Ultrasonic Pasteurization of Liquid Foods

Ultrasonic pasteurization is a non-thermal sterilization process to inactivate microbes such as E.coli, Pseudomonas fluorescens, Listeria monocytogenes, Staphylococcus aureus, Bacillus coagulans, Anoxybacillus flavithermus amongst many others to prevent microbial spoilage and achieve long-term stability of food and beverages.

Non-Thermal Pasteurization of Food & Beverages by Sonication

Ultrasonic pasteurization is a non-thermal alternative technology that is used to destroy or deactivate organisms and enzymes that contribute to food spoilage. Ultrasonication can be used to pasteurize canned foods, milk, dairy, eggs, juices, beverages with low alcohol content, and other liquid foods. Ultrasonication alone as well as ultrasound combined with elevated heat and pressure conditions (known as thermo-mano-sonication) can efficiently pasteurize juices, milk, dairy, liquid eggs and other food products. A sophisticated ultrasonic pasteurization treatment excels traditional pasteurization techniques as ultrasound does not adversely effect the nutrient content and physical characteristics of the treated food products. Using ultrasound or thermo-mano-sonication in order to pasteurize liquid food products can provide a nutrient-rich product with a higher quality than the traditional high-temperature short time (HTST) pasteurization method.
Research studies such as from Beslar et al. (2015) found that ultrasonic treatment can provide significant advantages for the processing of juices including enhanced quality factors, such as yield, extraction, cloudiness, rheological properties, and color as well as the shelf life.

Survival curves of Escherichia coli (a) and Staphylococcus aureus (b) in apple juice after Ultrasound Treatment (UT) at different temperatures and after Heat Treatment (HT) at the same temperatures.
picture and study: Baboli et al. 2015

How Does Ultrasonic Pasteurization Work?

Ultrasonic inactivation and destruction of microbes is a non-thermal technique, which means its main working principle is not based on heat. Ultrasonic pasteurization is caused mainly by the effects of acoustic cavitation. The phenomenon of acoustic / ultrasonic cavitation is known for its locally high temperatures, pressures, and respective differentials, which occur in and around the minute cavitation bubbles. Furthermore, acoustic cavitation generates very intense shear forces, liquid jets and turbulences. These destructive forces cause extensive damage on microbial cells, such cell perforation and disruption. Cell perforation and disruption are unique effects found in ultrasonically-treated cells caused mainly by the liquid jets generated by cavitation.

Ultrasonic pasteurization is based on acoustic cavitation and its hydrodynamic shear forces

Why Sonication Excels Traditional Pasteurization

The food and beverage industry applies conventional pasteurization widely to inactivate or kill microbes such as bacteria, yeast, and fungi to prevent microbial spoilage and to give their products a longer shelf-life and stability. Conventional pasteurization works by a short treatment at elevated temperatures of usually below than 100°C (212°F). The exact temperature and duration is normally adjusted to the specific food product and the microbes, which must be inactivated. The effectiveness of a pasteurization process is determined by the microbial inactivation rate, which is measured as log reduction. The log reduction measures the percentage of inactivated microbes at a specific temperature over a specific time. The conditions of the temperature treatment and the microbial inactivation rate are influenced by the type of microbes as well as the composition of the food product. Traditional heat-based pasteurization has several disadvantages ranging from insufficient microbial inactivation, negative effects on the food product as well as uneven heating through the treated product. Insufficient heating by to short pasteurization duration or too low temperature results in a low log reduction rate and subsequent microbial spoilage. Too much heat treatment can cause product deterioration such as burned off-flavours, and less nutrient density due to destroyed temperature-sensitive nutrients.

Disadvantages of Conventional Pasteurization

• can destroy or damage important nutrients
• can cause off-flavours
• high energy requirements
• ineffective against kill heat-resistant pathogens
• not applicable to every food product

Ultrasonic Pasteurization of Dairy

Sonication, thermo-sonication and thermo-mano-sonication habve been widely researched for the pasteurization of milk and dairy products. For instance, ultrasound was found to eliminate spoilage and potential pathogens to zero or to levels acceptable by South African and British milk legislation, even when initial inoculum loads of 5× higher than permitted were present before treatment. Viable cell counts of E. coli were reduced by 100% after 10.0 min of ultrasonication. Furthermore was shown that viable counts of Pseudomonas fluorescens were reduced by 100% after 6.0 min and Listeria monocytogenes was reduced by 99% after 10.0 min. (Cameron et al. 2009)
Research also demonstrated that thermo-sonication can inactivate Listeria innocua and mesophilic bacteria in raw whole milk. Ultrasound was shown to be a viable technology for pasteurization and homogenization of milk, exhibiting shorter processing times without important changes in pH and lactic acid content, along with better appearance and consistency when compared to conventional thermal treatment. These facts are advantageous in many aspects of dairy processing. (Bermúdez-Aguirre et al. 2009)

Ultrasonic Pasteurization of Juices and Fruit Purees

Ultrasonic pasteurization was applied as an efficient and rapid alternative pasteurization technique in order to inactivate Escherichia coli and Staphylococcus aureus in apple juice. When the pulp free apple juice was ultrasonically processed, the 5-log reduction time was 35 s for E. coli at 60degC and 30 s for S. aureus at 62degC. Although in the study was found that high pulp content made ultrasound less lethal to S. aureus, while it had no significant effect on E. coli, it should be noted that no pressure was applied. Sonication under elevated pressures significantly intensifies ultrasonic cavitation and thereby microbial inactivation in more viscous liquids. Ultrasound treatment had no significant effect on antioxidant activity determined by 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity, but it significantly increased the total phenolic content. The treatment also resulted in more stable juice with higher uniformity. (cf. Baboli et al. 2020)

Ultrasonic Inactivation of Gram-Positive and Gram-Negative Bacteria

Gram positive bacteria, such as Listeria monocytogenes or Staphylococcus aureus, are in general known to be more resistant than gram-negative bacteria and withstand pasteurization technologies such as PEF, HPP and mano-sonication (MS) for a longer treatment periods due to thicker cell walls. Gram-negative bacteria have two – one external and one cytoplasmic – lipid cell membranes with a thin layer of peptidoglycan among them, which makes them more susceptible to ultrasonic inactivation. On the other hand, gram-positive bacteria have only a single lipid membrane with a thicker peptidoglycan wall, which gives them more resistance against pasteurization treatments. Scientific investigations compared the effect of power ultrasound on gram-negative and gram-positive bacteria and found that it had a stronger inhibitory effect on gram-negative bacteria. (cf. Monsen et al. 2009) Gram-positive bacteria require more intense ultrasound conditions, i.e. higher amplitudes, higher temperatures, higher pressures and/or longer sonication time. Hielscher Ultrasonics’ power-ultrasound systems can deliver very high amplitudes and can be operated at elevated temperatures and with pressurizable flow-cell reactors. This allows for intense sonication / thermo-mano-sonication in order to inactivate even very resistant bacteria strains.

Ultrasonic Inactivation of Thermoduric Bacteria

Thermoduric bacteria are bacteria which can survive, to varying extents, the pasteurisation process. Thermoduric species of bacteria include Bacillus, Clostridium and Enterococci. “Ultrasonication at 80% amplitude for 10 min however, inactivated the vegetative cells of B. coagulans and A. flavithermus in skim milk by 4.53, and 4.26 logs, respectively. A combined treatment of pasteurization (63 degrees C/30 min) followed by ultrasonication completely eliminated approximately log 6 cfu/mL of these cells in skim milk.” (Khanal et al. 2014)

High-Performance Ultrasonic Pasteurization Equipment

Hielscher Ultrasonics is long-experienced in the application of power ultrasound in the food & beverage industry as well as many other industrial branches. Our ultrasonic processors are equipped with easy-to-clean (clean-in-place CIP / sterilize-in-place SIP) sonotrodes and flow-cells (the wet parts). Hielscher Ultrasonics’ industrial ultrasonic processors can deliver very high amplitudes. Amplitudes of up to 200µm can be easily continuously run in 24/7 operation. High amplitudes are important to inactivate more resistant microbes (e.g., gram-positive bacteria). For even higher amplitudes, customized ultrasonic sonotrodes are available. All sonotrodes and ultrasonic flow cell reactors can be operated under elevated temperatures and pressures, which allows for a reliable thermo-mano-sonication and highly effective pasteurization.
State-of-the-art technology, high-performance and sophisticated software make Hielscher Ultrasonics’ reliable work horses in your food pasteurization line. With a small footprint and versatile installation options, Hielscher ultrasonicators can be easily integrated or retro-fitted into existing production lines.
Please contact us know to learn more about the features and capability of our ultrasonic pasteurization systems. We would be glad to discuss your application with you!
The table below gives you an indication of the approximate processing capacity of our ultrasonicators: