【作者】 韩敏义；

【导师】 徐幸莲； 周光宏；

【作者基本信息】 南京农业大学 ， 食品科学， 2009， 博士

【副题名】低场NMR和拉曼光谱法

【摘要】 肌原纤维蛋白热诱导凝胶是决定肉制品品质的关键因素,对产品的质构、赋形、保水性和保留其他食品成分有重要作用。在肌肉凝胶形成过程中,蛋白质分子从天然状态到变性状态的转变包括二级、三级和四级构象变化。这些变化受温度、pH、蛋白质浓度、离子强度、不同肌肉类型及添加物的影响,决定了蛋白质热诱导凝胶的最终质构、微观结构及保水性。本研究以猪肉肌原纤维蛋白为原料,用低场核磁共振(nuclear magnetic resonance, NMR)、拉曼光谱、圆二色谱等方法研究了加热温度、离子强度、微生物转谷氨酰胺酶(microbial transglutaminase, MTG)和多聚磷酸盐等因素对肌原纤维蛋白凝胶功能特性及蛋白质生化特性的影响,探讨了蛋白质结构与功能特性之间的相关关系,为优化加工工艺条件,改善凝胶类肉制品的品质提供理论依据。具体研究内容和结果如下：1猪肉肌原纤维蛋白热诱导凝胶保水性与凝胶强度研究研究了温度、NaCl浓度、微生物转谷氨酰胺酶和磷酸盐对猪肉肌原纤维蛋白热诱导凝胶保水性(water holding capacity, WHC)和硬度影响。总体上看猪肉肌原纤维蛋白凝胶WHC随温度升高而显著降低；硬度随着温度上升而显著升高,到60℃时达最高；WHC和硬度随着NaCl浓度的增加而显著增加。当MTG浓度从0 U·g-1提高到2 U·g-1时,WHC从81.4%增加到95.6%,但MTG浓度继续增加时,差异不显著。加入MTG会显著提高凝胶硬度,当MTG浓度从0 U·g-1蛋白提高到2 U·g-1蛋白时,凝胶硬度从36.70 g增加到63.24 g,在浓度增加到4 U·g-1蛋白时,硬度达到最高值。六偏磷酸钠(sodium hexametaphosphate, HMP)、焦磷酸钠(sodium pyrophosphate, PP)和三聚磷酸钠(Sodium Tripolyphosphate, TPP)显著提高了凝胶保水性。HMP可以显著提高凝胶硬度,而PP和TPP会降低凝胶硬度。2猪肉肌原纤维蛋白热诱导凝胶低场NMR研究用低场NMR和扫描电镜研究了温度、NaCl浓度、MTG和磷酸盐对猪肉肌原纤维蛋白热诱导凝胶中水的状态分布和凝胶微观结构的影响。结果表明,拟合后不同温度NMR T2分布出现3个峰,分别对应于结合水(T21),不易流动水(T22)和自由水(T23)。三个峰对应的T2弛豫时间(峰顶所对应时间)分别为：T21：1.15～7.05 ms、T22：231.01～403.70 ms、T23：1629.75～2477.08 ms。T22随着温度上升显著下降,即随着温度上升向快弛豫方向移动,说明这种状态水移动性下降；随着温度升高,形成了孔径分布均匀的凝胶；拟合后不同浓度NaCl形成的热诱导凝胶T2弛豫时间分布为3～4个峰。在0.76～6.14 ms(T21)和16.30～43.29ms(T22a)有一个小峰,在76.65～174.75ms(T22b)有一个大峰,而且很宽,在1232.85～1629.75ms(T23)也有一个大峰；加入NaCl会使主要峰T22b向慢弛豫方向移动,会使此峰峰面积百分数显著增加,结合第一章结果,NaCl浓度增加使凝胶WHC增加,WHC增加对应的应该是这种状态的水。随着NaCl浓度增加,加热后形成的猪肉肌原纤维蛋白凝胶形成了多孔致密的结构。加入不同浓度MTG后,猪肉肌原纤维蛋白热诱导凝胶T2弛豫特征是在0.7～1.1ms(T21)和26～35 ms(T22a)的位置有一个小的分布,在170～230 ms(T22b)位置形成一个主要分布,最后在1400～1800 ms(T23)位置存在一个小峰。在猪肉肌原纤维蛋白凝胶中加入MTG对T22b有显著影响,最大峰的位置发生明显移动,形成更快弛豫,加入2 U·g-1MTG后T22b从226 ms降低到188 ms。然而,MTG从2 U·g-1浓度增加到8 U·g-1后,T22b没有进一步降低。NMR弛豫结果主成分分析也显示加MTG处理样品弛豫时间更短。MTG使猪肉肌原纤维蛋白热诱导凝胶微观结构更精细,导致了凝胶中水质子移动性变差,从而形成了更短T2弛豫时间。加入磷酸盐会使主要弛豫成分T22b明显增加,说明水的移动性加强了,且其峰面积占所有峰面积的百分比显著增加,可能对应于WHC增加。3猪肉肌原纤维蛋白热诱导凝胶过程中蛋白质结构变化研究用拉曼光谱和圆二色谱研究了温度、NaCl浓度、MTG和磷酸盐对猪肉肌原纤维蛋白热诱导凝胶过程中蛋白质结构变化的影响。结果表明,温度上升过程中,拉曼光谱545 cm-1显示一个弱峰,这个峰被指认为二硫键的反式-扭式-反式构象,这个条带归一化强度随着温度上升显著下降,说明二硫键伸缩振动的变化。在猪肉肌原纤维蛋白凝胶过程中,I850／I830的值在1.13～2.56之间,表明在凝胶过程中酪氨酸残基主要是暴露的,能够参与中度或弱的氢键。随着温度升高,α-螺旋含量显著下降,而β-折叠、β-转角及随机卷曲含量都有显著上升。拉曼光谱545 cm-1条带归一化强度随着NaCl浓度上升显著下降,而后又显著上升。不同NaCl浓度下,I850／I830比值在1.24～2.19之间,表明酪氨酸残基主要是暴露的,能够参与中度或弱的氢键。加入不同浓度NaCl后加热形成凝胶,各种二级结构含量没有显著变化.原因可能是在凝胶形成过程中NaCl作用被热作用抵消。归一化C-H伸缩振动区域峰高显著上升,峰面积在高浓度NaCl时有上升的趋势,但差异不显著。拉曼光谱分析显示加入MTG后蛋白质二级结构含量和蛋白质微环境发生显著变化。α-螺旋含量显著下降,而其他二级结构含量显著上升。高浓度HMP(0.25%)会使α-螺旋含量下降,β-折叠含量显著增加。PP对肌原纤维蛋白各种二级结构含量都没有显著影响,低浓度TPP(0.05%～0.15%)使α-螺旋含量显著增加,0.2%TPP会使β折叠含量上升到21%。4猪肉肌原纤维蛋白结构与热诱导凝胶功能特性的关系研究通过主成分分析和相关系数矩阵的方法研究猪肉肌原纤维蛋白结构与热诱导凝胶功能特性的关系。结果表明,温度变化过程中,原始变量主成分分析结果显示前三个主成分能够解释总体方差变异99.59%,WHC、硬度、T2弛豫时间和蛋白质二级结构含量有很强的相关性,低温处理和高温处理在样品主成分评分图上位置正好相反。主成分分析显示低浓度NaCl处理在第一主成分的右端,特征是低WHC、硬度和T22b、T23、T22b峰面积和低极性,而高浓度在第一主成分左端,共同特征是高WHC、硬度和高极性。从变量评分图上也可以看出不同指标随NaCl浓度变化情况。在MTG影响下,WHC与NMR质子弛豫性质和蛋白质二级结构有很强的相关性,硬度与NMR质子弛豫性质和色氨酸的包埋程度有关,说明结构与功能有很强的相关性。加入磷酸盐后保水性和硬度与蛋白质二级结构没有显著相关性。综上所述,温度升高和加入MTG使α-螺旋含量显著下降,β-螺旋含量显著上升,形成了更高的凝胶硬度。NaCl和磷酸盐使保水性提高,但与蛋白质二级结构含量没有显著相关性。温度和MTG影响下,凝胶孔径下降导致了T2弛豫时间降低,不易流动水移动性下降。更多还原

【Abstract】 The gel-forming properties of myofibrillar proteins are essential to the development of muscle-based products in that it contributes to textural properties, shaping the product, retaining water, and holding other food components in the product. During gelation, the molecular transition of the protein from its native state to the denatured state involves conformational changes in the quaternary, tertiary and secondary structures, which are influenced by pH, protein concentration, ionic conditions, vary muscle sources and additives. These changes determine the final structure and textural properties of the protein gels. The effect of temperature, ionic strength, microbial transglutaminase (MTG) and polyphosphate on the functional properties and biochemical characteristics of pork myofibrillar protein (PMP) gel were studied by combined low field nuclear magnetic resonance (NMR), Raman spectroscopy and circular dichroism spectra (CD) technique. The aim was to provide more insight into the functional properties of myofibrillar proteins, allowing the manipulation of processing conditions in order to obtain products with the desired structural and textural attributes. The detailed contents and results are shown as follows.1 Study on WHC and gel strength of PMP heat-induced gelationThe effect of temperature, NaCl, MTG and polyphosphate on WHC and hardness of PMP gel was investigated. The results showed the WHC of PMP gel decreased significantly with increasing of the temperature, however the hardness improved significantly and reached maximum at 60℃. The addition of NaCl produced a significant increase in the WHC and hardness of PMP gel. The enzymatic protein preparations had significantly higher WHC and hardness in comparison with the control system. The WHC and hardness increased from 81.4% and 36.70 g of control system to 95.6% and 63.24 g of the PMP gel containing 2 U·g-1 protein, respectively. However no further changes were observed for WHC when the level of MTG increased, and the hardness reached the maximum at the 4 U·g-1 MTG. The addition produced a significant increase in WHC for all polyphosphates and in hardness for HMP whereas decrease in hardness for PP and TPP.2 Study on functional properties of PMP gel by low field NMR methodThe effect of temperature, NaCl, MTG and polyphosphate on T2 relaxation times and microstructure of PMP gel was estimated by low field NMR and scanning electron micrograph (SEM). The results showed the NMR decay curve was fitted to three component which are attributed to bound (T21), immobile (T22) and free water state (T23) and the T2 relaxation times (peak time) of these three components were T21,1.15-7.05 ms; T22,231.01-403.70 ms and T23,1629-2477.08 ms, respectively. The position of the major component T22 clearly shifted lower relaxation times with increasing temperature, indicating limited water proton mobility, a fine and porous microstructure was observed due to the increasing temperature. The distributed water proton NMR T2 relaxation of different levels of NaCl after heat treatment characterized by two minor populations with relaxation times centered around 0.76-6.14 ms (T21) and 16.30-43.29 ms (T22a), a major population with a relaxation time of 76.65-174.75 ms (T22b) which was very broad. In addition, a broad, less well-defined population also appeared in the region between 1332.85 and 1629.75 ms (T23). The position of the major component clearly shifted higher relaxation times with increasing NaCl concentration, and integrated peak area proportion increased, which could explain the increasing WHC. Homogenous microstructure with decreasing pore size were introduced after the addition of NaCl and heat treatment. The distributed water proton NMR T2 relaxation of different levels of MTG characterized by two minor populations with relaxation times centered around 0.7-1.1 ms (T21) and 26-35 ms (T22a), a major population with a relaxation time of 170-230 ms (T22b) that was very broad. In addition, a broad, less well-defined population also appeared in the region between 1400 and 1800 ms (T23). The enzymatic preparation had significantly lower values of spin-spin time (T2). The major population T2 relaxation time was reduced from 226 ms (peak value) of the PMP gel containing no MTG to 188 ms of the PMP gel containing 2 U·g-1 protein. However no further decrease was shown when the dosage of MTG increased. The principal component analysis (PCA) also revealed the sample with MTG has shorter relaxation time. The position of the major relaxation component T22b clearly shifted towards longer relaxation times with increasing concentration of polyphosphate indicating increase of water proton mobility. And the integrated peak area proportion of the component increased with the adding of polyphosphate, which revealed that the increased WHC.3 Study on changes of the protein structure by Raman spectroscopy and CD The effect of different temperature, NaCl, MTG and polyphosphates on Raman spectroscopy and CD of PMP gel was examined. The normalized intensity of the band located near 545 cm-1 which is assigned to disulfide bonds in the trans-gauche-trans conformations decreased, indicating changes in disulfide bond stretching. The ratio of 1850/1830 ranged from 1.13 to 2.56 during heat-induced gelation, suggesting that the tyrosine residues of the samples were mainly exposed and able to participate in moderate or weak hydrogen bonding. Modifications in the amideⅠ(1650-1680 cm-1) and amideⅢ(1200-1300 cm-1) regions indicated a significant decrease in a-helix content, accompanied by a significant increase inβ-sheet,β-turn and random coil contents. The normalized intensity of the band located near 545 cm-1 which is assigned to disulfide bonds in the trans-gauche-trans conformations first decreased, and then increased, indicating changes in disulfide bond stretching. The ratio of 1850/1830 ranged from 1.24 to 2.19 due to different NaCl levels, suggesting that the tyrosine residues of the samples were mainly exposed and able to participate in moderate or weak hydrogen bonding. No significant differences of the protein secondary structure estimated were found as a function of NaCl concentration, maybe effect of NaCl on secondary structure before heat treatment was balanced by the heat treatment. The normalized intensity of the band assigned to C-H stretching vibration increased significantly, and there was a trend towards an increase in the integrated area of this band, although no significant changes were observed. Raman spectroscopy analysis indicated the occurrence of secondary structure and microenvironment changes due to MTG. Modifications in the amideⅠand amideⅢregions indicated a significant decrease in a-helix content, accompanied by a significant increase in other structures and a fine and porous microstructure was observed which limited the mobility of water proton and resulted in shorter T2 relaxation time due to the addition of the enzyme to PMP gel. The content of a-helix increased at the expense ofβ-sheet after the level of HMP reached 0.25%. No significant difference of secondary structure was demonstrated with adding of PP. However, a significant increase of content of a-helix andβ-sheet was observed after the TPP reached 0.15% and 0.2%, respectively.4 Study on the relationship of the protein structure and functional properties of PMP gel by PCA and correlation matrixThe relationship of the protein structure and functional properties of PMP gel was determined by PCA and correlation matrix. The PCA results of original parameters showed the first three principal components could explain 99.59% of the total variance, significant correlations were found between the WHC, hardness, T2 relaxation times and protein secondary structural changes of myofibrillar protein, the low and high temperature treatment located in the opposite position of the PCA score plots. The PCA results suggested the samples with low levels NaCl located in the right side of the PCA score plots, characterized by low WHC, hardness, T23, T24, peak3area proportion and polarity, whereas, the high levels NaCl located in the left side and characterized by the opposite properties. The correlation of the parameters was displayed in the PCA loading score plot. A strong correlation was observed between WHC, NMR T2 relaxation characteristics and secondary structure, and hardness strong related to NMR T2 relaxation characteristics, tryptophan buried or exposed, which indicated strong correlation between structure and functionality. No strong correlation between WHC, hardness and protein secondary structure was observed after phosphates were added.To sum up, a decreasing in a-helix content at the expense ofβ-sheet and shorter T22b relaxation time with the increasing of temperature and MTG resulted in higher hardness. The WHC improved with NaCl and phosphate added, which has no significant correlation with the content of protein secondary structures. The decrease of gel pore size leads to the decline of T2 relaxation times and mobility of immobile water for the samples of different temperature and MTG dosage.更多还原