Dr Mukul Sain1
1College of Dairy Science & Technology, RAJUVAS, Bikaner (Rajasthan)
Dr. P.S. Minz2
2ICAR-NDRI, Karnal (Haryana)
R.S. Pal2
1College of Dairy Science & Technology,
RAJUVAS, Bikaner (Rajasthan)
Introduction
Milk is a highly perishable food product, prone to microbial spoilage and chemical changes during storage and processing. To ensure milk safety and extend its shelf life, thermal treatments like pasteurization and sterilization are widely used. However, conventional heat treatments can negatively impact milk’s nutritional, sensory, and physicochemical properties due to uneven heating and prolonged exposure to high temperatures.
Ohmic heating has emerged as a promising alternative for high-temperature treatment of milk. It involves the use of electrical currents to generate heat directly within the milk through electrical resistance, resulting in rapid and uniform heating. This article examines the effect of ohmic heating on various aspects of milk quality, including nutritional composition, sensory attributes, microbiological stability, and physicochemical properties.
How Ohmic Heating Affects Milk Quality
Ohmic heating influences milk quality in several key areas:
1. Nutritional Composition
Milk contains essential nutrients such as proteins, fats, vitamins, and minerals. The effect of ohmic heating on these components depends largely on the temperature, heating rate, and exposure time.
1.1 Proteins
Proteins are highly sensitive to heat, especially whey proteins like β-lactoglobulin and α-lactalbumin.
- Ohmic heating causes less denaturation of whey proteins compared to traditional heating because of the rapid and uniform heat distribution.
- Studies show that heating milk using ohmic heating at 70°C–90°C for 15–30 seconds retains over 90% of native whey protein content.
- Caseins are more heat-stable, and ohmic heating at sterilization temperatures (135°C–150°C) does not significantly alter their structure.
✅ Advantages:
- Preservation of biologically active proteins improves the nutritional quality of milk.
- Reduced protein denaturation enhances the digestibility and bioavailability of amino acids.
1.2 Fats
Milk fat is composed mainly of triglycerides, phospholipids, and cholesterol.
- Ohmic heating causes minimal changes to fat globule size and structure due to the absence of localized overheating.
- The stability of milk fat globule membranes is preserved better with ohmic heating than with direct steam heating.
- Lipid oxidation, which can produce off-flavors, is significantly lower in ohmic heating due to shorter processing times and uniform heating.
✅ Advantages:
- Improved retention of fat globule integrity enhances the creaminess and mouthfeel of milk.
- Lower lipid oxidation preserves the natural flavor of milk.
1.3 Vitamins
Vitamins, particularly water-soluble ones like B-complex and vitamin C, are heat-sensitive.
- Ohmic heating at high temperatures (above 130°C) for short durations (less than 5 seconds) preserves more vitamins than traditional heating.
- Vitamin B2 (riboflavin) and B12 retention rates are over 85% in ohmic-heated milk compared to 70% in conventionally pasteurized milk.
- Fat-soluble vitamins (A, D, E, and K) remain stable due to reduced exposure to high temperatures.
✅ Advantages:
- Enhanced retention of vitamins improves the overall nutritional profile of milk.
- Reduced vitamin degradation increases the health benefits of processed milk.
1.4 Minerals
Milk is rich in minerals like calcium, phosphorus, and magnesium, which are critical for bone health and metabolic functions.
- Ohmic heating maintains the bioavailability of minerals better than conventional methods due to reduced thermal stress.
- No significant changes in mineral solubility are observed with ohmic heating at sterilization temperatures.
✅ Advantages:
- Improved mineral retention supports the nutritional value of milk.
- Enhanced calcium bioavailability improves the health benefits of milk.
2. Microbiological Stability
Microbial contamination is a major cause of milk spoilage. The effectiveness of ohmic heating in destroying pathogens and spoilage organisms contributes to improved microbiological stability.
2.1 Pathogen Reduction
- Ohmic heating effectively eliminates common milk pathogens such as Listeria monocytogenes, Escherichia coli, Salmonella spp., and Staphylococcus aureus.
- The rapid heating process ensures that all microorganisms are exposed to lethal temperatures uniformly, reducing the chance of microbial survival.
2.2 Spore Inactivation
- Heat-resistant spores from Bacillus and Clostridium species are difficult to eliminate using traditional pasteurization methods.
- Ohmic heating at UHT temperatures (135°C–150°C) for 2–5 seconds achieves a higher spore reduction rate due to the rapid and even heating.
✅ Advantages:
- Enhanced microbial reduction extends the shelf life of milk.
- Improved microbial stability reduces the need for preservatives.
3. Physicochemical Properties
The structure and stability of milk are influenced by heating conditions. Ohmic heating causes minimal disruption to milk’s natural structure.
3.1 pH and Acidity
- Conventional heating often lowers the pH of milk due to the formation of acidic compounds from protein denaturation and lactose breakdown.
- Ohmic heating results in minimal pH reduction due to shorter heating times and reduced protein denaturation.
- Milk treated with ohmic heating at UHT conditions retains a pH close to 6.6–6.8.
✅ Advantages:
- Better stability during storage and processing.
- Maintains the natural taste and texture of milk.
3.2 Color
- Milk color is primarily influenced by the presence of proteins and fat globules.
- Browning reactions (Maillard reactions) are less pronounced in ohmic heating due to rapid heating and minimal temperature gradients.
- Retention of natural whiteness enhances the visual appeal of milk.
✅ Advantages:
- Better visual quality increases consumer appeal.
- Minimal browning preserves milk’s natural appearance.
3.3 Texture and Viscosity
- Heating-induced protein unfolding and aggregation can increase milk viscosity.
- Ohmic heating minimizes these changes, leading to a smoother and more natural texture.
- Milk treated with ohmic heating at pasteurization or UHT conditions retains a viscosity comparable to raw milk.
✅ Advantages:
- Improved mouthfeel and sensory quality.
- Better processing properties for downstream dairy product manufacturing.
4. Sensory Quality
The taste, aroma, and mouthfeel of milk are critical factors influencing consumer acceptance. Ohmic heating preserves these sensory attributes better than traditional methods.
4.1 Flavor
- Conventional heating can produce “cooked” or “burnt” flavors due to protein and lactose degradation.
- Ohmic heating minimizes these reactions, preserving the fresh, creamy taste of milk.
- Reduced lipid oxidation also prevents off-flavors from developing.
4.2 Aroma
- Volatile compounds responsible for the aroma of milk are better retained with ohmic heating.
- Higher retention of lactones and aldehydes enhances the fresh smell of milk.
✅ Advantages:
- Fresher taste and aroma improve consumer acceptance.
- Better flavor stability during storage.
Comparison of Ohmic Heating with Conventional Methods
| Parameter | Conventional Heating | Ohmic Heating |
| Heating Uniformity | Uneven | Uniform |
| Nutrient Loss | High (especially vitamins) | Low |
| Microbial Reduction | Effective (but longer time needed) | More effective (due to rapid heating) |
| Flavor Changes | Burnt or cooked flavor possible | Fresh flavor retained |
| Lipid Oxidation | High | Low |
| Protein Denaturation | High | Low |
| Processing Time | Longer | Shorter |
| Energy Efficiency | Moderate | High |
Conclusion Ohmic heating is a highly effective method for the high-temperature treatment of milk, preserving its nutritional value, sensory quality, and microbiological stability. Its ability to provide rapid, uniform heating minimizes nutrient degradation, reduces the risk of lipid oxidation, and improves the texture and flavor of milk. Compared to conventional methods, ohmic heating offers superior microbial reduction and better retention of vitamins and proteins. The improved quality of ohmic-heated milk makes it an attractive option for both consumers and dairy processors. As technology advances, ohmic heating is expected to play a larger role in the future of dairy processing, combining efficiency with product excellence.