP, it will carry a net positive charge. This characteristic influences the molecule's solubility and behavior in solution.
Casein and Milk Stability
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Casein Micelles: Casein exists in milk as large aggregates called micelles. In milk, which has a normal pH of about 6.6, the casein micelles have a net negative charge. This negative charge causes the micelles to repel each other, contributing to the stability of milk.
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Precipitation: When the pH of milk is lowered to around 4.6 (the IEP of casein), the casein micelles lose their net charge and become unstable. This leads to the aggregation and precipitation of casein, which is observed as curdling.
Other Milk Components
While casein is the primary factor determining the behavior of milk near its isoelectric point, other milk components, such as whey proteins, also have their own isoelectric points. However, casein's dominance in mass and its structural role within the milk matrix make its IEP the most relevant for describing milk's overall behavior.
Summary
In summary, while milk is a complex mixture, the isoelectric point of casein (approximately 4.6) is the most relevant value for understanding the stability and behavior of milk, particularly in relation to curdling and precipitation.
