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.     

Changes in Temperature and Structure of Agar Gel as Affected by Sucrose During High-pressure Freezing

ABSTRACT: To determine the effects of high-pressure freezing, agar gel with 0, 5, 10, or 20% sucrose were frozen at 0.1 to about 686 MPa and -20 °C. Exothermic peaks were detected at 0.1, 100, 500 to about 686 MPa (freezing). However, at approximately 200 to 400 MPa, gel did not freeze but froze during pressure release. Thus, structure of gel frozen at approximately 200 to 400 MPa was better than other samples due to quick freezing. The phase transition from high-pressure-ices to ice I at -20 °C might have promoted the growth of ice crystals. With the addition of sucrose, the initial freezing temperature decreased and structural quality improved. Keywords: high pressure, agar gel, freezing, texture, ice crystals     

Effect of different freezing processes on the microstructure of Atlantic salmon (Salmo salar) fillets

Salmon fillets were frozen either by pressure shift freezing (PSF, 200 MPa, − 18 °C or 100 MPa, − 10 °C) or by air-blast freezing (ABF, − 30 °C, 1 m/s or 4 m/s) or direct-contact freezing, and then stored at − 20 °C for 6 months. The influence of these treatments on the microstructure of Salmon fillets was studied. The equivalent diameter of the intracellular ice crystals were 14.69 ± 4.11, 5.52 ± 2.11, and 30.65 ± 6.31 μm for the samples subjected respectively to PSF at 100, 200 MPa and ABF (− 30 °C, 4 m/s) after 2 days of storage. Smaller and more regular intracellular ice crystals were observed in fillets frozen by PSF (200 MPa) compared with PSF (100 MPa), ABF and direct-contact frozen ones. Significant differences were observed between the size of the ice crystals obtained after conventional freezing process and PSF. Large and extracellular ice crystals were observed in fillets frozen by ABF (1 m/s) and direct-contact frozen. Minimal changes in the size of ice crystals were observed during a 3 months storage.

Industrial relevance

This paper compares different freezing methods and subsequent frozen storage with respect to their effect on microstructures of salmon fillets. Pressure shift freezing at 200 MPa was superior to conventional freezing regarding small and regular ice crystal formation. Interestingly, during frozen storage for up to 3 months the high quality product obtained via pressure freezing at 200 MPa could be retained. For longer storage periods lower pressures (100 MPa) seem sufficient to achieve stable ice crystals.     

Changes in Temperature, Texture, and Structure of Konnyaku (Konjac Glucomannan Gel) During High-pressure-freezing

To determine the effects of high-pressure-freezing, changes in temperature, texture, and structure of konnyaku (a gel with high water content) were measured during freezing for 60 min at 0.1 to 700 MPa and -20 °C. During freezing at 0.1, 100, 500, 600, and 700 MPa, exothermic peaks were detected (konnyaku froze). However, at 200 to 400 MPa, exothermic peak was not detected and temperature rose when pressure was released at -20 °C; the supercooled konnyaku froze by pressure-shift-freezing. The coarse gel network observed in unfrozen konnyaku was compressed by freezing due to formation of ice crystals. The rupture stress increased and strain decreased in all frozen konnyaku. High-pressure-freezing was ineffective in improving the texture of frozen-then-thawed konnyaku.     

Quality changes during the frozen storage of sea bass (Dicentrarchus labrax) muscle after pressure shift freezing and pressure assisted thawing

Quality of frozen sea bass muscle stored (1, 3 and 5 months) at two levels of temperature (−15 and −25 °C) after a pressure shift freezing process (200 MPa) — PSF — and/or a pressure assisted thawing process (200 MPa) — PAT — was evaluated in comparison with samples frozen and thawed using conventional methods (air-blast AF and AT, respectively). Frozen storage of high-pressure treated samples did not significantly affect initial quality of frozen muscle. Thus, parameters related to protein denaturation and extractability, water holding capacity and color presented similar values than those obtained for not stored samples. In addition, the improvement of the microstructure achieved by PSF application remains unchanged during frozen storage. On the other hand, conventional treated samples experienced significant changes during frozen storage, such as protein denaturation, and water holding capacity and color modifications. Storage temperatures did not have influence in the quality of PSF and PAT samples, but it showed some effects in AF muscle.Industrial relevance: This work demonstrates the potential application and benefits of high pressure (HP) in the freezing and thawing of fish meat in comparison to conventional methods, due to an improvement on the cellular integrity of the tissue. Although some negative effects are produced during processing with HP, no additional modifications occur during the frozen storage. The studied methodologies seemed to be very suitable for fish freezing and thawing, especially for products which will be frozen stored and/or cooked.     

Inactivation kinetics of apple polyphenol oxidase in different pressure–temperature domains

The impact of high hydrostatic pressure and temperature on the stability of polyphenol oxidase (PPO) was studied in cloudy apple juice. Application of 200–500 MPa near room temperature or heat treatment at 45–55 °C at ambient pressure caused an increase of PPO activity of up to 65% in freshly squeezed apple juice. Combined pressure–temperature inactivation experiments with fully activated PPO (5 min treatment at 400 MPa and 20 °C) were carried out in the range of 0.1–700 MPa and 20–80 °C. Enzyme inactivation kinetics followed a 2.2 order reaction scheme at all pressure–temperature conditions tested. A polynomial model was successfully applied to describe the rate of PPO inactivation as a function of pressure and temperature and was used to construct a pressure–temperature isokinetic diagram. This diagram clearly showed synergistic effects of pressure and temperature on the inactivation of apple PPO at pressures above 300 MPa and antagonistic effects at lower pressures. Compared to ambient pressure conditions, temperatures required to inactivate PPO in apple juice were increased 10–15 °C at 100–300 MPa.

Industrial relevance

High pressure processing of fresh fruits is gaining popularity in the food industry because of its ability to inactivate microorganisms and some enzymes near room temperature with little impact on flavour or nutritional attributes of the food. However, quantitative data regarding the impact of process parameters on the target reaction are required to economically utilise this technology. This paper provides a mathematical model describing the combined effect of pressure, temperature and treatment time on the inactivation of PPO in cloudy apple juice.     

A comparative study of heat and high pressure induced gels of whey and egg albumen proteins and their binary mixtures

Large deformation rheological studies of either egg albumen or whey protein isolate (15% protein w/w) gels induced by heating at 90 °C for 30 min were compared to those induced by a range high pressures (400–800 MPa for 20 min). Gels made by heating indicated higher gel strength and Young's modulus values for whey protein from pressures of 400–600 MPa for 20 min but similar values at 650–800 MPa. In contrast, egg albumen showed no gelation below 500 MPa for 20 min, but there was an increase in both gel strength and Young's modulus with increasing pressure, although values remained lower than those of the heat-induced gels. A mixture of 10:5 whey/egg albumen showed the highest gel strength and Young's modulus for both heated and high pressure-treated (400–600 MPa) gels, although, the heated mixture had the highest values. Electron micrographs indicated that high pressure-treated gels had a porous aggregated network for egg albumen while whey proteins showed a continuous fine stranded network. The heated mixtures of whey:egg albumen (7.5:7.5) showed large dense aggregates whereas high pressure-treated mixtures produced smaller aggregates. Raman spectroscopy of both heated and high pressure-treated whey and egg albumen (15% w/w in D₂O pD7) and their binary mixtures (7.5:7.5, protein w/w) indicated changes in β-sheet structures in the Amide 111′ region (980–990 cm⁻¹); however, peak intensity was reduced for high pressure-treated samples. β-Sheet structure (1238–1240 cm⁻¹) present in heated whey was absent in high pressure-treated whey and in egg albumen. Involvement of hydrophobic regions was reflected by changes in the CH (1350 cm⁻¹) and CH₂ (1450 cm⁻¹) bending vibrations. In addition to the Trp residues at 760 cm⁻¹, there were broad peaks at 874–880 cm⁻¹ and tyrosine 1207 cm⁻¹ in the high pressure-treated samples. Disulphide bands (500–540 cm⁻¹) in heated whey and egg albumen proteins showed higher peak intensities compared to high pressure-treated samples. Differences in the experimental and theoretical spectra indicated changes in the hydrophobic regions, tyrosine (1207 cm⁻¹) and tryptophan (880 cm⁻¹) and CH₂ bending in high pressure-treated samples, whereas heated samples indicated marked changes in β-sheet structures (987 and 1238 cm⁻¹) as well as hydrophobic regions CH (1350 cm⁻¹) and CH₂ (1450 cm⁻¹) bending vibrations.     

PRESSURE-ASSISTED FREEZING AND THAWING: PRINCIPLES AND POTENTIAL APPLICATIONS

The phase diagram of water as a function of temperature and pressure delimits distinct crystalline ice forms with different specific volumes, melting temperatures, and latent heats of fusion. The melting temperature of ice I decreases to -22°C when pressure increases to 207.5 MPa. It is possible to freeze a biological or food sample under pressure (obtaining ice I, III, V, VI, or VII), to enhance ice nucleation by fast pressure release, to keep a sample at subzero temperatures without ice crystal formation, to generate pressure through freezing, to reach the glassy state of water by fast cooling under pressure, or to thaw a frozen sample under pressure below 0°C. Fast pressure release from -10 or -20°C and 100 or 200 MPa (with a prior cooling step under pressure), called “pressure-shift freezing,” induces significant supercooling (as detected by fast data acquisition) and enhances uniform ice nucleation throughout the sample. When freezing is then completed at atmospheric pressure, different microscopy techniques reveal numerous small ice crystals with no specific orientation or marked size gradient. Crystals are smaller in pressure-shift frozen gels than in similarly frozen oil-in-water emulsions. In the latter, increasing solute concentrations in the aqueous phase tends to reduce ice crystal size. Modeling is proposed for pressure-shift freezing, although the supercooling and nucleation steps are not taken into account. Both freezing under various pressure levels and pressure-shift freezing are reported for gels (mainly heat-induced protein gels), emulsions, and plant and animal tissues. In spite of some discrepancies, gel or tissue structure and texture are generally better maintained after thawing, as compared to control samples frozen by air blast or immersion in a cooling medium at 0.1 MPa. Less liquid exudation is also observed. However, some protein denaturation is detected (unfolding of myofibrillar proteins, toughening of meat or seafood), especially when the initial cooling step is carried out at a high pressure level for a long time. Pressure application at subzero temperature is found to inactivate only some enzymes, but causes a significant degree of microbial inactivation for several species of micro-organisms. Freezing gels or vegetables under pressure with the formation of ice III, V, or VI appears to maintain tissue structure and texture, but the mechanisms for these effects are not fully understood. Pressure-assisted thawing markedly enhances the rate of thawing, mainly due to a greater ΔT between the subzero thawing temperature and that of the heating medium. Specific packaging and equipment requirements for pressure-assisted freezing and thawing are discussed. Suggestions are made for further studies on high pressure-subzero temperature treatments, such as the influence of sample size and composition; the effects on cell membranes; the reduced need for blanching before freezing; the viability of pressure-shift frozen cells, embryos, or organs; the mechanisms of protein denaturation; and texture-promoting effects, especially in ice creams.     

Structural and Textural Changes in Kinu-Tofu Due to High-Pressure-Freezing

To determine the effect of high-pressure-freezing on quality, kinu-tofu (soybean curd) was frozen at 100 MPa (ice I), 200 MPa (liquid phase), 340 MPa (ice III), 400, 500, 600 MPa (ice V) or 700 MPa (ice VI) at ca. –20°C for 90 min. After reduction to atmospheric pressure, tofu was stored 2 days at –30°C then thawed at 20°C. Texture and structure were compared with kinu-tofu frozen (–20°C, –30°C or –80°C) at atmospheric pressure (0.1 MPa). The rupture stress and strain of tofu frozen at 0.1 MPa and 100 MPa increased, but that of tofu frozen at 200 MPa and 340 MPa was similar to untreated tofu. As pressure rose above 500 MPa, rupture stress increased. The ice crystals in tofu frozen at 200 MPa ～400 MPa were smaller than in tofu frozen at 100 MPa or 700 MPa. Thus, high-pressure-freezing at 200 MPa ～400 MPa was effective in improving the texture of frozen tofu.     

Influence of freeze-drying conditions on survival of Oenococcus oeni for malolactic fermentation

Malolactic fermentation (MLF) is an important process in wine production. Oenococcus oeni is most often responsible for MLF. Starter culture technology, involving the inoculation of O. oeni into wines, has been developed for inducing MLF. In this study, the effects of cell washing, pH of suspension medium, preincubation in sodium glutamate, initial cell concentration and freezing temperature on viability of freeze-dried O. oeni H-2 were investigated. The cell viability of samples without washing with potassium phosphate buffer was significantly lower than those samples undergone washing. When pH of suspension medium was 7.0 the cell survival was the highest. The cell viability was enhanced when the cells were preincubated at 25 °C before freezing. When 2.5% sodium glutamate was used as protective agent in suspension medium, the optimal initial cell concentration was 10⁹ CFU/ml. The cell viability increased by 21.6% as freezing temperature decreased from − 20 °C to − 65 °C. However, when the cells were frozen in liquid nitrogen (− 196 °C), the cell survival significantly decreased.     

PRESSURE-ASSISTED FREEZING AND THAWING: PRINCIPLES AND POTENTIAL APPLICATIONS

The phase diagram of water as a function of temperature and pressure delimits distinct crystalline ice forms with different specific volumes, melting temperatures, and latent heats of fusion. The melting temperature of ice I decreases to ?22°C when pressure increases to 207.5 MPa. It is possible to freeze a biological or food sample under pressure (obtaining ice I, III, V, VI, or VII), to enhance ice nucleation by fast pressure release, to keep a sample at subzero temperatures without ice crystal formation, to generate pressure through freezing, to reach the glassy state of water by fast cooling under pressure, or to thaw a frozen sample under pressure below 0°C. Fast pressure release from ?10 or ?20°C and 100 or 200 MPa (with a prior cooling step under pressure), called “pressure-shift freezing,” induces significant supercooling (as detected by fast data acquisition) and enhances uniform ice nucleation throughout the sample. When freezing is then completed at atmospheric pressure, different microscopy techniques reveal numerous small ice crystals with no specific orientation or marked size gradient. Crystals are smaller in pressure-shift frozen gels than in similarly frozen oil-in-water emulsions. In the latter, increasing solute concentrations in the aqueous phase tends to reduce ice crystal size. Modeling is proposed for pressure-shift freezing, although the supercooling and nucleation steps are not taken into account. Both freezing under various pressure levels and pressure-shift freezing are reported for gels (mainly heat-induced protein gels), emulsions, and plant and animal tissues. In spite of some discrepancies, gel or tissue structure and texture are generally better maintained after thawing, as compared to control samples frozen by air blast or immersion in a cooling medium at 0.1 MPa. Less liquid exudation is also observed. However, some protein denaturation is detected (unfolding of myofibrillar proteins, toughening of meat or seafood), especially when the initial cooling step is carried out at a high pressure level for a long time. Pressure application at subzero temperature is found to inactivate only some enzymes, but causes a significant degree of microbial inactivation for several species of micro-organisms. Freezing gels or vegetables under pressure with the formation of ice III, V, or VI appears to maintain tissue structure and texture, but the mechanisms for these effects are not fully understood. Pressure-assisted thawing markedly enhances the rate of thawing, mainly due to a greater ΔT between the subzero thawing temperature and that of the heating medium. Specific packaging and equipment requirements for pressure-assisted freezing and thawing are discussed. Suggestions are made for further studies on high pressure–subzero temperature treatments, such as the influence of sample size and composition; the effects on cell membranes; the reduced need for blanching before freezing; the viability of pressure-shift frozen cells, embryos, or organs; the mechanisms of protein denaturation; and texture-promoting effects, especially in ice creams.     

On the stability of oil-in-water emulsions to freezing

Oil in water emulsions (40 wt%) were prepared from a homologous series of n-alkanes (C10–C18). The samples were temperature cycled in a differential scanning calorimeter (two cycles of 40 °C to −50 °C to 40 °C at 5 °C min⁻¹) and in bulk (to −20 °C). The emulsions destabilized and phase-separated after freeze–thaw if the droplets were solid at the same time as the continuous phase and were more unstable if a small molecule (SDS or polyoxyethylene sorbitan monolaurate) rather than a protein (whey protein isolate or sodium caseinate) emulsifier was used. The unstable emulsions formed a self-supporting cryo-gel that persisted between the melting of the water and the melting of the hydrocarbon phase. Microscopy provides further evidence of a hydrocarbon continuous network formed during freezing by a mechanism related to partial coalescence which collapses during lipid melting to allow phase separation.     

Characterization of gelation time and texture of gelatin and gelatin–polysaccharide mixed gels

The effects of cooling rate, holding temperature, pH and polysaccharide concentration on gelation characteristics of gelatin and gelatin–polysaccharide mixtures were investigated using a mechanical rheometer which monitored the evolution of G′ and G″. At low holding temperatures of 0 and 4 °C, elastic gelatin gels were formed whereas a higher holding temperature of 10 °C produced less elastic gels. At slow cooling rates of 1 and 2 °C/min, gelling was observed during the cooling phase in which the temperature was decreased from room temperature to the holding temperature. On the other hand, at higher cooling rates of 4 and 8 °C/min, no gelation was observed during the cooling phase. Good gelling behavior similar to that of commercial Strawberry Jell-O^® Gelatin Dessert was observed for mixtures of 1.5 and 15 g sucrose in 100 ml 0.01 M citrate buffer containing 0.0029–0.0066 g low-acyl gellan. Also, these mixed gels were stronger than Strawberry Jell-O^® Gelatin Desserts as evidenced by higher G′ and gel strength values. At a very low gellan content of 0.0029 g, increasing pH from 4.2 to 4.4 led to a decrease in the temperature at the onset of gelation, G′ at the end of cooling, holding and melting as well as an increase in gel strength. The gelation time was found to decrease to about 40 min for gelatin/sucrose dispersions in the presence of 0.0029 g gellan at pH 4.2 whereas the corresponding time at pH 4.4 was higher (79 min). In general, the gelation time of gelatin/sucrose dispersions decreased by a factor of 2 to 3 in the presence of low-acyl gellan. The addition of low-acyl gellan resulted in an increase in the gelation rate constant from 157.4 to 291 Pa. There was an optimum low-acyl gellan content for minimum gelation time, this optimum being pH dependent. Addition of guar gum also led to a decrease in gelation time to 73 min with a corresponding increase in the gelation rate constant to 211 Pa/min though these values were not sensitive to guar gum content in the range of 0.008–0.05 g. The melting temperature of gelatin/sucrose/gellan as well as gelatin/sucrose/guar mixtures did not differ significantly from that of pure gelatin or Strawberry Jell-O^® Gelatin Desserts. At pH 4.2, the melting rate constant was highest at a low-acyl gellan content of 0.0029 g whereas the rate constant was insensitive to low-acyl gellan content at pH 4.4. Addition of guar did not seem to affect the melting temperature or the melting rate constant.     

Rheological behavior of aqueous dispersions of cashew gum and gum arabic: effect of concentration and blending

Rheological properties of cashew gum (CG) and gum arabic (AR), the exudate polysaccharides from Anacardium occidentale L. and Acacia, at different solutions (0.4–50% w/v) were studied. The intrinsic viscosity, [η], of CG in water at 20°C was ≈0.1 dl g⁻¹, while that of AR was ≈0.6 dl g⁻¹. The apparent viscosity of the unheated and the heated (at 80°C for 30 min) CG and AR solutions showed a progressive increase with increasing concentration. The flow curves of blends with equal viscosity solutions of AR/CG: 25/75, 50/50 and 75/25, showed no major interaction. The apparent viscosity (η_a) vs. shear rate data for both the AR and CG dispersions (4–50% w/v) exhibited shear-thinning characteristics at low shear rates (< about 10 s⁻¹) and Newtonian plateaus at shear rates >100 s⁻¹, and the Sisko model described well the η_a vs. data of all the dispersions.     

Effect of high-pressure versus conventional thawing on color, drip loss and texture of Atlantic salmon frozen by different methods

Atlantic salmon (Salmo salar) samples were frozen by conventional air freezing, plate freezing and liquid nitrogen (LN) freezing, and subjected to different thawing treatments: water immersion thawing (WIT) (4°C and 20°C) and high-pressure thawing (HPT) at 100, 150 and 200 MPa with water (containing 2 g oil/100 g) as pressure medium at 20°C. Temperature and phase change behavior of fish samples were monitored during freezing and thawing. The phase change point of frozen salmon was lowered to −14°C, −19°C and −25°C for the HPT processes at 100, 150 and 200 MPa, respectively. These phase change temperatures were lower than for pure ice at the same pressures possibly due to the presence of solutes in salmon. The HPT times were 22.6±1.4, 18.1±1.4 and 17.0±1.3 min at 100, 150 and 200 MPa, respectively, as compared with 26.6±2.1 and 94.3±3.4 min for the WIT process at 20°C and 4°C, respectively. Employing pressures above 150 MPa caused noticeable color changes in salmon during the HPT process and the product texture was significantly modified during HPT at 200 MPa. Different freezing rates prior to thawing resulted in differences in drip loss in salmon samples, but they did not induce specific color and texture changes. A significant (P<0.05) reduction of drip loss by the HPT process was observed only for the LN frozen samples in which mechanical cracking occurred and much of the drip appeared after WIT process. Drip loss formed during pressure thawing seems to be a complicated process, for which further studies are needed.     

Effect of high pressure combined with mild heat or nisin on inoculated bacteria and mesophiles of goat's milk fresh cheese

The effect of pressures ranging from 400 to 500 MPa combined with mild heat on Staphylococcus carnosus inoculated in fresh cheese and the concurrent use of 500 MPa and nisin to inactivate cheese indigenous populations has been studied. Staphylococcus carnosus counts could not be substantially decreased with treatments at 500 MPa at 10 or 25°C for 30 min, whereas treatments at 50°C for 5 min caused a reduction of 7-log₁₀cfu g⁻¹. Multiple-cycle treatments of 500 MPa and times between 15 and 30 min also improved the inactivation rate. Combination of 500 MPa and nisin was the most effective treatment to inactivate cheese indigenous microbiota. Inactivation of Bacillus subtilis spores inoculated in fresh cheese has also been studied. Germination treatments of 60 MPa at 40°C for 210 min followed by vegetative cells inactivation treatments of 500 MPa at 40°C for 15 min caused a lethality of 4·9-log₁₀ofB. subtilis , whereas the same combination of treatments applied at 25°C only caused a 2·7-log₁₀reduction.     

Effects of pressure-shift freezing and conventional freezing on model food gels

Summary Gels of agar, starch, ovalbumin, gelatin and an industrial β-lactoglobulin protein isolate, were frozen conventionally in a −30 °C freezer and by pressure-shift freezing at 200 MPa at −15 °C. Thawing was carried out conventionally at 20 °C and by the application of a pressure of 200 MPa. The microscopic structure and mechanical properties of the thawed gels were compared with those of the initial gels. Microscopic examination showed that pressure-shift freezing produces smaller and more uniform ice crystal damage than conventional freezing at −30 °C. The results also suggest that the freeze-thaw behaviour of food gels can be categorized into two general types: (1) gels which have a reduced gel strength as a result of mechanical damage to the gel microstructure caused by ice crystal formation, and (2) gels which have an enhanced gel strength, as a result of molecular structural changes that take place in the frozen state. Agar and gelatin were found to be typical of type (1) gels, whereas starch, β-lactoglobulin protein isolate and ovalbumin were found to be typical of type (2) gels. In the case of starch, retrogradation during thawing was found to be the most important factor.     

Effect of high-pressure induced ice I/ice III-transition on the texture and microstructure of fresh and pretreated carrots and strawberries

The effect of pressure treatments at −25 °C between 150 and 300 MPa, indicated as high-pressure induced crystallization (HPIC) processes if formation of ice III occurs during pressurization, on the texture and structure of frozen strawberries and carrots were studied. The formation of ice III, which has been proven to inactivate the microbial load of a frozen food, occurred when pressure was increased to 250 MPa or higher. Volume changes related to the formation of ice III affected the cell wall integrity of infused frozen strawberries and caused a 42–46% reduction of the fruit’s hardness. These textural and structural changes were not affected by the pressure holding time (30 s versus 10 min), and thus by partial thawing during the pressure holding time, and were absent in frozen fruits treated at pressures lower than 250 MPa. The structure and texture of frozen carrots were respectively not and only slightly altered during high-pressure–low-temperature (HP–LT) treatments at all pressure levels studied. However, if carrots were blanched (30 min at 60 °C, 2 min at 90 °C and a combination of both) prior to freezing, structural damages during pretreatment and freezing made the tissue, in terms of both structural and textural quality, unsuitable for a post-freezing HP–LT treatment. These observations should be taken in mind when analyzing the possibilities of HPIC processes as a tool for post-freezing microbial reduction when applied to tissue based systems.     

Sweetening power of aspartame in hydrocolloids gels: Influence of texture

Equivalent sweetness of aspartame relative to two sucrose concentrations (10% and 20% w/w) were determined in water and in hydrocolloids gels. The influence of the texture of three hydrocolloids gelled systems—gellan gum, κ-carrageenan, and κ-carrageenan/locust bean gum (LBG)—at two gums concentrations (0.3% and 1.2% w/w) on the equivalent sweetness of aspartame were then studied. For the three gelled systems, the increase in hydrocolloid concentration produced a significant increase in the true rupture stress and in the deformability modulus values. For both κ-carrageenan and mixed gels the true rupture strain values increased when increasing hydrocolloid concentration while for gellan gels, decreased. For the same hydrocolloid concentrations the κ-carrageenan/LBG gels showed the largest strain at rupture and gellan gels the smallest (most brittle). For both soft (0.3% gum) and hard (1.2% gum) gellan gels and κ-carrageenan gels, the concentrations of aspartame needed to deliver a sweetness intensity equivalent to that of gels with 10% sucrose (0.079–0.087% w/w) were similar to those obtained for aqueous solutions (0.084% w/v). For hard κ-carrageenan/LBG gels the corresponding concentration of aspartame was slightly lower. For all gelled systems the concentrations of aspartame needed to deliver a sweetness intensity equivalent to that of gels with 20% sucrose were higher for soft gels than for hard gels.     

Conventional freezing plus high pressure-low temperature treatment: Physical properties, microbial quality and storage stability of beef meat

Meat high-hydrostatic pressure treatment causes severe decolouration, preventing its commercialisation due to consumer rejection. Novel procedures involving product freezing plus low-temperature pressure processing are here investigated. Room temperature (20 °C) pressurisation (650 MPa/10 min) and air blast freezing (−30 °C) are compared to air blast freezing plus high pressure at subzero temperature (−35 °C) in terms of drip loss, expressible moisture, shear force, colour, microbial quality and storage stability of fresh and salt-added beef samples (Longissimus dorsi muscle). The latter treatment induced solid water transitions among ice phases. Fresh beef high pressure treatment (650 MPa/20 °C/10 min) increased significantly expressible moisture while it decreased in pressurised (650 MPa/−35 °C/10 min) frozen beef. Salt addition reduced high pressure-induced water loss. Treatments studied did not change fresh or salt-added samples shear force. Frozen beef pressurised at low temperature showed L, a and b values after thawing close to fresh samples. However, these samples in frozen state, presented chromatic parameters similar to unfrozen beef pressurised at room temperature. Apparently, freezing protects meat against pressure colour deterioration, fresh colour being recovered after thawing. High pressure processing (20 °C or −35 °C) was very effective reducing aerobic total (2-log₁₀ cycles) and lactic acid bacteria counts (2.4-log₁₀ cycles), in fresh and salt-added samples. Frozen + pressurised beef stored at −18 °C during 45 days recovered its original colour after thawing, similarly to just-treated samples while their counts remain below detection limits during storage.