Introduction

In recent years, the food industry is rapidly evolving and facing new consumers’ demands and global food trends. Nowadays, consumers are looking for healthier and safe food with minimal or no-added food preservatives and extended shelf life, together with the demand for more sustainable food resources (plant-based proteins, for example). Although conventional thermal processing methods are verified tools for ensuring microbiological food safety [18], the intense heat used in these methods can cause nutrient degradation (e.g., vitamins and volatile aroma compounds) and result in the formation of unwanted compounds and off-flavors, for example, acrylamide, chloropropanols, and furan [95, 127, 141]. Emerging technologies are often investigated as a replacement of conventional methods to minimize the effect of heat on food components while ensuring microbial safety and preserving nutrients as well as sensorial properties, and, in some cases, improve techno-functional properties. These technologies include high hydrostatic pressure (HHP), high-pressure homogenization (HPH), ohmic heating, pulsed electric fields (PEF), microwave heating, cold plasma, gamma irradiation, UV processing, and ultrasound [5, 40, 107, 134, 175, 183, 212].

In HHP processing, the pressure is applied uniformly and transmitted to the pre-packed product by the pressure-transmitting medium at ambient or subambient temperature for several minutes, without inducing a shearing effect. Although many products are commercially being treated with HHP for pasteurization (fruit and vegetable beverages, for example) [13, 130], this process is not continuous and thus only a relatively low processing volume is possible. On the other hand, HPH, also known as dynamic high-pressure homogenization or high-pressure valve homogenization, is an emerging continuous flow process technology enabling the homogenization and pasteurization, and in some cases sterilization, of fluids in one single step [122, 210], while the fluid is being subjected to high pressure for less than a second [166, 187].

Homogenization is a physical process in which a dispersed system, suspension or emulsion, is forced to flow at a high velocity through a narrow passage, a disruption valve, producing a smaller and narrow particle size distribution [12]. Conventional homogenization, usually up to 50 MPa [9], is widely utilized in the food industry to stabilize emulsions by preventing creaming and coalescence, to reduce particle size (dispersion) and to mix ingredients [12]. In contrast to (ultra) high-pressure homogenization ((U)HPH), conventional homogenization has no preservation effect on the treated fluid. A major difference between (U)HPH and conventional homogenization is the maximum pressure level reached, and it is dependent on the homogenizer design and characteristics such as gap size, seals, and valve geometry. UHPH reaches pressure levels up to 400 MPa [10, 210], while HPH reaches pressure levels between 50 and 200 MPa [63]. It should be noted that some authors differ on the cut-off point between HPH and UHPH [49, 166, 210]. High-pressure jet (HPJ) technology is a similar technology reported to reach up to 500–600 MPa by utilization of a nozzle (from diamond, sapphire, or ruby) restricting the flow and forcing the fluid to form a jet stream that hits the air around it and transforms the liquid into aerosols. Immediately after the nozzle, a heat exchanger is connected, allowing those aerosols to coalesce back to liquid by hitting the wall of the heat exchanger [72, 76, 77, 185].

(U)HPH has been demonstrated as a valuable tool with two main impacts, the first, mainly focuses on the physical changes of the fluid after being subjected to the treatment. Such changes have crucial importance in various applications, such as preparation and stabilization of emulsions and nanoemulsions, reduction in droplets, and particle size of emulsions and suspensions together with a narrower size distribution, changes in the techno-functional properties of proteins and polysaccharides, texture modification and changes, and improvement in rheological properties of fluids [9, 21, 70, 79, 122, 174, 210]. In the pharmaceutical, cosmetics, and biochemical industries, it can be a tool for handling solid lipid nanoparticles and crystalline solids, dispersions, emulsions with controlled droplet size, drug nanoparticles, and nanosuspension [24, 46, 88, 91, 131, 187, 188]. The effect of cell disruption induced by (U)HPH can also be used as a tool for improved extraction of intra-cellular compounds (e.g., proteins, enzymes, fatty acids) in bioengineering-related industries [11, 14, 90, 125]. The second major application is microbial inactivation. (U)HPH can induce a reduction of the microbial load of food products to the level of pasteurization and even sterilization [9, 42, 62, 100, 101, 134, 145, 210], depending on process parameters such as pressure level and the number of passes, process temperature (inlet and maximal temperature), high-pressure valve design, and the properties of the treated fluid itself [122, 187, 210].

(U)HPH applications for enzyme inactivation and improvement of techno-functional properties of food components have been also explored. More recently, an increased scientific focus has been given to the direct and indirect effects of (U)HPH on bioactive compounds. Currently, the majority of (U)HPH utilization in many aspects is still in a laboratory or pilot scale. The industrial usage of commercially available (U)HPH units is limited, due to operation, usually at the maximal achievable pressure levels with flow rates less than the industrial requirements and, in some cases, high energy consumption [49, 122, 166]. A comprehensive review of the main engineering aspects and the physics behind (U)HPH, along with opportunities in scaling up of the process to commercial scale, was recently published by Martínez-Monteagudo et al. [122]. The utilization of (U)HPH for pasteurization, sterilization, and enzyme inactivation, especially in the beverage industry, has been previously reviewed [9, 43, 46, 49, 63, 70, 122, 137, 154, 166, 187, 210], while others have reviewed the impact of (U)HPH on emulsion stability [16, 49, 52, 70, 122, 135, 174, 210]. In addition to the interest in the opportunities of (U)HPH in microbial inactivation, including patents on the topic [30, 117, 124], the technology was also studied regarding the influence on the techno-functional properties of proteins and polysaccharides and the manufacture of food products with improved functionality. The focus of this publication is an updated review of the engineering aspects of (U)HPH and the potential of this emerging technology for applications beyond microbial inactivation. Technological aspects, detailed descriptions, and working principles as related to applications in the development of novel food products are also discussed.

Principles of (Ultra) High-Pressure Homogenization

The design of high-pressure homogenizer usually consists of one or two stages restricting the fluid flow, depending on the desired application and the properties of the final product [70]. The recent developments of designs (intensifiers, different valve, and homogenization chamber geometries) and various high-pressure-resistant materials (e.g., ceramic, diamond, sapphire, seals) allow operating pressure levels of up to 400 MPa and process temperatures of up to 140–150 °C [10, 49, 166]. Commercially available units differ mostly in their high-pressure valve design affecting the pressure range and applicable flow rates at the laboratory, pilot, and industrial scale [49, 166]. Valve design and geometry were reported to influence vegetative microorganisms’ inactivation [45] and the formation of food nanoemulsions [47] even when the same pressure was used.

Two-stage (U)HPH systems are usually equipped with a positive displacement pump, intensifier, and heat exchangers before and/or after the high-pressure valve. The first stage is the high-pressure valve, and the second stage is the low-pressure valve. A schematic diagram of a two-stage (U)HPH system design is presented in Fig. 1. The fluid, depending on the processing objective (pasteurization, sterilization, emulsification), is pre-chilled or pre-heated to the desired inlet temperature by a heat exchanger. Then, it is pressurized by a pressure intensifier, to the required pressure, usually up to 400 MPa, and consequently, the fluid temperature rises due to hydrostatic compression. Afterward, the fluid is depressurized by passing through the high-pressure homogenization valve reaching after the first stage, a depressurization to 10–20 MPa. Due to the pressure drop, part of the kinetic energy is converted to heat, resulting in a temperature increase [49, 122, 210]. The second homogenization valve, the low-pressure valve, reduces the fluid pressure to atmospheric pressure and disrupts agglomerates that might have been formed during the first homogenization valve discharge. As the final product temperature can be high, immediate cooling by a heat exchanger is often employed after homogenization to minimize damage to thermolabile components.

Fig. 1
figure 1

A schematic flow diagram of a two-stage high-pressure homogenization processing system. Tr is the temperature of the fluid reservoir; Tin is the fluid inlet temperature before increasing pressure to the homogenization pressure p; Tp is the fluid temperature after hydrostatic compression; T1 is the fluid temperature after the high-pressure valve;T2 is the fluid temperature after the low-pressure valve; Tout is the final fluid temperature after cooling in the heat exchanger. Based on [46, 49, 70, 122, 210]

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Valve Design and Temperature Increase

Valve geometry and design are crucial parameters affecting the process performance and characteristics of the final product. During (U)HPH, the fluid is forced to flow through the high-pressure valve passing a minute orifice (width of a few micrometers), increasing the fluid’s velocity. The disruption of the fluid particles (emulsion or suspensions) is mostly influenced by the valve geometry and the homogenization pressure level selected. The fluid itself is exposed to high pressure for a very short period, less than a second, and therefore hydrostatic effects are relatively small [166, 187]. Adjusting the initial fluid temperature, the pressure, the number of passes, and valve design may help to achieve the desired physical changes and process temperature [122]. Various valve geometries and designs are commercially available aiming for specific final applications [122]. Different commercial high-pressure homogenizers exist for laboratory and pilot-scale and a few ones fitting industrial requirements [122, 166, 210]. The three main types of valve geometries are counter jet, radial diffusers, and axial flow valves. While comprehensive descriptions of available homogenization valves are available [46, 122, 187, 210], Fig. 2 describes the most reported valve geometries [46, 70]. A nozzle-type geometry (Fig. 2a) was reported to be effective in the applications of droplet disruption [178], microbial inactivation, and cell rupture [152, 159]. The conical piston valve (Fig. 2b) was reported to be effective for breaking cell structures [20, 61, 103, 126], preparation of nanosuspensions [128] and nanoemulsions [47], solid lipid nanoparticles [177], and microbial inactivation [45, 57, 169]. The ceramic needle and seat (Fig. 2c) were reported to be effective for cell disruption [42, 57, 169, 193], preparation of nanosuspensions [59, 74, 186], protein denaturation [136], and microbial inactivation [51, 190]. Microfluidics (Fig. 2d) was reported to effectively disrupt cells, with relatively larger particles compared with the conical piston [61], heat denaturation of proteins [80], and formation of nanoparticles [99], and fine emulsion formation [24].

Fig. 2
figure 2

Schematic representation of common high-pressure valves. a Nozzle. b Needle and seat. c Conical piston. d Microfluidics. e Ball and seat. f Sharp angle. Drawings do not represent real scale. Based on [46, 49, 70, 122, 210]

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Velocity and Temperature Increase

In (U)HPH, the fluid is simultaneously subjected to homogenization and pressure effects. As the fluid depressurizes after the first homogenization valve and passes through a minute gap, the fluid velocity increases due to the tubing size reduction along with the corresponding drop in fluid pressure [49]. The patterns of fluid flow together with the velocity profile have been previously reviewed [122]. The increment in fluid velocity results in intense turbulence contributing to the disruption of the fluid dispersed phase and the homogenization effect. Forcing the fluid to pass through the small gap while the valve restricts its motion results in shearing between the fluid and the valve seat. Shear has a large effect on the droplet disruption. It was previously reported that the inertial and surface forces could be characterized by the Weber number (We in Eq. (1)) described before for disruption of a particle in a laminar flow [122, 195]. But for (U)HPH, the relation between the surface tension depends on the pressure and the temperature during homogenization (Eq. (2)) [122].

$$ {W}_e=\frac{\rho \times {v}^2\times l}{\gamma } $$
(1)
$$ {\left(\frac{\partial \gamma }{\partial p}\right)}_{T,A}={\left(\frac{\partial V}{\partial A}\right)}_{p,T} $$
(2)

where ρ is the fluid density, v is the fluid velocity, l is the typical length, γ is the surface tension, p is the pressure of the system, V the total volume of the system, A total surface area, and T is the system temperature.

Mixing, emulsifying, and homogenization in the (U)HPH valves are considered to occur due to the turbulence created by the restricted fluid flow, leading to the increment in its velocity. The generated turbulence dissipates kinetic energy into heat, leading to an increase in fluid temperature [29, 78, 122]. The third phenomenon happening after the first (U)HPH valve is cavitation due to the sudden pressure drop [122]. Cavitation refers to the formation of air bubbles in the fluid and their subsequent collapse. Quantification and prediction of cavitation are possible using two dimensionless numbers: cavitation (Cv, Eq. (3)) and cavitation inception (Cin, Eq. (4)):

$$ {C}_v=\frac{p_2}{p_1} $$
(3)
$$ {C}_{in}=\frac{p_2-{p}_v\ }{0.5\times \rho \times {v_{\mathrm{max}}}^2} $$
(4)

where p2 is the pressure after homogenization valve, p1 is the pressure before homogenization valve, pv is the liquid vapor pressure, ρ is the fluid density, and vmax is the fluid maximum velocity. The combination of three phenomena, turbulence, high shear, and cavitation, is mainly responsible for the effect of mixing, dispersion, emulsification, and reduction in particle size [29, 78, 122].

The total increment in temperature reflects two factors: heat compression and the combined effect of shearing turbulence and mixing (as a result of the instantaneous pressure drop). The rapid pressurization generated during the pressure build-up in the intensifier causes heat compression and an increase in temperature at a range of 2–3 °C per 100 MPa, depending on the fluid characteristics while this increment is reversible upon pressure release [46, 49, 122]. Due to the isotactic pressure and the work done during the pressure discharge through the homogenization valves, the kinetic energy of the fluid is lost in the form of heat and the fluid temperature increases irreversibly by 14–25 °C per 100 MPa, depending on the physicochemical properties of the fluid, the fluid inlet temperature, and the homogenizer design [46, 49, 122, 166, 187, 210]. For example, it was reported that the range of increasing temperature per 100 MPa for bovine milk was 16.6–19.5 °C when the inlet temperature ranged from 4 to 50 °C [73, 139, 142, 182]. For fruit juices and plant-based beverages, the increment was reported to be 15.0–22.6 °C per 100 MPa when inlet temperature ranged from 4 to 75 °C [143, 180, 189]. The increment in fluid temperature is mostly dependent on the homogenization pressure at a specific inlet temperature [210]. This increment was described to be a linear or polynomial function of pressure, based on empirical models reviewed in previous works [46, 49, 122, 181]. A general equation (Eq. (5)) for estimation of the fluid temperature during the process has been suggested [122]:

$$ {T}_p={T}_{in}+\delta \times \Delta p+\xi \times \Delta p-\Sigma \mathrm{heat}\ \mathrm{loss} $$
(5)

where Tp is the fluid temperature during the process, Tin is the fluid inlet temperature, δ is the heat compression of the fluid, ∆p the hydrostatic pressure, ξ homogenization heat, and heat loss that can be negligible if the system is well insulated (mostly relevant for laboratory-scale equipment) [122].

Effect on Bioactive Compounds and Properties of Food Matrices

(U)HPH has been suggested as a tool for improving the techno-functional properties of food and food components, investigated mainly in fruit juices. The utilization of (U)HPH in fruit juices processing is a promising example of a combination of the versatile applications of (U)HPH. The technology allows to achieve a reduction of microbial load, preservation of quality properties (such as the content of thermally sensitive compounds) of fresh products, and affect the physical properties, such as phase separation, in one single process. Each matrix reacts differently when subjected to (U)HPH processing and the effect on product properties cannot be easily predicted. Every fruit has a unique composition, and cell fragmentation exposes and releases internal and wall constituents (e.g., pectins and proteins), increasing the potential for particle-particle and particle-serum interactions. Also, the cell wall has its structure, and consequently different resistance to shear forces [104, 116]. Published data on fruit juices and juice concentrates showed that rheological properties were modified (e.g., decrease in apparent viscosity and thixotropic behavior changes) [104, 105, 173, 202, 213], decrease in sedimentation during storage [96, 156, 161, 173, 202], decrease in particle size with increasing homogenization pressure [7, 17, 22, 23, 96, 104, 105, 161, 173, 213], but reaching a critical size below which particles could not be further degraded regardless of the initial size [7, 161, 173]. While HPH was reported to induce a reduction in the thixotropic behavior of most fruit juices and juice concentrates [104, 213], an increase in this behavior was observed for tomato juices [96]. As further described, (U)HPH can reduce the consistency of many polysaccharide solutions, thus suggesting that such a technology could also be used to reduce the consistency of juice concentrates with low pulp content. From an engineering point of view, a lower consistency leads to lower friction during processing and distribution, thus minimizing energy requirements [104, 105]. Concentrated orange juice (COJ) is a common industrial raw material for the juice industry. As it is a concentrated product, its high consistency requires high amounts of energy for processing and handling. Leite et al. [104] evaluated the effect of HPH treatment on the rheological properties and flow behavior occurring in COJ. The apparent viscosity, the consistency index, and the mean particle diameter were reduced, while an increase in the flow behavior index was observed. Thus, the utilization of HPH presents an opportunity to minimize the energy required during COJ processing and distribution due to the reduction in the product’s consistency.

The effect of (U)HPH on the preservation of some of the health-promoting and bioactive compounds such as vitamins, polyphenols, compared with the conventional heat treatments was previously reviewed [122, 210]. Recently, Sharabi et al. [167] investigated the possible improvement of milk nutritional quality during shelf life by testing the stability of ascorbic acid and riboflavin and antioxidant capacity in milk and model systems after HPH treatments. The results revealed reduced microbial load, almost unaffected concentration of ascorbic acid immediately after the treatment vs. rapid degradation during storage. The degradation rate of riboflavin, a light-sensitive molecule, was reduced by ~ 50%, depending on the homogenization pressure. The authors suggested that this outcome is an indirect effect of the (U)HPH on the degradation due to increased light scattering and absorbance of the wavelengths related to photo-oxidation of riboflavin. (U)HPH is known to disrupt plant matrices affecting both the functional properties of the products and the release of some bioactive compounds in the matrices. Juice beverage industry is facing several quality defects during storage, such as cloud loss, enzymatic browning, flavor changes, color loss, and deterioration of bioactive compounds [207]. Recent works have focused on these properties and the optional improvement by (U)HPH using different numbers of passes on some fruit beverages and nectars [85, 129, 156, 173, 180, 202, 207, 209]. Moscovici-Joubran et al. [129] studied the effect of different (U)HPH pressures and the number of cycles on the physicochemical, structural, and functional attributes of strawberry nectar. The study also tested the effect of a filtration step that partially removed large particles. As expected, the (U)HPH treatment reduced the particle size, but contrary to some previous publications on juices [96, 114, 156, 202], the stability against separation was also negatively affected, likely due to the destruction of a weak network. The total polyphenol concentration was not affected by the homogenization pressure itself but significantly enhanced with an increasing number of cycles at 200 MPa, whereas the color and the anthocyanins content were only slightly affected. Moscovici-Joubran et al. [129] demonstrated that the number of cycles has the potential of enhancing the extractable content of health-promoting compounds in processed matrices. The effect of (U)HPH on the antioxidant capacity of bioactive compounds was mainly investigated in food matrices, and specifically in fruit juices, and was reviewed before [81, 210]. In general, compared with the heat treatment, (U)HPH processing of juices was reported to better preserve antioxidant capacity [66, 85, 180, 191], total polyphenol content [66, 85, 180, 191], and bioactive compounds such as ascorbic acid [120, 167, 180, 191], carotenoids [66, 106, 180, 191], flavonoids [15, 191], and vitamins [19]. Toro-Funes et al. [184] investigated the effect of (U)HPH and various inlet temperatures on soy drink bioactive compounds, phytosterols, isoflavones, and tocopherols. The total phytosterols and isoflavones content in soy drink increased with increasing pressures and temperatures. As the inlet temperature increased, the total tocopherols decreased while an increase in the pressure level increased their content. Liu et al. [113] investigated the impact of HPH and different inlet temperature on physical stability and carotenoid degradation kinetics of carrot beverage during storage. The use of lower inlet temperature combined with the HPH preserved the color during storage better than the same pressure level with higher inlet temperature. The physical stability of all the tested samples decreased during storage. Yet, the stability of the samples treated at a higher temperature and high pressure decreased more than samples treated with lower inlet temperature combined with the high pressure. The combination of high pressure and high inlet temperature also assisted in improving the preservation of carotenoids during storage when compared with the other tested treatments. A study carried out by Loira et al. [115] tested the application of (U)HPH to process grape must before fermentation. They demonstrated that (U)HPH processing led to an elimination of grape microflora allowing the reduction of the added sulfite levels needed for controlling oxidation and thus improving wine’s aroma compared with sulfited wine.

Fernandez-Avila et al. [52] reviewed the potential and the current application of (U)HPH utilization for the production of nanostructures based on lipid, carbohydrates, protein, or protein-polysaccharide complexes loaded with biologically bioactive compounds such as vitamins and phenolic compounds. The bioaccessibility of bioactive compounds in model suspensions and food matrices as affected by HPH was reported in previous works and reviewed elsewhere [16, 27, 92, 122]. Some works published recently investigated the influence of HPH processing of food products on the bioaccessibility of some bioactive compounds such as carotenoids [110, 114, 201]. In general, HPH improved the bioaccessibility of total carotenoids [110, 201] and total polyphenols [201]. The addition of oil or emulsion before HPH treatment also enhanced the bioaccessibility of carotenoids without affecting the bioaccessibility of polyphenols [110, 201]. Liu et al. [110] studied the effects of HPH and the addition of oil on carotenoid bioaccessibility in carrot juice. They reported that the total carotenoids bioaccessibility was significantly higher when carrot juice was treated at pressures above 60 MPa and inlet temperature 70 °C. Liu et al. [114] studied the influence of HPH, number of cycles, and different inlet temperatures on the bioaccessibility of carotenoids in carrot juice, reporting that HPH increased the total carotenoids bioaccessibility. The physical stability was also improved while the effect of pressure level and number of cycles were more significant than inlet temperatures.

(U)HPH Effects on Polysaccharides

In the food industry, polysaccharides are extensively exploited to modify textural properties of fluids and semi-solid foods and used as stabilizers, thickeners, binders, suspending agents, emulsifiers, and gelling agents, depending on the functional properties of a given polymer [2, 133, 179]. When added to food, polysaccharides can modify hydration, solubility, rheological, and interfacial properties and can be used as texture enhancers [2, 70, 133, 179]. The rheological characteristics of food products are essential both for the process design and optimization (e.g., flow in pumps and pipelines) and for the final product itself as it affects product stability, quality prediction, sensory characteristics, and consequently consumer acceptance [146, 192]. Physicochemical properties, such as apparent viscosity, are associated with the polysaccharide molecular weight (Mw) distribution that can be affected by food processing [70]. Thus, many researchers have attempted to control polysaccharides molecular weight and make the distribution more uniform, mainly by the degradation of polysaccharides by enzymatic [87, 94, 97, 132], chemical [3, 41, 60, 67, 84, 93, 94, 151, 172], physical [1, 34, 38] methods, and also (U)HPH [70]. The latter showed a large effect on polysaccharides and their functionality. In (U)HPH processing, polysaccharides are subjected to the turbulent and shear forces [89]. Therefore, (U)HPH could influence the structure and the rheological properties of polysaccharides, which would thus influence their application in food systems [200]. The degree of physical change of polysaccharides during (U)HPH is influenced by processing parameters such as pressure and the number of homogenization cycles, and parameters related to the polysaccharide and the fluid such as solution pH and the polymer structure and concentration [89, 194]. Several studies explored the influence of (U)HPH on the rheological properties and the degradation of polysaccharides solutions. Studies investigating the influence of (U)HPH on polysaccharides and their techno-functional properties are summarized and presented in Table 1.

Table 1 The effect of (ultra) high-pressure homogenization on the characteristics of polysaccharides
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(U)HPH impacts on polysaccharides described in Table 1 indicate that the structure and conformation of polysaccharides (e.g., linear, branched) have a larger impact on the outcome of (U)HPH treatment than the polymer charge. Linear and stiff polysaccharides undergo depolymerization resulting in reduced polydispersity while globular-branched structures are less affected [194]. Also, pH has an impact on depolymerization, possibly reflecting the polymer conformation in solution during homogenization. At pH 6.3, (U)HPH induced conformational changes in pectins resulting in a more compact structure, while at pH 4.4, no conformational changes were noticed [170]. Besides, larger chitosan molecules were more susceptible to chain scission resulting in a narrower molecular weight distribution than that of original chitosan, indicating that large macromolecules were preferentially fragmented [86]. Similar results were reported also for pectins [170]. For a specific homogenization pressure, the molecular weight reduction occurs until a critical molecular weight is achieved and no further reduction occurs if the homogenization pressure is kept constant or reduced [71]. These results also suggest that additional homogenization cycles at constant pressure would only reduce polysaccharide molecular weight polydispersity by depolymerizing remaining polysaccharide strands having a molecular weight above the critical molecular weight for the specific homogenization pressure. In the case of rheological properties of the polysaccharide containing system, the thickening properties of polymers were also reduced due to the (U)HPH treatment likely related to the reduction in molecular weight [58].

Effect on Proteins

Enzymes in food matrices and their activity are an important parameter influencing shelf life, characteristics of the product, and the fate of bioactive compounds. (U)HPH was previously reported to activate, inactivate, and in some cases, not to affect enzyme activity, and the data has been summarized in previous reviews [49, 122, 210]. Proteins in food are technologically valuable due to their diverse techno-functional properties such as solubility, swelling, water holding capacity, foaming, emulsifying, and gelling capacity [211]. The use of proteins as emulsifying and foaming agents is based on their amphiphilic characteristics, and their ability to migrate to the interface depends on the conformation and the flexibility of the molecule [65, 121, 147]. Research on processing technologies changing the functionality of polymers, including proteins, is growing, partially due to the opportunity to reduce the use of stabilizers and emulsifier agents. (U)HPH is one of the technologies capable of achieving physical protein modifications [70]. The effect of (U)HPH on the protein’s secondary or tertiary structure is controversial. (U)HPH was reported as a useful tool for the disruption of protein quaternary structure [70]. Some studies suggest that (U)HPH up to 400 MPa has little impact on the secondary or tertiary structure when the solutions are immediately cooled after the homogenization [50, 64, 138,139,140] while others led to different findings [37, 203]. Various studies were conducted to understand the influence of (U)HPH on the techno-functional properties of proteins in suspension, such as foaming, solubility, and particle size distribution [26, 44, 109, 158, 205]. In this section, the focus will be mainly on the effect of (U)HPH on the techno-functional properties of plant-based proteins and more recent works dealing with proteins that possess poor water solubility. Protein unfolding can expose the inner hydrophobic regions increasing the potential for hydrophobic interactions between proteins, fat globules, and small particles finely distributed in the aqueous phase, creating a new oil-water interface that may favor the formation of particle aggregates [53, 150, 198]. (U)HPH impact on proteins, described in Table 2, indicate that (U)HPH has the potential to improve and modify techno-functional properties of proteins such solubility, emulsifying and foaming properties, particle size distributions, zeta potential, and rheological properties of proteins. Several works demonstrated that homogenization above a specific pressure might induce further unfolding of the protein and exposure of hydrophobic regions leading to protein aggregation and thus a reduction in some of the techno-functional properties [157, 158]. (U)HPH was also reported as a tool for enhancing proteins solubility [26, 168, 204, 205, 208].

Table 2 The effect of (ultra) high-pressure homogenization on proteins and polysaccharide-protein mixed systems
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Polysaccharides and Proteins Mixed Systems

Proteins and polysaccharides in the food matrix can improve physicochemical properties such as stability, rheological properties, and mouthfeel [48, 149, 160]. When these two ingredients are mixed in an aqueous phase, the interaction between them can result in segregation and/or association, depending on their concentration and net charge. If an electrostatic repulsive force occurs, phase separation of a protein and polysaccharide-rich phases will occur. In the case of attractive electrostatic force, phase separation of protein and polysaccharide-rich phase and a solvent-rich phase will occur [149]. Some physical processes can induce changes in protein structure, altering the ability to interact, and among them is (U)HPH. (U)HPH impact on polysaccharides and proteins mixed systems is also described in Table 2. The impact of (U)HPH on the interaction between electrostatically charged polysaccharides and neutral polysaccharide and protein was demonstrated to effect the complexes [83]. Such studies may help to understand how (U)HPH can manipulate protein-polysaccharide interactions and open the way for further research on the formation of new functional polysaccharide-protein aggregates\complexes, and opportunities in the stabilization of hydrophobic proteins in an aqueous medium.

Improvement of Techno-Functionality of Complete Food Systems by (U)HPH

Many studies investigated the possibility of processing liquid food products by (U)HPH to reduce the microbial load, improve techno-functional and sensory properties, reduce fat content, and reduce the use of stabilizers [6, 28, 119, 122, 138, 139, 143, 166, 210]. The application of (U)HPH in dairy products, including for pasteurization and improvement of functional properties, has been comprehensively reviewed [9, 46, 49, 63, 122, 135, 166, 187, 210]. Recent works on bovine milk developed a method for monitoring and predicting the structural and functional changes in milk during (U)HPH at different inlet temperatures by monitoring the formation of Maillard compounds using face fluorescence spectroscopy [111, 112]. The influence of (U)HPH milk treatment on the properties of cheese has been previously reviewed [46, 135, 187, 210]. For dairy-based yogurt, several studies investigated the utilization of (U)HPH on skim and whole milk for the production of set or stirred yogurt and its influence on yogurt properties. They reported that (U)HPH treatment resulted in the reduction of particle size, the formation of finer dispersion, as well as the denaturation of whey proteins and dissociation of casein micelles. Thus, yogurt properties such as water holding capacity, firmness, syneresis, consistency, and water retention were improved [75, 102, 162, 164, 165]. The impact of HPH on techno-functional properties of some complete food systems is presented in Table 3. HPJ was studied and shown to change the physiochemical properties of skim milk, skim milk powder, and whole milk [76, 77, 185]. These findings may allow the future utilization of HPJ to manufacture novel functional ingredients to minimize/replace the addition of emulsifiers or stabilizers to food products. (U)HPH was also reported as a tool for changing during shelf life the physiochemical properties, texture, and sensorial properties of dairy products such as cream and yogurt [28, 148, 163].

Table 3 The effect of (ultra) high-pressure homogenization on food system properties
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The demand for plant-based protein beverages and yogurt substitutes as an alternative to milk and milk-based yogurt is increasing in recent years [82, 118]. Plant-based dispersions often suffer from techno-functional limitations such as poor aqueous solubility, off-flavor, color and physical instability [4, 39, 82]. Thus, recent works have focused on the utilization of (U)HPH as a preservation technology for such products and studied the outcome on techno-functional characteristics of plant-based drinks, e.g., particle size, physical stability, color, and volatile compounds. As expected, (U)HPH considerably reduced the microbial load and particle size and extended the product shelf life [31, 32, 35, 55, 56, 68, 69, 171], improved product color, usually luminosity, compared with the one observed for conventional heat treatment [32, 56], and improved physical stability, mainly against creaming, due to the particle size reduction [31, 55]. Also, (U)HPH processing resulted in a similar, or sometimes decreased the off-flavors when compared with conventional thermal treatments [32, 56]. (U)HPH was found to improve characteristics of yogurt alternative prepared from the treated plant-based beverage, such as greater firmness, higher water holding capacity, and similar color compared with conventional heat treatment [36, 54].

Conclusion

(U)HPH is an emerging technology with potential applications in various areas of the food industry including pasteurization and sterilization, emulsion and suspension stabilization, and modification of the structure of whole matrices and specific biopolymers for the production of functional foods and novel ingredients. Major effects due to (U)HPH treatment are observed for polysaccharides, proteins, and their combination, leading to the formation of physically modified biopolymers with novel functionalities. The extent of these effects depends on a range of controllable processing parameters such as pressure, inlet temperature, valve design, solution pH, and others. To transform the current rich scientific data obtained from laboratory and pilot-scale work into commercial applications, the understanding of scaling up, energy consumption, and process cost estimation must improve. An additional possible driving force for the implementation of this technology could stem from a better understanding of (U)HPH effects on the bioaccessibility of bioactive compounds and the benefits of the modification of matrices and biopolymers when engineering the digestive fate of foods. From an engineering point of view, a more comprehensive understanding of the effect of valve type and geometry on the techno-functionality is also vital.