类PSE（Pale，Soft，and Exudative）鸡肉指的是具有肉色苍白、质软、汁液易流失等特点的一类异质肉，作为原料生产深加工制品，其凝胶和乳化特性差、出品率低，给肉制品加工产业造成巨大经济损失。酸碱处理（ISP）技术能修饰天然蛋白的原始结构，对凝胶和乳化特性产生影响，在肌肉蛋白加工领域具有潜在应用价值。然而，ISP技术用于改善类PSE鸡肉蛋白加工特性的有效处理条件还未建立，其中具体作用机制也尚不清晰。因此，探究ISP技术改善类PSE肉蛋白加工特性的作用及机制具有重要意义。本文首先研究不同溶解pH处理条件下类PSE鸡肉蛋白的凝胶特性，并分析在优势溶解pH条件下乳化凝胶品质变化，进一步筛选最适回收pH，综合明确用于改善类PSE肉蛋白凝胶特性的ISP处理条件；其次研究ISP处理对类PSE鸡肉蛋白的构象变化、聚集特性以及物化特性的影响，分析非功能性肌浆蛋白经ISP处理后在肌原纤维蛋白凝胶形成过程中的贡献作用，综合阐明ISP处理改善类PSE肉蛋白凝胶特性的内在机制；最后探究ISP处理后类PSE肉蛋白的氧化稳定性和体外模拟消化特性。综合上述研究，为ISP技术改善类PSE鸡肉蛋白的凝胶特性提供理论依据，并为ISP分离蛋白在食品中的应用提供新的思路。具体研究内容与结果如下：

1. 不同溶解pH条件对类PSE鸡肉分离蛋白凝胶特性的影响

为建立有效ISP策略用于改善类PSE鸡肉蛋白的凝胶特性，首先确定不同pH条件下蛋白的溶解度曲线，进而研究极酸性和极碱性pH作为溶解条件时类PSE鸡肉ISP分离蛋白的蛋白盐溶特性、总蛋白及盐溶蛋白组分，以及热诱导凝胶质构、保水性以及颜色特性。结果表明：在pH 2.0-12.0范围内溶解度曲线呈经典U型曲线，为保证蛋白回收率，选择pH 2.5-3.5，11.0-12.0为溶解pH作为后续研究条件。各处理组间总蛋白组分差异不明显，但ISP分离蛋白中盐溶蛋白组分与对照组发生明显改变，其中碱性pH相对酸性pH与对照组更为相近。碱性pH 11.0及酸性pH 3.5处理得到的类PSE分离蛋白的凝胶硬度由2158 N分别显著提高至2967 N及2441 N，蒸煮损失由8.9%显著下降（P< 0.05）至4.8%及6.5%，但凝胶弹性无显著变化（P> 0.05）。其余碱性条件对凝胶硬度均有积极影响，但酸性pH 2.5和3.0处理的类PSE肉蛋白凝胶质构特性下降。此外，pH 2.5及碱性pH处理均能显著提高蛋白凝胶的白度（P< 0.05），其中pH 3.5组白度值最高，并与11.0组无显著差异（P> 0.05）。综上，酸性溶解pH 3.5及碱性溶解pH 11.0均能提高类PSE鸡肉蛋白凝胶的质构、保水及颜色特性。

2. ISP处理对类PSE鸡肉蛋白O/W乳化凝胶特性的影响

为阐明ISP处理对不同蛋白浓度类PSE鸡肉蛋白乳化凝胶的影响，分别制备50、100和150 mg/g的ISP分离蛋白及类PSE鸡肉糜/大豆油O/W蛋白乳液，研究蛋白乳液的物理化学和流变学特性，比较热诱导乳化凝胶的质构特性、汁液保持能力以及颜色特性。结果如下：与对照组相比，ISP处理组在冷藏72 h过程中乳液稳定性上升，且乳滴分布更加细小均匀，分散性更强。所有处理组蛋白乳液表观粘度均符合Ostwald-de-Waele模型分布（R2> 0.9），经过ISP处理后表征蛋白乳液粘度的稠度系数K值显著升高（P< 0.05）。根据频率扫描结果，所有乳液体系均表现出“强凝胶”特性。此外，ISP处理组在加热过程中表现出与对照组不同的流变行为。蛋白浓度过高不利于ISP分离蛋白乳化凝胶的形成，即当蛋白浓度为50和100 mg/g时，ISP组凝胶硬度值显著较高于对照组（P< 0.05），然而当蛋白浓度为150 mg/g时，趋势相反。当对照组蛋白浓度从50 mg/g增加至150 mg/g时，对照组的总汁液流失（TEF）从30.3%持续下降到12.7%；然而，在ISP处理组中，当蛋白浓度从50 mg/g增加至100 mg/g时，TEF从21.9%显著下降至13.4%（P< 0.05），继续增加蛋白浓度至150 mg/g时，TEF则小幅上升至16.1%（P< 0.05）。总体而言，ISP处理能增加类PSE鸡肉蛋白乳液体系粘度，使乳液均匀分布，提高乳液稳定性，改善乳化凝胶质构和汁液保持能力；但是蛋白浓度过高（150 mg/g）对ISP分离蛋白乳化凝胶品质会有不利影响。

3. 不同回收pH条件对类PSE鸡肉分离蛋白凝胶特性的影响

为进一步优化ISP处理策略，提高ISP分离蛋白凝胶特性，选择鸡肉蛋白等电点pH 5.5及相对远离等电点pH 6.2为不同回收条件，以第一章中确定的酸性pH 3.5和碱性pH 11.0为溶解条件，比较四种ISP处理策略对回收率、蛋白溶解特性、溶解蛋白组分、体系粘度、聚集特性、表面疏水性以及凝胶特性的影响。结果表明：pH 6.2回收条件会导致回收率小幅下降（P< 0.05）；回收pH相较溶解pH对ISP分离蛋白中盐溶蛋白及水溶蛋白组分的影响小；碱处理过程虽然导致更多水溶蛋白流失，但是盐溶蛋白比例上升，电泳结果表明ISP处理过程导致肌球蛋白轻链的丢失。与pH 5.5回收处理组相比，pH 6.2回收组表面Zeta电位值显著降低（P< 0.05），导致更高程度的蛋白聚集，聚集体粒径显著增加（P< 0.05）；此外，pH 6.2回收组热诱导凝胶网络结构更加均匀，质构硬度显著升高（P< 0.05）。综合上述结果，碱性pH 11.0溶解结合pH 6.2回收条件能够更有效促进类PSE鸡肉蛋白凝胶网络形成，增加凝胶质构特性及保水性。

4. ISP处理改善类PSE鸡肉分离蛋白凝胶特性的分子热聚集行为机制

为解析ISP处理改善类PSE蛋白凝胶特性的分子机制，研究ISP处理前后蛋白在加热过程中的结构变化以及热焓变情况，包括色氨酸荧光强度，总巯基含量，分子间作用力，水分分布特性和流变学特性，并采用傅里叶红外分析仪测定加热前后二级结构比例。结果表明：经过ISP处理后，肌球蛋白及肌动蛋白的吸热变性峰向更低（P < 0.05）温度迁移，表明ISP处理降低了蛋白的热稳定性；ISP处理后色氨酸荧光吸收强度下降；加热前总巯基含量显著降低（P < 0.05），但加热后酸处理组与对照组总巯基含量差异不显著（P > 0.05）；pH 6.2回收条件能增加热诱导凝胶储能模量G'。ISP处理后二级结构中α-螺旋含量显著下降（P < 0.05），经酸溶解处理后，pH 5.5及6.2回收组α-螺旋含量由40.8%分别下降至34.9%及32.7%，经碱溶解处理后，pH 5.5及6.2回收分别下降至28.9%及29.4%；然而，加热后各组间α-螺旋含量无显著差异。根据分子间作用力变化，对照组中疏水相互作用随加热持续增加，但是ISP处理组中增加幅度减小，其中碱溶解处理pH 6.2回收组45°C前疏水相互作用呈现小幅下降。综上，ISP处理会引发类PSE肉蛋白分子结构的改变，导致更多有序的二级结构向无序迁移，疏水相互作用增加，巯基氧化形成二硫键；ISP处理引起的蛋白变性可能对加热诱导的蛋白变性产生拮抗作用，减缓蛋白变性速度，从而改变热诱导凝胶形成行为。

5. ISP处理改善类PSE鸡肉分离蛋白凝胶特性的肌浆蛋白作用机制

为进一步阐明ISP处理改善类PSE鸡肉分离蛋白凝胶特性机制，首先比较不同ISP处理策略下肌浆蛋白的微观形态变化、聚集特性及分子间作用力等；进一步研究ISP分离肌浆蛋白（ISP-SP）对ISP处理肌原纤维蛋白（ISP-MP）凝胶的贡献作用，包括热变性温度、流变学行为、质构特性及保水性等。结果表明：经过ISP处理后，圆球颗粒形态的SP在分子间作用力驱动下形成松散的漂浮状聚集物，不同回收pH对聚集体粒径影响显著（P< 0.05），ISP处理后SP悬浊液的流动特性无显著变化。加热过程中分子间作用力的变化受到ISP处理的影响，维持聚集体的主要作用力是疏水相互作用以及少量二硫键作用。与对照组SP不同，ISP-SP组疏水相互作用在加热过程中变化较小，ISP-SP本身无法形成有秩序的弹性结构，这可能与SP的蛋白结构及氨基酸分布有关。ISP-SP添加不改变MP的流变学行为趋势。将SP与MP混合后，由于SP流动性强且粒径小，对MP凝胶形成几乎没有正面作用；在MP中添加ISP-SP后，凝胶破断力由91.7 g增加至293.7 g，在ISP-MP中添加ISP-SP，凝胶破断力由275.7 g增加至304.5 g。此外，在MP中添加ISP-SP，可以使蒸煮损失由23.0%显著下降（P < 0.05）至6.8% ，有效减少水分流失。上述研究结果表明，ISP-SP添加在不影响MP热诱导凝胶形成秩序的前提下，能够显著增强类PSE鸡肉MP的凝胶特性。

6. ISP处理对类PSE鸡肉蛋白氧化稳定性的影响

为探究ISP处理对类PSE鸡肉蛋白氧化稳定性的影响，研究不同ISP处理条件下蛋白体系中促氧化剂含量，之后采用Fenton模拟氧化体系（Fe3+/H2O2），研究不同氧化强度（H2O2浓度分别为0，1和10 mM）对酸性及碱性ISP分离蛋白中羰基含量，水解氨基酸种类，巯基、席夫碱及自由氨基含量以及流变学特性的影响。结果表明：酸溶解处理及碱溶解处理均能够降低类PSE鸡肉体系中脂肪和色素类蛋白等促氧化剂含量，其中，酸溶解处理组中脂肪含量（g/100 g干物质）由对照组中6.21 显著下降（P < 0.05）至5.58，肌红蛋白含量（mg/g干物质）由3.08显著下降至0.37，去除效率更为显著。ISP处理对类PSE鸡肉中羰基含量和自由氨基损失均无显著影响（P > 0.05）；在氧化过程中，ISP分离蛋白产生更少的羰基衍生物，然而造成更高含量的巯基及自由氨基损失（P < 0.05）。在温和的氧化条件下（1 mM H2O2），ISP分离蛋白中含硫氨基酸更容易被转化形成二硫键。此外，ISP分离蛋白经10 mM H2O2氧化后，凝胶最终G'值更高。综合上述结果，经过ISP处理后促氧化剂含量下降，含硫氨基酸更易被氧化形成二硫键，但其他氧敏性氨基酸的氧化抵抗性增加，总体上在氧化环境中更能维持自身凝胶形成能力。

7. ISP处理对类PSE鸡肉蛋白体外消化特性的影响

为进一步探究ISP处理对类PSE鸡肉蛋白消化特性的影响，借助体外模拟胃肠消化体系，研究不同加热条件下（72 °C 20 min，100 °C 60 min）ISP分离蛋白的物化特性，胃肠阶段体外消化动力学特性以及各消化阶段蛋白组分变化等，并采用蛋白组学及多肽组学技术对各处理组间消化产物进行差异分析。结果表明：在高温长时间加热条件下，ISP处理增加保水及质构的优势更显著；与对照组相比，ISP处理显著改变蛋白结构，疏水基团充分暴露，然而，加热后蛋白结构差异较小，形成的热聚集体粒径大小也无显著差异（P > 0.05）；电泳结果表明，高温长时加热条件能够充分引起蛋白的氧化变性交联，形成非二硫键共价键。加热处理能够提高胃酶阶段的消化潜能和消化速度，与对照组相比，在模拟胃阶段，低温短时加热后ISP分离蛋白消化能力增加（P < 0.05），高温长时加热消化能力变化不显著（P > 0.05）；在模拟肠道阶段，除低温短时处理组外，不同处理组间消化特性相似；高温长时处理后，对照组与ISP处理组的最终消化产物相似，然而，低温短时处理后，，对照组与ISP处理组的最终消化产物差异显著；高温长时处理可能会导致肌浆蛋白与肌原纤维蛋白间形成交联聚集，降低其消化率。与对照组相比，ISP分离蛋白在消化后表现出更高的多肽丰度。综上可知，高温长时间加热下ISP分离蛋白凝胶特性最佳；低温短时加热能够增加ISP分离蛋白在模拟胃阶段的消化特性，提高消化产物多肽丰度，但在模拟肠道阶段与对照组无显著差异。

综上所述：ISP处理能够有效改善类PSE鸡肉蛋白的凝胶特性，溶解pH 11.0、回收pH 6.2时ISP分离蛋白的凝胶特性最佳。ISP处理能够通过修饰天然蛋白构象及调控聚集体形成影响蛋白热诱导凝胶形成行为，从而改善最终凝胶特性；ISP处理后的肌浆蛋白对提高类PSE鸡肉蛋白凝胶特性起到重要作用。在模拟氧化体系中，ISP分离蛋白表现出更好的氧化稳定性，有利于维持其天然凝胶形成能力；ISP处理结合低温短时加热能够提高类PSE鸡肉蛋白的消化特性，增加消化产物丰度。

High glycolytic potential and fast acidification rate, which are induced by pre-slaughter stress, co-lead to PSE-like chicken breast. As a kind of dysfunctional chicken fillets, PSE-like chicken obtained pale appearance, soft texture and high expressible moisture. As a raw material for meat product processing, it would lead to unsatisfied quality and low yield, which both resulted in economic loss. Isoelectric solubilization/precipitation (ISP) process has been applied in meat processing field. For now, ISP process was generally studied in marine-derived protein modification. It could change the native structural properties of protein by adjusting unfolding and refolding behavior, and affect the gelation and emulsification properties of protein, thus enhance the protein gel quality. Therefore, pursuing a proper ISP strategy for improving gelation properties of PSE-like chicken protein and elucidating the mechanism were meaningful.  In this paper, the effects of different solubilization pH on the quality of PSE chicken protein gel and emulsion gel were studied. Based on this, the optimal recovery pHs were studied to confirm the favorable ISP treatment strategy for improving the properties of PSE-like meat protein gel. Secondly, the conformational changes and aggregation properties of PSE-like meat proteins before and after ISP treatment were studied, and their physicochemical properties during heating process were compared. By analyzing the contribution of ISP-modified sarcoplasmic protein on the gelation properties of myofibrillar protein, the mechanism of ISP treatment on improving the properties of PSE gel was comprehensively elucidated. Finally, the oxidation properties and in vitrodigestibility of ISP-extracted protein were explored, providing a theoretical basis for the application of ISP isolated protein in meat product, and providing new ideas for improving the value of PSE-like chicken. Detailed contents and results are as follows:

1. Effects of solubilization pH on gelation properties of ISP-treated PSE-like chicken breast meat protein.

In order to establish an effective ISP strategy to improve the gel properties of PSE-like chicken protein, the solubility curves of proteins under different pH conditions were determined. Based on the results, the ISP-isolated protein was extracted, and the salt solubility, total protein and salt soluble protein profile of the ISP-isolated protein and the control PSE-like chicken meat paste were compared, while the gel texture, water retention and color properties of the heat-induced gel were also compared. The results showed that the solubility curve in the range of pH 2.0-12.0 was a classic U-shaped curve. In order to ensure the protein recovery, pH 2.5-3.5 and pH 11.0-12.0 were selected for subsequent study. There was no obvious difference in total protein profile among various treatment groups, but salt-solubilized protein profile of the ISP group was changed notably compared with the control group, and the alkali-extracted group was more similar than the acid group. The hardness of protein isolate treated by alkaline pH 11.0 and acidic pH 3.5 was significantly increased from 2158 N to 2967 N and 2441 N, respectively. In addition, cooking loss was significantly decreased (P < 0.05) from 8.9% to 4.8% and 6.5%, respectively. There was no significant (P < 0.05) change in gel springiness, while all the alkaline solubilization pH had a positive effect on the hardness of the gel, but pH 2.5 and 3.0 treatments were negative to the texture properties of PSE like meat protein gel. In addition, pH 2.5 and alkaline pH treatments significantly increased the whiteness of protein gel (P > 0.05), and the whiteness of pH 3.5 group showed the highest W*value, and there was no significant difference between the 11 groups (P < 0.05). These results indicated that the solubilization of pH 3.5 and pH 11 were beneficial to improving the texture, water retention and color characteristics of PSE chicken protein gel.

2. Effects of ISP processing on the O/W protein emulsions stabilized by isolated PSE-like chicken protein.

In order to clarify the effects of ISP treatment on PSE chicken protein emulsified gel, ISP-isolate protein and PSE-like chicken meat/soybean oil O/W protein emulsion were prepared under various protein concentrations (50, 100, 150 mg/g). Physicochemical and rheological properties of the protein emulsions before heating were studied. The textural properties, total expressible fluid retention ability and color characteristics of the thermal induced emulsion gels were compared. The results were as follows: compared with the control group, the stability of the emulsion increased in the ISP-treated group during the refrigeration period of 72 h, and the droplet distribution in the ISP group was more finer and more dispersed. The apparent viscosity of all protein emulsions fitted well with the Ostwald-de-Waele model distribution (R2> 0.9), and the Kvalue of the consistency coefficient of protein emulsion viscosity is significantly increased after ISP treatment (P < 0.05). According to frequency sweep tests, all emulsion systems, especially 100 and 150 mg/g protein concentration groups, showed ‘strong gel’ feature. In addition, the rheological behavior of ISP group was different from that of control group. Excessive protein concentration showed negative effects on the formation of ISP-isolated protein emulsion gel. When the protein concentration was 50 and 100 mg/g, the gel hardness of ISP group was significantly higher than that of the control group (P < 0.05), but when protein concentration was 150 mg/g, the trend was opposite. In addition, when the protein concentration of the control group increased from 50 to 150 mg/g, the total expressible fluid (TEF) of the control group decreased from 30.3% to 12.7%. However, in ISP group, when protein concentration increased from 50 to 100 mg/g, TEF decreased significantly from 21.9% to 13.4% (P < 0.05), while when protein concentration further increase to 150 mg/g, TEF increased slightly to 16.1% (P < 0.05). Overall, ISP treatment could increase the viscosity of PSE-like chicken protein emulsion system, make the emulsion more uniformly dispersing, improve the stability of emulsion, and enhance the emulsion gel texture and fluid retention ability. However, the high protein concentration (150 mg/g) will adversely affect the quality of ISP-treated emulsion gel.

3. Effects of recovery pH on gelation properties of ISP-treated PSE-like chicken breast meat protein.

To optimize the ISP process strategy and further improve the gel properties of ISP-isolate protein, pH 5.5 (isoelectric point) and pH 6.2 (relatively far from isoelectric point) were chosen as different recovery conditions, with acidic pH 3.5 and alkaline pH 11.0 for solubilization conditions. The effects of four ISP strategies on recovery yield, protein solubility, protein profile, viscosity, aggregation properties, surface hydrophobicity and gelation properties were compared. The results showed that the recovery yield decreased slightly for 6.2 groups (P < 0.05). Although alkali-treatment resulted in more water-soluble protein loss, the protein profile proportion of functional salt-soluble protein increased. Compared with the solubilization pH, the recovery pH has little effect on the salt-soluble and water-soluble protein profile in ISP groups. According to the SDS results, the loss of myosin light chain may be caused during ISP treatment. After ISP treatment, the surface hydrophobicity of PSE-like chicken protein increased significantly. Compared with pH 5.5 treatment group, pH 6.2 groups showed reduced zeta potential (P < 0.05), resulting in a higher degree of protein aggregation and a significant increase in particle size of protein aggregates (P < 0.05). In addition, compared with pH 5.5 groups, the heat-induced gel network formed by pH 6.2 recovered protein isolate was more homogeneous, and gel hardness increased significantly (P < 0.05). Cooking loss in acid treatment group increased significantly (P < 0.05). Based on the above results, alkaline pH 11 solubilization combined with pH 6.2 recovery can effectively promote the formation of PSE-like chicken gel network, increase the gel textural properties and water retention ability.

4. The mechanism of ISP process on modifying the functional properties of PSE-like chicken meat protein: a molecular perspective.

To study the effects of different ISP processing strategies on PSE like meat protein, the molecular changes of PSE-like protein induced by ISP treatment were tested. The structural changes and enthalpy changes of protein before and after ISP treatment were studied, including the fluorescence intensity of tryptophan (TFI), the content of total sulfhydryl group, intermolecular forces, water distribution characteristics, rheological characteristics, and the secondary structural contents of ISP-isolated protein and control group were measured by fourier infrared spectroscopy. The results showed that after ISP treatment, the enthalpy transition peak of myosin and actin migrated to a lower temperature (P < 0.05), which indicated that ISP treatment reduced the heat stability of protein; the TFI in ISP treatment group decreased compared with the control group, which indicated that the exposure degree of tryptophan increased, and the content of total sulfhydryl group decreased significantly (P< 0.05), indicating a certain amount of disulfide bonds formation. However, after heating, there was no significant difference in total sulfhydryl content between the acid treatment group and the control group (P > 0.05). For acid treatment group and alkali treatment group, pH 6.2-recovery could increase the elasticity of heat-induced gel. After ISP treatment, the content of α-helix of PSE-like chicken protein decreased significantly (P < 0.05). For acid group, the α-helix content of pH 5.5 and 6.2 recovered protein decreased from 40.8% to 34.9% and 32.7%, respectively; alkali-treated sample with pH 5.5 and 6.2 recovery decreased to 28.9% and 29.4%, respectively, however, there was no difference (P > 0.05) in α-helix content among groups after heating; according to the changes of intermolecular force, the hydrophobic interaction in control group continued to increase with the temperature rising, but the amplification of ISP-treated group decreased, among which for the alkali-treated pH 6.2 recovered group, the hydrophobic interaction decreased slightly before  45 °C. According to the above results, ISP treatment leads to the change of molecular structure of PSE-like chicken protein, more orderly secondary structure transforms to disorder stuctures, hydrophobic interaction increases, sulfhydryl was oxidized to disulfide bond, protein denaturation caused by ISP treatment may antagonize the heat-induced denaturation to some extent, and the heat-induced denaturation retards, thus changing the gel formation behavior.

5. The mechanism of ISP process on modifying the functional properties of PSE-like chicken meat protein: contribution of sarcoplasmic protein.

In order to further elucidate the mechanism of ISP treatment improving the gelation properties of PSE-like chicken protein, the micromorphological changes, aggregation characteristics and intermolecular forces of sarcoplasmic proteins under different ISP treatment strategies were compared, and then the contribution of ISP-modified sarcoplasmic protein (ISP-SP) to the gel quality of myofibrillar protein (MP) were studied, including thermal denaturation, rheological behavior, textural characteristics and water retention capacity. The results showed that SP in the shape of native spherical particles was driven to form loose floating aggregates under intermolecular force after ISP treatment, and different recovery pH had significant effect on the size of aggregates (P < 0.05), but there was no significant difference in the flow properties of SP suspension between ISP treatment group and control group. The main molecular forces maintaining these aggregates are hydrophobic interaction and a certain amount of disulfide bonds. The changes of intermolecular force during heating were affected by ISP treatment. Different from SP in control group, the hydrophobic interaction in ISP-SP group changed little during heating. Even after ISP treatment, SP alone cannot form orderly elastic network, which is determined by the protein structure and amino acid distribution of SP itself. ISP-SP addition did not change the rheological behavior curve of MP. After mixing the SP and MP system with or without ISP treatment, it was found that native SP had almost no positive effect on the gel formation of MP system due to its strong mobility and small molecular weight. While adding ISP-SP to MP, the gel breaking force increased from 91.7 g to 293.7 g, adding ISP-SP in ISP-MP, the gel breaking force increased from 275.7 g to 304.5 g. In addition, adding ISP-SP to MP could reduce cooking loss from 23.0% to 6.8% hence effectively reducing water loss (P < 0.05). The results showed that the addition of ISP-denatured sarcoplasmic protein did not affect the thermal behavior of MP gelation, but enhanced the gel quality of PSE-like MP.

6. Oxidative stability of ISP-isolated PSE-like chicken protein under hydroxyl radical generating system.

To study the oxidative stability of ISP-isolated PSE-like chicken protein, the effects of a hydroxyl radical generating system (Fe3+/H2O2) at different H2O2concentrations (0, 1 and 10 mM) on the chemical and structural properties of isoelectric solubilization/precipitation (ISP)-isolated PSE (pale, soft, exudative)-like chicken protein were investigated, including the content of pro-oxidant, the formation of carbonyl group, the types of hydrolyzed amino acids, the changes of content of sulfhydryl group, Schiff base and free amino acids, and the rheological properties of protein after oxidation. The results showed that acid treatment and alkali treatment could reduce the content of pro-oxidants in PSE-like chicken system, including lipids, pigments and myoglobin. The removal efficiency of acid treatment was higher. The content of lipid decreased significantly from 6.21 in the control group (P < 0.05) to 5.58 /100 g dry basis, and the content of myoglobin decreased significantly from 3.08 to 0.37 mg/g dry samples. ISP treatment had no significant effect on carbonyl formation and free amino loss in PSE-like paste (P > 0.05). However, in the simulated oxidation system, ISP-isolate protein generated fewer carbonyl derivatives (P < 0.05), but resulted in higher level (P < 0.05) of sulfhydryl and free amino loss. Accordingly, under mild oxidation conditions (1 mM H2O2), the sulfur-containing amino acids in ISP protein isolate were more easily transformed into disulfide bonds than those in the control group. In addition, compared with the control group PSE-like meat, the G' value of ISP isolate was higher after 10 mM oxidation. Based on the above results, after ISP treatment, the sensitivity of sulfur amino acids increased, and it was more easily oxidized to form disulfide bonds. However, oxidation sensitive amino acids increased their resistance, and better maintained their gel formation ability under the oxidation stress.

7. In vitrostimulated digestion properties of ISP-isolated PSE-like chicken protein.

In order to further explore the effect of ISP treatment on the digestibility of PSE-like chicken protein, the impacts of different heating strategies (72 °C, 20 min, 100 °C, 60 ℃) under in vitrosimulated gastrointestinal digestion model were studied. The differences of digestibility between ISP protein isolateS and PSE like paste in control group, including the changes of physicochemical properties related to digestibility, in vitrodigestibility kinetics in gastrointestinal stage, changes of protein profile, etc., and the differences of digested products were analyzed by proteomics and polypeptide peptideomics. The results showed that the advantage of ISP treatment in increasing water retention ability and textural properties was more significant under the condition of high temperature and long time heating; although the structure of protein changed greatly before heating, especially the exposure of hydrophobic groups, the difference of structure after heating was diminished, and the size of thermal aggregation particles formed was similar to that of the control group (P > 0.05); the results of SDS-page showed that 100 °C 60 min heating could cause oxidative denaturation of protein and form non-disulfide covalent bond; heating can significantly increase the digestion properties of PSE-like meat protein; analysis of digestion kinetic parameters shows that heating can improve the digestion potential and rate of gastric digestion stage, in which the digestion ability of ISP protein isolate after heating at 72 °C is higher than that of the control group (P< 0.05), but there was no significant difference in the 100 °C heating group (P > 0.05); in the simulated intestinal stage, except for the 72 °C treatment group, the digestion kinetics of different groups were similar; for the final digestion products, the control group and ISP treatment group showed similar characteristics under 100 °C treatment, but for the 72 °C and unheated group, the digestion products were more affected by the protein source; 100 °C 60 min heating may lead to the crosslinking and aggregation between sarcoplasmic protein and myofibrillar protein, so its digestibility will be reduced. Compared with the control group, ISP group showed higher peptide abundance before and after heating. In summary, ISP treatment can increase digestibility of pepsin of 72 °C 20 min heated PSE-meat protein and increase the eptide abundance of digestion products, but there is no significant difference between the ISP group and the control group under intestinal digestion stage. For ISP protein isolates, 72 °C 20 min heating is beneficial to increase its digestion properties, but after this heating strategy, the gel properties are decreaed than that of 100 °C 60 min treatment group.

In summary, ISP treatment can effectively improve the gelation properties of PSE-like chicken protein, and the solubilization pH of 11.0 combined with recovery pH of 6.2 was proved as optimal strategies. ISP treatment can modify the conformation of native protein and regulate the formation of protein aggregates to modify gel formation behavior, thereby affecting the final gelation properties. The ISP-modified sarcoplasmic protein can be distributed in the myofibrillar domained gel network, and plays an important role in improving the gel properties of PSE chicken protein. PSE-like meat protein with or without ISP treatment exhibited different oxidaition properties under hydroxyl radical generating system. ISP-treated protein retained higher level of gel forming ability under high oxidation stress compared to the control. ISP treatment combined with low-temperature short-term heating could increase the digestibility of PSE-like meat protein and increase the abundance of final digestion products.