Gel Firmness Research Articles

Thermal Syneresis Affected by Heating Schedule and Moisture Level in Surimi Gels

The extent of thermal syneresis in protein gelation is indicative of thermal and freeze-thaw stability as well as the network integrity of a protein gel. Thermal syneresis in Alaska pollock surimi gels was examined under different heating schedules (40 degrees C/20 min to 90 degrees C/30 min, 60 degrees C/20 min to 90 degrees C/30 min, and 90 degrees C/20 min to 90 degrees C/20 min) at varying moisture levels (80%, 82%, and 84%). The extent of syneresis and gel firming was monitored by centrifugation expressible moisture and penetration force, respectively. The occurrence of 2 distinct peaks as a function of time for both thermal syneresis and gel firming suggests that a multistage aggregation is involved in the formation of gel network. All syneresis preceded gel firming upon protein aggregation. Increasing the moisture content in the gel delayed the 2nd stage of protein aggregation. The 60 degrees C/20 min preheating followed by 90 degrees C/30 min postheating resulted in significantly greater thermal syneresis and gel weakening compared to 40 and 90 degrees C preheating. Changes of gel structure clearly reflected thermal syneresis when the size of water pores became smaller with initiation of network formation and progressively larger upon further heating. Thermal syneresis history during protein gelation can be used to predict thermal and freeze-thaw stability.

Tunable Thermoassociation of Binary Guanosine Gels

It is well-known that aqueous solutions of individual guanosine compounds can form gels through reversible self-assembly. Typically, gelation is favored at low temperature and acidic pH. We have discovered that binary mixtures of 5'-guanosine monophosphate (GMP) and guanosine (Guo) can form stable gels at neutral pH over a temperature range that can be tuned by varying the relative proportions of the hydrophobic Guo and the hydrophilic GMP in the mixture. Gelation was studied over the temperature range of 5-40 degrees C or 60 degrees C at pH 7.2 using visual detection, circular dichroism (CD) spectroscopy, and CD thermal melt experiments. Solutions with high GMP/Guo ratios behaved similar to solutions of GMP alone while solutions with low GMP/Guo formed firm gels across the entire temperature range. Most interesting were solutions between these two extremes, which were found to exhibit thermoassociative behavior; these solutions are liquid at refrigerator temperature and undergo sharp transitions to a gel only at higher temperatures. Increasing the GMP/Guo ratio and increasing the total concentration of guanosine compounds shifted the onset of gelation to higher temperatures (ranging from 20 to 40 degrees C), narrowed the temperature range of the gel phase, and sharpened the reversible phase transitions. The combination of self-assembly, reversibility, and tunability over biologically relevant temperature ranges and pH offers exciting possibilities for these simple and inexpensive materials in medical, biological, analytical, and nanotechnological applications.

Effect of milk solids concentration on the gels formed by the acidification of heated pH-adjusted skim milk

Reconstituted skim milk of 10–25% total solids was adjusted to pH values between about 6.2 and 7.1 and heated at 80 °C for 30 min. Gels were formed from the heated milks by slow acidification to pH 4.2 and the gelation process and final gels were analyzed for their rheological properties. At each milk concentration, the final acid gel firmness (final G′) and breaking stress could be changed markedly by manipulation of the pH during heating. The final gel firmness and breaking stress could also be modified by changing the concentration of the milk solids prior to heating and acidification. The results indicated that similar gel firmness and breaking stress could be achieved over a range of milk concentrations by control of the pH of the milk during heating. When expressed as a percentage change in final G′ or breaking stress relative to that obtained at the natural pH, plots of the change in final G′ or breaking stress versus pH fell close to a single curve, indicating that the same mechanism may influence the gelation properties at all milk concentrations. The final G′ and breaking stress were related to the denaturation and interaction of the whey proteins with the casein micelles, and the formation of non-sedimentable casein when the milk was heated.

Effect of transglutaminase on structure and syneresis of neutral and acidic sodium caseinate gels

The effect of transglutaminase on nuclear magnetic resonance (NMR) relaxation behaviour, porosity, firmness and syneresis of neutral and acidic sodium caseinate gels was studied. A drastic increase in porosity was observed to increase the amount of separating water up to 15% of gel weight, but structural homogeneity at the level of confocal laser scanning microscope (CLSM) resolution did not inhibit water separation in other acidic gels. Spin–spin relaxation time was reduced from 0.32 s (peak value) of the neutral transglutaminase-containing gel to 0.14 s of the corresponding acidic gel. The reduction was attributed to reduced mobility of protein protons in exchange with water protons in the system. With sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS–PAGE) analysis, more pronounced cross-linking of protein was observed at neutral than at acidic conditions, yet the neutral gel was weak compared with the acidic gel. It was assumed that, because of transglutaminase action, electrostatic repulsion was partially removed from the surface of sodium caseinate particles, allowing, e.g., hydrophobic interactions between the particles and gel formation. The mobility of proteins in the gel network was suggested to be related to its syneresis behaviour.

Physicochemical properties of cell wall materials from apple, kiwifruit and tomato

Cell wall materials (CWMs) were isolated from the fruit of ripe apple, kiwifruit and tomato using methods of isolation which maximised the water retaining capacity and viscosity generating properties of the CWMs. Aqueous suspensions of all three CWMs were able to form a gel-like matrix at a concentration of 1%. There was a dramatic enhancement in gel firmness of kiwifruit and tomato following a high shear treatment, but no such effect was apparent with apple CWM. Confocal microscopy showed that the shear-induced increase in viscosity was accompanied by fragmentation of the CWMs of kiwifruit and tomato which increased the available surface area for particle–particle and/or particle–solvent interaction. The viscosity of kiwifruit and tomato CWM dispersions was reduced in the presence of electrolytes indicating an important role for the double electrical layer in the gelling properties of the CWMs. The viscosifying properties of apple CWM were however independent of both shear and added electrolyte. This was attributed to the fact that CWM from apple resisted breakup under high shear. The greater connective integrity of the apple cell walls compared to that of kiwifruit and tomato is discussed in relation to differences in ripening induced changes to the pectic polysaccharides of the cell walls.

Creaminess in relation to consistency and particle size in stirred fat-free yogurt

Stirred yogurts with the same chemical composition but prepared using different procedures had different gel particle diameters and different rheological characteristics. Sensorial evaluations were performed and compared with instrumental analyses. The smoothness perceived both visually and orally varied with the size of the particles of the stirred gels. For products with small particle sizes, the consistency was related to viscoelastic parameters. It was shown that products perceived as creamy were firm. When particle size was above 150 μm, it seemed impossible to perceive creaminess. Nevertheless, results obtained from two commercial products containing fat and starter showed that creaminess perception could not be reduced to simple relationships between gel firmness and particle size in the case of stirred gels.

Physical, chemical and flavour quality of non-fat yogurt as affected by a β-glucan hydrocolloidal composite during storage

In this study, a possible use of β-glucan hydrocolloidal composite as a fat replacer in the manufacture of non-fat yogurts was investigated. The yogurts with added β-glucan composite were compared with non-fat yogurt without addition of β-glucan composite and the samples were analysed for physical, chemical and sensory attributes after 1, 7 or 15 d of storage. Fat and protein contents of the experimental yogurts were identical, while ash content differed. Addition of β-glucan composite did not show a significant change of pH, titratable acidity, acetaldehyde, volatile fatty acids and tyrosine contents at any storage time. Titratable acidity and tyrosine content increased significantly throughout storage. Gel firmness and water-holding capacity in the yogurts were not influenced by addition of β-glucan composite, but these variables decreased with storage time. Addition of β-glucan composite and storage time caused a decrease in whey separation. Viscosity values in the yogurts increased by addition of β-glucan composite and storage time. Sensory results indicated a preference for control yogurts; however, use of low levels of β-glucan composite in the production of non-fat yogurt gave satisfactory sensory scores. Yogurts containing 0.25% or 0.50% β-glucan hydrocolloidal composite were acceptable by expert panels and had scores similar to the control yogurt.

Influence of pressure release rate and protein concentration on the formation of pressure-induced casein structures

The formation of pressure-induced casein structures (600 MPa for 30 min at 30 degrees C) was investigated for different pressure release rates (20 to 600 MPa min-1) and casein contents (1 to 15 g/100 ml). Structures from liquid (sol) to solid (gel) were observed. The higher the protein content and the pressure release rate, the higher was the dynamic viscosity. A firm gel was built up at a casein content of 7 g/100 ml for a pressure release rate of 600 MPa min-1, while lower release rates resulted in less firm gels (200 MPa min-1) or liquid structures (20 MPa min-1). In a 5 g/100 ml casein solution and at a pressure release rate of 600 MPa min-1, casein aggregates were generated which were built from smaller casein particles with a larger hydrodynamic diameter and higher voluminosity than in the untreated solution. After a slow release rate casein micelles had a smaller hydrodynamic diameter and a lower voluminosity, but were similar in shape and diameter as compared with the micelles in solution before high pressure treatment.

Egg white drying: Influence of industrial processing steps on protein structure and functionalities

Spray-dried egg white is commonly used as a food ingredient for its foaming and gelling properties. However, these properties are obtained thanks to dry-heating of egg white powder, which is necessary to offset the harmful effects of spray-drying process on egg white functionality. The purpose of the present work is to identify the processing steps responsible for the damages to egg white functional properties, and to understand the mechanisms that occur in order to limit these effects and to reduce dry-heating time. Two trials were performed and the measurements of egg white protein conformation and gel firmness were significantly different from one trial to another, thus emphasizing great variations in raw material characteristics. In spite of this trial effect, processing steps significantly modified egg white foaming properties. The most critical step was the spray-drying one that strongly damaged foaming properties. During this step, heat transfers and air–product interface area rather than shear rates were responsible for these changes. Then, it was the pumping and filtering steps that had also a considerable effect, due to the generation of shear rates and stainless steel–product interfaces, responsible for foaming property damages. On the other hand, concentration and desugarization steps had an interesting improving effect on egg white foaming properties.

Assessment of the rennet coagulation of skim milk: A comparison of methods

Monitoring of rennet-induced coagulation of milk reconstituted from low-heat or medium-heat skim milk powders with or without calcium enrichment (6.25 mmol kg −1) was carried out by five analytical methods. The four types of milk samples allowed evaluation of these methods on substrates characterized by very different rennetability. Thirteen parameters describing the coagulation process were extracted from the various measurements for each sample and compared. The visual flocculation and gelation times evaluated by the Berridge test and dynamic small amplitude oscillatory rheometer (DSAOR), respectively, were not significantly different from specific parameters identified from near-infrared (NIR) spectra analysis. DSAOR, piezoelectric rheometer, and NIR spectrometer allowed and evaluation of the beginning of gel firming. It clearly appeared that NIR spectroscopy, an on-line sensor, could best assess rennet-induced coagulation of studied samples.

Prediction of meat emulsion stability using reflection photometry

Manufacture of finely comminuted meat products are operations that require improved control to produce stable products. Emulsion breakdown becomes evident during the cooking process, when it is too late for corrective actions. Two different emulsion formulations that produced high or low cooking loss tendencies were prepared. Emulsion ingredients were chopped and the mixture was sampled at different time intervals. CIELAB coordinates of each sample were measured and the samples cooked to determine cooking loss and gel firmness. L∗ values increased at the beginning of chopping, which was accompanied with increased gel firmness ( P < 0.01) and reduced cooking loss ( P < 0.001). After 8 min of chopping (emulsion temperature ⩾22 °C) a reduction in L∗ and b∗ values and in emulsion firmness was observed simultaneously with increasing cooking losses. These results suggest the feasibility of an on-line optical sensor technology to predict the optimum endpoint of emulsification in the manufacture of finely comminuted meat products.

Physicochemical Variables Affecting the Rheology and Microstructure of Rennet Casein Gels

The rheology and microstructure of a rennet casein system were studied in the pH range from 5.8 to 12.0 during cooling from 80 to 5 degrees C at four cooling rates: 0.5, 0.1, 0.05, and 0.025 degrees C/min. A dramatic increase in storage modulus with pH was observed during cooling at a fixed cooling rate. Continuous networks were formed for gels at pH 7.2 and above, while a discontinuous network was observed for gels below pH 6.5. The monotonic increase in storage modulus with pH could be correlated to the number of net (negative) charges and the strength of the hydrophobic interactions. At a higher pH, the protein micelles were larger due to weaker hydrophobic interactions and stronger repulsive electrostatic interactions resulting from more charges. When these protein micelles aggregated into flocs during cooling, the flocs had similar sizes at different pH values but a smaller fractal dimension at a higher pH. Consequently, for systems of the same protein and salt concentrations, more flocs were present in the gels at a higher pH, which subsequently generated more cross-links and a higher storage modulus. The pH also determined how the cooling rate affected the gel properties. At pH 5.8 and 6.5, the gels were firmest at the fastest cooling schedule, and the cooling rate did not show a trend in affecting the gel strength at the other three rates. On the other hand, a slower cooling rate generated a firmer gel at pH 7.2 and 12.0. The analysis of casein interactions suggests that the cooling rate affected the casein floc size only when repulsive interactions enabled a slow flocculation (at higher pH values) comparable with temperature change rates during cooling. For rennet casein gels of pH within the range of processed cheese products (pH 5.8 and 6.5), particle or cluster rearrangements created more uniform networks for gels cooled at slower schedules and weakened the structure.

Effect of Minerals on Casein Micelle Stability of Cows' Milk

The effects of minerals on casein micelle stability of individual cows' milk, throughout a complete lactation, were investigated. Calcium and calcium ions, magnesium, phosphorus, sodium, potassium and citrate contents were analysed, together with the following physical properties of milk; pH, ethanol stability, rennet clotting time and coagulum firmness. There was an inverse non-linear relationship between free calcium ion concentration and ethanol stability (ES; r=0.84). Rennet coagulation time showed a weaker relationship with free calcium ion concentration (r=0.44) but a stronger relationship with pH (r=0.66). In addition, samples containing higher amounts of free calcium ions produced a firmer gel. Citrate in natural samples acts as a stabilizing factor, as it slightly improves milk stability. Potassium, on the other hand, exhibited a negative correlation, but only with rennet clotting time (r=-0.52). Throughout lactation the average values were; free Ca2+ concentration 1.88 mM, pH 6.63, ES 83.2% and clotting time 13.6 min. The equilibrium relationship between pH and free Ca2+ concentration was investigated by adjusting milk pH from 5.9 to 7.1, using acid and alkali. There was a good inverse linear relationship between pH and log (free Ca2+) for individual milk samples, with a gradient of -0.62 and a standard deviation of 0.042.

Influence of transglutaminase protein cross-linking on the rennet coagulation of casein

The influence of protein cross-linking on the rennet coagulation of casein was investigated using a microbial transglutaminase (TG) preparation containing glutathione (GSH). As unheated milk is normally less reactive towards TG, GSH was applied in order to enable non-inhibited cross-linking. Thus, interactions between TG cross-linking and rennet coagulation and their impact on the renneting properties were studied independently from a pre-heat treatment of milk beyond pasteurisation. Protein cross-linking was carried out either prior to or simultaneously with the addition of rennet. Coagulation times t C and gel firmness assessed as the storage modulus G 60 min ′ could be related either to TG pre-incubation times or to TG concentrations. The results show that enzymatic cross-linking affects both the primary and the secondary stage of rennet coagulation. The impact of protein cross-linking on the primary enzymatic phase can be mainly related to increased coagulation times due to inhibition of the caseinomacropeptide release. Consequently, a higher extent of protein cross-linking leads to reduced gel firmness. The simultaneous reaction of TG and rennet, however, results in markedly increased yield after centrifugation due to the enhanced serum binding of the gel network stabilised by additional covalent bonds. In conclusion, the cross-linking has an intense impact on the entire renneting process leading to rennet gels with modified functional properties.

Tyrosinase-Aided Protein Cross-Linking: Effects on Gel Formation of Chicken Breast Myofibrils and Texture and Water-Holding of Chicken Breast Meat Homogenate Gels

The effects of Trichoderma reesei tyrosinase-catalyzed cross-linking of isolated chicken breast myofibril proteins as a simplified model system were studied with special emphasis on the thermal stability and gel formation of myofibrillar proteins. In addition, tyrosinase-catalyzed cross-linking was utilized to modify the firmness, water-holding capacity (WHC), and microstructure of cooked chicken breast meat homogenate gels. According to SDS-PAGE, the myosin heavy chain (MHC) and troponin T were the most sensitive proteins to the action of tyrosinase, whereas actin was not affected to the same extent. Calorimetric enthalpy (DeltaH) of the major thermal transition associated with myosin denaturation was reduced and with actin denaturation increased in the presence of tyrosinase. Low-amplitude viscoelastic measurements at constant temperatures of 25 degrees C and 40 degrees C showed that tyrosinase substantially increased the storage modulus (G') of the 4% myofibrillar protein suspension in the 0.35 M NaCl concentration. The effect was the most pronounced with high-enzyme dosages and at 40 degrees C. Without tyrosinase, the G' increase was low. Tyrosinase increased the firmness of the cooked phosphate-free and low-meat chicken breast meat homogenate gels compared to the corresponding controls. Tyrosinase maintained gel firmness at the control level of the low-salt homogenate gel and weakened it when both salt and phosphate levels were low. Tyrosinase improved the WHC of the low-meat and low-salt homogenate gels and maintained it at the level of the corresponding controls of phosphate-free and low-salt/low-phosphate homogenate gels. Microstructural characterization showed that a collagen network was formed in the presence of tyrosinase. Chicken myofibrillar proteins; protein modification; cross-linking; tyrosinase; gelation; thermal stability; texture; water-holding capacity; microstructure.

Development of a poloxamer analogs/carbopol-based in situ gelling and mucoadhesive ophthalmic delivery system for puerarin

Conventional ophthalmic solutions often eliminate rapidly after administration and cannot provide and maintain an adequate concentration of the drug in the precorneal area. To solve these problems, we developed a thermosensitive in situ gelling and mucoadhesive ophthalmic drug delivery system containing puerarin based on poloxamer analogs (21% (w/v) poloxamer 407/5% (w/v) poloxamer 188) and carbopol (0.1% (w/v) or 0.2% (w/v) carbopol 1342P NF). The combined solutions would convert to firm gels under physiological condition and attach to the ocular mucosal surface for a relative long time. The incorporation of carbopol 1342P NF not only did not affect the pseudoplastic behavior with hysteresis of the poloxamer analogs solution and leaded to a higher shear stress at each shear rate, but also enhanced the mucoadhesive force significantly. In vitro release studies demonstrated diffusion-controlled release of puerarin from the combined solutions over a period of 8 h. In vivo evaluation (the elimination of puerarin in tear and intraocular pressure-lowering effect) indicated the combined solutions had better ability to retain drug than poloxamer analogs or carbopol alone. It appears that ocular bioavailability can be increased more readily by using the in situ gelling and mucoadhesive vehicle.

Influence of prior acid treatment on acetylation of wheat, potato and maize starches

Functional properties of acid-thinned, acetylated, and acid-thinned acetylated wheat, potato, and maize starches were investigated. Total amylose content and the extent of amylose leaching were increased after acid-thinning in all the starches indicating creation of more linear segments. Acid treatment decreased the swelling factor of potato starch at all tested acid concentrations but slightly increased it in wheat and maize starches in low acid treatment. Acid-thinning increased gelatinization temperatures and enthalpy but greatly decreased peak viscosity of all the starches. Wheat and maize starches showed early viscosity increase after acid-thinning but it was delayed in potato starch. Potato starch produced firmer gels after acid-thinning but wheat starch gave weaker gels. For maize starch, gel hardness was increased with low concentration acid treatment. Unchanged melting enthalpy of amylose–lipid complex in acid-thinned starch reflects its resistance to acid hydrolysis. Amylopectin retrogradation of potato and maize starches was not affected by acid modification but it decreased in wheat starch. Acetylation decreased gelatinization temperature and enthalpy, amylopectin retrogradation and gel hardness, but increased swelling factor, amylose leaching and peak viscosity. Acid-thinning decreased the degree of substitution. Introduction of acetyl groups to acid-thinned starches decreased gelatinization and retrogradation transition parameters and produced very soft gels.

Structuring edible oil with lecithin and sorbitan tri-stearate

The gelation of edible oil by a mixture of lecithin and sorbitan tri-stearate (STS) was studied. The two components individually in oil do not give structure at concentrations between 6% and 20% w/w: viscous, pourable solutions are obtained. A synergetic effect is observed with their mixture, at specific ratios of lecithin:STS, between 40:60 and 60:40, when firm gels are obtained. The interaction of the two structurants was studied by varying concentration and ratio of lecithin:STS and evaluated through microscopy, texture analysis, X-ray diffraction (XRD), rheology and nuclear magnetic resonance (NMR). The gels are shear-sensitive, they irreversibly lose their firmness upon deformation. The gels collapse at about 30 °C but this phenomenon is reversible, since upon cooling they regain their firmness. Under polarised light, the microstructure of 50:50 lecithin:STS mixture in oil is given by needle-like crystals. Oil with pure STS and oil with lecithin and STS at a ratio 50:50 show similar X-ray scattering patterns. Both X-ray scattering patterns and NMR relaxation curves show that no solid or crystalline matter is present in samples containing only lecithin in oil. The analytical results lead to the hypothesis that the structure is mainly given by STS crystals, assuming needle or plate shape due to the lecithin action and connected by weak junctions, also due to lecithin.

Acid coagulation properties and suitability for yogurt production of cows’ milk treated by high-pressure homogenisation

The effects of high-pressure homogenisation (HPH) of cows’ milk were investigated for suitability for yogurt manufacture, compared with the processes currently applied in industry. Milk at different inlet temperatures (30 °C or 40 °C) was subjected to HPH treatment at 100, 200 or 300 MPa (one stage) and 130, 230 or 330 MPa (two-stage). HPH-treated milk was compared with milk heat-treated (90 °C for 90 s) and homogenised at 15 MPa, and with milk treated under the same thermal conditions and also fortified with 3% skim milk powder. Milk treated at 300 or 200 MPa showed higher gel strengths on coagulation, higher gel firmness in texture analysis, less syneresis and lower titratable acidity compared with conventionally treated milk fortified with 3% skim milk powder.

Effect of transglutaminase on rheological properties and microstructure of chemically acidified sodium caseinate gels

The mechanical properties and microstructure of 2.7% and 4.5% sodium caseinate gels chemically acidified by glucono- δ-lactone (GDL) and cross-linked by microbial transglutaminase (TG) were studied. The acidification was performed at different temperatures. According to SDS–PAGE TG clearly caused polymerisation of caseinate irrespective of the treatment temperature (4–50 °C), The cross-linking of the proteins was more extensive at temperatures 22–50 °C. Low amplitude viscoelastic measurements showed that 4.5% caseinate gels acidified at 50 °C were formed much faster than gels acidified at 22 °C. TG only slightly increased the time of gelling. Control gels prepared without TG at temperatures of 4, 22, 37 and 50 °C were mechanically weak. Examination of the control gels with a confocal laser scanning microscope showed that gels formed at 37 and 50 °C were coarse and porous with large cavities between particle aggregates, whereas those formed at 22 °C were much more homogeneous. The TG-treated and acidified sodium caseinate dispersions formed firm gels, indicating cross-linking of casein proteins. Interestingly, the strongest gels were formed at 22 and 37 °C. TG treatment improved the homogeneity of the gel structure at temperatures of 37 and 50 °C. The hardness of TG-treated gels acidified at 4 °C increased during 1 week of storage.