ROLE OF pH IN GEL FORMATION OF WASHED CHICKEN MUSCLE AT LOW IONIC STRENGTH

This work was designed to test the hypothesis that it is not solubilization of the myofibrillar proteins per se that is required to form good gels at low salt concentrations, but the protein‐containing structures must be disorganized. Gels were made from washed minced chicken breast muscle at 0.15, 0.88, and 2.5% sodium chloride. The gels made with varying salt concentrations were evaluated either at pH 6.0–6.5 or pH 7.0–7.4. Strain values, an indicator of protein quality, were high only at neutral pH in the gels containing 0.15 or 0.88% salt. At 2.5% salt, strain values of gels made at acid pH were superior to those at the low salt concentrations at acid pH, but inferior to gels with 2.5% salt at neutral pH. Poor gels were obtained at 0.15% salt and low pH whether or not there was an intermittent adjustment to neutral pH. A neutral salt wash markedly increased the water content of the mince, suggesting that solubility‐inhibiting proteins were removed. Good quality gels were obtained in the absence of any detectable solubilization of myosin and only minimal solubilization of actin.     

Gelation of β-Lactoglobulin: Effects of Sodium Chloride and Calcium Chloride on the Rheological and Structural Properties of Gels

Heating β-lactoglobulin solutions at pH 8 causes an increase in viscosity, but self-supporting gels were not formed unless salts, such as sodium chloride or calcium chloride, were added. The rheological and textural properties and gel strength were markedly affected by salt concentration. Thus, gels of maximum compressive strength were obtained with sodium chloride and calcium chloride at concentrations of 200 and 10mM, respectively. Increasing the concentration of sodium chloride resulted in the formation of soft gels which released water easily. Calcium chloride strengthened β-lactoglobulin gels by forming crossbridges. However, above 10 mM it tended to enhance coagulation rather than gelation. This was confirmed by election microscopy of the gel matrix.     

Gelling properties of whey protein isolate: influence of calcium removal by dialysis or diafiltration at acid or neutral pH

Dialysis of whey protein isolates (WPI) removed much more calcium when carried out at an acid pH (close to 4.0) than at neutral pH. Diafiltration at acid pH was also effective. The characteristics of thermally-induced gels prepared from WPI dialysed at acid or neutral pH were studied at pH 3.75 or pH 7.0, respectively, and at calcium concentrations ranging from 0 to about 60mM (with addition of calcium chloride). The water-holding capacity (WHC) and elasticity of gels increased with decreasing calcium concentration, at both pHs. Gel firmness was maximum at 10–20 mM calcium. The solubility of the protein constituents of WPI gels in a pH 8.0 buffer was high in the case of acid gels (especially at calcium concentrations lower or equal to 20 mM) and low for neutral gels at all calcium concentrations. Protein solubility values in the presence or absence of denaturing and reducing agents reflect the existence of intermolecular disulphide bonds in neutral gels and their absence in acid gels.     

钠盐替代物对全蛋液功能特性的影响

为探究氯化钾和氯化镁替代氯化钠改善全蛋液功能特性的可行性,评估不同浓度的氯化钠（0.2、0.4、0.8 mol/L和1.6 mol/L）、氯化钾（0.2、0.4、0.8 mol/L和1.6 mol/L）和氯化镁（5、10、20 mmol/L和40 mmol/L）对全蛋液理化和功能特性的影响。结果表明：盐的加入显著影响了全蛋液的功能特性。添加0.8 mol/L和1.6 mol/L的氯化钾后,泡沫稳定性分别提高了21.4%和21.6%。添加20 mmol/L和40 mmol/L的氯化镁后,乳化稳定性分别提高了14.7%和24.1%,同时蛋白质的溶解性也提高了12.7%和13.8%。此外,添加40 mmol/L的氯化镁后,全蛋液的凝胶持水性和弹性均未被破坏,硬度显著增高,同时全蛋液的颜色变化程度较低且不会引起全蛋液的pH值出现显著变化。综合来看,氯化镁更具有替代氯化钠改善全蛋液功能特性的潜力。研究结果将为钠盐替代物加入全蛋液中提供理论依据。     

Thermal scanning rheology of myofibrillar proteins from muscles of defined fibre type

Emulsion meat products are made with little regard to rheological properties of different muscles. Here the rheology of gels made from three classes of muscle defined by myosin type (fast twitch, slow twitch and heart) are compared. Myofibrillar fractions were prepared from representative bovine muscles-cutaneus trunci, masseter and heart-by a procedure that removed connective tissue, fat and sarcoplasmic proteins. Complicating effects of nongelling agents were thus minimized. Fractions were mixed with NaCl and pyrophosphate at concentrations typical of those used in processed meats. Rigidity and elasticity of the gelling mixtures were monitored during heating from 10 to 84°C over a range of pH values. Several indices of gelation showed that masseter and heart (slow muscle group) gelations were similar to each other but distinct from cutaneus trunci (fast) gelation. Gelation temperature was 10°C lower for fast than slow, perhaps explainable by one of two hypotheses: differential salt extraction or lower thermal stability of fast myosin rod. Cutaneus trunci gels were also more rigid at all pH values. Slow group gels suffered from proteolysis, which was combated with cathepsin inhibitors. For both fast and slow groups, rigidity increased as pH decreased, analogous to results reported for purified myosin in dilute solution. Texture also changed. Below pH 5·7 slow group gels were brittle and granular, with low water-holding capacity. Above that pH, gels were elastic and smooth. The equivalent point for cutaneus trunci gels was pH 5·5. Around pH 7, pyrophosphate had a fluidizing effect at pregelation temperatures, but elasticity data indicated that this ion plays no part in final gelation.     

Heat Gelation Properties and Protein Extractability of Beef Myofibrils

At a heating rate of 1^oC/min suspensions (pH 6.0) of isolated beef myofibrils were found to start forming gels at 43-56^oC, as detected by dynamic rheological measurements. The increase in gel storage modulus levelled off at temperatures > 65^oC. At medium to high (0.3-0.6M) concentrations of sodium chloride, addition of pyrophosphate (plus magnesium chloride) had the following effects: (1) both the protein extractability of non-heated myofibrils and the storage moduli of heat-induced gels were markedly increased; (2) the apparent activation energy for gel formation was decreased. Increasing concentrations of sodium chloride, up to 0.5-0.6M, increased the protein concentration of the liquid phase of the gels.     

Effect of protein concentration, pH, lactose content and pasteurization on thermal gelation of acid caprine whey protein concentrates

The influence of pH (4.5-6.5), sodium chloride content (125-375 mM), calcium chloride content (10-30 mM), protein concentration (70-90 g/l) and lactose content on the gel hardness of goat whey protein concentrate (GWPC) in relation to the origin of the acid whey (raw or pasteurized milk) was studied using a factorial design. Gels were obtained after heat treatment (90 degrees C, 30 min). Gel hardness was measured using texture analyser. Only protein concentration and pH were found to have a statistically significant effect on the gel hardness. An increase in the protein concentration resulted in an increase in the gel hardness. GWPC containing 800g/kg protein formed gels with a hardness maximum at the pHi, whereas GWPC containing 300 g/kg protein did not form true gels. Whey from pasteurized milk formed softer gels than whey from raw milk. A high lactose content (approximately 360 g/kg) also reduced the gelation performance of GWPC.     

Timed Emulsification Studies with Chicken Breast Muscle: Whole Muscle, Low-Salt Washed Muscle and Low-Salt Soluble Proteins

Emulsion formation with chicken breast muscle was investigated using timed emulsification. Emulsions were made at a fixed oil/water ratio of 2:1 by Omni-mixing for various lengths of time between 0 and 5 min; then the emulsions were centrifuged and the aqueous layer analyzed for protein. Emulsions formed using whole muscle or muscle washed with low molarity salt solution and suspended in buffered (10 mM sodium phosphate, pH 7.0) 0.6M NaCl were superior in stability upon centrifugation to those made with muscle in distilled water, buffered 0.025M NaCl or buffered 0.05M NaCl. Size of insoluble protein pellets from the centrifuged emulsions decreased as emulsification time increased. Little emulsifying effect of the low-salt soluble (sarcoplasmic) protein, defined as those proteins extracted with buffered 0.05M NaCl, was observed in the presence or absence of high-salt solubilized protein.     

Effects of Magnesium Chloride Combined with Tetrasodium Pyrophosphate on Meat Emulsion Characteristics

The effects were determined of adding various combinations of magnesium and sodium chloride plus tetrasodium pyrophosphate on the stability, pH, solubilized protein, chopping times and textural characteristics of raw and cooked meat emulsions.The results of adding different combinations of magnesium and sodium chloride were quite similar whether ionic strength or chloride ion concentration was kept constant.Emulsion stability and raw emulsion pH were reduced with increased proportion of added magnesium chloride. However,the level of solubilized protein found in raw emulsions increased with increasing proportions of magnesium chloride.     

Fish Myosin Aggregation as Affected by Freezing and Initial Physical State

ABSTRACT: Fish myosin obtained from Tilapia nilotica was solubilized in 20 mM Tris-HCl, pH 7.0, with 0.6 M KCl (solution model system), or suspended without salt (suspension model system). Changes in % soluble protein, Ca²⁺-ATPase activity, and total and reactive -SH groups during frozen storage were evaluated. Frozen induced aggregation of fish myosin showed different behavior depending upon its initial physicochemical state. When myosin was solubilized prior to frozen storage, head-to-head interactions seemed to be more involved in protein aggregation with a strong participation of disulfide bonds. On the contrary, a preferentially side-to-side mechanism might be involved in the aggregation of myosin upon suspension, with a minor interaction of -SH groups.     

GEL CHARACTERISTICS OF β-LACTOGLOBULIN, WHEY PROTEIN CONCENTRATE AND WHEY PROTEIN ISOLATE

The gelation characteristics of β-lactoglobulin, whey protein isolate and whey protein concentrate at varying levels of protein (6–11%), sodium chloride (25–400 mM), calcium chloride (10–40 mM) and pH (4.0–8.0) were studied in a multifactorial design. Small scale deformation of the gels was measured by dynamic rheology to give the gel point (°C), complex consistency index (k*), complex power law factor (n*) and critical strain (γ_c). The gel point decreased and turbidity increased with increasing calcium level. The denaturation temperature measured by differential scanning calorimetry was measured at higher pH values. Large scale deformation at 20% and 70% compression was measured using an Instron Universal Testing machine. The true protein level had the largest effect on the stress required to produce 20% and 70% compression and on the consistency (k*) of the gels.     

Curtailing Oxidation‐Induced Loss of Myosin Gelling Potential by Pyrophosphate Through Shielding the S1 Subfragment

In muscle food processing, where oxidation is inevitable, phosphates are usually added to improve water binding. This present study attempted to investigate the interactive roles of protein oxidation and pyrophosphate (PP) during thermal gelation of myosin. Myosin isolated from pork muscle was solubilized in 0.5 M NaCl at pH 6.2 then oxidatively stressed with an iron‐redox cycling system that produces hydroxyl radicals with or without 1 mM PP and 2 mM MgCl₂ at 4 °C for 12 or 24 h then heated to 50 °C at 1.3 °C/min. Protein conformational stability was measured by differential scanning calorimetry, and covalent cross‐linking was examined by sodium dodecyl sulfate–polyacrylamide gel electrophoresis following chymotrypsin digestion. The binding of PP to myosin suppressed disulfide bond formation in myosin subfragments 1 and 2 and partially inhibited oxidation‐initiated cross‐linking of heavy meromyosin during myosin gelation with a lesser effect on light meromyosin. In the presence of PP, myosin exhibited less loss of conformational integrity upon oxidation than myosin without PP. Rheological analysis from 20 to 75 °C indicated up to 32% decreases (P < 0.05) in elastic modulus (G′) of myosin gels due to oxidation. However, the presence of 1 mM PP, which also lowered the gelling capacity of myosin, inhibited the oxidation‐induced G′ by nearly half (P < 0.05). These results suggest that the protection of myosin head from oxidative modification by PP can be a significant factor for the minimization of gelling property losses during cooking of comminuted meats.     

FACTORS AFFECTING THE SODIUM CHLORIDE EXTRACTABILITY OF MUSCLE PROTEINS FROM CHICKEN BREAST, TROUT WHITE AND LOBSTER TAIL MUSCLES

The amount of protein extracted from chicken breast muscle at low salt (0–50 mM NaCl) increased as the salt concentration of the extracting solutions increased. The addition of 10 mM sodium phosphate buffer pH 7 (P_i) caused a marked increase in protein extractability at all salt concentrations. A particular polypeptide chain of about 150,000 daltons appeared to be particularly sensitive to the extraction conditions. At high salt (0.6M NaCl, 50 mM sodium phosphate buffer pH 7.0) a second extraction still contained significant amounts of protein. The amount of protein extracted was maximized at a 1/20 dilution. On the other hand, the protein extract-ability of trout white muscle, showed a smaller P_i effect and very little dependence on low salt concentration. The protein extractability of lobster flexor muscle showed little change with either increased salt or P_i. For all three muscles extraction over time with either high or low salt remained essentially constant after the first day with the most protein being extracted from lobster muscle and the least from chicken muscle.     

Hydroxyl Radical and Ferryl-Generating Systems Promote Gel Network Formation of Myofibrillar Protein

ABSTRACT: The objective of the study was to examine how oxidatively induced protein cross-linking would influence the gelation properties of myofibrillar protein (MP) under meat processing conditions. MP suspensions in 0.6 M NaCl at pH 6 were treated with an iron-catalyzed oxidizing system (IOS: 10 μM FeCl₃, 0.1 mM ascorbic acid, 0.05 to 5 mM H₂O₂) or a H₂O₂-activated metmyoglobin oxidizing system (MOS: 0.01 to 0.1 mM metmyoglobin/H₂O₂) that produced hydroxyl radical and ferryl species, respectively. Both oxidizing systems promoted MP thermal gelation, which was evidenced by rapid protein–protein interaction and the enhancement in storage modulus (elasticity) of the gel network as revealed by dynamic rheological testing in the 20 to 74 °C temperature range. This gelation-enhancing effect was attributed to the shift of myosin aggregation in the early stage of heating from predominantly head–head association (nonoxidized control samples) to prevalently tail–tail cross-linking through disulfide bonds. However, both hardness and water-holding capacity of chilled gels tended to decline when MP was exposed to ≥1 mM H₂O₂ in IOS and to all concentrations of metmyoglobin in MOS. Microscopic examination confirmed a more porous structure in oxidized gels when compared with nonoxidized protein gels. The results demonstrated that mild oxidation altered the mode of myosin aggregation in favor of an elastic gel network formation, but it did not improve or had a negative effect on water-binding properties of MP gels. Practical Application: Mild oxidation promotes protein network formation and enhances gelation of myofibrillar protein under normal salt and pH conditions used in meat processing. This oxidative effect, which involves disulfide linkages, is somewhat similar to that in bakery product processing where oxidants are used to improve dough performance through gluten protein interaction.     

Myosins from red and white bovine muscles: Differences measured by turbidimetric,calorimetric and rheological techniques

The aim of this study was to elucidate the functional performance of the most abundant protein component in meat, ie myosin, which is recognised as important for binding in meat products. As several genetic variants of skeletal myosin exist, myosins from two bovine muscles of different fibre type composition, M masseter and M cutaneus trunci were compared with respect to filament forming properties and denaturation characteristics. The principal methods used were turbidimetric measurements, which were used to monitor filament formation, calorimetry and rheology. The myosin systems were examined at two different salt levels (0.2 and 0.6 M NaCl) and at pH 5.5–7.0. The method of preparing myosin suspensions/solutions was also examined. Differences in the filament-forming process for the two myosins were detected. Measurements of turbidity revealed that at conditions of low pH and low ionic strength white myosin had a higher ultimate turbidity compared with red myosin. Early in the transition from low to high turbidity, red myosin had a higher turbidity than white myosin corresponding to reduced solubility. The turbidity increased with time of storing the myosin suspensions/solutions. This change was attributed to formation of filaments and further association of filaments. White myosin had a smaller apparent enthalpy of denaturation than red myosin. The calorimetric measurements recorded in 0.2 M NaCl suggested that the head and the rod of white myosin were less stable than the corresponding parts of red myosin. However, exceptions to this rule were found at pH 6.0. In 0.6 M NaCl the identification of the transitions for red myosin was more difficult. The method of preparing myosin suspensions affected calorimetric and rheological measurements. In 0.6 M NaCl and pH 6.0 calorimetric thermograms of both myosins were affected by the preparation method. At pH 5.5 this change was interpreted as caused by denaturation promoted by the dilution/rapid titration technique compared with dialysing the systems to pH 5.5 from pH 7. Differences in the filaments formed might, however, also contribute to the variations seen in the calorimetric ther-mograms. The gelling properties of white myosin were most sensitive to the preparation method used. Systems prepared by dialysis gave stronger heat-induced gels than those prepared by ‘dilution’. White myosin always produced stronger gels than red myosin independent of the preparation technique. The rheological properties (at 80°C) of red myosin were less affected by the preparation method than were those of white myosin. At lower temperatures, however, there was more variation in the shapes of the rheological thermograms (? versus temperature) for red myosin than in the corresponding thermograms of white myosin.     

Formation of two types of gels from bovine myosin

Myosin was isolated from bovine m. semimembranosus and gels were formed by heat treatment at different pH values and ionic strengths. The gels were subjected to rigidity measurements and their microstructure was studied by scanning electron microscopy. This article provides evidence that myosin can form two completely different gel structures in the pH range 5.5–6.0, depending on ionic strength. Fine stranded gel structures were formed at low ionic strength (0.25M KCl), whereas coarsely aggregated gel structures were formed at high ionic strength (0.6M KCl). The fine stranded structure had a higher rigidity than the coarsely aggregated structure. It was found that all fine strand myosin gels were formed from turbid solutions and the aggregate gels from clear solutions. When the pH was lowered to 4 in 0.6M KCl a strand-type gel structure formed spontaneously on dialysis, even without heat treatment. This structure did not change in character on heating. It was concluded that the conditions required for the formation of strand-type myosin gels were already present before the heat treatment and that the strands were made up of myosin filaments at certain pH and ionic strength combinations, which produced a turbid solution. The strand-type structures were considered specific with regard to myosin interactions which was not the case for the aggregated structures. Variation of the heating temperature in the range 55 to 65°C had no major effect on the type of structure formed.     

Chicken Skin Composition As Affected By Aqueous Washing

Broiler skin was washed in either 0.1M sodium chloride (pH 7.0) or 0.5% sodium bicarbonate buffer (pH 8.4) to reduce water-soluble proteins and fat. Washing increased moisture and total protein content (P<0.05) of skin. Ash content was lower (P<0.05) in skin washed in 0.5% sodium bicarbonate buffer compared to unwashed and 0.1M sodium chloride washed skin. Fat content decreased (P<0.05) following washing. Salt extractable proteins and collagen content remaining in skin increased following washing (P<0.05). Water-soluble proteins remaining in washed skin were lower (P<0.05). SDS-PAGE gel patterns for salt extractable proteins remaining in skin after washing revealed more intense bands in regions of myosin (200K daltons), M-proteins (165K daltons) and C-protein (135–140K daltons) and 31-45K daltons.     

Heat-induced Gelation of Chicken Gizzard Myosin

Chicken gizzard myosin solution formed a gel when heated above 40°C. The rigidity of the gel was constant above 65°C. Maximum pH for gel formation was 5.9 at 0.6M and 5.7 at 0.15M KCl. Higher rigidity of the myosin gel was observed at low ionic strength than at high ionic strength. Rigidities of myosin at 0.6M KCl increased by (mg/mL)^2.5 and at 0.15M (mg/mL)^{1, 4} myosin concentration. The strength of gizzard myosin gels was comparable to that of myosin gels from chicken breast muscle under similar conditions.     

ROLE OF IONIC STRENGTH IN BIOCHEMICAL PROPERTIES OF SOLUBLE FISH PROTEINS ISOLATED FROM CRYOPROTECTED PACIFIC WHITING MINCE

Biochemical characteristics of Pacific whiting muscle proteins extracted at acidic, neutral and alkaline conditions were investigated as affected by various ionic strength levels. The protein solubility at pH 4 declined, as NaCl was added up to 200 mM, due to protein aggregation through hydrophobic interactions. In contrast, at pH 7 and 10, solubility increased as NaCl was added up to 400 mM after which it remained constant. Changes in total SH content and S_owere highly related to the different molecular weight distributions of the soluble proteins. At pH 4, myosin heavy chain (MHC) was soluble as evidenced by the presence of MHC in the soluble fraction, even though degraded molecules were shown at IS 10–100 mM, and became completely insoluble at IS ≥ 150 mM. At pH 10, the density of the MHC band gradually increased as IS increased and the formation of high MW polymers was observed at IS ≥ 150 mM.     

Conformational and rheological changes in catfish myosin during alkali-induced unfolding and refolding

Changes in the conformation of catfish (Ictalurus punctatus) myosin due to (i) cations (ii) alkaline pH and (iii) salt addition were determined using circular dichroism, tryptophan fluorescence, differential scanning calorimetry and hydrophobicity studies. The relation between conformation and storage modulus (G′) of alkali treated myosin was studied. Two types of bases, NaOH and KOH were used for unfolding myosin under three alkaline conditions, pH 11.0, 11.5 and 12.0. Myosin, unfolded under alkali conditions was immediately refolded by adjusting pH back to 7.3. Subjecting myosin to alkaline conditions and subsequent readjustment to pH 7.3 increased the G′ of thermally treated myosin. G′ was affected by the presence or absence of salt during alkali treatments. When salt was present during alkali unfolding of myosin, the added salt stabilized the conformation of myosin against alkali unfolding and denaturation. In the absence of salt or when salt was added after refolding, myosin showed significantly higher denaturation and high G′ on heating and cooling. Among the different alkaline pH values, myosin treated at pH 11.0 showed higher G′. The type of anions influenced the conformation of myosin and the strength of gels. Treatment of myosin with KOH resulted in greater denaturation and higher gelling ability (G′) compared to NaOH.