【作者】 胡昊；

【导师】 潘思轶；

【作者基本信息】 华中农业大学 ， 食品科学， 2014， 博士

【摘要】 大豆蛋白作为大豆产业中的主要产品之一,在食品生产中应用广泛。在大豆蛋白的众多性质中,凝胶性是重要又独特的性质。近年来,高场强超声波技术在食品工业中的应用受到越来越多的关注,已有研究表明超声技术能够改善大豆蛋白的功能特性。但是,利用高场强超声技术,系统全面地对大豆蛋白凝胶性进行改善的研究并不多见。本研究首先以高场强超声波技术作为改性手段改性大豆蛋白。然后选取三种不同类型的大豆蛋白冷热凝胶(“内酯豆腐”热凝胶、“传统豆腐”热凝胶和转谷氨酰胺酶冷凝胶)作为成胶模型,模拟凝胶形成。研究发现高场强超声作用显著提高了以上三种大豆蛋白凝胶的凝胶性质。接着,为了探索利用大豆蛋白凝胶作为营养物及药物包埋载体的可能性,我们选取其中的冷凝胶(以避免对热敏性营养物或者药物的破坏)作为包埋载体,对核黄素进行了包埋和体外消化实验。结果显示超声处理显著减缓了核黄素在消化液中的释放速率。最后,将超声波分别作用于大豆7S和1lS蛋白后发现,超声对7S性质的改变比11S更显著。其具体结果如下：(1)高场强超声波(20kHz,200-600W下,15或者30分钟)作用能改变商用大豆分离蛋白的功能特性。超声波作用降低大豆分离蛋白溶液的储存模量和损失模量,使蛋白溶液体现出更像液体的流变性。扫描电镜显示,超声作用使冻干后的大豆分离蛋白的片状聚合物增大,并且超声作用后大豆分离蛋白溶液的游离巯基含量从9.13±0.44μmol g-1可溶蛋白增加至(?)18.08±0.39μmol g-1可溶蛋白,表面疏水性由1400增加到4200,在不同溶液中的溶解性也增加。超声后,蛋白分子间的非共价作用可能减少并转化为静电相互作用。高场强超声作用能够改变商用大豆分离蛋白二级结构；并且打开蛋白分子或者聚合体,使巯基和疏水基团暴露,改变蛋白三级结构；超声还可能改变蛋白的聚合形式,使蛋白聚合物部分解体。(2)模拟“内酯”豆腐生产工艺,以10%(w/v)大豆分离蛋白,内酯加入量为0.3g/100mL来制作大豆分离蛋白内酯凝胶。对比未被超声作用的大豆分离蛋白和高场强超声波(20kHz,400W,5-40分钟)处理之后的大豆分离蛋白内酯凝胶发现：高场强超声显著提高商用大豆分离蛋白葡萄糖酸内酯凝胶的持水性、凝胶强度和凝胶坚固性(G’)。其中,被超声处理20分钟的凝胶持水性、凝胶强度和凝胶G’最高,分别为95.53±0.25%,60.90±2.87g和96340Pa。此外,超声预处理还降低了内酯凝胶的巯基含量和蛋白分子间的非共价相互作用。扫描电镜显示,超声预处理后的凝胶空间结构致密、均一,而未处理凝胶含有很多大小不均匀的孔状结构。流变研究显示,在凝胶形成的各个阶段里,降温过程对超声处理后的凝胶强度增强作用最明显,而保温过程对未被超声处理的凝胶强度增强作用最明显。超声降低了蛋白溶液粒度,Pearson相关性分析显示蛋白溶液粒径大小与内酯凝胶持水性、凝胶强度和凝胶G’显著负相关。(3)模拟“传统豆腐”生产工艺,以10%(w/v)大豆分离蛋白,硫酸钙加入量为20mM来制作大豆分离蛋白钙离子凝胶。对比未被超声作用的大豆分离蛋白和高场强超声波(20kHz,400W,5-40分钟)处理之后的大豆分离蛋白钙离子凝胶发现：高场强超声显著提高商用大豆分离蛋白硫酸钙凝胶的持水性和凝胶强度。被超声处理40分钟的凝胶持水性和凝胶强度最高。并且,超声预处理还降低了钙凝胶的巯基含量。扫描电镜显示,超声预处理改变了钙离子凝胶的三维结构,经过超声预处理的钙离子凝胶空间结构致密、均一,而未处理凝胶含有很多大小不均匀的孔状结构。对大豆分离蛋白在95℃加热10分钟以后,超声预处理仍然在一些重要性质方面体现出差异,比如：粒度分布改变,蛋白粒度降低,表面疏水性和巯基含量增加。(4)超声提高内酯和钙离子凝胶性的机理总结如下：超声作用降低了大豆分离蛋白粒度,并且在此过程中将原本包埋于大豆分离蛋白内部的疏水基团和巯基暴露到分子或者聚合物表面。接下来的加热处理进一步促进了蛋白粒度的降低和活性基团的暴露。超声波还可能促使蛋白形成可溶性聚合物,这些可溶性蛋白聚合物可能在凝胶形成时的加热过程中形成难溶聚合物。接着凝固剂加入,促进聚合物形成。超声对大豆分离蛋白产生的结构和构象的改变可能会有益于分子间疏水相互作用和分子间二硫键的形成,最终形成致密、均一的网状空间结构。(5)探究了超声预处理对大豆分离蛋白转谷氨酰胺酶冷凝胶凝胶性的影响,发现超声能够增强其凝胶性。选取此种冷凝胶为包埋载体包埋核黄素,因为冷环境能够更好地保护热敏性营养物质或药物。高场强超声波(20kHz,400W)处理40分钟之后,包埋核黄素的大豆分离蛋白冷凝胶凝胶产量由6.02增加到11.27；凝胶强度由11.4g增加到37.5g;包埋率也由88.8%增加到100%。并且经过40分钟超声处理之后核黄素在模拟胃液或者肠液中的释放速率减慢。通过对凝胶的进一步研究发现：凝胶的肿胀性降低,抗腐蚀性增加,空间结构也呈现出更加致密的网状结构。聚丙烯酰胺电泳显示超声预处理增加了大豆蛋白凝胶的交联度,促进了共价交联反应的发生。拉曼光谱显示,超声预处理后,蛋白凝胶多肽链和氨基酸残基的微环境以及化学性质发生了变化,意味着三级结构改变了。(6)用高场强超声(20kHz,400W下5-40分钟)处理从脱脂大豆粉中提取的7S和11S。超声作用后,在pH=7.0,0.05M的Tris缓冲液中,7S的粒度由73.3nm减少至51.6nnm,浊度(2%)由0.602减少到0.147,表面疏水性由856增加到1060,溶解性从85%增加到93%,乳化活性指数由34.1m2/g增加到53.7m2/g,乳化稳定指数由9.7分钟增加到52.6分钟。对11S而言,超声后11S在Tris缓冲液中的浊度降低,但对其粒度和乳化稳定指数影响不大。除此之外,11S的表面疏水性和溶解度在前20分钟时降低但之后又增加。7S和11S的巯基含量都在超声之后降低。超声没有改变7S和11S的二级结构,但它增加了非还原性电泳在高分子量处聚集物的含量,并且拉曼光谱显示芳香和脂肪族氨基酸侧链的微环境也发生了改变。从以上在Tris缓冲液中粒度、浊度、溶解度和乳化性的改变,不难发现,超声对7S的影响比11S更加显著。超声对7S蛋白聚集物的解聚集,可能是造成以上变化的主要原因。但同时,我们应该注意到11S在Tris缓冲液中聚集程度高,这也可能导致超声对11S功能性质影响不大。更多还原

【Abstract】 Soybean protein which has already been widely used in food processing area is an important product in soy industry. Among all the properties of soy protein, gelation property is important and unique. During the recent years, high intensity ultrasound (HIU) technology has attracted a lot of attentions. Moreover, some recent researches have pointed out that HIU can change the physicochemical properties of soy protein. However, to the best of our knowledge, few systematic researches on using HIU to improve the gelation property of soy protein have been reported.In this study, the first step is to use HIU to change the physicochemical properties of soy protein. Then three kinds of gelation models, namely,"glucono-deta-lactone (GDL) tofu" heated gel,"traditional" tofu heated gel and TGase induced cold gel, were chosen. It was observed that HIU increased the gelation property of the above three kinds of gels. After that, in order to develop the soy protein macro-hydrogel as drug or nutritional compound carrier, TGase induced cold gel was chosen to encapsulate riboflavin. The reason for choosing TGase gel is because this gel can be formed at mild temperature thus can protect a lot of heat sensitive materials. In vitro experiments showed that40min HIU reduced the release speed of riboflavin obviously. Finally, soy7S and11S were treated by HIU and we found that the effects of HIU on7S were more profound than those of11S. Our specific findings were listed below:(1) The effects of low-frequency (20kHz) HIU at varying power (200,400or600W) and time (15or30min) on functional and structural properties of reconstituted soy protein isolate (SPI) dispersions were examined. HIU treatments reduced both the storage modulus and loss modulus of SPI dispersions and formed more viscous SPI dispersions (fluid character). Moreover, HIU treatment significantly decreased the consistency coefficients and increased the flow behaviour index of SPI dispersions. Scanning electron microscopy of lyophilized HIU SPI showed different microstructure with larger aggregates compared to non-treated SPI. No significant change was observed in the protein electrophoretic patterns by SDS-PAGE. However, free sulfhydryl content (SH), surface hydrophobicity and protein solubility of SPI dispersions were all increased with HIU treatment. Differences in solubility profiles in the presence versus absence of denaturing (0.5%sodium dodecyl sulfate and6M urea) and reducing (mercaptoethanol) agents suggested a decrease in non-covalent interactions of SPI in dispersion after HIU. Secondary structure analysis by circular dichroism indicated lower a-helix and random coil in SPI treated at lower power, in contrast to higher a-helix and lower β-sheet in SPI treated with higher power (600W). HIU resulted in partial unfolding and reduction of intermolecular interactions as demonstrated by increases in free sulfhydryl groups and surface hydrophobicity, leading to improved solubility and fluid character of SPI dispersions, while larger aggregates of HIU SPI in the dry state were formed after lyophilization.(2)HIU (20k Hz,400W) pre-treatments of SPI improved the water holding capacity (WHC), gel strength and gel firmness (final elastic moduli) of glucono-δ-lactone induced SPI gels (GISG). Sonication time (0,5,20, and40min) had a significant effect on the above three properties.20min HIU-GISG had the highest WHC (95.53±0.25%), gel strength (60.90±2.87g) and gel firmness (96340Pa), compared with other samples. Moreover, SH groups and non-covalent interactions of GISG also changed after HIU pre-treatments. The HIU GISG had denser and more uniform microstructures than the untreated GISG. Rheological investments showed that the cooling step (reduce the temperature from95℃to25℃at a speed of2℃/min) was more important for the HIU GISG network formation while the heat preservation step (keep temperature at95℃for20min) was more important for the untreated GISG. HIU reduced the particle size of SPI and Pearson correlation test showed that the particle size of SPI dispersions was negatively correlated with WHC, gel strength and gel firmness.(3) HIU (20kHz at400W for5,20or40min) pre-treatments of SPI changed the particle distribution and reduced particle size of SPI dispersions. Surface hydrophobicity and free SH content of SPI increased with HIU time. Free SH content of CaSO4-induced SPI gels (CISG) and protein solubility in the presence of SDS and urea decreased after HIU pretreatments, suggesting HIU facilitated disulfide bond formation during CISG formation. HIU resulted in more uniform and denser gel network, WHC and gel strength of CISG. Furthermore, WHC and gel strength were positively correlated with free SH content of heated SPI and surface hydrophobicity of unheated SPI, and negatively correlated with particle size of heated SPI and free SH content of CISG. In conclusion, HUS induced structural changes in SPI molecules, leading to different microstructure and improved WHC and gel strength of CISG.(4)The mechanism of HIU improvement of gelation properties of GISG and CISG could be summarized as follow: HIU reduced the particle size of soy protein, meanwhile, the hydrophobic and SH groups were exposed from the interior of SPI aggregate or molecular to the surface. The following heat step further reduced the particle size and exposed active groups. Moreover, HIU facilitated the formation of soluble protein aggregates which might be formed as non-soluble aggregates during the heating process. After that, coagulates were added and aggregates were formed. HIU changes the structures and conformation of SPI, which may accelerate the formation of intermolecular hydrophobic interactions and S-S bonds, finally resulting in dense and uniform3D structure.(5) HIU increased the gelation property of TGase induced cold SPI gel (TISG). TISG was used as control release model to encapsulate riboflavin because TISG was a cold gel which can protect heat sensitive materials.40min HIU (20kHz,400W) increased the gel yield from6.02to11.27, increased the gel strength from11.4g to37.5g and increased the encapsulate efficiency from88.8%to100%. Moreover,40min HIU reduced the riboflavin release speed in simulated gastric or intestinal fluid. Further investigations showed that the swell property of TISG reduced while the anti-erosion property increased. SDS-PAGE indicated that HIU increased the cross-link degree of SPI when treated by TGase. Raman spectroscopy revealed that HIU pretreatment of TISG changed the microenvironment of polypeptide and the chemistry of amino acid side chain, indicating the modification of tertiary structure.(6) The effects of HIU (20kHz at400W for5,20or40min) on soybean P-conglycinin (7S) and glycinin (11S) fractions were investigated in this study. HIU decreased turbidity and particle size of7S in0.05M Tris-HCl buffer at pH7.0, while it increased surface hydrophobicity, solubility, emulsifying activity (EAI) and emulsion stability (ESI). Similarly, HIU of soybean glycinin (11S) decreased turbidity while increasing EAI but it had minimal effects on particle size and ESI. Furthermore, surface hydrophobicity and solubility of11S decreased during the first20min of HIU but then increased upon longer treatment. The SH groups of both7S and11S fractions decreased after HIU. HIU did not change7S or11S secondary structure, but it slightly increased the percentage of high molecular-weight aggregates under non-reducing SDS-PAGE, and changed the microenvironment of aromatic and aliphatic side chains as observed by Raman spectroscopy of freeze-dried samples. The physicochemical changes of11S and especially of7S proteins induced by HIU treatment may contribute to improved applications of soy proteins in food products.更多还原