Peptone is a complex mixture of protein hydrolysates—containing peptides, amino acids, and inorganic salts—that functions primarily as a non-penetrating, extracellular cryoprotectant. While its protective effects have been recognized for decades, its use is largely empirical and is most established in the preservation of microorganisms and, more recently, in embryo cryopreservation as a protein-free alternative.

Mechanism of Cryoprotection

The precise mechanism by which peptone protects cells is not fully understood, but evidence points to a combination of physical and nutritional roles.

Peptide-Mediated Physical Protection: The protective activity is attributed to specific peptide fractions within the hydrolysate. Studies with T4 bacteriophage found that protection against freezing damage was concentration-dependent, while protection against eutectic injury (damage from the phase change of solutes) was strongly dependent on peptide size. This suggests that peptides can modulate the physicochemical environment during freezing.

Nutritional and Buffering Support: Peptone supplies a rich source of nutrients, including B vitamins and minerals, that can support cell recovery and viability following freeze-drying stress. The amino acids and buffering properties of protein hydrolysates contribute to their effectiveness in enhancing preservation.

Osmotic and Membrane Effects: As a mixture of solutes, peptone contributes to the osmotic balance of the freezing medium. However, its action is distinct from dedicated osmolytes, and some peptone fractions can be inhibitory; for example, a lactalbumin hydrolysate was found to strongly inhibit the growth of damaged bacteria, indicating that not all peptone components are protective.