Milk proteins, primarily casein and whey proteins, function as non-penetrating (extracellular) cryoprotectants. They are most widely recognized for their role in sperm cryopreservation, where skim milk is a standard component of extenders, but their protective mechanisms are applicable to other biological systems where membrane stabilization and osmotic support are critical.

Mechanism of Cryoprotection

Milk proteins protect cells through a combination of physical and biochemical mechanisms that operate in the extracellular space.

• Membrane Stabilization and Coating: The casein micelles and whey proteins in skim milk are considered the major protective constituents. They form a protective layer that shields the cell membrane, helping to preserve structural integrity and normal morphology during freezing and thawing. Proteomic studies indicate that this protection involves the modulation of an energy-membrane protein network, stabilizing the membrane against cryoinjury.
• Osmotic Regulation: The lactose naturally present in milk is a key hydrophilic component that helps maintain osmotic pressure, preventing osmotic shock and protecting the cell wall from freeze damage.
• Antioxidant and Chelating Activity: Milk proteins can chelate heavy metal ions that would otherwise catalyze oxidative damage. They also help reduce lipid peroxidation, a major contributor to cryoinjury. For example, the addition of 2.8% skim milk decreased lipid peroxidation in post-thaw goat sperm by 29%.
• pH Buffering: Milk proteins contribute to buffering the pH of the freezing medium, providing a more stable chemical environment during the freeze-thaw transition.