Hemoglobin-mediated lipid oxidation of protein isolates obtained from cod and haddock white muscle as affected by citric acid,calcium chloride and pH

Acid or alkali solubilization followed by isoelectric precipitation can be used to isolate proteins with good functional properties from muscle tissue. Both acid (pH 3.0) and alkali (pH 10.5) treatment of the muscle decreased lipid oxidation catalyzed by hemoglobin. Citric acid and calcium chloride improved the oxidative stability of both acid- and alkali-solubilized muscle protein isolates when added to the homogenized muscle before separating the membrane or directly to the isolated membranes in the assay. Citric acid may have functioned in part by lowering the pH and destroying preformed peroxides. Exposing the muscle and the hemoglobin together at pH 3 promoted lipid oxidation, while addition of citric acid/calcium chloride or press juice to washed cod prior to solubilization inhibited lipid oxidation even when the tissue and hemoglobin were acidified together.     

Stability and antimicrobial efficiency of eugenol encapsulated in surfactant micelles as affected by temperature and pH

Growth inhibition of four strains of Escherichia coli O157:H7 (H1730, F4546, 932, and E0019) and Listeria monocytogenes (Scott A, 101, 108, and 310) by eugenol encapsulated in water soluble micellar nonionic surfactant solutions (Surfynol 485W) adjusted to pH 5, 6, and 7 and incubated at 10, 22, and 32 degrees C was determined. Concentrations of eugenol ranged from 0.2 to 0.9% at a surfactant concentration of 5%. Antimicrobial activity was assessed using a microbroth dilution assay. Eugenol encapsulated in surfactant micelles inhibited both microorganisms at pH 5, 6, and 7. At pH 5, some inhibition occurred in the absence of eugenol, i.e., by the surfactant itself (optical density at 24 h for L. monocytogenes = 0.07 and optical density at 24 h for E. coli O157:H7 = 0.09), but addition of >0.2% eugenol led to complete inhibition of both microorganisms. Inhibition of L. monocytogenes and E. coli O157:H7 decreased with increasing pH, that is, the minimum inhibitory concentration was 0.2, 0.5, and 0.5% of micellar encapsulated eugenol solutions at pH 5, 6, and 7, respectively. The encapsulated essential oil component in surfactant micelles was effective at all three temperatures tested (10, 22, and 32 degrees C), indicating that the activity of encapsulated eugenol was not affected by high or low (refrigeration) temperatures. Overall, strains of E. coli O157:H7 were more sensitive than strains of L. monocytogenes. Improved activity was attributed to increased solubility of eugenol in the aqueous phase due to the presence of surfactants and improved interactions of antimicrobials with microorganisms.     

The effect of NaCl on survival of Shigella flexneri in broth as affected by temperature and pH

Shigella, a major foodborne pathogen, survives well in salt-containing environments. However, systematic data are scarce. We studied the behavior of Shigella flexneri 5348 in brain heart infusion broth (pH 4 to 6) containing 0.5 to 8% NaCl. Stationary-phase cells were inoculated into sterile media at initial concentrations of 6 to 7 log10 CFU/ml and incubated at 12 to 37 degrees C. Bacterial population sizes were determined periodically by plate counts. Survivor curves were derived from plate count data by using a two-phase linear model to determine lag times and slopes of the curves, from which decimal reduction times (D-values) and times to a 4-log10 inactivation (T4D) were calculated. In media of pH 6, the bacteria grew in the presence of < or = 6% NaCl at 19 and 37 degrees C and in the presence of < or = 7% NaCl at 28 degrees C. In media of pH 5, growth was observed in the presence of < or = 2, < or = 4, < or = 4, and 0.5% NaCl at 37, 28, 19, and 12 degrees C, respectively. Growth did not occur and bacterial populations gradually declined in media of pH 4. While NaCl had a major effect on growth, bacterial survival was affected to a lesser extent. Lag times decreased with increasing NaCl levels; however, the effect on D-values and T4D values was less pronounced. The average T4D values for media of pH 4 containing 0.5 to 6% NaCl were 4, 13, 23, and 61 days at 37, 28, 19, and 12 degrees C, respectively. These results show that S. flexneri is salt tolerant and suggest that salty foods may serve as vehicles for infection with this bacterium.     

Amino acid composition,foaming, emulsifying properties and surface hydrophobicity of mustard protein isolate as affected by pH and NaCl

Amino acid composition, protein hydrophobicity, foaming and emulsifying properties of mustard protein isolate at pH 3, 5, 7 and in 0.05 and 0.1 m NaCl were studied. Glutamic (19.18 ± 0.30%) and aspartic (7.49 ± 0.11%) acids were the dominants. Foaming ability was enhanced by NaCl. Time to reach 75 mL foam was 23% higher in water than in NaCl. Drained volume after 10 min was concentration dependent and was the lowest in 0.05 and 0.1 m NaCl at protein concentration of 2.5% and 5%. The emulsifying properties were pH and concentration dependent, and the best results were obtained at pH 3, corresponding to the highest positive charge density of the protein surface. The highest emulsion stability (90.22 ± 3.52%) was obtained in 0.05 m NaCl and 5% protein concentration, whereas the lowest (63.00 ± 1.06%) was in water at all protein concentrations. Protein hydrophobicity was low and depended of pH but not of NaCl.     

酯化改性淀粉纳米晶稳定的Pickering乳液及其油脂氧化稳定性

通过辛烯基琥珀酸酐（OSA）与淀粉纳米晶（SNC）的酯化反应获得OSA改性淀粉纳米晶（OSA-SNC）,以OSA-SNC为乳化剂,橄榄油为油相,制备出Pickering乳液,并探讨了OSA-SNC浓度、水相p H对乳液稳定性的影响。此外,通过与Tween-20稳定的乳液进行对比,研究了乳化剂浓度、水相p H对乳液油脂氧化稳定性的影响。研究表明,Pickering乳液的稳定性随着乳化剂浓度或者水相p H的增加而得到提高。OSA-SNC稳定乳液的油脂氧化稳定性显著优于Tween-20稳定的乳液;随着乳化剂浓度的增加,Tween-20稳定乳液的POV升高,而OSA-SNC稳定乳液的POV降低;Tween-20稳定乳液的POV随着水相p H的增大先降低再升高,而OSA-SNC稳定乳液的POV随着水相p H的增大而升高。      

食品乳状液中脂质氧化的影响因素及抑制方法

脂质氧化是造成食品乳状液质量下降的一个主要原因。乳化体系中脂质氧化的机理及影响氧化的因素与均相体系中脂质的氧化有很大不同。综述了油脂、氧、抗氧化剂、界面特性、油滴大小以及体系中各成分对食品乳状液中脂质氧化的影响,对于了解食品乳状液中脂质氧化的机理和有效防止食品乳状液中脂质氧化具有实际意义。     

Inhibition of lipid oxidation by encapsulation of emulsion droplets within hydrogel microspheres

The purpose of this study was to assess whether the oxidation of polyunsaturated lipids could be inhibited by encapsulating them within protein-rich hydrogel microspheres (size range 1-100 μm). Filled hydrogel microspheres were fabricated as follows: (i) high methoxy pectin, sodium caseinate, and casein-coated lipid droplets were mixed at pH 7, (ii) the mixture was acidified (pH 5), (iii) casein was cross-linked using transglutaminase, (iv) the pH was adjusted to pH 7. Samples were stored in the dark at 55 °C and were monitored for lipid hydroperoxide formation and headspace propanal. Oxidation of fish oil (1% vol/vol) in the microspheres was compared with that in oil-in-water emulsions stabilised by either sodium caseinate or Tween 20. Emulsions stabilised by Tween 20 oxidised faster than either microspheres or emulsions stabilised by casein, while microspheres and the casein stabilised emulsion showed similar oxidation rates. Results highlight the natural antioxidant properties of food proteins.     

Protein and lipid oxidation in Longissimus dorsi and dry cured loin from Iberian pigs as affected by crossbreeding and diet

Lipid and protein oxidation in Longissimus dorsi (LD) and dry-cured loins from pigs with different genetic (pure Iberian (IBP), Iberian female × Duroc male (IB × D) and Duroc female × Iberian male (D × IB)) and feeding backgrounds (free rearing on acorn and pasture (MON), concentrates high in oleic acid and supplemented with 250 ppm of vitamin E(HOVE) and control concentrates (CON)) were investigated. Diet influenced the fatty acids profile from PL and α- and γ-tocopherol contents of LD. IBP–MON pigs showed the lowest malonaldehyde (MDA) values at 200 min of iron induced muscle oxidation. Dry-cured loins from IBP–HOVE pigs had significantly (p < 0.05) higher values of TBARS than those from the other batches. Neither the diet nor crossbreeding affected hexanal counts in dry-cured loins. Protein carbonyl content showed a similar trend to that observed for MDA values in LD, suggesting a protective role of tocopherol against lipid and protein oxidation. The positive and significant correlations between iron induced lipid oxidation in LD (200 min) and carbonyl content in LD and dry-cured loin (R²: 0.55 and R²: 0.52, respectively, p < 0.01) support the relationship between lipid and protein oxidation.     

Lipid oxidation in chicken as affected by cooking and frozen storage

Oxidative rancidity in fresh, frozen and cooked chicken breast and leg meat was evaluated by measuring malondialdehyde (MDA) in fat from meat with an improved 2-thiobarbituric acid (TBA) assay with antioxidant protection, and by measuring the relative fluorescent products of organic and aqueous layers from Folch extracted meat. Fresh samples were frozen for 3 and 6 months at ?18 °C and cooked in convection and microwave ovens. Frozen storage for 3 and 6 months either before cooking or after convection and microwave cooking substantially increased MDA concentrations in fat from meat, whereas cooking was more effective in generating fluorescent products. There were no significant differences in free MDA concentrations or TBA numbers in chicken meat between convection and microwave cooking methods. The certain secondary fluorescent products were significantly higher in meat cooked by convection oven. The initial levels of either MDA or fluorescent products in meat are of primary importance in determining the final MDA and fluorescence levels after processing.     

Muscle lipids, vitamins E and A, and lipid oxidation as affected by diet and RN genotype in female and castrated male Hampshire crossbreed pigs

The objectives of this study were to investigate the polar and neutral lipid fatty acid composition, content of retinol, -tocopherol, γ-tocopherol and level of oxidation of the pig muscle (M. longissimus dorsi). Female and castrated male Hampshire crossbreeds were produced in two systems. One group was raised indoors with a more polyunsaturated diet and the other raised outdoors with a more saturated diet. The level of polyunsaturated fatty acids in muscle was higher in the indoor females compared with the outdoor females, indoor castrated males and outdoor castrated males. The increased level of polyunsaturated fatty acids in the muscle, which was accompanied by a relatively low content of -tocopherol, increased the susceptibility to lipid oxidation in the form of MDA (malondialdehyde) in the indoor female pigs. Finally, the level of polyunsaturated fatty acids in the polar lipids was affected by the RN genotype, and this difference was dependent on sex. In conclusion, diet has a major effect on the fatty acid composition and oxidation stability in pork muscle, but additional factors such as sex and RN genotype might also contribute.     

Survival of staphylococcus aureus and Escherichia coli as affected by ethanol and NaCl

Growth of three strains of Staphylococcus aureus and two strains of Escherichia coli on nutrient agar (NA) supplemented with ethanol and NaCl was investigated. S. aureus did not grow on NA containing > or =10% ethanol (wt/wt) combined with > or =0% NaCl (wt/wt), or 7.5% ethanol combined with 7.5% NaCl. Neither E. coli nor E. coli O157:H7 grew on NA containing > or =7.5% ethanol combined with > or =0% NaCl, 5% ethanol combined with > or =2.5% NaCl, or > or =5% NaCl combined with > or =0% ethanol. It is apparent that NaCl enhanced the inhibitory effect of ethanol on growth of S. aureus and E. coli When cells were suspended in nutrient broth containing 12.5, 20, or 40% ethanol combined with NaCl, viable cells decreased with an increase of ethanol concentration. Ethanol sensitivity among strains and between genera varied in a limited range. When the cells were exposed to 20% ethanol in combination with 5% NaCl, S. aureus and E. coli lost viability after 30 and 10 min, respectively. When treated with 40% ethanol combined with > or =0% NaCl, all test strains lost viability within 5 min.     

Survival,growth, and inactivation of acid-stressed Shigella flexneri as affected by pH and temperature

A study was done to determine the survival, growth, and inactivation characteristics of unadapted, acid-adapted, and acid-shocked Shigella flexneri 2a cells as affected by pH and temperature. The pathogen was grown at 37 degrees C for 18 h in tryptic soy broth containing no glucose (TSBNG) (unadapted cells) and TSBNG supplemented with 1% glucose (TSBG) (acid-adapted cells). Cells grown in TSBNG were acid-shocked by adjusting 18-h cultures to pH 4.5+/-0.05 with lactic acid. All three cell types were separately inoculated into tryptic soy broth (6.6-7.0 log(10) cfu/ml) containing 0.25% glucose (TSB) acidified to pH 3.5-5.5 with lactic acid and incubated at 4, 12, 21, 30, and 48 degrees C for up to 144 h. Overall, inactivation of S. flexneri cells at low pH was enhanced with an increase in incubation temperature. All three types of cells survived for 144 h at 4 degrees C in TSB acidified to pH 3.5, compared to < 24 h at 30 degrees C and 2 h at 48 degrees C. The population of all three cell types increased significantly (alpha = 0.05) within 24 h when cells were incubated at 12, 21, or 30 degrees C in TSB at pH 5.0, 5.5, or 7.3. Prior exposure of the S. flexneri to an acidic environment (acid-adapted or acid-shocked cells) resulted in increased resistance to extreme acid and temperature conditions. Acid-adapted cells decreased by approximately 2.5 log(10) cfu/ml when incubated at 4 degrees C for 144 h, compared to a 6-log(10) reduction in control (unadapted) cells. When cells were exposed to low pH (3.5-4.5) and high temperature (48 degrees C), significantly higher (alpha = 0.05) populations were recovered on tryptic soy agar (TSA) than on TSA supplemented with 4% NaCl (TSAS), indicating that a portion of S. flexneri cells were injured. Results show that the ability of S. flexneri to survive and grow at a given pH is influenced by previous exposure to acidic environments and by incubation temperature.     

Buffer pH and pKa values as affected by added glycerol and sucrose

Many properties of foods and beverages depend upon being formulated to the proper pH. Should this pH shift unexpectedly, the stability and quality of the product could be affected. Although a pH-lowering effect of sucrose added to phosphate buffer has been observed, an explanation for the phenomenon has not been provided. The purpose of this study was to determine why the addition of non-ionic polyols changed buffer pH. Titration curves of citrate and phosphate buffers, with and without added sucrose or glycerol were constructed. Apparent pK_a values were determined from titration curves and by measuring the pH values of equimolar buffer solutions. Sucrose appeared to lower the apparent pK_a of phosphate buffer, but not citrate buffer. Glycerol appeared to have no effect on the apparent pK_a value of phosphate buffer while causing a slight increase in the values for citrate buffer. These results parallel the effects of the polyols on buffer pH; the largest change was a pH decrease from adding sucrose to phosphate buffer. A pK_a change appears responsible for this pH-lowering effect, possibly due to differences in solute–water interactions and changes to the hydration sphere around the buffer anions. Potential pH changes from food additives should be recognized during product development in order to optimize product quality and shelf life.     

Growth kinetics and cell morphology of Listeria monocytogenes Scott A as affected by temperature,NaCl, and EDTA

Growth kinetics and morphological characteristics of Listeria monocytogenes Scott A grown under stress conditions induced by increasing levels of NaCl and EDTA were studied as a function of temperature. L. monocytogenes Scott A was inoculated into brain heart infusion broth (pH 6) at 19, 28, 37, and 42 degrees C. Test cultures contained NaCl (at concentrations of 4.5, 6.0, and 7.5%) or EDTA (at concentrations of 0.1, 0.2, and 0.3 mM); control cultures contained 0.5% NaCl. Growth curves were fitted from plate count data by the Gompertz equation, and growth kinetics parameters were derived. Stationary-phase cells were examined by scanning and transmission electron microscopy. Generation times (GTs) and lag phase duration times (LPDs) increased as additive levels were increased. The bacterium grew at all NaCl levels. At 37 and 42 degrees C, growth was slow in media containing 7.5% NaCl, and no growth occurred in media containing 0.3 mM EDTA. Temperature was a major factor in certain stress conditions that led to cell elongation and loss of flagella. Cells in control media at 28 degrees C grew as short rods (0.5 by 1.0 to 2.0 microm), while at 42 degrees C most cells were 4 to 10 times as long. Higher levels of NaCl at higher temperatures resulted in longer and thicker cells. At 28 degrees C, 0.1 mM EDTA had little effect on growth kinetics and morphology; however, 0.3 mM EDTA caused a sixfold increase in GT and LPD and loss of flagellae, with most cells being two to six times as long as normal. Cell length did not correlate with growth kinetics. The results of this study suggest that the effect of altered morphological characteristics of L. monocytogenes cells grown under stress on the virulence and subsequent survival of these cells should be investigated.     

Oxidative stability of soybean oil in oleosomes as affected by pH and iron

The oxidative stability of oil in soybean oleosomes, isolated using the Enzyme-Assisted Aqueous Extraction Process (EAEP), was evaluated. The effects of ferric chloride, at two concentration levels (100 and 500 μM), on lipid oxidation, was examined under pH 2 and 7. The peroxide value (PV) and thiobarbituric acid-reactive substance (TBARS) value of oil, in oleosome suspensions stored at 60 °C, were measured over a 12 day period. The presence of ferric chloride significantly (P < 0.05) affected the oxidative stability of oil in the isolated oleosome, as measured by the PV and TBARS. Greater lipid oxidation occurred under an acidic pH. In the pH 7 samples, the positively charged transition metals were strongly attracted to the negatively charged droplets. However, the low ζ-potential and the high creaming rate at this pH, may have limited the oxidation. Freezing, freeze–drying or heating of oleosomes have an insignificant impact on the oxidative stability of oil in isolated soybean oleosomes. Manufacturers should be cautious when adding oleosomes as ingredients in food systems containing transition metal ions.     

Colour and lipid oxidation changes in dry-cured loins from free-range reared and intensively reared pigs as affected by ionizing radiation dose level

The effect of irradiation (0, 5 and 10 kGy) of vacuum-packaged Iberian dry-cured loin slices from pigs fed on concentrate (CON) or free-range reared (FRG) was studied in relation to colour changes, TBA-RS and hexanal content. Both, ionizing radiation and type of loin had a significant effect on the instrumental colour parameters of the samples. Irradiation resulted in significantly higher a*-values in both sets of loins, indicating a redder colour. Numerically calculated total colour difference (ΔE) changes were significantly less intense in CON vacuum-packaged dry-cured loin slices than in FRG samples and changed significantly at 10 kGy dose levels in both types of samples. TBA-RS numbers were significantly affected by irradiation dose and type of loin and increased linearly with dose in both types of slices. Increments in TBA-RS numbers in FRG loin slices was dose-dependent and was closely related to the type of dry-cured loin. Irradiation of dry-cured loin slices significantly increased hexanal contents in both groups of loins and the increases were dose-dependent and greater in FRG samples than in CON samples. Differences in the characteristics of the raw material and initial lipid oxidation level could play an important role in the irradiation-induced changes in vacuum-packaged dry-cured loin slices.     

NaCl浓度和pH对肉糜中脂肪微粒蛋白吸收量及凝胶特性的影响

研究了不同浓度(0.2mol/L和0.6mol/L)NaCl和pH(6.0和6.5)对肉糜中脂肪微粒表面蛋白吸收量、乳化稳定性、凝胶硬度和微观结构的影响。研究结果表明,高浓度(0.6mol/L)NaCl能够显著提高肉糜中脂肪微粒表面蛋白吸收量和单位界面膜蛋白吸收量,有效地改善肉糜乳化稳定性、凝胶硬度和微观结构等指标(P<0.05)。但在相同浓度(0.2mol/L和0.6mol/L)NaCl条件下,较大pH(6.0和6.5)对脂肪微粒表面蛋白吸收量、单位界面膜蛋白吸收量、乳化稳定性、凝胶硬度和微观结构等指标无作用效果(P>0.05)。因此,本实验认为0.6mol/LNaCl(pH6.0和6.5)的提取条件下能够充分剪切肌肉蛋白和脂肪,肉糜乳化性能好。      

Impact of surfactant type, pH and antioxidants on the oxidation of methyl linoleate in micellar solutions

The oxidation of methyl linoleate micelles has been studied, aiming at elucidating the effect of various variables, including surfactant type, pH and antioxidants. The progress of the methyl linoleate oxidation was evaluated by measurement of conjugated dienes hydroperoxides (CDHP) and malondialdehyde (MDA). It was shown that the oxidative stability of methyl linoleate micelles was influenced by surfactant type, with oxidative rate being greater in SDS micelles than in Tween 20 micelles. Besides, the methyl linoleate micelles at pH 6.8 had greater rates of lipid oxidation than their pH 3.0 counterparts. Moreover, the incorporation of VE and VC in the methyl linoleate micelles successfully slowed the formation of hydroperoxides and their subsequent decomposition product MDA. However, the antioxidant activities of VE and VC were related to their concentrations.