Calcium sulphate‐induced soya bean protein tofu‐type gels: influence of denaturation and particle size

Soya bean protein isolate (SPI) dispersions (7.25%, w/v) were heated at 65, 75, 85 or 90 °C for different time periods to produce SPI aggregates with diverse degrees of denaturation and particle size to investigate the effects on calcium sulphate (CaSO₄)‐induced tofu‐type gel. The results revealed that gel hardness and water‐holding capacity correlated positively with the degree of denaturation of glycinin (11S) and the particle size of the SPI aggregates. The formed gels showed more uniform and denser network structures with increasing degrees of denaturation and particle size of SPI. Hydrophobic interaction was speculated to be the crucial factor for the retention of gels prepared by SPI whose degree of denaturation by 11S was lower than 4.35%. However, disulphide bonds probably played a more important role in the retention of gels generated by SPI with the 11S denaturation degree of >84.47%. Moreover, the bulk density of the protein aggregates might determine the gel structures to a certain extent.     

Extreme pH treatments enhance the structure-reinforcement role of soy protein isolate and its emulsions in pork myofibrillar protein gels in the presence of microbial transglutaminase

Alkali (pH₁₂) and acid (pH_1.5) pH-treated soy protein isolates (SPI) were incorporated (0.25–0.75% protein) into sols of myofibrillar protein (MP, 3%, in 0.6 M NaCl at pH 6.25) with or without 0.1% microbial transglutaminase (TG) to investigate the potential as meat processing ingredients. Static and dynamic rheological measurements showed significant enhancements of MP gelling ability by the inclusion of pH_1.5-treated as well as preheated SPI (90 °C, 3 min). A 7-h incubation with TG accentuated the gel-strengthening effect by these SPI samples. The B subunit in 11S of SPI was the main component manifesting structure reinforcement in the mixed gels. The MP gelling properties were also greatly improved (P < 0.05) by the addition of 10% canola oil emulsions stabilized by pH-treated SPI. The principal force in the MP gels incorporated with pH-treated SPI was hydrophobic patches; in the presence of TG, cross-linking of previously dissociated A and B subunits of 11S was also intimately involved.     

Structural and mechanical characterization of κ/ι-hybrid carrageenan gels in potassium salt using Fourier Transform rheology

The mechanical and structural properties of κ/ι-hybrid carrageenan gels obtained at various concentrations in the presence of 0.1 m KCl were studied with Fourier Transform rheology (FTR) and cryoSEM imaging. FTR data show that gels formed at concentration below 1.25 wt% exhibit a strain hardening behavior. The strain hardening is characterized by a quadratic increase of the scaled third harmonic with the strain and a third harmonic phase angle of zero degree. Both features are weakly depending on the concentration and conform to predictions from a strain hardening model devised for fractal colloidal gels. However, the phase angle of the third harmonic reveals that κ/ι-hybrid carrageenan gels obtained at higher concentrations show shear thinning behavior. Colloidal gel models used to extract structural information from the concentration scaling of gel equilibrium shear modulus G₀ and the strain dependence of FTR parameters suggest that κ/ι-hybrid carrageenan gels are built from aggregating rod-like strands (with fractal dimension x = 1.13) which essentially stretch under increasing strain. The mechanically relevant structural parameters fairly match the gel fractal dimension (d = 1.66) obtained from the cryoSEM analysis.     

Cold‐set whey protein gels induced by calcium or sodium salt addition

Cold‐set whey protein isolate (WPI) gels formed by sodium or calcium chloride diffusion through dialysis membranes were evaluated by mechanical properties, water‐holding capacity and microscopy. The increase of WPI concentration led to a decrease of porosity of the gels and to an increase of hardness, elasticity and water‐holding capacity for both systems (CaCl₂ and NaCl). WPI gels formed by calcium chloride addition were harder, more elastic and opaque, but less deformable and with decreased ability to hold water in relation to sodium gels. The non linear part of stress–strain data was evaluated by the Blatz, Sharda, and Tschoegl equation and cold‐set gels induced by calcium and sodium chloride addition showed strain‐weakening and strain‐hardening behaviour, respectively. The fractal structure of the gels indicated a weak‐link behaviour. For WPI gels results suggest intrafloc links, formed at heating step, which were more rigid than the interfloc links, promoted by salt addition.     

In vitro digestibility of rare sugar (D-allulose) added pectin–soy protein gels

Confectionery gels are known to be high-caloric products due their high sugar content. Changing their formulations by substituting the sugar with alternative natural sweeteners and functionalising them, the addition of proteins has gained attention. Understanding the rate of digestion of these products is also important for selecting the appropriate formulation. In this study, in vitro gastric digestion behaviour of the gels formulated with D-allulose, a low-calorie rare sugar, soy protein isolate (SPI) (1%, 2.5%) and pectin (4%) were examined. Digestion decreased the hardness of the gels (P < 0.05), but, at 2.5% SPI concentration. Moisture content of the samples increased after digestion and presence of SPI induced higher water uptake. At the end of 2 h of digestion, 1% soy protein isolate containing gels had the highest brix values showing that after a certain concentration, soy protein isolate governed the system due to improved soy protein–pectin interaction or due to improved gelation with Maillard reaction. NMR relaxometry experiments further confirmed the changes in the gels with the increase in T₂ values. Power law model was fitted for the dissolution behaviour using the ^oBrix values of the digestion medium. Dissolution of sugar and the contribution of SPI to the gel network were clearly observed in SEM images. Results showed that these gels had the potential to slow down the emptying rate of stomach thus could lead to ‘fullness’ for a longer time.     

Characterization of plasma protein gels by means of image analysis

In this study, image analysis is presented as a powerful tool to analyse changes in the microstructural properties of protein gels. Several methods were used to elucidate changes in plasma protein gels induced by changes in the pH conditions, visually noticeable but difficult to assess objectively. Fractal analysis was used to measure the degree of self-similarity, lacunarity analysis to study the distribution of pores within the protein network and the spatial variability of SEM images to describe the compactness of the gel. It was found that the lacunarity measurements and the cut-off of the autocorrelation functions complemented the fractal dimension (D_f) and the average pore size, the most used parameters in describing the microstructure of protein gels. Correlations between macroscopic properties and microstructural attributes revealed that an increase in textural attributes, such as hardness, springiness and cohesiveness, and in the capacity of gels to retain water, entailed an increase of D_f and the compactness of gels, but a decrease in the pore size and the heterogeneity of gaps.     

Effects of combined high-pressure and heat treatment on the textural properties of soya gels

SPI, 7S, and 11S globulin at 12% (w/v) protein concentration, at neutral pH, did not form gels when heat-treated (90 °C, 15 min) or when high pressure-treated (300–700 MPa), except for the 11S, which formed a gel when heat-treated. The combination of heat and pressure (that is heating the solutions in a water bath and then pressure-treating at room temperature or the reverse sequence), led to differences: when heat-treatment was before high-pressure treatment, only the 11S fraction formed a self-standing gel; however, when the solutions were pressurised before heat treatment, all the proteins formed self-standing gels. The textural and water-holding properties were measured on the gels formed with the three different soy proteins.     

Cold-set gelation of high pressure-treated soybean proteins

The ability of soybean proteins to form cold-set gels, using high pressure (HP) processing as denaturing agent and calcium incorporation was evaluated. Different protein preparations were assayed: soybean protein isolate (SPI), a β-conglycinin enriched fraction (7SEF) and a glycinin enriched fraction (11SEF). 7SEF formed aggregated gels with low water holding capacity whereas 11SEF did not form self-standing gels. SPI formed the better gels: ordered and with high water holding capacity. SPI gels were relatively soft and their visual aspect, rheological and texture properties, and water holding capacity depended on HP level (400–600 MPa), CaCl₂ concentration (0.015–0.020 mol L⁻¹) and protein concentration (85–95 g L⁻¹), thus gels with different characteristics may be obtained. The gels comprised a three dimensional network stabilized by non-covalent interactions with spaces filled of proteins in aqueous solution. The results indicate that it is possible to use HP and subsequent calcium incorporation to form self-standing cold-set gels from soybean proteins. These gels may be of interest to incorporate heat-labile compounds or probiotics during the gelation step.     

Quantitative analysis of confocal laser scanning microscopy images of heat-set globular protein gels

Gels of the globular protein β-lactoglobulin were made by heating solutions at pH 7 and different NaCl concentrations (C_s). The influence of the ionic strength on the gel structure was studied by confocal laser scanning microscopy (CLSM). For C_s < 0.2 M the images were homogeneous, but at higher NaCl concentrations micro-phase separation was observed. The protein concentration in the two phases was determined from the images. It is shown how CLSM images can be quantitatively analysed in terms of the pair correlation function of the protein concentration fluctuations, yielding results that can be directly compared to those obtained from light scattering. The transition between so-called finely stranded and particulate gels is explained by a switch from net repulsive to net attractive interaction between growing protein aggregates.     

Influence of inulin/oligofructose on the acid‐induced cold aggregation and gelation of preheated soy proteins

BACKGROUND: In recent years inulin‐type prebiotics have attracted much attention due to consumers' awareness of the health benefits of functional foods. Currently no information is available about the possible texture‐modifying effect of these non‐ionizable polar carbohydrates in different soy‐based food systems. In this study, the effect of inulin/oligofructose on the cold aggregation and gelation of preheated soy protein isolate (SPI) and its fractions (7S, 11S, and their mixture), induced by glucono‐δ‐lactone (GDL), were evaluated by turbidity (A₆₀₀) and dynamic rheological measurements. RESULTS: Oligofructose significantly delayed the aggregation of all protein samples and decreased the end‐point optical density of 11S fraction and SPI. Inulin, a long‐chain fructan, only delayed the aggregation of 7S globulin and reduced the capacity of aggregation (A₆₀₀) of SPI. While oligofructose showed no significant effect, the addition of 5% (w/v) inulin enhanced the gelation of SPI and the 7S/11S mixture, which was demonstrated by the increase in gel storage modulus up to 13.6% and 10.1% (P < 0.05), respectively. CONCLUSION: Inulin was found to enhance the viscoelastic properties of GDL‐induced cold‐set soy protein gels. It is expected that ‘functional’ cold‐set gel products with improved texture can be prepared from preheated soy proteins and inulin. Copyright © 2009 Society of Chemical Industry     

Gel—sol transition in gellan gum solutions. II. DSC studies on the effects of salts

The thermal properties of sodium form gellan gum solutions with and without sodium chloride, potassium chloride, calcium chloride and magnesium chloride were studied by differential scanning calorimetry (DSC). The DSC cooling or heating curves for 1% gellan gum solutions without salt showed a single exothermic or endothermic peak at ~30°C. DSC cooling curves showed a single exothermic peak, with the setting temperature (Ts) shifting to progressively higher temperatures with increasing concentration of the added NaCl or KCl. At low concentrations of NaCl or KCl, DSC heating curves showed a single endothermic peak; however with more addition of salt the endothermic peak gradually developed a bimodal character and eventually split into more than two distinct peaks. The onset of detectable splitting occurred at a high salt concentration which coincided with that at which elastic gels are formed at even a higher temperature as was observed by viscoelastic measurements. With a sufficient addition of monovalent cations the endothermic curve became again a single peak shifting to higher temperatures. In the presence of divalent cations, although Ts shifted to higher temperatures with increasing concentration of added CaCl₂ or MgCl₂, the melting temperature (Tm) in heating DSC curves shifted to higher temperatures (up to a certain temperature) and then shifted to lower temperatures with increasing concentration of salt. With increasing concentration of CaCl₂ or MgCl₂, the exothermic and endothermic enthalpies estimated for a main peak increased up to a certain salt concentration and then decreased; however many other peaks were observed at higher temperatures. The endothermic peaks for gels with excessive divalent cations were too broad to be resolved from the baseline; in contrast the exothermic peaks were much sharper and readily recognized. In comparing thermal properties with rheological properties, gellan gum solutions with excessive divalent cations form firm gels on cooling to below the setting temperature, and then it was difficult to remelt them. This was quite different from the behaviour of thermoreversible gels formed in the presence of monovalent cations. It seems that the mechanism of gel formation in gellan gum with divalent cations is markedly different from that with monovalent cations.     

Effect of starch, sucrose and their combinations on the mechanical and acoustic properties of freeze-dried alginate gels

Alginate gels (2%) with and without incorporated starch and/or sucrose were freeze-dehydrated to produce cellular sponges. The stress–strain relationships of these dried gels were all irregular and jagged, typical of brittle cellular solids. The stiffness was assessed in terms of the fitted (‘smoothed’) apparent engineering stress at two pre-selected engineering strain levels (40 and 60%) using a polynomial model. Their brittleness was assessed in terms of the apparent fractal dimension of their stress-strain curves using the compass and box counting algorithms which yield the Richardson's and Kolmogorov's dimensions, respectively. The presence of starch in the dried gels solid matrix invariably increased their stiffness, but not in a manner that could be predicted on the basis of a stochiometric relationship or density increase. Sucrose, in contrast, could have the opposite effect suggesting that its presence in the solid matrix might interfere with its mechanical integrity. All the dried gels had a ‘rich’ acoustic signature as judged by the magnitude of their apparent fractal dimension, which was determined by the ‘blanket’ algorithm. But if there were differences between the different dried gels' acoustic signatures, they were too subtle to be detected by the apparent fractal dimension alone.     

Effects of Varying Levels of Chloride Salts on Clostridium Botulinum Toxin Production in Turkey Frankfurters

Inhibitory effects of sodium chloride (NaCl), potassium chloride (KCl), and magnesium chloride (MgCl₂) on C. botulinum inoculated turkey frankfurter emulsions were determined. Ionic strengths (I.S.) of 0.42, 0.55, and 0.68 were compared (equivalent to 2.5%, 3.25%, and 4.0% NaCl). Sodium nitrite levels were constant at 150 ppm. Inoculated emulsions (10³ spores/g) were incubated and maintained at 27°C. In-creasing NaCl from I.S. 0.42 to 0.68 delayed toxin production from 4 days to at least 40 days. KCl was almost as effective as NaCl at I.S. 0.42, but inferior at higher levels. MgCl₂ did not demonstrate any inhibitory effect. Fifty percent substitution of 2.5% NaCl with KCl or MgCl₂ generally reduced time for toxin production to occur.     

The influence of gelation rate on the physical properties/structure of salt-induced gels of soy protein isolate–gellan gum

The cold-set gelation of soy protein isolate (SPI)-gellan gum was induced by the addition of salts (KCl or CaCl₂) using two different procedures: the direct addition of salts (fast gelation) or the diffusion of salts through a membrane (slow gelation). The mechanical properties, syneresis and microstructure of the mixed gels were evaluated, as well as for gellan and SPI gels. The mixed gels induced by calcium diffusion were stronger and more deformable than gels induced by the direct addition of calcium, while the opposite occurred for potassium-induced gels. All the mixed gels were macroscopically homogeneous, but at the microscopic level two independent networks could be observed. These two separate networks were more evident for the calcium-induced gels, and the structural characteristics depended strongly on the concentration of the protein and the polysaccharide. However an organized microstructure with the formation of microtubes surrounded by other network was only observed for the mixed gels induced by calcium diffusion at the higher protein/polysaccharide (10:1) ratio. Thus besides the composition and concentration of the biopolymers, the results showed that the type of salt and its velocity of incorporation led to gels with different structures and consequently different mechanical properties.     

Gelation behaviour and rheological properties of acid-induced soy protein-stabilized emulsion gels

The protein concentration dependence on the rheological properties of acid-induced gels formed with unheated and heated soy protein-stabilized emulsions (UHSPE and HSPE) was investigated at different acidification temperatures. Pre-heat treatment on soy protein solutions resulted in a higher storage modulus (G′) and a shorter gelation time (t_gel) of acid-induced emulsion gels. A maximum in tan δ was observed in the UHSPE gels but no maximum was detected in the HSPE gels. Increasing the acidification temperature decreased the G′ and t_gel. The dependence of the G′ on the protein concentration (c) can be scaled with a power law: G′ ∼ c^A. The exponent (A) increased with pre-heat treatment and acidification temperature. The experimental data.fitted the fractal scaling model (G′ ∼ φ^A) and the simple time- scaling model above very well for the acid-induced soy protein-stabilized HSPE gels with varying oil volume fraction. The large deformation and fracture properties were significantly affected by soy protein concentration, pre-heat treatment, acidification temperature and volume fraction of oil droplets (p < 0.05).     

The influence of additives on properties of heat‐induced gels from salt‐soluble proteins extracted from goose

Calcium chloride (CaCl₂) and phosphates are important additives to improve product quality during meat processing. Response surface methodology was used to study the influence of CaCl₂ and phosphates on the hardness, water‐holding capacity (WHC) and ultra‐structure of salt‐soluble goose meat protein gels. The results show that the hardness and WHC of salt‐soluble protein gels increased significantly when CaCl₂ concentration was increased and phosphates were added. Scanning electron microscopy showed that tetrasodium pyrophosphate (TSPP) and sodium tripolyphosphate (STPP) had a greater impact on the cross‐linking and pore diameter of the gel networks than sodium hexametaphosphate (SHMP). At the 0.02 m and 4:3:2 of CaCl₂ concentration and the ratio of TSPP, SHMP and STPP, hardness and WHC values were 114.55 gf and 96.65%, which corresponded to the prediction value of our model. Further results showed that the hardness and WHC of gels reached the maximum with 0.3% of phosphates levels.     

Mobility and redistribution of waters within bighead carp (Aristichthys nobilis) heat-induced myosin gels

Water-holding capacity is closely related to gel microstructure, and is a very important quality trait in surimi and surimi product. The changes in the secondary structure, gel microstructure, and the migration of water in bighead carp (Aristichthys nobilis) myosin gel induced by different temperatures (50–90°C) were investigated. The α-helical structure of myosin decreased at temperatures of 40°C or higher. The fractal dimension of the gels increased at 40, 50, and 60°C, but decreased at temperatures over 60°C. The pore size of the gels increased with temperatures up to 50°C, decreased at 60°C, and then increased with temperatures up to 90°C again. The transverse relaxation times also varied; T₂₁ remained constant at temperatures over 40°C; T₂₂ decreased at temperatures lower than 50°C, increased at 60°C, and then decreased with temperatures up to 90°C; and T₂₃ increased at temperatures lower than 50°C and then remained constant until 90°C. Principal component analysis showed that the proportion of T₂₂ water (PT₂₂) was inversely correlated with the unfolding of myosin, whereas directly correlated with the pore size. The proportion of T₂₃ water (PT₂₃) was positively correlated with the fractal dimensions of the gels, whereas negatively correlated with the pore size. The migration of the secondary layer of water was mainly caused by hydrophobic force and the physical space formed by the myosin backbone, and the migration of water within the third layer was mainly caused by capillary pressure. Therefore, the mobility and redistribution of waters depend on the water retention mechanism, which is determined by the physical structure of gels. This study provides further information about the relationship between the NMR data, gel microstructure, water mobility, and distribution.     

Effects of Oligosaccharides and Soy Soluble Polysaccharide on the Rheological and Textural Properties of Calcium Sulfate-Induced Soy Protein Gels

This study investigated the rheological and textural properties of calcium sulfate (CaSO₄)-induced gels formed by soy protein containing oligosaccharides (sucrose, raffinose, and stachyose) and soy soluble polysaccharide. The results showed that the hardness and water holding capacity of the gels were significantly strengthened (P < 0.05) by the incorporation of oligosaccharides at the 1, 3, and 5% (w/v) levels and soy polysaccharide at the 0.3 and 0.5% (w/v) levels. The storage modulus after the temperature cycle, frequency sweep, creep recovery tests, and large deformation properties demonstrated that the affixion of above cosolutes enhanced the rigidity (elastic properties) of gels. Confocal scanning laser microscopy (CLSM) analysis indicated that the cosolutes also improved the microstructures of the gels and herein the gels containing stachyose exhibited the greatest compactness. The gel solubility in different buffers verified the speculation that the protein-protein interactions were enhanced by the incorporation of cosolutes and hence accounted for the improvement of the gel properties. The results of this study would potentially promote the use of health-promoting raffinose, stachyose, or soy polysaccharide and facilitate the development of novel soy protein-based gel products with firmer texture and higher nutritional value.     

Effect of protein concentration on whey protein gels obtained by a two-stage heating process

 The influence of protein concentration on the properties of gels obtained by a two-stage heating process was determined. In the first stage, whey protein dispersion (3–10%) was heated at pH 8.0, and in the second stage it was diluted to 3% protein, adjusted to pH 7.0 and heated again. Increased protein concentration in the first stage of polymerization resulted in the gels obtained in the second stage having a lower phase angle, increased storage modulus and increased hardness. Increased protein concentration also resulted in gels with an increased optical density, which suggests thathigher protein concentration leads to more and larger aggregates. Gels obtained from dispersions preheated at a higher protein concentration had higher permeability coefficient (B _gel) values. The increase in B _gel suggests that the higher protein concentration increased the size of the aggregates, which in a second stage of heating formed a gel matrix with a larger pore size. Received: 11 February 1999     

Myofibrillar Protein Solubility of Model Beef Batters as Affected by Low Levels of Calcium, Magnesium and Zinc Chloride

Preblended composites of semimembranosus and adductor muscles were stored 12h at 4°C with 2.0% NaCl, 0 or 0.05% CaCl₂, MgCl₂, or ZnCl₂ and 0 or 0.4% sodium tripolyphosphate (STPP). Model systems were formulated to contain 30% fat (high fat; HF) or 10% fat (low fat; LF). Divalent salts lowered extract pH and ZnCl₂ elicited the greatest reduction. At both fat levels, CaCl₂ increased and ZnCl₂ decreased protein solubility, compared to the control (p<0.05). Myosin was not detected in ZnCl₂-treated HF and LF batters without STPP and in the presence of STPP, MgCl₂ and ZnCl₂ increased myosin concentration at both fat levels (p<0.05). Zinc chloride increased actin concentration in HF batters; whereas, MgCl₂ decreased soluble actin in LF batters (p<0.05). Magnesium chloride (0.05%) increased soluble proteins in LF batters containing 0.4% STPP by increasing myosin extractability.