Gelation of single heated vs. double heated whey protein isolate

Gelation of single and double heated whey protein dispersions was investigated using Ca²⁺ as inducing agents. Whey protein isolate (WPI) dispersions (10% w/w) were single heated (30 min, 80 °C at pH 7.0) or double heated (30 min, 80 °C at pH 8.0 and 30 min, 80 °C at pH 7.0) and diluted to obtain the desired protein and/or calcium ions concentration (4–9% and 5–30 mm, respectively). Calcium ions were added directly or by using a dialysis method. Double-heated dispersions gelled faster at lower protein and calcium ion concentrations than single-heated dispersions. Gels obtained from double-heated dispersions had lower values of shear strain and shear stress at fracture than gels obtained from single-heated dispersions. Double heating caused a significant complex modulus (G*) increase at 4% WPI and 15 mm calcium ions in comparison with gels obtained from single-heated dispersion. Less significant differences between gels made from double and single-heated dispersions were observed at 6% WPI, however a higher value of complex modulus was obtained for 8% protein gels from the single-heated solution. Native and non-reduced SDS–PAGE did not show clearly the effect of different procedures of heating on the quantities of polymerised proteins. Proteins in double-heated dispersions had higher hydrophobicity. Increased calcium concentration caused decreased protein hydrophobicity for both single and double-heated solutions.     

Effect of polysaccharides with different ionic charge on the rheological,microstructural and textural properties of acid milk gels

The effects of addition of polysaccharides with different ionic charge on rheology, microstructure, texture and water holding capacity (WHC) of acid milk gels were studied and compared to that of gelatin addition. Similar to gelatin, starch (neutral) and xanthan gum (anionic) did not prevent milk gelation in the first 30 min of the acidification stage, even at high concentrations, and the typical casein network in acid milk gels could still be seen from electron micrographs; gelling and melting of these hydrocolloids were observed during the cooling and heating stages at specific concentrations. On the other hand, two neutral polysaccharides, guar gum (≥ 0.05%) and locust bean gum [LBG] (≥ 0.1%) inhibited milk gelation from the beginning of the acidification stage; the microstructure of the gel was modified greatly and no gelling/melting was observed during the cooling or heating stages. Another anionic polysaccharide, carrageenan, induced earlier milk gelation at low concentration (≤ 0.05%), but inhibited gelation entirely at high concentration (0.2%); inflections at ~ 27 °C and 21 °C were also observed during the cooling and heating stages at 0.05% concentration. The gel microstructure was not changed greatly, but showed smaller particle size at a carrageenan concentration of 0.05% than control sample. None of the polysaccharides showed as much improvement in WHC of the milk gels as gelatin did. Hence, xanthan and starch were found to be closer to gelatin in their effect on acid milk gels compared to guar gum, LBG and carrageenan.     

Structural and rheological properties of amaranth protein concentrate gels obtained by different processes

The heat-induced gelation of amaranth protein concentrates (APCs) by three processes was studied. The first was the conventional process for isolating protein (standard process-st), the second included an acid washing step prior to protein extraction (acid washing process-aw) and the third included heating (50 °C) during the alkaline extraction step (heat process-ht). The dispersions (12%, w/v) were heated to 55–90 °C and assessed by rheological measurements made under small deformations, whereas the gels obtained by heating at 70, 80 or 90 °C/30 min were subjected to uniaxial compression measurements (TPA and mechanical properties). The rheological parameters associated with the network structure, elasticity modulus (E) and storage modulus (G′), increased with increasing gelation temperature. For the APCst and APCht gels, protein aggregation occurred in two steps, whereas for APCaw, gelation occurred in a single step. The APCht gels showed the highest fracturability, hardness and adhesiveness, followed by the APCst and APCaw gels (p < 0.05). Similar results were obtained for the mechanical properties at fracture. Increasing the heat treatment temperature from 80 to 90 °C resulted in a more structured matrix with greater water-holding capacity as compared to gels obtained at 70 °C, and these properties were influenced by the extraction processes used to obtain the APCs. Heat extraction (APCht) improved the gelation and water-holding properties, whereas the acid treatment had the opposite effect.     

Heat stability of oil-in-water emulsions formed with intact or hydrolysed whey proteins: influence of polysaccharides

The heat stability of emulsions (4 wt% corn oil) formed with whey protein isolate (WPI) or extensively hydrolysed whey protein (WPH) products and containing xanthan gum or guar gum was examined after a retort treatment at 121 °C for 16 min. At neutral pH and low ionic strength, emulsions stabilized with both 0.5 and 4 wt% WPI (intact whey protein) were stable against retorting. The amount of β-lactoglobulin (β-lg) at the droplet surface increased during retorting, especially in the emulsion containing 4 wt% protein, whereas the amount of adsorbed α-lactalbumin (α-la) decreased markedly. Addition of xanthan gum or guar gum caused depletion flocculation of the emulsion droplets, but this flocculation did not lead to their aggregation during heating. In contrast, the droplet size of emulsions formed with WPH increased during heat treatment, indicating that coalescence had occurred. The coalescence during heating was enhanced considerably with increasing concentration of polysaccharide in the emulsions, up to 0.12% and 0.2% for xanthan gum and guar gum, respectively; whey peptides in the WPH emulsions formed weaker and looser, mobile interfacial structures than those formed with intact whey proteins. Consequently, the lack of electrostatic and steric repulsion resulted in the coalescence of flocculated droplets during retort treatment. At higher levels of xanthan gum or guar gum addition, the extent of coalescence decreased gradually, apparently because of the high viscosity of the aqueous phase.     

Characterization of emulsion stabilization properties of quince seed extract as a new source of hydrocolloid

The capability of seed extracts in stabilizing emulsions has particularly received interest in recent years. Upon soaking quince seeds into water, biopolymers inside the seeds are extracted to water, forming mucilage. This study investigates the physical stability, rheology and microstructure of oil (sunflower oil) in water emulsions, stabilized by 2% (w/v) whey protein isolate with varying concentrations of xanthan and quince seed gum. Quince seed gum resulted in emulsions with smaller low-shear viscosities and shear thinning capabilities compared to the same concentrations of xanthan. Quince seed gum emulsions with concentrations ≤ 0.1 (w/v), displayed rapid creaming due to bridging flocculation. Despite the difference in apparent viscosities, for gum concentrations < 0.2 (w/v), both gums demonstrated comparable stability with xanthan gum in general yielding marginally more stable emulsions. Gum concentrations > 0.3 (w/v) resulted in physically stable emulsions even after 5 months. Overall, quince seed gum displayed significant emulsification and stabilization properties.     

Rheology of whey protein isolate-xanthan mixed solutions and gels. Effect of pH and xanthan concentration

Flow properties at pH 5.5-7.5 of whey protein isolate (WPI)-xanthan solutions containing 0-0.5 w/w% xanthan were studied by viscosimetry, although rigidity and fracture properties of the corresponding heat-set gels (90°C, 30 min) were determined by uniaxial compression. All the studied solutions displayed generalized shearthinning flow behaviour. Synergistic WPI-xanthan interactions has been revealed by observing that rheological parameters [σ_msf, K, n, η (γ)] characterizing blends were larger than those calculated from the two separated solutions. Such a behaviour was attributed to segregative phase separation of whey proteins and xanthan. Effects of xanthan on WPI-xanthan gel properties both depended on pH and xanthan concentration. Simultaneous increased xanthan concentration and decreased pH inhibited gelation of WPI-xanthan blends. Regarding gel strength, synergistic WPI-xanthan interactions were observed at pH >7.0 and low xanthan concentration (0.05 or 0.1 w/w%). Antagonism between the two macromolecules occurred at low xanthan concentration and pH ≤6.5, and high xanthan concentration (0.2 or 0.5 w/w%) at all pH tested. Low xanthan concentration rendered mixed gels more brittle than protein gels, and high xanthan concentration decreased pH effects on gel stress-strain relationships. The balance between strong thermal aggregation of concentrated whey proteins - in presence of incompatible xanthan -, high viscosity of blends and repulsive surface forces of protein molecules was thought to be at the origin of WPI-xanthan gel mechanical properties.     

Gelation properties of salt soluble meat protein and soluble wheat protein mixtures

Salt soluble meat proteins (SSMP) and commercially available soluble wheat proteins (SWP) were characterised by SDS-polyacrylamide gel electrophoresis, differential scanning calorimetry (DSC) and small and large deformation testing. DSC scans indicated transitions similar to those of native actomyosin for the salt soluble meat extract whereas SWP did not indicate any transitions between 20 and 120 °C. Small deformation tests on SWP indicated a G′/G″ crossover gelation temperature of 90 °C and weak gels as judged by frequency sweeps. In contrast, SSMP gelled at 40 °C and formed strong gels on heating to 90 °C. However, on autoclaving at 120 °C, 20% SWP in distilled water produced strong elastic gels with little syneresis, compared with the more brittle gels produced with 20% (w/w) SSMP as indicated by large deformation testing. Mixtures of the two proteins in the ratio SSMP/SWP (15:5) gave strong elastic gels similar to the SWP gels. Even the presence of very small amounts of SWP in the mixture, e.g. SSMP/SWP 20:1 trebled the elastic modulus compared with a SSMP gel and reduced syneresis. This was probably due to the close association of SWP with actomyosin strands as viewed by transmission electron microscopy. However, increased levels of SWP in the mixture, for example SSMP/SWP 10:10 ratio, resulted in the separation of the two protein phases as shown by phase contrast microscopy, and consequently led to lower G′ values in the mixed gels. The addition of 20 mM chloride salts showed that potassium reduced the shear modulus, sodium had no effect and calcium enhanced the shear modulus for SWP gels formed at 120 °C. In contrast, SSMP gels were stronger in the presence of potassium, followed by sodium and calcium.     

Rapid (microwave) heating rate effects on texture,fat/water holding,and microstructure of cooked comminuted meat batters

Comminuted and gelled, fat-containing meat products such as frankfurters and luncheon meats are commercially processed by heating relatively slowly (for up to 2 h or more) to an endpoint of about 70 °C prior to cooling. This study compared such a slow, ramp heating regime (0.5 °C/min), terminated at 70 °C, to rapid, square-wave cooking (one step: rapid 100 °C/min heating to 70 °C endpoint, plus isothermal holding prior to cooling, or two-step: rapid heating to 50 °C, holding, then rapid heating to 70 °C plus holding prior to cooling) on meat batter gel properties (fracture and small strain rheology, microstructure, cook loss, and expressible water). The results indicated that a rapid cooking process, with its inherent advantages of reduced process time, lower equipment footprint, and more efficient use of energy, can produce a product nearly equivalent in textural properties and cook yield to one processed by traditional smokehouse cooking when the cook value of the processes is similar and an intermediate (near 50 °C) holding step is included (two-step rapid heating). One-step rapid heating negatively affected gel structural homogeneity and water/fat holding properties of fat-containing gels.     

Influence of pH and NaCl on rheological properties of rennet-induced casein gels made from UF concentrated skim milk

Milk concentrates are used in the production of cast cheese. The effects of pH (5.19–6.21) and NaCl concentration (0, 1.75% and 3.50%, w/w) on the rheological and microstructural properties of rennet-induced casein gels made from ultra filtered skim milk (19.8%, w/w casein) were investigated. Low pH and high NaCl concentration reduced the development rate of the gel elasticity after coagulation of the ultra filtrated skim milk. Strain at fracture and stress at fracture from uniaxial compression of casein gels 48 h after coagulation showed maximum and minimum values at pH ∼5.8 and 5.29, respectively. Young's modulus from uniaxial compression of the same gels was almost constant between pH 5.52 and 6.21 but much lower at pH 5.28. Addition of NaCl resulted in increased Young's modulus in the interval pH 6.21–5.52. As pH decreased, the level of colloidal calcium phosphate decreased concomitantly, giving less cross-links in the casein network and partly explaining the changes in the rheological properties. Increased ionic strength by adjusting pH and addition of NaCl also influenced rheological results. The microstructure examined with confocal laser-scanning microscopy was unaffected by the changes in pH and concentrations of NaCl in the range studied as revealed by image analysis and calculations of two- and three-dimensional data from micrographs.     

Influence of EDTA and citrate on thermal stability of whey protein stabilized oil-in-water emulsions containing calcium chloride

The influence of calcium ions and chelating agents on the thermal stability of model nutritional beverages was examined. Oil-in-water emulsions (6.94% (w/v) soybean oil, 0.35% (w/v) WPI, 0.02% (w/v) sodium azide, 20 mM Tris buffer, 0–10 mM CaCl₂, and 0–40 mM EDTA or citrate, pH 7.0) were stored at temperatures between 30 and 120 °C for 15 min. The particle size, particle charge, creaming stability, rheology, and free-calcium concentration of the emulsions were then measured. In the absence of chelating agents, appreciable droplet aggregation occurred in emulsions held at temperatures from 80 to 120 °C, which led to increased emulsion particle diameter, shear-thinning behavior, apparent viscosity, and creaming instability. Addition of chelating agents to the emulsions prior to heating decreased, but did not prevent, droplet aggregation in the emulsions. EDTA was more effective than citrate in decreasing droplet aggregation. Heat treatment increased the amount of chelating agents required to prevent droplet aggregation in the emulsions. Free-calcium concentration and droplet surface potential was independent of heat-treatment temperature, indicating that the performance of the chelating agents in binding calcium ions was not affected by the heat treatment. It was suggested that increased hydrophobic attractive interactions between the droplets occurred during heating, which induced droplet aggregation.     

X-ray diffraction analysis of lactose crystallization in freeze-dried lactose–whey protein systems

Water sorption, time-dependent crystallization and XRD patterns of lactose and lactose–WPI mixtures were studied with glass transition data. The results indicated that the sorbed water of lactose–WPI mixtures was fractional and water content of individual amorphous components in lactose–WPI mixtures at each a_w from 25 °C to 45 °C could be calculated. Crystallization occurred in pure lactose whereas partial crystallization was typical of lactose–WPI mixtures (protein content ≤ 50%) at intermediate and high a_w (> 0.44 a_w) from 25 °C to 45 °C. The extents of crystallization were significantly delayed by WPI. The T_g values of lactose–WPI systems showed the composition-dependent property in systems and might indicate the occurrence of phase separation phenomena during 240 h storage. XRD showed no anhydrous β-lactose and mixed α-/β-lactose with molar ratios of 4:1 crystals in crystallized lactose–WPI systems (70:30 and 50:50 solids ratios). Reduced crystallization in the presence of WPI was more pronounced possibly because of reduced nucleation and diffusion during crystal-growth. The present study showed that WPI could present an important role in preventing sugar crystallization.     

Gel mechanical properties of milk whey protein–dextran conjugates obtained by Maillard reaction

The effect of Maillard reaction on the mechanical properties of whey protein isolate (WPI) heat-induced gels was evaluated. WPI and dextran (15–25 kDa) conjugates were obtained by controlled dry heating during storage at 60 °C and 63% relative humidity for 2, 5 and 9 days. Changes in browning intensity and content of free amino groups were used to estimate the Maillard reaction. A decrease in free amino groups of WPI was observed when increasing polysaccharide concentration and reaction time. An increase in both a* and b* CIE Lab colour parameters indicated the development of a reddish-brown colour, typical of the Maillard reaction. Uniaxial compression and stress relaxation tests were performed to measure the mechanical properties of mixed and conjugate gels. Maillard reaction significantly modified the mechanical properties of WPI/DX gels, and even prevented fracture when conjugate gels were subjected to 80% deformation in uniaxial compression test.     

Stability of flocculated particles in concentrated and high hydrophilic solid layer-by-layer (LBL) emulsions formed using whey proteins and gum Arabic

The objective of the present study was to investigate flocculation in layer-by-layer (LBL) emulsion systems with high total solids content and deflocculation at various pH conditions, and the effects of whey protein isolate (WPI) concentration and total solids content on the stability of LBL emulsions. WPI (1.96% (1WPI) or 10.71% (10WPI), w/w in water) was prepared in water and high-pressure homogenized with sunflower oil (10%, w/w, of total emulsion). Gum Arabic (0.15%, w/w, in total emulsion) was added to assemble electrostatically on WPI at oil particle interfaces at pH 3.5 using aqueous citric acid (10% w/w) forming LBL emulsion. The ζ-potential measurements showed charge reversal upon addition of gum Arabic solution into single layer (SL) emulsion confirming the formation of LBL interface. Trehalose:maltodextrin mixture (1:1, w/w, total emulsion, 28.57% (28) or 57.14% (57), w/w, in water) was used in the continuous phase. The high total solids content of the system results in depletion flocculation of the particles leading to bridging flocculation without coalescence as deflocculation into individual particles occurred with increasing pH from pH 3.5 to pH 6.5 in 10WPI systems. Deflocculation was evident in 10WPI-28 and 10WPI-57 as found from a decreased ζ-average diameter and visually under microscope. Coalescence was observed in 1WPI systems. Viscosity of the systems was significantly (P < 0.05) increased with higher total solids content. Accelerated destabilization test showed that systems at higher WPI and total solids contents exhibited the highest stability against creaming. Deflocculation in LBL systems can be controlled by pH while high solids in the aqueous phase provide stability against creaming.     

Comparison of properties of oil-in-water emulsions stabilized by coconut cream proteins with those stabilized by whey protein isolate

Coconut cream protein (CCP) fractions were isolated from coconuts using two different isolation procedures: isoelectric precipitation (CCP1-fraction) and freeze–thaw treatment (CCP2-fraction). The ability of these protein fractions to form and stabilize oil-in-water emulsions was compared with that of whey protein isolate (WPI). Protein solubility was a minimum at ∼pH 4, 4.5 and 5 for CCP1, CCP2, and WPI, respectively, and decreased with increasing salt concentration (0–200 mM NaCl) for the coconut proteins. All of the proteins studied were surface active, but WPI was more surface active than the two coconut cream proteins. The two coconut cream proteins were used to prepare 10 wt% corn oil-in-water emulsions (pH 6.2, 5 mM phosphate buffer). CCP2 emulsions had smaller mean droplet diameters (d₃₂ ≈ 2 μm) than CCP1 emulsions (d₃₂ ≈ 5 μm). Corn oil-in-water emulsions (10 wt%) stabilized by 0.2 wt% CCP2 and WPI were prepared with different pH values (3–8), salt concentrations (0–500 mM NaCl) and thermal treatments (50–90 °C for 30 min). Considerable droplet flocculation occurred in the emulsions near the isoelectric point of the proteins: CCP2 (pH ∼ 4.3); WPI (pH ∼ 4.8). Emulsions with monomodal particle size distributions, small mean droplet diameters, and good creaming stability could be produced at pH 7 for WPI, but CCP2 produced bimodal distributions at this pH. The CCP2 and WPI emulsions remained relatively stable to droplet aggregation and creaming at NaCl concentrations ⩽50 and ⩽100 mM, respectively. In the absence of salt, both CCP2 and WPI emulsions were quite stable to thermal treatments (50–90 °C for 30 min).     

Texture and structure of pressure-shift-frozen agar gel with high visco-elasticity

To determine the effects of sucrose and high-pressure-freezing, two kinds of agar gel were compared; A gel with high visco-elasticity and B gel, an ordinary dessert gel. Both agar gels with 0, 5, 10 or 20% sucrose were frozen at 0.1–686 MPa and −20 °C. They were frozen during pressurization, and exothermic peaks were detected at 0.1, 100, 600 and 686 MPa and −20 °C (freezing). However, at 200 MPa, they did not freeze but froze with released pressure (pressure-shift-freezing). Thus, the amount of syneresis from gel pressure-shift-frozen at 200 MPa was smaller than that from gel frozen at other pressures. Also, amount of syneresis from A was smaller than B. In addition, compared to control gels, the appearance of 0% sucrose–agar gels frozen at 0.1, 100, 600 and 686 MPa differed greatly due to syneresis and a volumetric shrinkage of the gel. It was apparent that the rupture stress of the gels decreased, strain and size of ice crystals increased and quality declined. Conversely, due to quick freezing, the texture and structure of both A and B pressure-shift-frozen at 200 MPa were better than the other pressure-treated gels and gels frozen in freezers (−20, −30 or −80 °C) at atmospheric pressure. Consequently, pressure-shift-freezing was more effective. However, texture, structure and syneresis of A were somewhat better than that of B. It was found that the addition of sucrose to the gel was effective in improving the quality of frozen agar gels.     

Highly-porous mannitol particle production using a new templating approach

A new method has been successfully developed to create highly-porous powder templates through the spray drying of mannitol, using citric acid as the templating agent and then removing the citric acid by ethanol washing of the spray-dried powders. It has been demonstrated that textural properties, such as the surface area and the pore volume of the resultant mannitol, can be tuned by varying the concentrations of citric acid, whey protein isolate (WPI), and Gum Arabic. The resulting powder structure is a mannitol network with significant porosity and high surface area of 8.0 ± 0.4 m²/g and total pore volume of 0.075 ml/g. The scanning electron microscope micrographs were consistent with the Brunauer–Emmett–Teller (BET) surface area measurements. It has been found that higher concentrations of WPI, more than 0.5% w/w, have a detrimental effect on the porosity of the spray-dried powders from the solutions of mannitol/WPI with 2% (w/w) citric acid. For Gum Arabic, more than 1% (w/w) Gum Arabic causes lower porosities due to the lower citric acid removal by ethanol washing. The results of this study allow excipient powders to be produced with high specific surface areas.     

Absorption of whey protein isolated (WPI)-stabilized β-Carotene emulsions by oppositely charged oxidized starch microgels

β-Carotene is a natural antioxidant that is beneficial for human health. β-Carotene is well known for its aqueous insolubility and its sensitivity to environmental stimuli. To achieve the targeted delivery of β-Carotene to human intestine, a microgel system was developed. The microgel is made of oxidized potato starch polymers, which are obtained by chemical cross-link process by sodium trimetaphosphate (STMP). At pH < pI (WPI: pH 5), as in the acidic condition in stomach, the positively charged WPI emulsified β-Carotene nanoemulsion droplets were absorbed by the negatively charged starch microgel particles, which prevented the early release of β-Carotene in stomach. At pH > pI, as in the intestinal condition, WPI–β-Carotene nanoemulsion droplets and microgel particles both carry negative charges. Under this condition, β-Carotene will be released from microgel. In this study, we investigated the absorption capacity of β-Carotene nanoemulsion droplets by oxidized starch microgel particles of various degrees of oxidation (DO) and the cross-link density (R_{cross-linker/polymer(w/w)}) is investigated as a function of pH and salt concentrations. We found that DO30% with R_{cross-linker/polymer(w/w)} = 0.1 was the optimal gel type for nanoemulsion droplets absorption, and pH 3 and ionic strength of 0.06 M were the optimal conditions for nanoemulsion droplets absorption. The swelling capacity of the microgel particles rather than its ζ-potential was dominant in governing the absorption capacity. The in vitro release experiment under stimulated gastrointestinal fluids suggested that β-Carotene emulsions droplets remained stable at the gastric condition and were majorly released under the intestinal condition. The results suggested that the oxidized starch microgel could be used to prevent the early release of β-Carotene in the stomach and target delivery of them to the intestine.     

Gelation characteristics of tropical surimi under water bath and ohmic heating

Gelation characteristics of tropical surimi, namely threadfin bream (TB), bigeye snapper (BS), goatfish (GF) and lizardfish (LF) prepared in the absence and presence of 10 g kg^?1 egg white proteins were evaluated using either ohmic (OH) or water bath (WB) heating. LF and GF surimi exhibited higher endogenous proteolytic activity than BS and TB. Ohmic heating markedly minimized proteolysis of LF and GF surimi as evidenced by a reduction of trichloroacetic acid (TCA)-soluble oligopeptide content of gels and more retention of myosin heavy chain (MHC). Ohmic heating increased breaking force and deformation of TB and BS surimi by 1.3 and 1.6 times, respectively, as compared to water bath heating. However, TB surimi gels heated by a higher applied voltage gradient of 16.7 V cm^?1 exhibited lower breaking force than those heated at 6.7 V cm^?1. Gels heated ohmically contained lower total sulfhydryl concentration, indicating the greater extent of disulfide bond formation as compared to gels heated in a 90 °C water bath. The rapid heating method with shorter heating time could improve water holding capacity and preserve color of tropical surimi gels when compared to water bath heating.     

Mechanical and thermal characteristics of amaranth starch isolated by acid wet-milling procedure

Effects of soaking conditions during acid wet-milling of amaranth grain on mechanical and thermal properties of amaranth starch were investigated. A factorial design based on temperature (40–60 °C) and SO₂ concentration (0.1–1.0 g/L) was used. Dynamic oscillatory tests involved heating and cooling cycles (25–90 °C) with tempering at 90 °C followed by a frequency sweep (0.1–20 Hz) at constant temperature. Thermal properties of starch suspensions were based on DSC tests. Viscoelastic modulus and thermal properties were affected by both factors, being significant the interaction effect. Maximum values of viscoelastic modulus at the end of heating and cooling steps were determined for samples corresponding to soaking conditions of 50 °C and 1.0 g/L SO₂. Based on Ross-Murphy index from frequency sweep analysis it was found that paste behavior occurs at 60 °C and 0.1 g/L, while at 47.4 °C and 0.72 g/L starch suspension gave a strong gel. Onset (61.7 °C) and peak (66.2 °C) temperatures showed a minimum at 50 °C and 1.0 g/L. As SO₂ concentration decreased, the end of gelatinization took place at lower temperature. Maximum gelatinization enthalpy (12.7 J/g) was found at 40 °C and 0.1 g/L SO₂.     

Influence of ohmic heating/osmotic dehydration treatments on polyphenoloxidase inactivation,physical properties and microbial stability of apples (cv. Granny Smith)

The combined effect of ohmic heating (OH) and osmotic dehydration (OD) with vacuum impregnation (VI), on the polyphenoloxidase (PPO) inactivation, physical properties and microbial stability of apples stored at 5 °C or 10 °C was analyzed. The treatments were performed using a 65% (w/w) sucrose solution and with ohmic heating at 13 V/cm at 30 °C, 40 °C or 50 °C for 90 min. Examination of the dehydrated samples showed that the water loss and the solid gain were greater with the OD/OH and VI/OH treatments at 50 °C. PPO was completely inactivated by the OD/OH and VI/OH treatments at 50 °C. There was a correlation between the PPO activity, the color change and the browning index of the treated and stored samples; the values for these parameters were stable when PPO was inactivated. The lowest loss of firmness and color was obtained with the VI/OH treatment at 50 °C. The shelf-life of the apples treated with VI/OH at 50 °C and stored at 5 °C was extended to more than 4 weeks. Therefore, the VI/OH treatment at 50 °C was determined to be the best process for dehydrating apples.Industrial relevanceThe aim of this research was 1 to study the combined effect of ohmic heating (OH) and osmotic dehydration (OD) with vacuum impregnation (VI) on the polyphenoloxidase inactivation and microbial stability of osmotically dehydrated apples stored at either 5 °C or 10 °C. Two technologies, OH and OD were performed at 30, 40 or 50 °C with an electric field intensity of 13 V/cm and conventional heating for 90 min. The results showed a correlation between the PPO activity, the color change and the browning index of the treated and stored samples; the values were stable when PPO was inactivated. PPO was completely inactivated by the OD/OH and VI/OH treatments at 50 °C. The shelf-life of the apples treated was extended to more than 4 weeks. Under the investigated conditions, VI/OH treatment at 50 °C and stored at 5 °C may be considered the better minimal processing that preserves the fresh-like properties.