豆腐凝胶形成机理及水分状态与品质关系研究
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摘要
我国大豆资源丰富,豆制品的加工有着2000年多年的历史。豆腐是最受欢迎的豆制品之一,其蛋白质含量丰富,不含胆固醇,具有许多优良的加工品质。另外,豆腐中富含的异黄酮对癌症有抑制作用。大豆蛋白的凝胶特性是豆制品加工中的重要特性之一。蛋白质凝胶中,蛋白质和水的相互作用决定和维持蛋白质三维结构,它不但影响蛋白质的功能性,还影响食品的感官评定。有关豆腐的报道相对集中的是大豆蛋白的组成和结构、大豆蛋白的功能特性、凝胶形成的机理以及工艺等方面,而对豆腐中水分状态与凝胶的质构特性的关系则鲜见报道。因此,研究影响豆腐凝胶形成的影响因素及其水分状态与凝胶的质构特性的关系,对有效控制凝胶形成过程,开发高品质豆腐制品具有重要的理论和实践意义。
钟祥豆腐是湖北著名的特产之一。本文旨在研究豆腐的凝胶化条件,着重探讨大豆蛋白与金属离子和油脂的交互作用对豆腐凝胶形成的影响以及豆腐中水分状态与凝胶的结构和质构特性的关系,为豆腐品质和稳定性的研究与新产品的开发提供理论依据和实验基础。主要研究结果如下:
1、豆腐凝胶形成的基础研究本研究采用SPI模拟豆腐复合体系中凝胶形成的过程,探讨分子间作用力对多因素诱导的大豆蛋白凝胶体系的影响情况,实验结果表明,在石膏凝固SPI过程中,静电相互作用、疏水作用、氢键相互作用以及巯基转换作用对豆腐凝胶的形成起重要作用。
2、大豆蛋白凝胶形成过程中蛋白质的变化大豆磨浆后加热前,溶液中蛋白质的α-螺旋、β-折叠以及β-转角含量均为0,蛋白质几乎完全以无规卷曲形式存在。煮沸后冷却到80℃,点浆后充分搅拌,α-螺旋含量增大。点浆后20min,α-螺旋和β-转角略有增大,β-折叠明显减少,无规卷曲明显增多。点浆后50min,稳定的凝胶网络基本形成。压榨后形成的豆腐,α-螺旋含量减少,β-折叠含量升高。同时,凝胶网络中的非可冻结水含量升高,可冻结结合水和自由水含量下降。
3、金属离子和油脂对大豆蛋白凝胶中水分状态的影响不同的金属离子对大豆蛋白凝胶的溶涨率和平衡水含量的影响不同。随着Na~+、Mg~(2+)以及油脂浓度的增加,石膏凝固SPI凝胶的溶涨率和平衡水含量均降低,而则随着Ca~(2+)浓度的增加使得石膏凝固SPI凝胶的溶涨率和平衡水含量升高。
随着金属离子的加入,石膏凝固SPI凝胶的可冻水含量下降。在非冻结水方面,随着Ca~(2+)浓度的增大,W_(nf)值越来越大。但是Na~+和Mg~(2+)对凝胶中非冻结水含量影响情况相反。另外,金属离子的加入会缩短凝胶形成时间,说明金属离子促进凝胶的形成。
4、石膏凝固SPI凝胶水分状态与显微结构的关系通过电镜分析观察到,随着金属离子浓度的增大,凝胶孔洞直径逐渐减小,凝胶的W_(fro)值逐渐减小。认为随着大分子链聚集更为紧密,形成的凝胶网络孔洞直径越小,其形成的网格空间就会越小,产生的空间阻力会变大,能够进入孔洞而被束缚的水就越少,从而凝胶的可冻结水含量下降。
5、金属离子和油脂对石膏凝固SPI凝胶二级结构的影响石膏凝固SPI凝胶体系中,α-螺旋结构、β-折叠结构、β-转角结构以及无规卷曲结构的比例分别为4.9%、33.0%、14.6%以及47.5%。钠、镁和钙三种金属离子的加入,使得体系的α-螺旋结构的比例略有提高,而β-折叠结构的比例相对下降。另一方面,随着金属离子浓度的增大,蛋白分子链连接更加紧密,使得它们之间的空间位置发生很大变化,分子链层与层之间距离减小,破坏了β折叠的层状结构,因此β折叠结构的比例下降。油脂的加入,使得整个体系的α-螺旋结构、β-折叠、β-转角以及无规卷曲结构的比例分别为4.0%、26.0%、22.6%以及47.4%。说明油脂的加入使得体系中的α-螺旋结构和β-折叠结构的比例略有下降。
6、金属离子和油脂对石膏凝固SPI凝胶形成过程及其质构特性的影响本文用大豆分离蛋白模拟传统豆腐的凝胶形成过程,研究了金属离子和油脂对大豆蛋白凝胶形成过程及质构特性的影响。结果表明,Na~+在0～0.050mol/L时,随着浓度的增大,凝胶形成时间缩短,在0.050～0.075mol/L时,凝胶形成时间延长。Ca~(2+)的浓度在0～6.0mmol/L之间时,随着浓度的增大,凝胶形成时间明显减少。Mg~(2+)的浓度在0～0.030mol/L范围内,随着浓度的增大,凝胶形成时间缩短。总的来说,随着金属离子浓度的增加,阴离子对蛋白质构象的影响导致凝胶特性的变化变得复杂。金属离子加入后,SPI凝胶的硬度减小,并且随着金属离子浓度的增大,弹性和粘附性逐渐减小;当Ca~(2+)浓度逐渐增大时,SPI凝胶的粘附性逐渐减小。Na~+、Mg~(2+)、Ca~(2+)对SPI凝胶的内聚性影响均不大。
随着油脂浓度的逐渐增大,豆腐凝胶的硬度逐渐减小,弹性逐渐增大,粘附性明显减小,内聚性略微增大。
7、石膏凝固SPI凝胶质构特性与结构的关系钠、镁和钙三种金属离子的加入,使得SPI凝胶的α-螺旋结构的比例略有提高,而β-折叠结构的比例相对下降。随着油脂浓度的逐渐增大,豆腐凝胶的硬度逐渐减小,弹性逐渐增大,粘附性明显减小,内聚性略微增大。油脂的加入使得体系中的α-螺旋结构和β-折叠结构的比例均略有下降。油脂加入后,在凝胶网络表面会形成一层油脂分子膜,包裹凝胶网络的油脂分子膜起到了锁水的作用,使凝胶体系回复能力加强,所以凝胶的弹性增大。
8、石膏凝固SPI凝胶水分状态与质构特性的关系随着Na~+、Mg~(2+)、Ca~(2+)三种金属离子浓度的增大,凝胶的W_(nf)与W_(fro)值逐渐减小,凝胶的硬度和弹性逐渐减小,粘附性变化无明显规律,内聚性影响不大。其原因可能是W_(nf)值的降低,凝胶孔径中含水量减小,使得水分对凝胶网络的支撑与稳定作用降低,所以其硬度和弹性降低。在粘附性方面,随着Ca~(2+)浓度的增大,凝胶的W_(nf)值逐渐减小,凝胶粘附性逐渐减小,可能是因为Ca~(2+)形成钙桥原因。其他两种金属离子的水分含量变化对凝胶粘附性影响未出现规律性变化,它们对粘附性的影响有待进一步研究。
9、不同脱水工艺条件对豆腐的水分状态的影响随着压力的增大和压榨时间的延长,凝胶网络中水分不断被压出,豆腐凝胶含水量、平衡水分含量、可冻结水含量以及溶胀率均明显降低,非冻结水的含量略有提高。从外观情况看,压榨时间为20min,压力采用700g和900g时豆腐外形好。在10min到40min的范围内,压榨时间对豆腐的影响不明显。
10、水分状态对豆腐凝胶质构特性的影响随着凝胶压榨压力的不断增大,凝胶体系可冻结水分含量减少,凝胶硬度不断增大,弹性基本呈增大趋势,粘附性先增大后减小,内聚性呈增大趋势。
随着压榨时间的延长,粘附性一直减小,凝胶中水分不断析出,网格不断缩小,使得网格中水分不再处于饱和状态,所以在这段时间内,水分不断减少,其硬度、弹性都呈下降趋势。因此认为刚制作的凝胶水分含量和状态不是最佳品质状态。豆腐凝胶的含水量及其水分状态对豆腐的可食用性和品质稳定性有着重要的影响作用。本实验显示,在豆腐制备中,凝胶特性最佳时的豆腐中网络结构孔径的大小在11.4μm左右,这个时候承载的水分量使得豆腐的外观、硬度以及弹性较好。
11、豆腐稳定性的影响因素影响豆腐稳定性的因素有温度、水分含量、盐浓度等。温度对豆腐稳定性的影响明显。4℃的样品将室温(25℃)下的样品保质期延长9d,还可以有效的减少失水率,保持豆腐的品质。
不同含水量对豆腐的稳定性也有一定影响。对市售非发酵豆制品进行稳定性实验,结果显示,随着样品含水量的减少,样品的室温(25℃)下的保质期由6d减少到3d。
盐浓度为0.1%时,豆腐保质期延长1d,失水率降低4%左右,说明盐的加入可以抑制霉菌的生长,有一定锁水的作用,从而延长食品的保质期。而当盐浓度增加至0.4%时,凝胶无法形成,由此可见,盐浓度过大会影响变性后蛋白质分子链重新构建网络。
Our country is rich in soybean resources.Soybean products have a history of more than 2000 years.Tofu is one of the most popular soybean products.It contains much protein,but without cholesterol.And it has many excellent processing qualities.What's more,tofu is rich of isoflavone which plays an important role in restraining cancers. The gel characteristic of soybean protein is one of the important characteristics in the process of soybean produce.In the protein gel,the interaction of protein and water decides and maintains the three-dimensional structure of protein.It not only influences the function of protein,but also influences the food sensory evaluate.
The reports about tofu focus on the compose and structure of soybean proteins, function characteristic of soybean proteins,the gel-forming mechanism and processing technology,but little on the relationship between the state of water in tofu and the gel's texture properties.Thus,studies on affecting factors of tofu gel forming and the relationship between the state of water and the gel's texture properties are very important to effectively control the gel forming and develop tofu products with high quality. Zhongxiang Tofu is one of the special local products in Hubei.Setting conditions and relative ground works of tofu gel,the interaction between metal ion,cooking oil and soybean protein and its influence on tofu gel forming,the relationship between the state of water in tofu and the gel's structure and texture properties were investigated.The results were as follows:
1.Basis studies on Tofu gel forming Gel forming and effects of molecular forces on gelation process and texture properties of soy protein gels induced by multifold factors were studied with the soy protein isolate simulating the gelation process of traditional tofu.It was shown that during course,the electrostatic forces,hydrophobic interactions and hydrogen bonds and sulphydryl conversion function have given important contribution for the gelation process of soybean protein gels coagulating by plaster.
2.Protein's change in the gelation process of soybean protein After soybean's milling,there were not anyα-helix,β-sheet andβ-turn and the secondary structure of soybean protein was mainly coil.When the boiled soybean milk was cooled to 80℃, plaster was put into it and the content ofα-helix was larger.Twenty minutes later,the content ofα-helix andβ-turn was a little larger,the content ofβ- sheet was clearly smaller and the content of coil was larger.Twenty minutes later,steady gel net came into being on the whole.After press,tofu came into being and content ofα-helix was smaller and content ofβ- sheet was larger.At the same time,the content of non- freezing water increased and the content of frozen water decreased in the gel net.
3.Effects of metal ion and cooking oil on the state of water of soy protein gels Different metal ions had different effects on the swelling ratio and equilibrium water content of soy protein gels.With the concentrations of Na~+,Mg~(2+) and cooking oil increasing,the swelling ratio and equilibrium water content of soy protein gels descended.On the other hand,the swelling ratio and equilibrium water content of soy protein gels increased with the increasing of the concentrations of Ca~(2+).
With the adding of metal ion,the content of frozen water soy protein gels descended. With the concentrations of Ca~(2+) increasing,the W_(nf) value was more and more large. However,the effect of Na~+ and Mg~(2+) on the content of non- frozen water in the gel was opposite.On the other hand,the time for gel formation was shorten with the addition of metal ions,which mean that metal ions could promote the formation of gel.
4.The relationship between the state of water and the microstructure in the SPI gel by plaster Through the analysis of electron microscope,it was observed that with the increasing of the concentration of metal ions,the gel's hole diameter decreased and the W_(fro) value of gel gradually decreased.We believe that with the macromolecular chain becoming closer together to form a gel network,its hole diameter became smaller and the space would be smaller,space resistance would be larger,and the combined water that could be tied into the holes would be less and as a result the content of frozen water decreased.
5.Effects of metal ion and cooking oil on the secondary structure of SPI gels by plaster In the SPI gels by plaster,the proportions ofα-helix,β-sheet,β-turn and random coil were 4.9%、33.0%、14.6%and 47.5%.After the addition of Na~+,Mg~(2+) and Ca~(2+),the proportions ofα-helix slightly increased,while theβ-sheet structure declined in the proportion.On the other hand,with the increasing of the concentration of metal ions, protein molecular chain connected more closely,resulting in great changes in the space. In the molecular chain,the distance between the levels reduced,damaging layered structure ofβ-sheet,therefore the proportion ofβ-sheet structure decreased.
The addition of cooking oil made the proportion ofα-helix,β- sheet,β-turn and random coil in the entire system were 4.0%,26.0%,22.6%and 47.4%.It shown that the addition of cooking oil made the proportion of theα-helix andβ-sheet structure decreased slightly in the system.
6.Effects of metal ion and cooking oil on gelation process and texture properties of SPI gels by plaster Effects of metal ion and cooking oil on gelation process and texture properties of soy protein gels were studied with the soy protein isolate simulating the gelation process of traditional bean curd.It was shown that when the concentration of Na~+ was between 0 and 0.05mol/L,the clotting time was shorten with the addition of natrium ion and when the concentration of Na~+ was between 0.05 and 0.075mol/L,the clotting time was prolonged with the addition of natrium ion.When the concentration of Ca~(2+) was between 0 and 6.0mmol/L,the clotting time was obviously shorten with the addition of calcium ion.When the concentration of Mg~(2+) was between 0 and 0.03mol/L, the clotting time was shorten with the addition of magnesium ion.To sum up,the change of gel characteristic was complex with the addition of the concentration of metal ion because of the effects on protein structure from negative ion.
After the addition of metal ions,the hardness of SPI gel reduced and with the concentration of metal ions increasing,the springiness and adhesiveness decreased. When the concentration of Ca~(2+) gradually increased,the adhesiveness of SPI gel gradually reduced.Na~+,Mg~(2+) and Ca~(2+) had not obvious effect on the cohesiveness of SPI gel.
With the increasing of the concentration of the cooking oil,the hardness of tofu gel gradually decreased,springiness and cohesiveness increased slightly and adhesiveness decreased significantly.
7.The relationship between the texture properties and structure of SPI gel by plaster The addition of sodium ion,magnesium ion and calcium ion made the proportion ofα-helix structure slightly improve,while the proportion ofβ-sheet structure relatively decline.
With the increasing of the concentration of cooking oil,the hardness of tofu gel gradually decreased,the springiness increased,adhesiveness decreased clearly and the cohesiveness increased slightly.The addition of cooking oil made the proportion ofα-helix structure andβ-sheet structure declined.In our opinion,a layer of lipid molecules membrane came into being on the surface of the gel network after the addition of oil.The membrane played a role in locking water to strengthen the reversion ability of gel system and increased the springiness of the SPI gel.
8.The relationship between the texture properties and structure of SPI gel by plaster Generally,the addition of sodium ion,magnesium ion and calcium ion made the W_(nf) and W_(fro) value declined and the hardness and springiness of the gel declined.The change of adhesiveness was ruleless and the cohesiveness had not clear change.
It was presumed that the reduction of W_(nf) value and the content of water in the hole of the gel reduced the role of support and stability,so the hardness and springiness of the gel declined.
On the aspect of adhesiveness,with the increasing of the concentration of Ca~(2+),the W_(nf) value of the gel gradually decreased,the adhesiveness of the gel decreased,possibly because of the formation of calcium bridges.The changes of water content were ruleless with the change of the other two metal ions and their impact on the adhesiveness should be studied further.
9.Effects of different dehydration conditions on the state of water in tofu In the course of tofu dehydration,with the increasing of the stress and press time,the water in gel network have been extruded continuously.The water content,equilibrium water content,frozen water content and swelling ratio in tofu gel declined clearly and the non-frozen water content in tofu gel increased slightly.From the appearance of the tofu, when the press time was 20 minutes and stress was 700g and 900g,tofu had good shape. When press time changed from 10 minutes to 40 minutes,press time had not obvious effect on tofu.
10.Effects of the state of water on the texture properties of tofu gel With the increasing of pressure,in the gel system the frozen water content reduced,the hardness of the gel increased continuously,the springiness and cohesiveness tended to increase and the adhesiveness declined after increasing.
With the pressure time prolonged,in the gel system the adhesiveness declined all the while.The water in the gel separated out.At the same time,the network shrank continuously and the water in the network became unsaturated.Therefore during this time period,the water content decreased,the hardness and springiness tended to declined. We believe that the quality of the gel that has just been produced was not the best quality status.
The water content and the water state of tofu gel had an important effect on the edibility and stability of quality.This experiment showed that during the facture of Tofu in small sample,the diameter of the hole in the network structure of tofu 11.4μm when the quality was the best.With this water content,the appearance,hardness and springiness of Tofu was better.
11.The factors of the stability of tofu The factors affecting the stability of tofu had temperature,water content,salt concentration and so on.This research showed that the temperature had obvious effect on the stability of tofu.The shelf-life of samples at 4℃were prolonged for 9 days relative to the samples at room temperature and at the same time the water loss rate was reduced and the quality was maintained 4℃.
Different water contents had some effects on the stability of tofu.The results of stability experiment on non-fermented soybean products from the market showed that with the decreasing of water content of samples,the shelf-life reduced from six to three days at room temperature.
When the concentration of salt was 0.1%,the shelf-life of tofu was prolonged for one day and the water loss rate was reduced for about 4%.
It showed that the addition of salt could suppress the growth of mildew,salt had a certain role of locking water and holding shape and the shelf life of foods was extended. When the concentration of salt increased to 0.4%,the gel could not come into being.It was presumed that the network rebuilding from the protein chain after protein denaturalization would be held back if the concentration of salt was great excessively.
钟祥豆腐是湖北著名的特产之一。本文旨在研究豆腐的凝胶化条件,着重探讨大豆蛋白与金属离子和油脂的交互作用对豆腐凝胶形成的影响以及豆腐中水分状态与凝胶的结构和质构特性的关系,为豆腐品质和稳定性的研究与新产品的开发提供理论依据和实验基础。主要研究结果如下:
1、豆腐凝胶形成的基础研究本研究采用SPI模拟豆腐复合体系中凝胶形成的过程,探讨分子间作用力对多因素诱导的大豆蛋白凝胶体系的影响情况,实验结果表明,在石膏凝固SPI过程中,静电相互作用、疏水作用、氢键相互作用以及巯基转换作用对豆腐凝胶的形成起重要作用。
2、大豆蛋白凝胶形成过程中蛋白质的变化大豆磨浆后加热前,溶液中蛋白质的α-螺旋、β-折叠以及β-转角含量均为0,蛋白质几乎完全以无规卷曲形式存在。煮沸后冷却到80℃,点浆后充分搅拌,α-螺旋含量增大。点浆后20min,α-螺旋和β-转角略有增大,β-折叠明显减少,无规卷曲明显增多。点浆后50min,稳定的凝胶网络基本形成。压榨后形成的豆腐,α-螺旋含量减少,β-折叠含量升高。同时,凝胶网络中的非可冻结水含量升高,可冻结结合水和自由水含量下降。
3、金属离子和油脂对大豆蛋白凝胶中水分状态的影响不同的金属离子对大豆蛋白凝胶的溶涨率和平衡水含量的影响不同。随着Na~+、Mg~(2+)以及油脂浓度的增加,石膏凝固SPI凝胶的溶涨率和平衡水含量均降低,而则随着Ca~(2+)浓度的增加使得石膏凝固SPI凝胶的溶涨率和平衡水含量升高。
随着金属离子的加入,石膏凝固SPI凝胶的可冻水含量下降。在非冻结水方面,随着Ca~(2+)浓度的增大,W_(nf)值越来越大。但是Na~+和Mg~(2+)对凝胶中非冻结水含量影响情况相反。另外,金属离子的加入会缩短凝胶形成时间,说明金属离子促进凝胶的形成。
4、石膏凝固SPI凝胶水分状态与显微结构的关系通过电镜分析观察到,随着金属离子浓度的增大,凝胶孔洞直径逐渐减小,凝胶的W_(fro)值逐渐减小。认为随着大分子链聚集更为紧密,形成的凝胶网络孔洞直径越小,其形成的网格空间就会越小,产生的空间阻力会变大,能够进入孔洞而被束缚的水就越少,从而凝胶的可冻结水含量下降。
5、金属离子和油脂对石膏凝固SPI凝胶二级结构的影响石膏凝固SPI凝胶体系中,α-螺旋结构、β-折叠结构、β-转角结构以及无规卷曲结构的比例分别为4.9%、33.0%、14.6%以及47.5%。钠、镁和钙三种金属离子的加入,使得体系的α-螺旋结构的比例略有提高,而β-折叠结构的比例相对下降。另一方面,随着金属离子浓度的增大,蛋白分子链连接更加紧密,使得它们之间的空间位置发生很大变化,分子链层与层之间距离减小,破坏了β折叠的层状结构,因此β折叠结构的比例下降。油脂的加入,使得整个体系的α-螺旋结构、β-折叠、β-转角以及无规卷曲结构的比例分别为4.0%、26.0%、22.6%以及47.4%。说明油脂的加入使得体系中的α-螺旋结构和β-折叠结构的比例略有下降。
6、金属离子和油脂对石膏凝固SPI凝胶形成过程及其质构特性的影响本文用大豆分离蛋白模拟传统豆腐的凝胶形成过程,研究了金属离子和油脂对大豆蛋白凝胶形成过程及质构特性的影响。结果表明,Na~+在0～0.050mol/L时,随着浓度的增大,凝胶形成时间缩短,在0.050～0.075mol/L时,凝胶形成时间延长。Ca~(2+)的浓度在0～6.0mmol/L之间时,随着浓度的增大,凝胶形成时间明显减少。Mg~(2+)的浓度在0～0.030mol/L范围内,随着浓度的增大,凝胶形成时间缩短。总的来说,随着金属离子浓度的增加,阴离子对蛋白质构象的影响导致凝胶特性的变化变得复杂。金属离子加入后,SPI凝胶的硬度减小,并且随着金属离子浓度的增大,弹性和粘附性逐渐减小;当Ca~(2+)浓度逐渐增大时,SPI凝胶的粘附性逐渐减小。Na~+、Mg~(2+)、Ca~(2+)对SPI凝胶的内聚性影响均不大。
随着油脂浓度的逐渐增大,豆腐凝胶的硬度逐渐减小,弹性逐渐增大,粘附性明显减小,内聚性略微增大。
7、石膏凝固SPI凝胶质构特性与结构的关系钠、镁和钙三种金属离子的加入,使得SPI凝胶的α-螺旋结构的比例略有提高,而β-折叠结构的比例相对下降。随着油脂浓度的逐渐增大,豆腐凝胶的硬度逐渐减小,弹性逐渐增大,粘附性明显减小,内聚性略微增大。油脂的加入使得体系中的α-螺旋结构和β-折叠结构的比例均略有下降。油脂加入后,在凝胶网络表面会形成一层油脂分子膜,包裹凝胶网络的油脂分子膜起到了锁水的作用,使凝胶体系回复能力加强,所以凝胶的弹性增大。
8、石膏凝固SPI凝胶水分状态与质构特性的关系随着Na~+、Mg~(2+)、Ca~(2+)三种金属离子浓度的增大,凝胶的W_(nf)与W_(fro)值逐渐减小,凝胶的硬度和弹性逐渐减小,粘附性变化无明显规律,内聚性影响不大。其原因可能是W_(nf)值的降低,凝胶孔径中含水量减小,使得水分对凝胶网络的支撑与稳定作用降低,所以其硬度和弹性降低。在粘附性方面,随着Ca~(2+)浓度的增大,凝胶的W_(nf)值逐渐减小,凝胶粘附性逐渐减小,可能是因为Ca~(2+)形成钙桥原因。其他两种金属离子的水分含量变化对凝胶粘附性影响未出现规律性变化,它们对粘附性的影响有待进一步研究。
9、不同脱水工艺条件对豆腐的水分状态的影响随着压力的增大和压榨时间的延长,凝胶网络中水分不断被压出,豆腐凝胶含水量、平衡水分含量、可冻结水含量以及溶胀率均明显降低,非冻结水的含量略有提高。从外观情况看,压榨时间为20min,压力采用700g和900g时豆腐外形好。在10min到40min的范围内,压榨时间对豆腐的影响不明显。
10、水分状态对豆腐凝胶质构特性的影响随着凝胶压榨压力的不断增大,凝胶体系可冻结水分含量减少,凝胶硬度不断增大,弹性基本呈增大趋势,粘附性先增大后减小,内聚性呈增大趋势。
随着压榨时间的延长,粘附性一直减小,凝胶中水分不断析出,网格不断缩小,使得网格中水分不再处于饱和状态,所以在这段时间内,水分不断减少,其硬度、弹性都呈下降趋势。因此认为刚制作的凝胶水分含量和状态不是最佳品质状态。豆腐凝胶的含水量及其水分状态对豆腐的可食用性和品质稳定性有着重要的影响作用。本实验显示,在豆腐制备中,凝胶特性最佳时的豆腐中网络结构孔径的大小在11.4μm左右,这个时候承载的水分量使得豆腐的外观、硬度以及弹性较好。
11、豆腐稳定性的影响因素影响豆腐稳定性的因素有温度、水分含量、盐浓度等。温度对豆腐稳定性的影响明显。4℃的样品将室温(25℃)下的样品保质期延长9d,还可以有效的减少失水率,保持豆腐的品质。
不同含水量对豆腐的稳定性也有一定影响。对市售非发酵豆制品进行稳定性实验,结果显示,随着样品含水量的减少,样品的室温(25℃)下的保质期由6d减少到3d。
盐浓度为0.1%时,豆腐保质期延长1d,失水率降低4%左右,说明盐的加入可以抑制霉菌的生长,有一定锁水的作用,从而延长食品的保质期。而当盐浓度增加至0.4%时,凝胶无法形成,由此可见,盐浓度过大会影响变性后蛋白质分子链重新构建网络。
Our country is rich in soybean resources.Soybean products have a history of more than 2000 years.Tofu is one of the most popular soybean products.It contains much protein,but without cholesterol.And it has many excellent processing qualities.What's more,tofu is rich of isoflavone which plays an important role in restraining cancers. The gel characteristic of soybean protein is one of the important characteristics in the process of soybean produce.In the protein gel,the interaction of protein and water decides and maintains the three-dimensional structure of protein.It not only influences the function of protein,but also influences the food sensory evaluate.
The reports about tofu focus on the compose and structure of soybean proteins, function characteristic of soybean proteins,the gel-forming mechanism and processing technology,but little on the relationship between the state of water in tofu and the gel's texture properties.Thus,studies on affecting factors of tofu gel forming and the relationship between the state of water and the gel's texture properties are very important to effectively control the gel forming and develop tofu products with high quality. Zhongxiang Tofu is one of the special local products in Hubei.Setting conditions and relative ground works of tofu gel,the interaction between metal ion,cooking oil and soybean protein and its influence on tofu gel forming,the relationship between the state of water in tofu and the gel's structure and texture properties were investigated.The results were as follows:
1.Basis studies on Tofu gel forming Gel forming and effects of molecular forces on gelation process and texture properties of soy protein gels induced by multifold factors were studied with the soy protein isolate simulating the gelation process of traditional tofu.It was shown that during course,the electrostatic forces,hydrophobic interactions and hydrogen bonds and sulphydryl conversion function have given important contribution for the gelation process of soybean protein gels coagulating by plaster.
2.Protein's change in the gelation process of soybean protein After soybean's milling,there were not anyα-helix,β-sheet andβ-turn and the secondary structure of soybean protein was mainly coil.When the boiled soybean milk was cooled to 80℃, plaster was put into it and the content ofα-helix was larger.Twenty minutes later,the content ofα-helix andβ-turn was a little larger,the content ofβ- sheet was clearly smaller and the content of coil was larger.Twenty minutes later,steady gel net came into being on the whole.After press,tofu came into being and content ofα-helix was smaller and content ofβ- sheet was larger.At the same time,the content of non- freezing water increased and the content of frozen water decreased in the gel net.
3.Effects of metal ion and cooking oil on the state of water of soy protein gels Different metal ions had different effects on the swelling ratio and equilibrium water content of soy protein gels.With the concentrations of Na~+,Mg~(2+) and cooking oil increasing,the swelling ratio and equilibrium water content of soy protein gels descended.On the other hand,the swelling ratio and equilibrium water content of soy protein gels increased with the increasing of the concentrations of Ca~(2+).
With the adding of metal ion,the content of frozen water soy protein gels descended. With the concentrations of Ca~(2+) increasing,the W_(nf) value was more and more large. However,the effect of Na~+ and Mg~(2+) on the content of non- frozen water in the gel was opposite.On the other hand,the time for gel formation was shorten with the addition of metal ions,which mean that metal ions could promote the formation of gel.
4.The relationship between the state of water and the microstructure in the SPI gel by plaster Through the analysis of electron microscope,it was observed that with the increasing of the concentration of metal ions,the gel's hole diameter decreased and the W_(fro) value of gel gradually decreased.We believe that with the macromolecular chain becoming closer together to form a gel network,its hole diameter became smaller and the space would be smaller,space resistance would be larger,and the combined water that could be tied into the holes would be less and as a result the content of frozen water decreased.
5.Effects of metal ion and cooking oil on the secondary structure of SPI gels by plaster In the SPI gels by plaster,the proportions ofα-helix,β-sheet,β-turn and random coil were 4.9%、33.0%、14.6%and 47.5%.After the addition of Na~+,Mg~(2+) and Ca~(2+),the proportions ofα-helix slightly increased,while theβ-sheet structure declined in the proportion.On the other hand,with the increasing of the concentration of metal ions, protein molecular chain connected more closely,resulting in great changes in the space. In the molecular chain,the distance between the levels reduced,damaging layered structure ofβ-sheet,therefore the proportion ofβ-sheet structure decreased.
The addition of cooking oil made the proportion ofα-helix,β- sheet,β-turn and random coil in the entire system were 4.0%,26.0%,22.6%and 47.4%.It shown that the addition of cooking oil made the proportion of theα-helix andβ-sheet structure decreased slightly in the system.
6.Effects of metal ion and cooking oil on gelation process and texture properties of SPI gels by plaster Effects of metal ion and cooking oil on gelation process and texture properties of soy protein gels were studied with the soy protein isolate simulating the gelation process of traditional bean curd.It was shown that when the concentration of Na~+ was between 0 and 0.05mol/L,the clotting time was shorten with the addition of natrium ion and when the concentration of Na~+ was between 0.05 and 0.075mol/L,the clotting time was prolonged with the addition of natrium ion.When the concentration of Ca~(2+) was between 0 and 6.0mmol/L,the clotting time was obviously shorten with the addition of calcium ion.When the concentration of Mg~(2+) was between 0 and 0.03mol/L, the clotting time was shorten with the addition of magnesium ion.To sum up,the change of gel characteristic was complex with the addition of the concentration of metal ion because of the effects on protein structure from negative ion.
After the addition of metal ions,the hardness of SPI gel reduced and with the concentration of metal ions increasing,the springiness and adhesiveness decreased. When the concentration of Ca~(2+) gradually increased,the adhesiveness of SPI gel gradually reduced.Na~+,Mg~(2+) and Ca~(2+) had not obvious effect on the cohesiveness of SPI gel.
With the increasing of the concentration of the cooking oil,the hardness of tofu gel gradually decreased,springiness and cohesiveness increased slightly and adhesiveness decreased significantly.
7.The relationship between the texture properties and structure of SPI gel by plaster The addition of sodium ion,magnesium ion and calcium ion made the proportion ofα-helix structure slightly improve,while the proportion ofβ-sheet structure relatively decline.
With the increasing of the concentration of cooking oil,the hardness of tofu gel gradually decreased,the springiness increased,adhesiveness decreased clearly and the cohesiveness increased slightly.The addition of cooking oil made the proportion ofα-helix structure andβ-sheet structure declined.In our opinion,a layer of lipid molecules membrane came into being on the surface of the gel network after the addition of oil.The membrane played a role in locking water to strengthen the reversion ability of gel system and increased the springiness of the SPI gel.
8.The relationship between the texture properties and structure of SPI gel by plaster Generally,the addition of sodium ion,magnesium ion and calcium ion made the W_(nf) and W_(fro) value declined and the hardness and springiness of the gel declined.The change of adhesiveness was ruleless and the cohesiveness had not clear change.
It was presumed that the reduction of W_(nf) value and the content of water in the hole of the gel reduced the role of support and stability,so the hardness and springiness of the gel declined.
On the aspect of adhesiveness,with the increasing of the concentration of Ca~(2+),the W_(nf) value of the gel gradually decreased,the adhesiveness of the gel decreased,possibly because of the formation of calcium bridges.The changes of water content were ruleless with the change of the other two metal ions and their impact on the adhesiveness should be studied further.
9.Effects of different dehydration conditions on the state of water in tofu In the course of tofu dehydration,with the increasing of the stress and press time,the water in gel network have been extruded continuously.The water content,equilibrium water content,frozen water content and swelling ratio in tofu gel declined clearly and the non-frozen water content in tofu gel increased slightly.From the appearance of the tofu, when the press time was 20 minutes and stress was 700g and 900g,tofu had good shape. When press time changed from 10 minutes to 40 minutes,press time had not obvious effect on tofu.
10.Effects of the state of water on the texture properties of tofu gel With the increasing of pressure,in the gel system the frozen water content reduced,the hardness of the gel increased continuously,the springiness and cohesiveness tended to increase and the adhesiveness declined after increasing.
With the pressure time prolonged,in the gel system the adhesiveness declined all the while.The water in the gel separated out.At the same time,the network shrank continuously and the water in the network became unsaturated.Therefore during this time period,the water content decreased,the hardness and springiness tended to declined. We believe that the quality of the gel that has just been produced was not the best quality status.
The water content and the water state of tofu gel had an important effect on the edibility and stability of quality.This experiment showed that during the facture of Tofu in small sample,the diameter of the hole in the network structure of tofu 11.4μm when the quality was the best.With this water content,the appearance,hardness and springiness of Tofu was better.
11.The factors of the stability of tofu The factors affecting the stability of tofu had temperature,water content,salt concentration and so on.This research showed that the temperature had obvious effect on the stability of tofu.The shelf-life of samples at 4℃were prolonged for 9 days relative to the samples at room temperature and at the same time the water loss rate was reduced and the quality was maintained 4℃.
Different water contents had some effects on the stability of tofu.The results of stability experiment on non-fermented soybean products from the market showed that with the decreasing of water content of samples,the shelf-life reduced from six to three days at room temperature.
When the concentration of salt was 0.1%,the shelf-life of tofu was prolonged for one day and the water loss rate was reduced for about 4%.
It showed that the addition of salt could suppress the growth of mildew,salt had a certain role of locking water and holding shape and the shelf life of foods was extended. When the concentration of salt increased to 0.4%,the gel could not come into being.It was presumed that the network rebuilding from the protein chain after protein denaturalization would be held back if the concentration of salt was great excessively.
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51. Danji Fukushima. Structure of plant storage proteins and their function[J]. Food Reviews International, 1991 ,7(3): 353-381
52. Enscore D J ,Hopfraberg H B,Stannett V T. Effect of Particle Size on the Mechanism Controlling n-Hexane Sorption in Glassy Polystyrene Microspheres[J]. Polymer,1977,(18): 793-806
53. Ellepolaa S W, Choib S M , Phillipsc D L, et al. Raman spectroscopic study of rice globulin [J]. Journal of Cereal Science, 2006, 43 (1) :8529
54. Gang L, Xiong Y L. Gelation of Chicken muscle Myofibrillar Proteins treated with protease inhibitors and phosphates[J]. J Agric Food Chem, 1997,61: 3437—3442
55. GuanY L, Shao L, Yao K D. A study on correlation between water state and swelling kinetics of chitosan-based hydrogels[J]. JAPPl Polym Sci, 1996, 61:2325-2335
56. H.L.Wang, E.W.Swain, W.F.Kwolek, et al. Effect of soybean varieties on the yield and qualityoftofu[J]. Cereal Chem, 1983,60(3): 245-248
57. Hajime Hatta et al. Turbidity and hardness of a heat - induced gel of hen egg ovalbumnin[J]. Agric Bio Chem, 1985 , 50(8): 2083-2089
58. Heertje I. Structure and function of food products: a review[J]. Food Struct, 1993,12: 343-346
59. J.Y. Lu, Eloise Carter, and R. A. Chung. Use of Calcium Salts for Soybean Curd Preparation[J]. Journal of Food Science , 1980 ,45: 32-34
60. Kato Akio, NakaShury I. Hydrophobixity determined by a fluoresscene probe method and its correlation with surface properties of protein[J]. Biochcimica et Biophisica Acta, 1980, 624: 13-20
61. Kinsella J E. Functional Properties of Soy Proteins[J]. J. A. O. C. S, 1979, 56: 242
62. Kohyama K etal. Rheological Characteristics and Gelation Mechanism of Tofu(Soybean Curd ). J Agric. Food Chem, 1995, 43: 1808-1812
63. L I Khomutov, N A Lashek, N M Ptitchkina, and E R. Morris. Temperature-composition phase diagram and gel properties of the gelatin-starch-water system[J]. Carbohydrate Polymers, Volume 28, Issue 4,December 1995, 341-345
64. Lee W F, Yuan W. Thermoreversible hydro gels XI. Effect of salt on the swelling properties of the(N-isopropylamide-oo-sodiurn 2-acrylamido-2-methylpropyl sulfonate)copolymerichydrogels[J]. J AppI Polym Sci. 2001, 79: 1675-1684
65. Lee, C.M., Filipi, I, Xiong, Y.L, Smith, D, Regenstein, J, Damodaran, S., Ma,C.Y.,Haque, Z.U. Standardized failure compression test of protein gels from a collaborative study[J]. Journal of Food Science, 1997, 62: 1163-1166
66. Li-Jun Wang , Dong Li, Eizo Tatsumi ,Zhi-Sheng Liu , Xiao Dong Chen, Li-Te Li. Application of two-stage ohmic heating to tofu processing[J]. Chemical Engineering and Processing, 2007,46: 486-490
67. Lim,B.T., de Man,J.M,Deman,L.et al. Yield and quality of tofu as affected by soybean and soymilk characteristics: calcium sulfate coagulant. J. of Food Science,1990,55(4): 1088-1092
68. Liu W, Yao K. What causes the unfrozen water in polymers: hydrogen bonds between water and polymer chains[J]. Polymer, 2001, 42: 3943-3947
69. Liu Y, Huglin M. Observations by DSC on bound water structure in some physically crosslinked hydrogels[J]. Polym Int, 1995, 37: 63-67
70. M.P.Ji, T.D.Cai and K.C.Chang. Tofu yield and texture properties from three soybean cultivars as affected by Ratios of 7S and 11 S proteins[J]. J. Food.Sci. 1999,64(5): 763-767
71. Maria C P, Maria C A. Rheological properties of acidic soybean proteingels: salt addition effect[J]. Food Hydrocolloids, 1999, 13: 167-176
72. Masahiko N., sakamoto H., Toiguchi S. Retort- resis tant Yofu Prepared by incubation with microbial tyansglutaminase[J]. Food Hydrocoiloids, 1996,10:41-44
73. MURPHY PA, CHEN H P, HAUCK C C, et al. Soybean protein composition and tofu quality[J]. Food Technology, 1997, 51(3): 86.
74. Murphy, P A, Chen, H P, Hauck C C, Wilson LA. Soybean protein composition and tofu quality[J]. Food Technology, 1997,51(3), 86-88, 110
75. Nagano T, Mori H, Nishinari K. Effect of heating and cooling on the gelation kinetics of 7S globulin from soybeans[J]. Agric.Food Chem. 1994,42,1415 — 1419
76. NAKAMURA T, UTSUMI S, KITAMURA K, et al. Cultivar difference in gelling characteristics of soybean glycinin[J]. J Agri Food Chem, 1984, 32: 647.
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