Pressure-induced Gel Research Articles

Pressure-induced gelation of blended milk and pea protein suspensions

The pressure-induced (400 MPa for 15 min) gelation of blended skim milk (SMP) and pea protein concentrate (PPC) suspensions (15% w/w) at different ratios (SMP:PPC = 10:0, 8:2, 6:4, 5:5, 4:6, 2:8, 0:10) were studied. Blending milk and pea proteins reduced the gel strength markedly, indicating that milk and pea proteins do not interact with each other to the same extent. The secondary structure of the proteins was not significantly changed after HP treatment. The PPC dominating gels (0:10 and 2:8) had a large increase in surface hydrophobicity indicating higher exposure of hydrophobic clusters. Based on a differential solubility method it was established that the SMP dominating gels are govern mostly by electrostatic and hydrophobic interactions, while disulphide bonds, hydrogen bonds and electrostatic interactions contributed most to the PPC dominating gel networks. The weak 5:5 gel was established solely by hydrophobic interaction between milk and plant proteins. It is suggested that in the milk protein dominant gel (8:2), the milk proteins formed the primary gel structure likely filled with the pea protein secondary gel network and/or aggregates, whereas the inverse occurred in the pea protein dominant gel (2:8). Replacing milk proteins with pea proteins offers a great potential for produce innovative dairy products using high pressure processing, like for example dairy desserts with a neutral pH and textural properties similar to yogurt.

High-Performance Hydrogels via Alternate Compression–Decompression

Novel bioapplications of hydrogels draw huge attention to the development of strong and tough hydrogels that are easily obtainable with less chemical additions. Here, we demonstrate that pressure, a basic thermodynamic quantity, could enable poly(vinyl alcohol) (PVA) to effectively form a gel. The resulting hydrogels by alternate compression–decompression (ACD) have tunable superior mechanical properties compared with conventional freeze-thawed (FT) PVA hydrogels. The microstructures of the ACD hydrogels under varying parameters, including compression rate, cycles, and holding time, reveal that either a slow compression rate or cyclic compressions favor the physical cross-linking of such single polymer networks. The mechanical results display a wide range of ultimate tensile strength, tensile strain, and maximum compressive strength of ∼0.3 to 2.5 MPa, ∼200 to 550%, and ∼3 to 7 MPa, respectively. Moreover, the load resistance of such hydrogels can be further trained by low-cyclic strain hardening to a maximum compressive strength of ∼50 MPa, followed by a desirable recovery. Upon cyclic compression, the ACD hydrogels exhibit consistent energy dissipation behaviors. More importantly, the effect of modulation of pressure on the hydrogel’s mechanical properties is very likely universal for other hydrogel systems due to the basic mechanism of pressure-induced gelation for the polymers discussed.

Structure and function of pea, lentil and faba bean proteins treated by high pressure processing and heat treatment

Utilization of pulse proteins in food products is increasingly explored, but the effect of processing on their quality and functionality is not well known. The effects of high pressure processing (HPP; 600 MPa, 5 °C, 4 min) and heat treatment (95 °C, 15 min) on the structure and functionality of pulse (lentil, pea, faba bean) proteins were evaluated, at protein concentrations characteristic for fortification of beverages (5 g/100 g) and gel formation (15 g/100 g). Structural changes were investigated by differential scanning calorimetry, rheological analyses, and surface hydrophobicity measurements. Sample solubility, water holding capacity, emulsifying and foaming properties were determined and compared to untreated controls. HPP and heat denatured pulse proteins and increased their surface hydrophobicity (p < 0.05). This led to changes in emulsifying and foaming, but the extent of these changes varied by protein type and treatment. Both treatments led to strong gels for 15 g/100 g samples, with heat-induced gels having greater strength (G’) than pressure-induced gels (p < 0.05). Both treatments resulted in higher water holding capacity for 15 g/100 g samples, but lower solubility for 5 g/100 g samples (p < 0.05) compared to untreated controls. These changes can have a significant influence on pulse products and may facilitate new pulse product innovations.

Comparison of Thermal and High-Pressure Gelation of Potato Protein Isolates.

Potato protein isolate (PPI), a commercial by-product of the starch industry, is a promising novel protein for food applications with limited information regarding its techno-functionality. This research focused on the formation of both thermal and high-pressure gels at acidic and neutral pH levels. Our results reveal that physical gels are formed after 30 min by heat at pH 7 and pH 3, while pressure (300–500 MPa) allows the formation of physical gels only at pH 3, and only when the system crosses 30 °C by adiabatic heating during pressurization. Texture profile analysis (TPA) revealed that gel hardness increased with both gelation temperature and pressure, while water-holding capacity was lower for the pressure-induced gels. The proteins released in the water-holding test suggested only partial involvement of patatin in the gel formation. Vitamin C as a model for a thermally liable compound verified the expected better conservation of such compounds in a pressure-induced gel compared to a thermal one of similar textural properties, presenting a possible advantage for pressure-induced gelation.

Mechanical and Functional Properties of Unwashed Barramundi (Lates calcarifer) Gels as Affected by High-Pressure Processing at three Different Temperatures and Salt Concentrations

ABSTRACTChanges in gelling properties of unwashed barramundi gels with 1% and 2% added salt, then pressurized at 300, 400, and 500 MPa at ≤10°C and 50°C for 10 min were studied. Pressure-induced unwashed barramundi gels exhibited a lower gel strength and poorer mechanical properties, such as lower hardness and springiness, as compared to heat-induced gels. Comparable water holding capacity to heat-induced gels was only obtained at 2% salt concentration and ≥400 MPa pressure. Scanning electron microscope images also showed that barramundi minced muscle with 2% added salt and pressurized at ≥400 MPa had a finer microstructure with smoother surface compared to heat-induced gels.

Changes in structural, functional and antioxidant properties induced by high pressure on quinoa flour

The quinoa flour (QF) dispersions (flour/water = 1:2) were subjected to high-pressure treatment (300, 450 and 600 MPa for 15 min) at ambient temperature, and thereafter, assessed for the structural changes using rheometry, calorimetry, microscopy, and diffractometry. The pressurization influenced the hydration property of QF significantly. The quinoa starch gelatinized completely by melting the amylopectin crystallites at 600 MPa as confirmed by the thermal, rheological and diffractometric studies. The mechanical rigidity of the pressure-induced gel improved considerably by increasing the pressure level. Further heating to the HP-treated QF dispersions (< 600 MPa) improved the mechanical rigidity significantly. The protein denaturation peaks remained unaffected by the pressure treatment. The antioxidant activity of QF increased at 450 MPa. The results obtained in this research could be useful for the development of quinoa-based food product under high-pressure.

The Combined Effect of High Hydrostatic Pressure and Calcium Salts on the Stability, Solubility and Gel Formation of β-Lactoglobulin.

Stability, aggregation and gelation of β-Lactoglobulin are affected by high pressure and salts of the Hofmeister series. Little is known about their combined effects on structure formation processes of β-Lactoglobulin, mainly because many salts of the series are not suitable for use in food. Here, we investigate the effect of calcium salts on the strength of pressure-induced gels, inspired by the fact that high pressure and salts change the water structure in a similar way. We find that the larger the applied pressures, the higher the strength of the gels. In addition to pressure, there is a significant influence by the type of anions and the amount of added calcium salts. Gel strength increases in the order CaCl2 < Ca (NO3)2 < CaI2. This trend correlates with the position of the salts in the Hofmeister series. The results are explained by analogy with the thermal aggregate formation by taking reaction rates for unfolding and aggregation, as well as specific/non-specific salts effect into consideration.

In situ observation of heat- and pressure-induced gelation of methylcellulose by fluorescence measurement

In situ observation of heat- and pressure-induced gelation of methylcellulose (MC) aqueous solution has been studied by using dynamic viscoelastic and fluorescence measurements. The storage modulus G′ decreased gradually first on heating, and then leveled off in the temperature range from 25°C to 75°C. Methylcellulose solutions were subjected to pressures up to 450MPa by using a high pressure cell. It was indicated that the microviscosity showed a dramatic change in the vicinity of the phase transition point. The T vs. P phase diagram of methylcellulose aqueous solution was constructed, and it indicated that the melting point was an increasing function of pressure. In situ microscopic observation of pressure-induced gelation of methylcellulose aqueous solution was also performed with a microscope, and it could be seen that gel phase of the sample disappeared and sol formed gradually. Pressure-released study by fluorescence measurement also showed that the phase transition of MC solution was reversible.

Effect of high-pressure on calorimetric, rheological and dielectric properties of selected starch dispersions

Effects of high-pressure (HP) treatment on the rheological, thermal and dielectric properties of the four selected starch dispersions (two modified starches, one native and one resistant) were evaluated. Differential scanning calorimetry (DSC) and oscillatory rheometry were employed to assess the extent of starch gelatinization and the developed gel rigidity (G′) of starch gels after HP treatment. It was observed that starch dispersions gelatinized completely at 500MPa with a 30-min holding time. The HP-treated starch samples exhibited predominantly solid-like (G′>G″) behavior except for the resistant starch. Pressure-induced gel rigidity differed significantly among starch samples. The G′ of starch gels increased with the pressure (400–600MPa) in the studied frequency range (0.1–10Hz) except for the native starch where a marginal decrease was recorded at similar condition. The holding time (15–30min) and concentration (20–25% w/w) significantly attributed towards gel rigidity of starch samples. Measurement of dielectric properties of HP-treated samples over the frequency range 450–4450MHz indicated differences in the dielectric constant (ɛ′), loss factor (ɛ″) and penetration depth among starch gels. Pressure did not show any effect on dielectric property of the resistant starch sample. Power penetration depth decreased significantly with frequency and with the pressure.

A comparative in vitro study of the digestibility of heat- and high pressure-induced gels prepared from industrial milk whey proteins

We undertook this study to compare the digestibility of heat- and high pressure-induced gels produced from whey protein isolate (WPI). To simulate in vivo gastrointestinal digestion of WPI gels, a pepsin–trypsin digestion system was used. The in vitro protein digestibility of WPI gels induced by high pressure (400 MPa and 30 min; P-gel) and those induced by heat (80°C and 30 min; H-gel) was compared using a protein concentration of 0.14 g mL−1. The in vitro protein digestibility of P-gels was significantly greater than that of H-gels (p<0.05). The size-exclusion chromatography profiles of the hydrolysates showed that the P-gel generated more and smaller peptides than natural WPI and H-gels. Furthermore, Sodium dodecyl sulfate–polyacrylamide gel electrophoresis analysis showed some soluble disulfide-mediated aggregation in the P-gel, while there was more insoluble aggregation in the H-gel than the P-gel. The P-gel was more sensitive to proteinase than the H-gel, which was related to the content of S–S bonds, and this in turn could be attributed to the differences in the gelation mechanism between the H-gel and P-gel.

Comparison of the gel-forming ability and gel properties of α-lactalbumin, lysozyme and myoglobin in the presence of β-lactoglobulin under high pressure

α-Lactalbumin (α-La) and lysozyme (LZM) each contain four disulfide bonds but no free SH group, whereas myoglobin (Mb) possesses no disulfide bond or free SH group. In this work, the pressure-induced gelation of α-La, LZM and Mb in the absence and in the presence of β-lactoglobulin (β-Lg) was studied. Solutions of α-La, LZM and Mb (1–24%, w/v) did not form a gel when subjected to a pressure of 800 MPa and circular dichroism analysis revealed that both α-La and LZM are pressure-resistant proteins. In the presence of β-Lg (5%, w/v), however, a pressure-induced gel formed for α-La and LZM (each 15%, w/v) but not for Mb (15%, w/v). One- and two-dimensional SDS-PAGE demonstrated the disulfide cross-linking of proteins was responsible for the gelation. Although α-La and LZM are homologous and have the same disulfide bond arrangement, the texture and appearance of the gels formed from α-La/β-Lg and LZM/β-Lg were markedly different even when induced under the same experimental conditions. Microscopic analysis indicated that phase separation occurs during the gelation of LZM/β-Lg but not during the gelation of α-La/β-Lg. NMR relaxation measurement revealed that the association of water molecules with the protein matrix in the α-La/β-Lg gel is tighter compared to that in the LZM/β-Lg gel. These results indicate that the gel-forming ability of a globular protein under high pressure is related to the primary structure of the protein, and that the gel properties depend on the cross-linking reaction and on the phase behavior of protein dispersion under high pressure.

Effect of High Pressure on Physicochemical Properties of Meat

The application of high pressure offers some interesting opportunities in the processing of muscle-based food products. It is well known that high-pressure processing can prolong the shelf life of meat products in addition to chilling but the pressure-labile nature of protein systems limits the commercial range of applications. High pressure can affect the texture and gel-forming properties of myofibrillar proteins and, hence, has been suggested as a physical and additive-free alternative to tenderize and soften or restructure meat and fish products. However, the rate and magnitude at which pressure and temperature effects take place in muscles are variable and depend on a number of circumstances and conditions that are still not precisely known. This review provides an overview of the current knowledge of the effects of high pressure on muscle tissue over a range of temperatures as it relates to meat texture, microstructure, color, enzymes, lipid oxidation, and pressure-induced gelation of myofibrillar proteins.

Study on the Subgel-Phase Formation Using an Asymmetric Phospholipid Bilayer Membrane by High-Pressure Fluorometry

The myristoylpalmitoylphosphatidylcholine (MPPC) bilayer membrane shows a complicated temperature-pressure phase diagram. The large portion of the lamellar gel (L(β)'), ripple gel (P(β)'), and pressure-induced gel (L(β)I) phases exist as metastable phases due to the extremely stable subgel (L(c)) phase. The stable L(c) phase enables us to examine the properties of the L(c) phase. The phases of the MPPC bilayers under atmospheric and high pressures were studied by small-angle neutron scattering (SANS) and fluorescence spectroscopy using a polarity-sensitive fluorescent probe Prodan. The SANS measurements clearly demonstrated the existence of the metastable L(β)I phase with the smallest lamellar repeat distance. From a second-derivative analysis of the fluorescence data, the line shape for the L(c) phase under high pressure was characterized by a broad peak with a minimum of ca. 460 nm. The line shapes and the minimum intensity wavelength (λ″(min)) values changed with pressure, indicating that the L(c) phase has highly pressure-sensible structure. The λ″(min) values of the L(c) phase spectra were split into ca. 430 and 500 nm in the L(β)I phase region, which corresponds to the formation of a interdigitated subgel L(c) (L(c)I) phase. Moreover, the phase transitions related to the L(c) phase were reversible transitions under high pressure. Taking into account the fluorescence behavior of Prodan for the L(c) phase, we concluded that the structure of the L(c) phase is highly probably a staggered structure, which can transform into the L(c)I phase easily.

The mechanism of high pressure-induced gels of rabbit myosin

Abstract Rabbit myosin (20 mg/mL of 0.6 M NaCl, pH 6.5) was subjected to various pressures (100–400 MPa) at 20 °C for 10 min to investigate pressure-induced gel mechanism by measurements of protein surface hydrophobicity, sulfhydryl contents, molecular size by native polyacrylamide gel electrophoresis, ultra-structure using SEM, and viscoelastic properties by dynamic rheology. Surface hydrophobicity and sulfhydryl groups had little increase at 100 and 200 MPa, but had significant increase at 300 and 400 MPa. Ultra-structure revealed that gels below 200 MPa were filament structure with many small cavities, while gels upon 300 MPa were globular aggregates with big cavities. Dynamic rheological measurements indicated that gels were formed at 400 MPa. Mechanism of myosin gel formation in pressure treatment was that myosin was unfolded, following the exposure of hydrophobic and buried sulfhydryl groups, then was denatured (electrophoresis showed the heavy chain became very weak) and associated to form gels. Industrial relevance In recent years, high pressure treatment of food has received interest as an alternative to heat treatment for gel formation. Gel-type meat products of high pressure processing are still not commercialized. This is basically due to lack of information about high pressure-induced gel. Myosin is the protein responsible for the gelling capacity of muscle systems. The processing characteristics of gels treated by high pressure were improved. The results of this experiment showed that myosin denatured and formed gels at pressure 400 MPa holding for 10 min. This information will help industry to adopt high pressure to produce meat products.

Kinetics of microstructure formation of high-pressure induced gel from a whey protein isolate

The kinetic process of pressure-induced gelation of whey protein isolate (WPI) solutions was studied using in situ light scattering. The relationship of the logarithm of scattered light intensity (I) versus time (t) was linear after the induced time and could be described by the Cahn-Hilliard linear theory. With increasing time, the scattered intensity deviated from the exponential relationship, and the time evolution of the scattered light intensity maximum Im and the corresponding wavenumber qm could be described in terms of the power-law relationship as Im~fβ and qm~f−α, respectively. These results indicated that phase separation occurred during the gelation of WPI solutions under high pressure.

Effects of pH and ionic strength on the rheology and microstructure of a pressure-induced whey protein gel

The effects of pH and ionic strength (I) on the properties of a pressure-induced gel from a whey protein isolate (0.2 g mL −1) were studied using a series of buffers with different pH and ionic strength (I). The rheological properties and water-holding capacity (WHC) of the gel changed in a complex pattern with pH and I but could be successfully modelled using an artificial neural network (ANN) model. The degree of cross-linking of S–S bonds was higher at pH 8.0 than at pH 5.0; consequently, a rigid gel was formed at alkaline pH values. The geometric dimensions of the microstructure decreased with increasing pH and decreasing ionic strength, and the WHC mainly depended on the geometric properties of the microstructure. These results indicate that pH and ionic strength influenced the gel properties by controlling the cross-linking reaction and phase separation that occur during gelation. They also confirmed the good potential application of ANN in studies of gelation.

Kinetics of phase separation during pressure-induced gelation of a whey protein isolate

The kinetic process of pressure-induced gelation of whey protein isolate (WPI) solutions (20–28%, w/v) was studied using in situ light scattering. The gelation of WPI solutions could be induced by pressurization at 250 MPa, a pressure lower than that reported in other studies. The gelation time decreased with increasing WPI concentration and followed an exponential rule. The relationship of the logarithm of scattered light intensity ( I) versus time ( t) was linear after the induced time and could be described by the Cahn–Hilliard linear theory. With increasing time, the scattered intensity deviated from the exponential relationship, and the time evolution of the scattered light intensity maximum I m and the corresponding wavenumber q m could be described in terms of the power-law relationship as I m ∼ t β and q m ∼ t − α , respectively. These results indicated that phase separation occurred during the gelation of WPI solutions under high pressure.

Comparison of heat- and pressure-induced gelation of β-lactoglobulin aqueous solutions studied by small-angle neutron and dynamic light scattering

The gelation mechanism of β-lactoglobulin (bLG) aqueous solutions was investigated by dynamic light scattering (DLS) and small-angle neutron scattering (SANS). Temperature- and pressure-jump experiments, respectively, abbreviated as T-jump (from 20 to 75°C; T-jump) and P-jump (from 0.1 to 315MPa) were carried out and the time evolution of gel structure was monitored by DLS and SANS as a function of time. The gelation threshold was determined by DLS as the point when nonergodicity appeared. In the case of T-jump, a rapid increase of the time-average scattered intensity, 〈I〉T, and a steep decrease of the initial amplitude of the intensity–intensity time correlation function, σI2, were observed at the gelation threshold. On the other hand, P-jump showed a gradual increase of the 〈I〉T and a continuous decrease of the σI2. It was revealed by SANS that bLG underwent thermal denaturation, resulting in a formation of gels consisting of densely aggregated unfolded bLG oligomers. On the other hand, the pressure-induced gels were found to be a fractal aggregates consisting of primary particles of bLG monomers. The difference in the gel structure as well as gelation mechanism between bLGs treated by T-jump and P-jump is discussed in comparison with T-induced and P-induced microphase separation of amphiphilic block copolymers in water [Osaka N, Shibayama M. Phys Rev Lett 2006;96:048303].

Effects of Sugars on the Cross-Linking Formation and Phase Separation of High-Pressure Induced Gel of Whey Protein from Bovine Milk

The effects of sugars (xylose, arabinose, fucose, fructose, galactose, glucose, sorbitol, maltose, sucrose, and lactose; 0-20% w/v) on the properties of the pressure-induced gel from a whey protein isolate (20%, 800 MPa, 30 degrees C, 10 min) were studied. All the sugars decreased the hardness, breaking stress and water-holding capacity of the gel at the same concentration of 55.5 mM. Increasing the sugar content changed the microstructure of the gel from a honeycomb-like structure to a stranded structure, while the strand thickness was progressively reduced. These results suggest that sugars decreased the degree of intermolecular S-S bonding of proteins and non-covalent interaction, and restrained the phase separation during gelation under high pressure.

高圧極限環境を利用した食品加工における新展開 牛乳のホエータンパク質とムコ多糖混合系からの高圧誘導ゲルのレオロジー特性と微細構造

This study concerns the rheological properties and microstructure of pressure-induced gels (800 MPa, 30°C, 10 min) from the mixture of whey protein isolate (WPI) and mucopolysaccharide (10% or 20% chitosan (CN), 0.25% or 0.5% hyaluronic acid (HA) or 3∼10% chondroitin sulfate (ChS) were added in 20% w/v WPI). CN increased the hardness and breaking stress of gel and restrained the phase separation during gelation under high pressure. HA decreased the hardness and breaking stress of gel, and converted the microstructure to a coarser structure formed with micro-particles. ChS did not affect the rheological properties of gel, but such gel has similar effect to the microstructure of gel with HA. The results suggest that coacervation in the presence of an anionic polysaccharide is triggered by whey proteins gelation under high pressure.