Sound Waves Could Pasteurize Milk Without Boiling It Away

Milk has a paradoxical reputation. It is one of the most nutritionally complete foods humans consume, delivering eighteen essential nutrients including vitamin D, calcium, and potassium, yet it is also one of the most fragile. Its rich blend of water, fat, protein, and lactose makes it an ideal growth medium for bacteria, and its history is punctuated by outbreaks of foodborne illness traced to pathogens such as Escherichia coli O157:H7, Salmonella Enteritidis, Campylobacter jejuni, and Listeria monocytogenes. For more than a century, the dairy industry has relied on heat to make milk safe, using regimes like low-temperature long-time pasteurization at 63 degrees Celsius for thirty minutes, high-temperature short-time treatment at 72 to 75 degrees Celsius for fifteen seconds, or ultra-high temperature processing at 135 to 150 degrees Celsius for a few seconds. Heat works, but it comes at a cost: degradation of vitamins, alteration of flavor, and loss of bioactive compounds. A new review published in Food Science of Animal Resources argues that sound waves, used alone or in combination with gentle heat, may offer a way to pasteurize milk and milk-based beverages while preserving more of what makes them valuable.

The review, authored by Rina Yu, Sin-Young Park, Prabhathma Yasasvi Rathnayake, and colleagues at Chungnam National University, Kongju National University, and the Korea Institute of Materials Science, systematically compares how different ultrasound systems and thermal conditions inactivate microbes in milk and milk-derived drinks. The authors frame their analysis around a simple but consequential observation: the antimicrobial power of ultrasound varies enormously depending on the equipment configuration, the processing parameters, and, crucially, how much heat is generated or deliberately added during treatment. By sorting the literature into three processing modes, cavitation-driven non-thermal ultrasound, ultrasound with natural temperature rise, and thermally assisted ultrasonic treatment, the review provides one of the clearest pictures yet of where sound-based pasteurization stands on the path from laboratory bench to dairy plant floor.

The physics behind ultrasonic pasteurization is striking. A transducer converts electrical energy into acoustic energy, sending longitudinal pressure waves through the liquid. These waves create alternating regions of compression and rarefaction, and in the rarefaction phase the liquid is literally torn apart, forming microscopic bubbles. At sufficiently high acoustic amplitudes, these bubbles undergo inertial cavitation: they expand rapidly and then collapse violently. The collapse generates localized hotspots with temperatures estimated to reach up to 5,500 degrees Celsius and pressures up to 50,000 kilopascals, conditions that exist for only fractions of a microsecond but are enough to wreak havoc on nearby microorganisms. The asymmetric collapse of bubbles produces shockwaves, microjets, and microstreaming currents that physically batter bacterial cell walls and membranes, tearing open structures that would otherwise protect the cell.

But the physical assault is only half the story. The extreme conditions inside collapsing bubbles also split water vapor into hydrogen and hydroxyl radicals, reactive oxygen species that penetrate bacterial cells through membrane pores and attack intracellular biomolecules. Hydroxyl radicals can cleave the DNA double helix, induce base modifications, and trigger lipid peroxidation in the phospholipid bilayer by reacting with polyunsaturated fatty acids. The result is oxidative degradation of proteins, lipids, polysaccharides, and nucleic acids, culminating in cell death. A related phenomenon, sonoluminescence, in which collapsing bubbles emit light across a broad spectrum, can further amplify microbial inactivation when a photocatalytic sonosensitizer is present, because the emitted light excites the sensitizer and generates additional reactive species. Together, these mechanical and chemical mechanisms make cavitation a genuinely multi-pronged antimicrobial weapon rather than a single-mode killer.

The review emphasizes that not all ultrasound equipment delivers this weapon equally well. Probe-type systems, in which a sonotrode is immersed directly in the sample, concentrate energy intensely at the tip and deliver the highest power intensity. Bath-type systems, which use sandwich transducers mounted on tank walls and rely on water as a coupling medium, suffer from attenuation of the ultrasound in the water, resulting in power intensity roughly one hundred times lower than probe systems. Both configurations irradiate from a single direction, producing non-uniform energy distribution within the treated volume. Continuous-flow systems, a newer development, address this weakness by circulating the product through a treatment zone, and some cylindrical designs generate ultrasonic energy in a 360-degree pattern, giving circulating milk stronger and more uniform exposure. These systems also allow operators to tune flow rate and number of treatment cycles alongside the usual power, amplitude, and frequency settings, a flexibility that matters greatly for industrial scale-up.

The experimental record assembled in the review shows how strongly outcomes depend on thermal conditions. Under controlled room-temperature conditions, where cooling systems or ice baths suppress heating, ultrasound alone achieves moderate inactivation. Mudgil and colleagues found that treating camel milk with probe-type ultrasound at 160 watts and 20 kilohertz for ten minutes reduced total aerobic bacteria by approximately 4.16 log CFU per milliliter, Staphylococcus species by 3.40 log, and lactic acid bacteria by 1.50 log. Chouliara’s group reported roughly 2 log reductions in total viable and psychrotrophic counts in milk after sixteen minutes at 200 watts and 24 kilohertz without added heat. In a Kinnow whey beverage, fifteen minutes of 400-watt, 20-kilohertz treatment cut total aerobic bacteria by about 1 log and molds and yeasts by 1.61 log. These are meaningful reductions, but they fall short of what regulators demand.

When the natural temperature rise of ultrasound is allowed to accumulate rather than suppressed, the numbers improve dramatically. Treating raw milk with probe-type ultrasound at 19 kilohertz and 475 watts for 158 seconds raised the temperature to 54 degrees Celsius and reduced mesophilic bacteria by about 2.6 log and lactic acid bacteria by 2.1 log. More strikingly, Shamila-Syuhada and colleagues showed that raw milk treated at 200 watts and 24 kilohertz for fifteen minutes reached 60 degrees Celsius at the higher amplitude tested, and at that point reductions of approximately 8 log CFU per milliliter were recorded for Staphylococcus aureus, Listeria monocytogenes, Salmonella Typhimurium, E. coli, Pseudomonas fluorescens, and two Lactobacillus species. Scudino’s team demonstrated the same principle through energy density: 1 kilojoule per milliliter raised raw milk to only 34 degrees Celsius with no detectable bacterial reduction, 3 kilojoules per milliliter reached 55 degrees Celsius and achieved a 1.8 log reduction, and 5 kilojoules per milliliter pushed the temperature to 76 degrees Celsius, delivering a 4 log reduction in just over four minutes. In chocolate milk, an energy density of 3 kilojoules per cubic centimeter at 400 watts and 19 kilohertz produced a 3.56 log reduction with a final temperature of only 42 degrees Celsius.

Thermally assisted ultrasound, combining acoustic cavitation with deliberately applied mild heat, offers perhaps the most pragmatic route to regulatory compliance. Van Hekken and colleagues applied continuous-flow ultrasound at dual frequencies of 16 and 20 kilohertz and 1,200 watts to raw milk held at 54 degrees Celsius for fourteen minutes, achieving reductions of 3.36 log in total mesophilic bacteria and 4.88 log in psychrotrophic bacteria, whereas the same treatment at 42 degrees Celsius showed no bactericidal effect at all. In human milk, bath-type ultrasound at 60 degrees Celsius reduced Staphylococcus aureus by approximately 4.58 log, while treatment at 20 degrees Celsius had no effect. The synergy arises because mild heat weakens the protective function of the bacterial cell envelope and increases cavitation efficiency, making cells simultaneously easier to rupture mechanically and more vulnerable to radical attack.

Yet the review is candid about the obstacles standing between these promising numbers and commercial reality. Under the Codex Alimentarius, any validated milk pasteurization technology must achieve at least a 5 log CFU per milliliter reduction of non-spore-forming pathogenic bacteria, and only a handful of ultrasound studies have crossed that threshold using ultrasound alone. Pushing intensity high enough to meet the standard risks damaging the product: ultrasound-generated radicals promote lipid oxidation and create volatile compounds responsible for metallic, burnt, and rubbery off-flavors in treated milk. There are also safety concerns specific to the equipment itself, since probe erosion can release metallic particles into the product and ultrasound accelerates electrochemical corrosion of stainless steel in the presence of chlorides. The authors argue that the future lies in continuous-flow systems suitable for large-scale processing, hurdle combinations of ultrasound with mild heat, pulsed electric fields, ultraviolet treatment, or natural antimicrobials, and real-time digital process control that can hold both microbial safety and sensory quality within tight tolerances.

What emerges from the review is not a verdict but a roadmap. Ultrasound is safe, non-toxic, relatively inexpensive to build, and well suited to liquid foods, and its antimicrobial mechanisms are now well characterized at the cellular level. The decisive variable is heat: whether it is suppressed, harvested, or deliberately supplied determines whether ultrasound is a gentle quality-preserving treatment or a genuine pasteurization technology. As consumer demand grows for fresh-flavored, minimally processed dairy, and as outbreaks linked to pasteurization failures continue to surface, the pressure to validate sound-based alternatives will only intensify. The next few years of equipment engineering and regulatory data generation will determine whether the dairy industry’s next pasteurizer hums rather than hisses.

Subject of Research: Ultrasonic pasteurization of milk and milk-based beverages using different ultrasound systems and thermal conditions

Article Title: A review of ultrasonic pasteurization in milk and milk-based beverages using different ultrasound systems and thermal conditions: from mechanistic insights to regulatory challenges

Article References: A review of ultrasonic pasteurization in milk and milk-based beverages using different ultrasound systems and thermal conditions: from mechanistic insights to regulatory challenges. (n.d.). https://doi.org/10.1007/s44463-026-00105-5

Image Credits: AI Generated

DOI: 10.1007/s44463-026-00105-5

Keywords: ultrasound, pasteurization, milk, dairy, acoustic cavitation, food safety, microbial inactivation, non-thermal processing, reactive oxygen species, continuous-flow ultrasound, foodborne pathogens, dairy processing