Introduction
Ohmic heating is a modern thermal processing technology that applies an electric current directly through a conductive food matrix to generate heat. In addition to its rapid and uniform heating capability, ohmic heating induces electroporation—a phenomenon where electric fields create temporary or permanent pores in the cell membranes of microorganisms. Electroporation enhances microbial inactivation and can also influence the functional and structural properties of milk.
This article explores the mechanisms behind electroporation during ohmic heating, its effects on microbial reduction, and its potential applications in dairy processing. Understanding how electroporation contributes to the overall efficiency of ohmic heating provides insights into improving milk safety and quality while maintaining nutritional integrity.
What is Electroporation?
Electroporation is a biophysical process in which an external electric field disturbs the lipid bilayer of a cell membrane, leading to the formation of nanopores or micropores. If the electric field strength exceeds a certain threshold, the membrane’s structural integrity is compromised, resulting in increased permeability or complete cell rupture.
Mechanism of Electroporation
Electric Field Application
When an electric field is applied to a cell, the electric potential difference across the membrane increases. The transmembrane potential (∆Vm) is influenced by the strength and duration of the applied electric field.
Membrane Reorganization
The lipid bilayer becomes destabilized as charged molecules in the membrane align with the electric field. This creates localized stress, leading to the formation of transient or permanent pores.
Permeability Increase
Small molecules (e.g., ions, water) pass through the pores, altering cellular homeostasis. Larger molecules may also penetrate the membrane depending on pore size and stability.
Cell Death or Recovery
If the electric field strength is low, the pores may reseal, allowing cell recovery. If the electric field strength is high or exposure is prolonged, irreversible pore formation leads to cell lysis and death.
Mathematical Basis of Electroporation
The critical transmembrane potential (∆Vc) required to induce electroporation is estimated by:
ΔVc=Ec⋅d
Where:
∆Vc = critical transmembrane potential (V)
Ec = critical electric field strength (V/m)
d = membrane thickness (m)
For most bacterial cells, the critical transmembrane potential is approximately 0.2–1.0 V. The electric field strength required for electroporation in food matrices typically ranges from 0.5 to 10 kV/cm.
Electroporation During Ohmic Heating
In ohmic heating, the application of an electric current through a conductive medium (such as milk) creates an electric field that promotes the electroporation of microbial cells. Unlike pulsed electric field (PEF) treatment, ohmic heating combines both electrical and thermal effects, enhancing the overall efficiency of microbial inactivation.
- Combined Electrical and Thermal Stress
The electric field induces pore formation in microbial membranes, increasing cell vulnerability.
The heat generated by ohmic heating causes protein denaturation and enzyme inactivation within the cell. This synergistic effect increases the microbial inactivation rate compared to conventional heating or PEF alone.
- Electroporation Threshold in Milk
The conductivity of milk influences the effectiveness of electroporation. Milk has a conductivity of approximately 0.4–0.7 S/m at room temperature, which increases with heating due to ion mobility. Typical electric field strengths used in ohmic heating for microbial inactivation range between 1–5 kV/cm.
- Influence of Temperature on Electroporation
Higher temperatures reduce membrane rigidity, making it more susceptible to electroporation.
Ohmic heating at temperatures between 70°C and 90°C enhances electroporation efficiency without significant nutrient loss.
Effects of Electroporation on Microbial Inactivation
Electroporation during ohmic heating significantly improves microbial reduction in milk:
- Inactivation of Pathogenic Bacteria
Ohmic heating combined with electroporation effectively destroys common milk-borne pathogens, including:
Listeria monocytogenes; Escherichia coli; Salmonella spp; Staphylococcus aureus.
Bacterial cell walls and membranes are weakened by pore formation, leading to leakage of intracellular components and cell death. Studies show that ohmic heating at 70°C–90°C with an electric field strength of 3–5 kV/cm achieves up to a 5-log reduction in bacterial counts.
- Spoilage Organisms and Heat-Resistant Spores
Heat-resistant spores from species such as Bacillus and Clostridium are difficult to eliminate with conventional pasteurization. Electroporation increases membrane permeability, allowing heat to penetrate the spore core more effectively. Ohmic heating at UHT conditions (135°C–150°C) combined with an electric field strength of 5–10 kV/cm results in a 2–3 log reduction in heat-resistant spores.
- Yeasts and Molds
Yeasts and molds are typically more resistant to heat than bacteria due to their cell wall structure. Electroporation facilitates the entry of lethal heat into fungal cells, disrupting internal structures. Ohmic heating at 70°C–85°C with electric fields above 2 kV/cm results in up to a 4-log reduction in yeast and mold counts.
4. Viruses
Electroporation increases the susceptibility of viral envelopes to heat damage. While viruses are less affected by electroporation alone, the combined thermal and electrical stress in ohmic heating enhances viral inactivation.
Effects of Electroporation on Milk Quality
Electroporation during ohmic heating can also affect milk’s nutritional and sensory properties:
- Protein Structure
Whey proteins (e.g., β-lactoglobulin) are less denatured by ohmic heating due to reduced overall heating time. Casein micelles remain intact, maintaining milk’s natural viscosity and texture.
- Fat Stability
Electroporation does not significantly affect fat globule size or membrane integrity. Lower lipid oxidation compared to conventional heating enhances flavour stability.
3. Vitamin Retention
Enhanced heat penetration reduces overall heating time, preserving heat-sensitive vitamins.
Electroporation has no direct adverse effect on fat-soluble vitamins.
- pH and Acidity
Electroporation does not alter milk’s pH under typical ohmic heating conditions. Minimal formation of acid byproducts preserves milk’s natural taste.
Applications of Electroporation in Dairy Processing
Electroporation during ohmic heating has practical applications in dairy processing:
✅ 1. Pasteurization and Sterilization
Faster microbial reduction with improved nutrient retention. Lower processing temperatures reduce protein and vitamin degradation.
✅ 2. Milk Protein Modification
Electroporation-induced changes in casein micelle structure can improve the texture and gelling properties of yogurt and cheese.
✅ 3. Lactose Hydrolysis
Increased membrane permeability facilitates enzyme penetration for lactose hydrolysis in low-lactose milk production.
✅ 4. Probiotic Viability
Controlled electroporation can improve probiotic delivery and viability in fortified milk products.
Conclusion
Electroporation induced by ohmic heating is a powerful mechanism for improving microbial inactivation and enhancing milk quality. The combined electrical and thermal stress improves the efficiency of high-temperature treatment while preserving the nutritional and sensory properties of milk. The ability to control pore formation and cell membrane disruption makes electroporation a valuable tool for improving the safety, shelf life, and functional properties of milk in modern dairy processing.