Micro-jet vs. Homogenization: A Comparative Analysis of Concepts and Functionality

As consumer demand grows for minimally processed, clean-label foods that retain nutritional and sensory qualities, non-thermal processing technologies are gaining significant attention. Among these, homogenization and microfluidics have emerged as highly promising processing technologies due to their ability to improve food texture and stability while avoiding the adverse effects of thermal processing. This chapter compares conventional homogenization and microjet technology, both of which utilize high pressure to reduce particle size, enhance emulsion stability, and inactivate harmful microorganisms and enzymes. Homogenization relies on forcing liquid flow through a narrow channel to generate shear forces, turbulence, and cavitation, whereas microjet technology employs a more advanced interactive chamber design to achieve precise collisions of microflows under extremely high pressure, thereby producing finer and more uniform dispersions. This paper will explore the fundamental mechanisms, processing parameters, and physicochemical effects of each method and conduct a comparative analysis of their impact on food quality. Finally, their applications in the food industry will be discussed to elucidate their practical advantages and limitations. Overall, this paper aims to provide readers with a comprehensive understanding of how these two technologies promote sustainable, high-quality food processing in modern industrial settings.

Introduction

As public awareness of food nutrition continues to grow, consumer preferences have shifted from heat-processed foods to more nutrient-rich, non-heat-processed foods, which better preserve the sensory characteristics of food. These technologies extend shelf life while preserving the food’s phytochemicals, sensory characteristics, and nutritional value.

A breakthrough technology in the food processing industry is homogenization, which uses external forces to disperse particles. By overcoming internal cohesive forces, homogenization reduces particle size in food. When particle size is reduced to the micron/nanometer scale, the food’s structural, physicochemical, and functional properties undergo changes. Due to the increased surface area at the micron scale, properties such as solubility, mouthfeel, water absorption, reaction rates, and flavor release are enhanced. Homogenization is a physical process that converts large particles into micron- or nanoscale particles by forcing liquid or emulsion products through valves or narrow channels. This process significantly increases the fluid velocity, leading to a drop in pressure within the valve gap and the generation of considerable shear stress. After the fluid exits the gap, the jet disperses and collides with the inner surface of the homogenizer, converting the kinetic energy of the flow into turbulence and cavitation. Homogenization technology can be used to inactivate microorganisms, improve emulsion stability, inactivate enzymes, and facilitate the preparation of nanoemulsions. This technology is energy-efficient, produces no harmful byproducts, and is environmentally friendly, making it a green technology.

Although microjet homogenization is a homogenization method, its principles differ from those of traditional homogenization techniques. It combines high-pressure homogenization, shock flow technology, and water jet technology. In the microjet process, the inlet fluid flows through an interaction chamber, where it is divided into multiple microjets that collide with one another. This process uniquely generates high pressure, vibrations, high shear rates and velocities, and hydrodynamic cavitation. Compared to other traditional methods, it is precisely this combination of forces that enables the production of uniform products.

Therefore, this paper will explore the basic principles, mechanisms, and operating parameters of homogenization and microjet technology. We will conduct a comparative analysis of the two, focusing on their effectiveness in particle size reduction, emulsion formation and stability, effects on physicochemical and functional properties, and the inactivation of microorganisms and enzymes. Furthermore, we will explore their applications in various food systems, such as dairy products, beverages, sauces, and nutritional and health supplement formulations, with a focus on analyzing the advantages and disadvantages of each method. The ultimate goal of this paper is to provide readers with a comprehensive understanding of how to apply these two non-thermal processing technologies to meet the evolving demands of the modern food industry for functional, clean-label, and long-shelf-life foods, without compromising their nutritional and sensory qualities.

Principle of Operation

Conventional Homogenization—Definition and Principles

Homogenization refers to the process of using pressure to force a liquid through a valve, thereby achieving a uniform particle size distribution for particles suspended in the liquid. Homogenizers can process fluid matrices at pressures ranging from 20 to 100 MPa and are primarily used to reduce particle size, thereby improving emulsion stability and preventing uneven emulsification and coalescence in the pharmaceutical, dairy, and cosmetics industries. A homogenizer consists of a high-pressure pump that forces particles through a fine tube, significantly reducing particle size and ensuring uniform distribution. Gas-filled bubbles (or voids) form within the fluid and expand (Figure 1). When local pressure is reduced without changing the temperature, the collapse of these voids can cause various local stresses to be transmitted to interfaces or particles (including microorganisms).

Figure 1: Conventional Homogenization

High turbulence, cavitation, strong shear forces, and sudden pressure gradients are key principles of homogenization (Figure 2). When the liquid to be homogenized enters the gap, its velocity immediately increases, causing a drop in pressure and an increase in shear stress within the valve gap. The pressure drop leads to the formation of a cavitation zone. The single-phase liquid flow in the turbulent region transitions to a multiphase liquid-vapor flow in the cavitation zone, where flow velocities can reach 100 m/s or higher. As the flow velocity decreases, the pressure inside the radial diffusion valve rises, causing the cavitation bubbles to implode, which results in a sudden transition from supersonic to subsonic flow.

Figure 2: Principles and Applications of Homogenization

Shear Effects

The fluid is forced through a tiny orifice inside the homogenizing valve, thereby generating shear forces. Shear occurs both between the fluid and the valve seat and within the fluid itself. The shear within the fluid is referred to as plane stress, while the shear between large particles and the valve surface is referred to as tensile flow stress. The stretching effect increases the shear force between food particles and the valve surface as particle velocity increases, leading to reduced stability of droplets during elongation. Additionally, high shear stress inhibits hydrophobic interactions and the formation of aggregates.

Turbulence

The fluid passage within the valve suddenly narrows by a factor of 100 to 1,000, creating a significant velocity gradient. This results in irregular fluid motion, which is characteristic of turbulence. Due to momentum effects, the diffusion coefficient increases, and kinetic energy is converted into thermal energy through vortices generated during the turbulent process, which helps break up particles and reduce their size.

Cavitation

Cavitation is the third phenomenon that occurs when the pressure of the flowing liquid drops abruptly. It describes the process by which bubbles or cavities form in the fluid and eventually collapse. Cavities are generated in regions of low pressure and high flow velocity. The formation of bubbles and their eventual collapse can lead to a breaking effect. Studies have shown that the shock waves generated by cavitation can reduce particle size and destroy cells. Two dimensionless numbers can be used to quantify and predict cavitation: the cavitation number (C_v, Equation (1)) and the cavitation initiation number (C_(in), Equation (2)):

Here, ρ is the fluid density, pv is the vapor pressure of the liquid, vmax is the maximum fluid velocity, p1 is the pressure before the homogenizing valve (upstream pressure), and p2 is the pressure after the homogenizing valve (downstream pressure). A key factor in homogenizing valve design is the cavitation number; for standard homogenizers, the cavitation number typically ranges from 0.1 to 0.3.

Heat Generated During Homogenization

Although homogenization is considered a non-thermal processing technique, adiabatic heating inevitably causes food products to heat up. Adiabatic heating results from turbulence, shear, and mixing during the homogenization process. Temperature control is critical for preventing overheating and the resulting loss of heat-sensitive molecules in food ingredients after homogenization.

Classification of Homogenizers

There are various types and shapes of homogenization valves available on the market. The geometry and design of the valve have a significant impact on the performance of the homogenizer and the characteristics of the final product. Homogenizer valves are designed to increase fluid velocity by allowing the fluid to flow through a small orifice between the valve stem and seat. Before striking the valve wall or impact ring, the fluid travels a certain distance and diverges at a specific angle. Depending on the intended application, there is a wide variety of homogenizer valves available on the market, each with different shapes, which are primarily classified into the following categories:

Radial Diffuser Type

The most commonly used high-pressure diffusion devices in industry are radial diffusers, also known as homogenizing valves. A typical radial diffuser consists of a valve face that deflects the fluid by 90 degrees and an axially movable valve seat, which can adjust both the slit width and the homogenization pressure at a constant flow rate. This design can achieve pressures of up to 150 MPa at both laboratory and industrial scales.

Axial-Flow Nozzle Type

This valve allows pressurized fluid to enter axially through an orifice that reduces the flow cross-section by a factor of three. This reduction in orifice size increases the fluid velocity, causing it to travel less than 2 millimeters before striking the needle valve and seat. The needle valve and annular seat are separated by a microscopic gap less than 2.5 millimeters in length and less than 10 micrometers in width. Pressure can be adjusted by closing the gap as the needle valve is drawn closer to the seat. The forward and backward movement of the needle valve is controlled by a pneumatic actuator. The advantage of this valve is that it can operate at different pressures while maintaining a stable flow rate. The flow rate is determined by the capacity of the booster. Currently, this valve can be used for fluid delivery at both the laboratory and pilot scales.

Reverse-Jet Disperser

The reverse-jet disperser operates on the principle of the collision of two turbulent flows. In the two microchannels separating the two fluid streams, the fluid velocities are each accelerated by approximately tenfold. Pressure then forces the two fluid streams to collide head-on. The region where the collision occurs is called the interaction chamber. Pressure is released by directing the fluid to an outlet with a diameter approximately nine times that of the microchannel.

Microjet Technology

Definition and Principles

Microjet technology is another non-thermal technique that utilizes a device called a “microjet,” which is based on a geometrically designed high-pressure interaction chamber (Figure 3). Initially applied in the pharmaceutical and cosmetics industries to prepare ultrafine emulsions, microjet technology has since found widespread application in the food industry as well. Microjet devices prepare stable and extremely fine emulsions by applying mechanical energy at extremely high pressures (up to 200 MPa). The functional properties of food macromolecules, including starch and fiber—such as porosity, oil and water-holding capacity, and solubility—change depending on the applied pressure, the number of processing cycles, the inherent characteristics of the food ingredients, and their concentration.

Figure 3: Schematic Diagram of a Microjet

Due to its operating principle, microjet technology offers significant advantages over traditional homogenization techniques. Furthermore, because there are no moving parts in its interaction chamber, it requires less maintenance than standard homogenizers. Microjet technology can also operate at higher pressures and produce very fine emulsions. It combines high-pressure homogenization, impact flow technology, and water jet technology. In microjet technology, liquid feedstock is introduced into an interaction chamber, where it is divided into two or more microflows that collide at speeds of up to 400 m/s. This method produces a unique combination of high velocity and shear rate, vibration, pressure reduction, and hydrodynamic cavitation. Unlike other traditional methods, it is precisely this specific combination of forces that results in a homogeneous product. Inertial forces arising from cavitation and turbulence control the fragmentation and homogenization processes, which primarily occur within the interaction chamber.

Principle of Operation

Although microjet devices are also a form of homogenization, their design and operation differ. A microjet system consists of the following main components: a hydraulic power system that drives the microjet device. The pressure from the hydraulic power system is amplified to an optimal level within the interaction chamber via a booster pump. The interaction chamber is a continuous microchannel that promotes turbulent mixing and energy dissipation. High-velocity liquid undergoes intense Y- or Z-shaped impingement within the chamber. In a Y-shaped chamber, two liquid streams collide with the channel walls and with each other. In a Z-shaped chamber, the liquid is forced into a zigzag microchannel and collides with the channel walls. Z-shaped chambers are typically used for the disruption of cellular structures and the dispersion of solid materials in suspensions, while Y-shaped chambers are primarily used for liquid-liquid dispersion. Process parameters (temperature, pressure, and number of cycles), the interaction chamber (channel design and diameter), and the type of material being processed all affect the efficiency of the microjet process. Once cooling is complete, the product can be collected in an output container. The sudden pressure drop at the outlet of the interaction chamber causes the fluid passing through the microjet process to expand, thereby loosening the originally tightly packed particle structure and creating voids or cavitation within the fluid.

When using pressure-type homogenizers, the movement of the homogenization valve causes pressure fluctuations throughout the process, and the fluid typically contains large particles across a wide size range. In contrast, microjet homogenizers pump multiphase fluids through the microchannels of the interaction chamber under high pressure, simultaneously exposing the multiphase fluid to high shear forces. Heat exchangers can be used to regulate product temperature, and microjet homogenizers can operate in either batch or continuous modes. A stable pressure distribution is achieved through a booster pump and a fixed geometry. At the target pressure—which remains constant throughout most of the compression stroke—the product passes through the microjet homogenizer, resulting in smaller particle sizes and a narrower particle size distribution.

Applications in the Food Industry

According to reports, homogenization and microjet technology are primarily used in food applications for (a) preservation and safety assurance by reducing microbial load and inactivating enzymes, (b) assisting in the extraction and stabilization of bioactive compounds, and (c) enhancing functional properties related to technological applications and health benefits.

Emulsification

The primary application of homogenization is to disperse immiscible liquids—typically an aqueous phase and an oil phase—into the continuous phase as droplets to form a stable colloidal dispersion. When a coarse oil dispersion in an aqueous phase containing an emulsifier undergoes high-pressure homogenization, the oil droplets can be reduced to submicron (less than 1 μm) or nanoscale (less than 200 nm) droplets. Due to their narrow particle size distribution, these submicron or nanoscale emulsions exhibit significantly reduced aggregation during storage and are less prone to gravitational separation, thereby extending their shelf life. The only drawback of homogenization is that coalescence and aggregation may occur after the valve outlet, which is attributed to Brownian motion. However, this can be avoided by using a two-stage homogenization system, which prevents particle aggregation and rapidly stabilizes the dispersed particles discharged from the first stage of homogenization. To achieve the production of kinetically stable submicron/nanoscale emulsions, researchers have examined homogenization pressure levels, the number of homogenization passes, the homogenization phase, the initial temperature of the processed fluid, and other emulsion formulation parameters.

Inactivation of Enzymes and Microorganisms

Homogenization serves as an alternative to thermal processing for inactivating microorganisms and enzymes, thereby enhancing food safety and extending shelf life while minimizing the negative effects associated with traditional methods, such as non-enzymatic browning and nutrient loss. During homogenization, structural changes caused by high temperature and pressure inactivate microorganisms and enzymes. This process and its effectiveness are influenced by various factors, including homogenization pressure, the types of microorganisms and enzymes, processing time, valve geometry, inlet temperature, and the number of homogenization cycles.

Food enzymes are crucial in food processing because they catalyze a variety of food processing reactions and contribute to the processing and improvement of the final product’s quality. Enzymes are used to produce various dietary components and important metabolites, which can serve as functional and nutritional supplements. However, some endogenous food enzymes can also trigger various food spoilage processes (such as browning, rancidity, and colloidal instability), leading to a decline in food quality and a shortened shelf life. The globular protein structure of enzymes is fragile and highly susceptible to damage from various physical, chemical, and biological factors, which often alter their natural conformation. Due to the influence of physical factors—including the unfolding of the protein’s natural three-dimensional structure and the resulting conformational changes (which alter the enzyme’s active site)—homogenization can lead to enzyme inactivation. The primary conformational changes caused by intense shear and the resulting cavitation are triggered by the bonds that maintain the polypeptide chains in their natural 3D structure.

Homogenization may inactivate microorganisms by disrupting microbial cell structures and releasing intracellular contents due to the intense mechanical stress generated by pressure. However, relying solely on homogenization to inactivate microorganisms is insufficient to meet food safety standards. Therefore, it is necessary to study the effectiveness of combining homogenization with other microbial inactivation techniques.

Encapsulation

The technique of encapsulating active ingredients within a suitable carrier is called encapsulation, which helps enhance the nutritional properties of food. Bioactive substances (vitamins, flavonoids, and polyphenols) are abundant in fruits, vegetables, nuts, oils, and herbs. Plants produce these substances for defense, signal transduction, and attracting mates. These substances are commonly referred to as plant secondary metabolites and exert both toxicological and pharmacological effects on humans and animals. These compounds help combat cancer, heart disease, and other illnesses. However, the bioavailability and bioaccessibility of these bioactive compounds are limited, and they are highly susceptible to chemical degradation, particularly under conditions of high temperature, high pH, high oxygen concentration, and exposure to light. By encapsulating bioactive components within food systems, the homogenization process can be utilized to enhance the nutritional value of the diet. When homogenization generates intense disruptive forces that break particles down to a manageable size, it becomes easier to encapsulate specific components within a suitable medium. Mechanical stress, associated heating, and interactions with emulsifiers can all influence the effectiveness of this process and the bioavailability of bioactive molecules.

Increased Bioavailability

Homogenization of liquid foods alters the bioavailability (the amount of ingested nutrients absorbed by the intestine and entering the bloodstream) or bioaccessibility (the amount of ingested nutrients released from the food matrix and available for intestinal absorption) of bioactive compounds. Most studies have found that bioavailability is enhanced due to the release of phytochemicals from within the food matrix. In other cases, changes in chemical structure lead to alterations in biological function. Furthermore, homogenization reduces particle size and increases surface area, making lipophilic bioactive components more soluble and thereby improving bioavailability.

Assisted Extraction of Bioactive Compounds

The food industry is committed to developing commercially viable functional foods that are rich in bioactive compounds, which are highly valued for their nutritional value and biological activity (such as antioxidant or antibacterial activity). In recent years, growing consumer concern regarding food safety, health, natural ingredients, the environment, and sustainability has driven an increase in demand for bioactive compounds from natural sources. To release intracellular compounds and improve the extraction yield of bioactive compounds, it is necessary to disrupt the cell walls of plants. Traditional extraction techniques (such as leaching and boiling) require prolonged high-temperature treatment, which may degrade bioactive components and reduce their biological activity. In recent years, green and efficient extraction technologies, such as homogenization and microjet technology, have been employed to assist in the extraction of bioactive compounds. Homogenization promotes the release of bioactive compounds by reducing particle size and increasing surface area. Studies have shown that homogenization can improve the extraction yield of various intracellular compounds due to its high cell disruption efficiency. The only drawback is that it leads to the non-selective release of intracellular chemicals and the dispersion of cell debris, which complicates subsequent separation procedures.

Limitations

Traditional high-pressure homogenizers have certain limitations:

· The types of emulsions that can be processed are limited. Systems containing particles, such as Pickering emulsions (particle-stabilized) and nanoparticle carrier systems, may cause wear in the homogenization unit.

· In processes such as dairy homogenization, where the volume fraction of the dispersed phase is high, agglomeration and coalescence are likely to occur. Compared to rotor-stator homogenizers, the high energy input and short residence time in pressure-type homogenizers lead to more frequent coalescence.

· To be suitable for high-pressure homogenizers, emulsifiers must meet certain requirements, such as rapid adsorption kinetics. For example, emulsifiers commonly used in the food industry (such as proteins or biopolymers) are sensitive to pressure or temperature, making it more difficult to form emulsions at higher temperatures.

· A aseptic filling line is a prerequisite for pressure homogenization following sterilization. Products that cannot be pumped or have excessively high viscosity cannot be processed. Furthermore, to compete with ultra-high-temperature (UHT) systems on an industrial scale, energy recovery must be implemented to improve efficiency and environmental sustainability.

The following factors limit its application in the food industry:

· Conventional microjet homogenizers have small reaction chambers (<300 μm). These devices cannot process substances prone to rapid precipitation, such as wheat bran and starch, which limits their ability to support continuous production.

· Microjet homogenizers are ideal for processing small batches of high-value products, such as pharmaceuticals and cosmetics.

· Although microjet homogenizers and high-pressure (valve-type) homogenizers have the same processing capacity, microjet homogenizers are currently more expensive than high-pressure homogenizers. Unlike traditional valve-type homogenizers, large-scale production using microjet technology requires larger and more expensive equipment. Consequently, high manufacturing costs limit its application in the food industry.

· The nonlinear characteristics of the flow channels within the microjet interaction chamber are a common cause of material residue and cleaning difficulties. Since it is difficult to monitor microbial contamination within the flow channels, aseptic processing techniques have also garnered significant attention.

Conclusions and Future Prospects

Both homogenization and microjet homogenization can significantly improve product quality across multiple sectors of the food industry. Although traditional homogenization remains the industry standard due to its reliability, scalability, and cost-effectiveness, emerging methods such as microjet homogenization are gaining significant attention for their superior emulsification efficiency and ability to produce emulsions with narrow particle size distributions and fine, stable textures.

Microjetting technology is particularly well-suited for high-value, functional, or sensitive formulations, as it can significantly improve the physical stability, bioavailability, and sensory quality of emulsions. However, depending on the matrix composition, high pressure and high temperature may lead to protein denaturation, fat coalescence, or emulsion instability; therefore, the use of microjetting technology requires strict control, and overprocessing should be avoided. Conventional homogenization techniques have been proven to effectively extend the shelf life of dairy and plant-based beverages, sauces, and other emulsified products, enhance microbial safety, and improve texture, striking a better balance between performance and practicality for common food processing applications. However, structural disruption and changes in mouthfeel remain significant potential drawbacks.

Combining homogenization and microjet technology with other cutting-edge technologies—such as membrane filtration, pulsed electric fields, or ultrasonic treatment—holds promise for pioneering new hybrid processing methods that strike a balance between product integrity, energy efficiency, and performance. The industrial application of these technologies also depends on the continued development of energy recovery systems, equipment design, and scaled-up processes. While homogenization will continue to dominate many large-scale food applications, microjet technology will find widespread use in niche markets and high-value product lines. This aligns with current trends toward personalized nutrition, clean-label requirements, and the development of functional foods. From a research perspective, both technologies remain essential tools for developing next-generation food systems, optimizing processes, and innovating ingredients.

As a specialized equipment supplier, we offer high-pressure homogenizers and microjet homogenizers covering the entire spectrum from laboratory and pilot-scale to industrial production. Our solutions combine the scalability advantages of traditional homogenization processes with the precise emulsification capabilities of microjet technology, making them suitable for process development and scale-up of both standard food systems and high-value, sensitive formulations. We help both research and production sectors fully leverage the application potential of these two homogenization technologies, enabling the successful implementation of process innovations for functional foods and clean-label products.