Oxidative modification of amino acids in porcine myofibrillar protein isolates exposed to three oxidizing systems

Susceptibility of amino acids in myofibrillar protein isolate (MPI) exposed to three oxidizing matrixes commonly encountered in muscle foods was compared. MPI suspensions (20 mg protein/mL) in 15 mM piperazine-N,N bis(2-ethane sulphonic acid) buffer (pH 6.0) were oxidized with an iron-catalyzed oxidizing system (IOS, 0.01 mM FeCl₃, 0.1 mM ascorbic acid, 0.0–10.0 mM H₂O₂), a lipid-oxidizing system (LOS, 0.0–10.0 mM linoleic acid, 3750 units of lipoxidase/mL), or a metmyoglobin (MetMb) oxidizing system (MOS, 0.0–0.5 mM H₂O₂/MetMb) for 24 h at 4 °C. Changes were quantitatively analyzed by determining amino acids on a reverse-phase liquid chromatographic (LC) system. In IOS, the amount of cysteine, methionine and tyrosine decreased (P < 0.05) with increasing [H₂O₂]. In LOS, only cysteine and methionine were lowered at increasing linoleic acid concentrations. In MOS, the quantity of alanine, cysteine, glycine, histidine, leucine and lysine, as well as the total amount of amino acids were significantly reduced at high concentrations of MetMb/H₂O₂. The results suggest that under typical meat processing conditions, iron- and metmyoglobin-catalyzed reactions play a major role in the oxidation of amino acids in muscle proteins.     

Pumpkin oil cake protein isolate films as potential gas barrier coating

This work was aimed to investigate the potential of PuOC protein isolate (PuOC PI) in preparation of biodegradable films at different pH values (2–12) and plasticizer content (0.3–0.6 g glycerol/g PuOC PI). Results showed that films could be formed in a wild range of pH, except at pH = 4–8. Films with 0.4 and 0.5 g glycerol/g PuOC PI were suitable for further analysis. The pH of film-forming solutions influenced all examined film’s characteristics. The amount of added glycerol significantly affected (p < 0.05) tensile strength, elongation at break and solubility of the films. Gas permeability of films with 0.4 g glycerol/g PuOC PI showed that these films represents an excellent barrier for O₂, N₂, CO₂ and air. Obtained films have improved elongation at break and gas permeability characteristics compared to PuOC biodegradable films, thus they could be used as gas barrier stretch coating.     

Concentration effects of hydroxyl radical oxidizing systems on biochemical properties of porcine muscle myofibrillar protein

The objective of the study was to determine the dose-dependency of myofibrillar protein oxidation on oxidizing ferric ion. Pork myofibrillar protein isolates (MPI) were suspended in 15 mM piperazine-N,N bis(2-ethane sulfonic acid) (PIPES) buffer (pH 6.0) with 0.6 M NaCl, and incubated at 4 °C for 24 h with two levels of ferric ion (0.01 and 0.1 mM FeCl₃) at eight concentrations of hydrogen peroxide (0.00–10 mM H₂O₂). In both high and low [FeCl₃] oxidizing systems, the Ca-ATPase activity steadily increased with the H₂O₂ concentration. On the other hand, K-ATPase activity, protein carbonyl content, and 2-thiobarbituric acid-reactive substances increased with H₂O₂ up to 1.0 mM, and then gradually declined. Protein unfolding and loss of myosin heavy chain occurred continuously with increasing H₂O₂ concentrations. All changes, except for K-ATPase activity, were generally more rapid and extensive in the high [FeCl₃] oxidizing system. Overall, the biochemical changes in MPI exposed to ferric iron-oxidizing systems were more pronounced at high [FeCl₃] than at low [FeCl₃], but the pattern of the biochemical alterations appeared to be independent of the FeCl₃ concentration.     

Hydroxyl Radical-Stressed Whey Protein Isolate: Functional and Rheological Properties

The objective of the study was to examine the sensitivity of whey protein functionality to oxidizing radicals. Whey protein isolate (WPI) was oxidatively stressed by incubation at 20 °C for 3, 5, and 10 h in hydroxyl radical-generating media containing 0.1 mM ascorbic acid, 0.1 mM FeCl₃, and 1–10 mM H₂O₂. Protein solubility decreased (P?<?0.05) with increasing H₂O₂ concentrations and oxidation time. Surface properties of WPI, including both emulsifying and foaming activities, exhibited significant improvements (P?<?0.05) at H₂O₂ concentrations up to 5 mM and oxidation time up to 5 h. The longer oxidation time or higher H₂O₂ concentrations tended to diminish the surface functionality. However, the oxidative stress, though decreasing the onset gelling temperature, had a general detrimental effect on WPI gelation (hardness, springiness, and storage modulus). The results indicated opposing effects of oxidation on WPI: detrimental to hydrodynamic properties (solubility, gelation) but beneficial to surface properties (emulsification, foaming).     

Brewster angle microscopy of adsorbed protein films at air–water and oil–water interfaces after compression,expansion and heat processing

Recent Brewster angle microscopy (BAM) observations of adsorbed films of proteins at the air-water (A–W) and oil–water (O–W) interfaces are reviewed and compared. At the A–W interface β-lactoglobulin (β-L) and ovalbumin (OA) were studied at pH 7 and 5. At the O–W interface β-L, α_s1-, β- and κ-caseins were studied at pH 7. The adsorbed films were periodically subjected to compression and expansion cycles such that the film area was typically varied between 125% and 50% of the original film area. At the A–W interface, little structure was observable on compression or expansion or aging, especially at pH 7. For ovalbumin at pH 5, some cracks and ridges in the films appeared. But, for both β-L and OA, such features became much more obvious on addition to the interface of a low area fraction (<0.01%) of 20 μm polystyrene latex (PS) particles. With particles present the structuring was also more obvious at pH 5 (closer to the protein isoelectric point) than at pH 7, and for greater adsorption times and/or higher bulk protein concentration (C_b). Particle addition was not necessary to highlight folding and ridges that occurred at the O–W interface for β-L. After heating to 80 and 90 °C, β-L films adsorbed from low C_b (0.005 wt%) even greater film structuring was evident. However, β-L films adsorbed from higher C_b (≥0.05 wt%) showed fewer, but more pronounced ridges and cracks. The caseins at the O–W interface showed comparatively little evidence of structuring, either before or after heating. A measure of the dilatational elastic modulus of the films correlated with the observed variations in the structural integrity of the films. Clearly, protein films subjected to these types of thermal and mechanical perturbations can become highly inhomogeneous, depending on the type of protein and the bulk solution conditions, with implications for the stability of the corresponding foams and emulsions. This is in agreement with earlier computer simulations. Protein films at the O–W interface (particularly after heating) appear to be more resilient and less aggregated than films at the A–W interface.     

Oxidation desensitizes actomyosin to magnesium pyrophosphate-induced dissociation

This study aimed to establish the influence of protein oxidation on the ability of magnesium pyrophosphate (PP) to dissociate actomyosin. Actomyosin isolated from pork muscle then suspended in 0.1 M NaCl at pH 6.2 was oxidatively stressed with 10 μM FeCl₃/0.1 mM ascorbate/1 mM H₂O₂ for 6 or 12 h at 4 °C. Protein oxidation was evidenced by the loss of myosin and actin, the concomitant formation of disulphide-cross-linked polymers, and elevated myosin ATPase activity. The intrinsic viscosity of oxidized actomyosin had a weaker response to PP-Mg²⁺ than that of non-oxidized actomyosin, indicating the suppression of actomyosin dissociation. Moreover, oxidized actomyosin solutions were devoid of small particles (<10 nm) and the stressed actomyosin exhibited weaker binding of PP-Mg²⁺ than non-oxidized, which further suggested a reduced myosin–PP interaction and subsequent dissociation of the actomyosin complexes.     

Partial purification and preparation of bovine lactoperoxidase and characterization of kinetic properties of its immobilized form incorporated into cross-linked alginate films

Lactoperoxidase (LPS), purified directly from bovine rennet whey by Toyopearl-SP cation-exchange chromatography and lyophilized by using dextran as supporting material, maintained almost 70 and 60% of its activity after almost 2 and 5 months storage at −18 °C, respectively. Incorporation of the prepared LPS into alginate films between 0.08 and 0.69 mg/cm² (516–4325 U/cm²) caused the immobilization of most of the enzyme and gave films with LPS activity between 0.05 and 2.8 U/cm², determined in the presence of 8 μM H₂O₂. Between 2 and 24 μM H₂O₂ concentrations, a two-fold increase in H₂O₂ concentration caused 1.5–2.5-fold increase in LPS activity of films incorporated with 0.24–0.28 mg/cm² (1200 U/cm²) LPS. The Q₁₀ and E_a of immobilized enzyme activity between 4 and 16 °C were 1.69 and 34.6 kJ/mol, respectively. However, in the 16–30 °C range, the temperature change had almost no effect on LPS activity of films. The optimal activity of immobilized LPS was observed at pH 6.0, but the enzyme maintained 30–85% of its activity between pH 3.0 and 7.0. The immobilized LPS also had a high stability between pH 4.0 and 6.0. The results of this study showed the good potential of LPS-incorporated alginate films in forming a natural antimicrobial mechanism in different foods.     

Formation and functionality of whey protein isolate–(kappa-, iota-, and lambda-type) carrageenan electrostatic complexes

The formation of electrostatic complexes between whey protein isolate (WPI) and (κ-, ι-, λ-type) carrageenan (CG) was investigated by turbidimetric measurements as a function of pH (1.5–7.0), biopolymer weight-mixing ratio (1:1–75:1 WPI:CG) and NaCl addition (0–500 mM) to better elucidate underlying mechanisms of interaction. Emulsion stabilizing effects of formed complexes was also studied to assess their potential as emulsifiers. Complex formation followed two pH-dependent structure-forming events associated with the formation of soluble (pH_c) and insoluble (pH_?1) complexes. For both the WPI–κ-CG and WPI–ι-CG mixtures, pH_c and pH_?1 occurred at pH 5.5 and 5.3, respectively, whereas in the WPI–λ-CG mixture values were slightly higher (pH_c = 5.7; pH_?1 = 5.5). In all mixtures, maximum turbidity was found to occur near pH 4.5, before declining at lower pHs. Biopolymer mixing ratios corresponding to maximum OD was found to occur at the 12:1 ratio for both the WPI–κ-CG and WPI–λ-CG mixtures, and 20:1 ratio for WPI–ι-CG mixture. The addition of NaCl disrupted complexation within WPI–κ-CG mixtures as levels were raised, whereas when ι-CG and λ-CG was present, complexation was enhanced up to a critical Na⁺ concentration before declining. Adsorption of CG chains to the small WPI–WPI aggregates during complexation was proposed to be related to both the linear charge density and conformation of the CG molecules involved. Emulsion stability in the mixed systems (12:1 mixing ratio), regardless of the CG type (κ, ι, λ), was significantly higher than individual WPI solutions indicating enhanced ability to stabilize the oil-in-water interface.     

Characterisation of an acidic peroxidase from papaya (Carica papaya L. cv Tainung No. 2) latex and its application in the determination of micromolar hydrogen peroxide in milk

An acidic peroxidase isoform, POD-A, with a molecular mass of 69.4 kDa and an isoelectric point of 3.5 was purified from papaya latex. Using o-phenylenediamine (OPD) as a hydrogen donor (citrate–phosphate as pH buffer), the optimum pH for the function of POD-A was 4.6, and the optimum temperature was 50 °C. The peroxidase activity of POD-A toward hydrogen donors was both pH- and concentration-dependent. Under optimal conditions, POD-A catalysed the oxidation of OPD at higher rates than pyrogallol, catechol, quercetin and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS). The chemical modification reagents N-bromosuccinimide and sodium azide significantly inhibited POD-A activity. The results of kinetic studies indicated that POD-A followed a ping-pong mechanism and had a K_m value of 2.8 mM for OPD. Using CPC silica-immobilised POD-A for the determination of micromolar H₂O₂ in milk, the lower limit of determination was 0.1 μM, and the recoveries of added H₂O₂ were 96–109%.     

Segregative interactions and competitive binding of Ca in gelling mixtures of whey protein isolate with Naκ-carrageenan

Gelling mixtures of Na⁺κ-carrageenan with whey protein isolate (WPI) at pH 7.0 have been studied rheologically and by differential scanning calorimetry (DSC), with comparative measurements for the individual constituents of the mixtures. The concentration of WPI was held fixed at 10.0 wt% and carrageenan concentration was varied in the range 0.05–3.0 wt%. Ca²⁺ cations, which have been shown previously to be particularly effective in inducing gelation of κ-carrageenan, were introduced as CaCl₂. The concentration of CaCl₂ used in most of the experiments was 8 mM, but other concentrations were also studied. Mixtures were prepared in the solution state at 45 °C, and showed no evidence of either phase separation or complex formation. Rheological changes were monitored by low-amplitude oscillatory measurements of storage modulus, G′, during (i) cooling (1 °C/min) and holding at 5 °C, to induce gelation of the carrageenan in the presence of non-gelled WPI; (ii) heating and holding at 80 °C to dissociate the carrageenan network and induce gelation of WPI; (iii) cooling and holding again at 5 °C, to give composite networks with both components gelled; and (iv) re-heating to 80 °C to dissociate the carrageenan network. Gel structure was characterised further by creep–recovery measurements at the end of each holding period, and by torsion measurements at 5 °C, before and after thermal gelation of WPI.     

Characterisation of binding of iron to sodium caseinate and whey protein isolate

The fortification of dairy products with iron is an important approach to delivering iron in required quantities to the consumer. The binding of iron (ferrous sulfate) to two commercial milk protein products, sodium caseinate and whey protein isolate (WPI), dissolved in 50 mM HEPES buffer, was examined as a function of pH and iron concentration. Sodium caseinate had more sites (n = 14) than WPI (n = 8) for binding iron, and the affinity of caseinate to bind iron was also higher than that of WPI. These differences were attributed to the presence of clusters of phosphoserine residues in casein molecules, which are known to bind divalent cations strongly. The amount of iron bound to sodium caseinate was found to be independent of pH in the range 5.5–7.0, whereas acidification (pH range 7.0–3.0) caused a marked decrease in the amount of iron bound to WPI.     

Effects of hydrogen peroxide and Fenton’s reagent on the properties of film from cuttlefish (Sepia pharaonis) skin gelatin

Properties of film from cuttlefish (Sepia pharaonis) ventral skin gelatin without and with partial hydrolysis (1.2% degree of hydrolysis), as influenced by H₂O₂ and Fenton’s reagent at different levels, were investigated. Films treated with H₂O₂ (0.01–0.04 M) and Fenton’s reagent [H₂O₂ (0.01–0.04 M) + FeSO₄ (0.001–0.004 M)] had higher tensile strengths (TS) but similar or lower elongations at break (EAB), compared with the control film (p < 0.05). Slight differences in water vapour permeability (WVP) were observed for all films. Films treated with Fenton’s reagent had a lower L^∗-value but higher a^∗-, b^∗- and ΔE^∗-values, while films treated with H₂O₂ had lower b^∗-values (p < 0.05), than had the control film. Cross-linking was pronounced in films treated with H₂O₂ or Fenton’s reagent and was associated with increased heat stability. Films treated with Fenton’s reagent had the lowest solubility in water (p < 0.05). However, fragmentation more likely took place when Fenton’s reagent (at a higher level) was used. Generally, similar results were noticeable between films from gelatin with and without partial hydrolysis. Thus, H₂O₂ and Fenton’s reagent directly affected the properties of film from cuttlefish skin gelatin, regardless of hydrolysis.     

Emulsion properties of algae soluble protein isolate from Tetraselmis sp.

To study possible applications of microalgae proteins in foods, a colourless, protein-rich fraction was isolated from Tetraselmis sp. In the present study the emulsion properties of this algae soluble protein isolate (ASPI) were investigated. Droplet size and droplet aggregation of ASPI stabilized oil-in-water emulsions were studied as function of isolate concentration (1.25–10.00 mg/mL), pH (3–7), and ionic strength (NaCl 10–500 mM; CaCl₂ 0–50 mM). Whey protein isolate (WPI) and gum arabic (GA) were used as reference emulsifiers. The lowest isolate concentrations needed to reach d₃₂ ≤ 1 μm in 30% oil-in-water emulsions were comparable for ASPI (6 mg/mL) and WPI (4 mg/mL). In contrast to WPI stabilized emulsions ASPI stabilized emulsions were stable around pH 5 at low ionic strength (I = 10 mM). Flocculation only occurred around pH 3, the pH with the smallest net droplet ζ-potential. Due to the charge contribution of the anionic polysaccharide fraction present in ASPI its droplet ζ-potential remained negative over the whole pH range investigated. An increase in ionic strength (≥100 mM) led to a broadening of the pH range over which the ASPI stabilized emulsions were unstable. GA emulsions are not prone to droplet aggregation upon changes in pH or ionic strength, but much higher concentrations are needed to produce stable emulsions. Since ASPI allows the formation of stable emulsions in the pH range 5–7 at low protein concentrations, it can offer an efficient natural alternative to existing protein–polysaccharide complexes.     

Structure-modifying alkaline and acidic pH-shifting processes promote film formation of soy proteins

The effect of pH-shifting, a process that induces the molten globule state in proteins, on the film-forming potential of soy protein isolate (SPI) at different temperatures was investigated. Partial unfolding at pH 1.5 or 12, followed by refolding at pH 7.0, was performed to alter the protein structure. Glycerin-plasticised films were prepared from pH-treated SPI at ambient temperature (20 °C), or by heating at 50, 60, 70, or 80 °C (30 min). Tensile strength (TS), elongation at break (EAB), water vapour permeability (WVP), protein solubility (pH 3–7), and non-participating proteins of films were analysed, and the film microstructures were examined. The pH₁₂-treated SPI spontaneously formed a transparent, slightly yellowish film at 20 °C, which had the greatest EAB, while pH_1.5-treated and native SPIs required preheating at 50 and 70 °C, respectively, to form a film. Heating generally decreased solubility and WVP but increased TS. Films formed from both pH₁₂- and pH_1.5-treated SPIs were more elastic (up to 2-fold greater in EAB, P < 0.05) than the film formed from untreated SPI despite slightly reduced TS and WVP. Electrophoresis revealed disulphide bonds between A and B subunits of glycinin being a dominant force in pH₁₂- and pH_1.5-treated SPI films, while noncovalent forces were abundant in untreated SPI films. The pH₁₂-treated SPI film consisted of more interactive protein strands than other SPI films, which seemed to explain its superior elastic properties.     

Physicochemical properties of lactoferrin stabilized oil-in-water emulsions: Effects of pH,salt and heating

Lactoferrin is a globular protein from bovine milk with an unusually high isoelectric point (pI > 8), which may lead to novel functional properties in foods and other products because it is cationic across a wide pH range. In this study, we investigated the influence of pH (2–9), NaCl addition (0–200 mM), CaCl₂ addition (0–200 mM), and thermal processing (30–90 °C, 20 min) on the stability of lactoferrin (LF) stabilized oil-in-water emulsions. At ambient temperature, the emulsions were stable to droplet aggregation at low pH (pH ≤ 6), but exhibited some aggregation at pH ≈ pI (pH 7–9). The thermal stability of the emulsions depended on pH, holding temperature, and thermal history. When LF-coated droplets were heated in distilled water, and then their pH was adjusted in the range 2–9, they were highly unstable to aggregation at pH 7 and 8. On the other hand, when the pH was altered in the range 2–9 first, and then they were heated, the LF-coated droplets were highly unstable to aggregation at pH ≥ 5 when heated above 50 °C. The stability of the emulsions to salt addition depended on pH and salt type, which was attributed to counter-ion binding and electrostatic screening effects. For NaCl, emulsions were stable from 0 to 200 mM at pH 3 and 9, but aggregated at ≥100 mM at pH 6. For CaCl₂, emulsions were stable from 0 to 200 mM at pH 3, but aggregated with ≥150 mM CaCl₂ at pH 6 and 9. These results have important implications for the formulation and production of emulsion-based products using lactoferrin as an emulsifier.     

Impact of surface deposition of lactoferrin on physical and chemical stability of omega-3 rich lipid droplets stabilised by caseinate

There is considerable interest in developing delivery systems to encapsulate and protect chemically labile lipophilic food components, such as omega-3 rich oils. In this study, multilayer emulsion-based delivery systems were prepared consisting of omega-3 rich oil droplets coated by either caseinate (Cas) or lactoferrin–caseinate (LF–Cas). Surface deposition of LF onto Cas-coated oil droplets was confirmed by ζ-potential measurements. Emulsions containing lactoferrin and caseinate had better physical stability to pH changes and salt addition (pH 3–7, 0–50 mM CaCl₂ at pH 7) than those containing only caseinate (pH 5–7, 0–2 mM CaCl₂ at pH 7). The addition of LF also retarded the formation of lipid oxidation markers (hydroperoxides and thiobarbituric acid reactive substances) in the emulsions. The ability of LF to enhance both the physical and chemical stability of protein-stabilised emulsions is useful for the fabrication of delivery systems designed for utilisation within the drug and food industries.     

Chemical, Physical, and Gel-forming Properties of Oxidized Myofibrils and Whey- and Soy-protein Isolates

Myofibrils, oxidized with FeCl₃/H₂O₂/ascorbate, exhibited an increase in carbonyls and amines, SH→SS conversion, peptide scission, myosin polymerization, and a decrease in thermal stability and gel‐formation ability. Amino‐acid side chains of whey‐protein isolates (WPI) and soy‐protein isolates (SPI) were also modified during oxidation, but the thermal stability of WPI or SPI was not significantly altered. Oxidation increased elasticity of SPI gel but not that of WPI gel. Similarly, oxidation promoted interactions of myofibrils with SPI but not with WPI, resulting in > 30% increases in elasticity of the myofibril/SPI composite gel over its nonoxidized control. Hence, in processed meats where oxidation occurs, the presence of soy proteins may enhance the functionality of myofibrillar proteins.     

Influence of interfacial composition on oxidative stability of oil-in-water emulsions stabilized by biopolymer emulsifiers

The objective of this research was to evaluate the influence of storage pH (3 and 7) and biopolymer emulsifier type (Whey protein isolate (WPI), Modified starch (MS) and Gum arabic (GA)) on the physical and oxidative stability of rice bran oil-in-water emulsions. All three emulsifiers formed small emulsion droplets (d₃₂ < 0.5 μm) when used at sufficiently high levels: 0.45%, 1% and 10% for WPI, MS and GA, respectively. The droplets were relatively stable to droplet growth throughout storage (d₃₂ < 0.6 μm after 20 days), although there was some evidence of droplet aggregation particularly in the MS-stabilized emulsions. The electrical charge on the biopolymer-coated lipid droplets depended on pH and biopolymer type: −13 and −27 mV at pH 3 and 7 for GA; −2 and −3 mV at pH 3 and 7 for MS; +37 and −38 mV at pH 3 and 7 for WPI. The oxidative stability of the emulsions was monitored by measuring peroxide (primary products) and hexanal (secondary products) formation during storage at 37 °C, for up to 20 days, in the presence of a pro-oxidant (iron/EDTA). Rice bran oil emulsions containing MS- and WPI-coated lipid droplets were relatively stable to lipid oxidation, but those containing GA-coated droplets were highly unstable to oxidation at both pH 3 and 7. The results are interpreted in terms of the impact of the electrical characteristics of the biopolymers on the ability of cationic iron ions to interact with emulsified lipids. These results have important implications for utilizing rice bran oil, and other oxidatively unstable oils, in commercial food and beverage products.     

Effect of microbial transglutaminase treatment on thermal stability and pH-solubility of heat-shocked whey protein isolate

Whey protein isolate (WPI) dispersions (5% protein, pH 7.0) were subjected to heat-shock at 70 °C for 1, 5 and 10 min. The heat-shocked WPI dispersions were treated with microbial transglutaminase (MTGase) enzyme, and thermal properties and pH-solubility of the treated proteins were investigated. Heat-shocking of WPI for 10 min at 70 °C increased the thermal denaturation temperature (T_d) of β-lactoglobulin in WPI by about 1.5 °C. MTGase treatment (30 h, 37 °C) of the heat-shocked WPI significantly increased the T_d of β-lactoglobulin by about 6.3–7.3 °C when compared with heat-shocked only WPI at pH 7.0. The T_d increased by about 13–15 °C following pH adjustment to 2.5; however, the T_d of heat-shocked WPI was not substantially different from heat-shocked and MTGase-treated WPI at pH 2.5. Both the heat-shocked and the heat-shocked-MTGase-treated WPI exhibited U-shaped pH-solubility profiles with minimum solubility at pH 4.0–5.0. However, the extent of precipitation of MTGase-treated WPI samples at pH 4.0–5.0 was much greater than all heat-shocked and native WPI samples. The study revealed that while MTGase cross-linking significantly enhanced the thermal stability of β-lactoglobulin in heat-shocked WPI, it caused pronounced precipitation at pH 4.0–5.0 via decreasing the hydrophilic/hydrophobic ratio of the water-accessible protein surface.     

Oxidation in HiOx-packaged pork Longissimus muscle predisposes myofibrillar and sarcoplasmic proteins to N-nitrosamine formation in nitrite-curing solution

The effect of meat protein in situ oxidation on the formation of N-nitrosodiethylamine (NDEA) was investigated. Fresh minced pork was untreated (Con) or treated with 700 mg/kg α-tocopherol (Toc) or 300 mg/kg tea polyphenols (PPE), packaged in a HiOx atmosphere (78.8% O₂, 18.8% CO₂, 2.4% N₂), then stored at 2 ± 1 °C for up to 10 days. Crude myofibrillar (MP) or sarcoplasmic (SP) protein (20 mg/mL) extracted from stored meat was reacted with 43 μM sodium nitrite at 80 °C for 1 h. Lipid oxidation was totally inhibited in PPE pork but increased in Con and Toc samples after 10 days. There was significant protein oxidation (losses of sulfhydryls, formation of protein carbonyls) in both MP and SP in all samples during storage. However, the Con group suffered more extensive protein oxidation than Toc and PPE and produced more NDEA (P < 0.05), indicating that protein oxidation promoted nitrosation.