Effect of thermal pretreatment of raw soymilk on the gel strength and microstructure of tofu induced by microbial transglutaminase

The influence of thermal pretreatment of raw soymilk on the gel hardness and microstructure of tofu, induced by microbial transglutaminase (MTGase), was investigated in this paper. Modulated differential scanning calorimetry analysis showed that individual proteins in soymilk were to a various extent denatured by different thermal pretreatments. The viscosity of the soymilk and the gel hardness of MTGase-induced tofu were more highly related with the heating rate (up to 90 °C) than the mode of heating. At any enzyme concentration of MTGase, the tofus prepared from soymilk heated at 75 °C for 10 or 30 min showed highest gel hardness among all tested ones (P?0.05). Scanning electron microscopy analysis indicated that the microstructure of the tofu from soymilk heated at 75 °C for 30 min had a unique coral-like structure, much more continuous and homogenous than that from soymilk at 95 °C for 5 min. These results confirmed that the appropriate heat pretreatment (e.g. in the present, at 75 °C for 10-30 min) remarkably improved the gel strength of tofu by means of MTGase, and strengthened the tofu gel structure.     

Effect of microbial transglutaminase on the rheological and textural characteristics of black soybean packed tofu coagulating with Agar

The objective of this study was to investigate the effects of microbial transglutaminase (MTGase) on the rheological and textural characteristics of black soybean packed tofu containing agar as the coagulant. Results showed that the addition of MTGase increased the gelation temperature of soymilk, and produced a firmer and more elastic packed tofu with low cooking loss. Dynamic rheological and textural parameters of the tofu were significantly affected by enzyme concentration, incubation temperature and time. Both G′ and η∗ were positively correlated to the hardness, gumminess and adhesiveness of packed tofu, thus the rheological parameters (G′ and η∗) were good indicators of the texture of the packed tofu. Results suggested that good quality packed tofu could be produced by mixing 90 g black soymilk solid and 2 g agar powder in 1 L of water and incubating with 10 g MTGase for 30 min at 55 °C.     

Stability of soybean seed composition and its effect on soymilk and tofu yield and quality

Soymilk was made from 10 soybean [Glycine max (L.) Merr.] lines grown at three locations for 2 years, using an 18:1 water/soy protein ratio. Tofu was made with either glucono-delta-lactone (GDL) or calcium sulphate dihydrate (CS). Genotype and year effects were substantially greater than location effects on soybean protein content and seed composition; soymilk and tofu yield, solids levels, and pH; and tofu colour, hardness, and firmness. Genotype by location and genotype by year interaction effects were minor relative to the genotype and year effects. Yield of soymilk, GDL tofu, and CS tofu, which averaged 7.39 l, 6.29 kg, and 6.15 kg per kg soybeans, respectively, were all positively correlated with seed protein and stachyose, and negatively correlated with seed oil, free sugar, sucrose and remainder content. Seed protein was positively correlated with tofu hardness and firmness, while seed oil, free sugar, sucrose, and remainder content were generally negatively correlated with tofu quality parameters. Results of stepwise regression analysis showed that seed protein was the major determinant of soymilk yield and solids content, while soymilk yield was, in turn, the major factor determining GDL tofu yield. Procedures used in making soymilk and tofu play a major role in determining which seed component has a major effect on soymilk and tofu yield and quality.     

The viscoelastic properties of soybean curd (tofu) as affected by soymilk concentration and type of coagulant

The viscoelastic properties of different types of tofu were investigated. Soymilk concentrations were 5, 6, 7, 8 and 9%. Coagulants used were 30 mm CaSO₄ or 30 mm glucono‐delta‐lactone (GDL). As the concentration of soymilk was increased viscosity and handling difficulties increased. A high concentration of soymilk in tofu gave a high break stress and produced hard tofu. The four‐element Burgers model fitted the creep behaviour and both viscous and elastic parameters could be acquired from model analysis, reflecting changes in elasticity and viscosity of tofu. The constant viscous parameter in the model increased with increasing soymilk concentration. The viscous parameters of viscoelastic materials like tofu gel, obtained from small deformation tests, seemed to correlate, to some extent, with the break stress obtained from large deformation tests. For hard tofu production increasing the soymilk concentration within a certain range and the partial replacement of calcium sulphate coagulant by GDL could be effective options.     

Effect of inulin on the rheological properties of silken tofu coagulated with glucono-δ-lactone

This study investigated the rheological properties of inulin-containing silken tofu coagulated with glucono-δ-lactone (GDL) upon heating. Inulin (Raftiline^® HP-gel) was added to a soy protein isolate-enriched cooked soymilk at 0%, 1%, 2%, 3% and 4% (w/v) levels along with 0.4% (w/v) GDL to prepare acid-induced silken tofu. Gelation was induced by heating the soymilk mixture from 20 to 90 °C at a constant rate (1 °C/min) or isothermally at 90 °C for 30 min. The gelling properties were measured with dynamic small-deformation mechanical analysis and static large-deformation compression tests. The rheological changes in soymilk during gelation were dependent upon both the pH decline (hydrolysis of GDL) and the specific temperature of heating. Control samples heated to 50 °C, with the pH lowered to 5.95, started to gel, showing a rapid increase in storage (G′) and loss (G″) moduli afterwards. The addition of 2% inulin lowered the on-set gelling temperature by 2.8 °C and improved (P < 0.05) both rheological parameters of the tofu gel as well as hardness and rupture force (textural profile analysis) of the formed silken tofu. The results indicated that inulin enhances the viscoelastic properties of GDL-coagulated silken tofu, and the textural effect of inulin is an added benefit to its current application mainly as a prebiotical ingredient in food.     

DRY TOFU CHARACTERISTICS AFFECTED BY SOYMILK SOLID CONTENT AND COAGULATION TIME1

The effects of soymilk solid content (°Brix) and coagulation time on the yield, proximate composition, and textural characteristics of dry tofu were investigated. Soymilk °Brix significantly (p < 0.05) influenced tofu yield, fracturability, hardness, springiness, cohesiveness, and chewiness. Among soymilk °Brix of 7.1, 7.8, 8.4, and 9.2 tested, °Brix 9.2 (9:1 water/bean ratio) produced the lowest yield (201g/100g bean), but the highest fracturability, hardness, cohesiveness, springiness, and chewiness. Coagulation time for 5 min resulted in a significantly lower tofu yield but higher cohesiveness than coagulation for 15 min. The fracturability and hardness of tofu from 9.2 °Brix soymilk coagulated for 5 min were greatest (18.0 and 19.6 kg, respectively). Lower soymilk °Brix and longer coagulation time produced tofu with a significantly higher moisture content. Dry tofu contained approximately 75–78% moisture (wet basis), 55–58% protein (dry basis), and 20–23% fat (dry basis).     

The properties and the related protein behaviors of oil bodies in soymilk preparation

Oil bodies (OBs), one of the major components of soymilk, are very important for the properties of soymilk and its related products. In this study, the properties and the related protein behaviors of OBs in the soymilk preparation were systematically examined. Raw soymilk OBs could keep their natural integrities after grinding, and they were bound by many soybean proteins (mainly β-conglycinin, glycinin, and Gly m Bd 30K) that could be removed by pH 11.0 washing. Heated soymilk OBs, including heat-induced coalesced OBs (>1 μm) and modified OBs (around 400 nm), were strongly bound by β-conglycinin and glycinin that even could not be removed by pH 11.0 washing. The oleosins (24 kDa, P29530 and P29531; 18 kDa, C3VHQ8) of raw soymilk OBs were hydrolyzed to 16 kDa polypeptides (on SDS–PAGE gel) when exposed to room temperature, while immediately heating, the raw soymilk could prevent the oleosin hydrolysis. Then, the isolated raw and heated soymilk OBs revealed different dispersion stabilities as a function of pH. At last, tofu curds were made from two reconstituted soymilks containing non-lipid soymilk and raw or heated soymilk OBs (pH 11.0 isolation), and one trend was found that tofu curds containing heated soymilk OBs had lower breaking stress and Young’s modulus than those containing raw soymilk OBs. It is considered that this study is meaningful for designing new strategies to improve the qualities of soymilk and its related products.     

Impact of microbial transglutaminase on gelling properties of Indian mackerel fish protein isolates

Impacts of microbial transglutaminase (MTGase) (0–0.6 units/g sample) on gel properties of Indian mackerel unwashed mince, surimi and protein isolates with and without prewashing were studied. Generally, lower myoglobin and lipid contents were found in protein isolate with and without prewashing, compared to those of unwashed mince and surimi (P < 0.05). Protein isolate had the decreased Ca²⁺-ATPase and protein solubility, indicating protein denaturation. When MTGase was incorporated, breaking force and deformation of all gels markedly increased, especially as MTGase levels increased (P < 0.05). At the same MTGase level, gel from protein isolate with prewashing exhibited the highest breaking force and deformation (P < 0.05). The addition of MTGase could lower the expressible moisture content of most gels. No change in whiteness of gel was observed with the addition of MTGase (P > 0.05), but gel from protein isolate gels had decreased whiteness as MTGase at high level was added. The microstructure of protein isolate gels without prewashing showed a similar network to unwashed mince gels, whilst a similar network was observed between surimi gel and gel from protein isolate with prewashing. Nevertheless, a larger void was noticeable in gels from protein isolates. All gels incorporated with MTGase (0.6 units/g) showed a slightly denser network than those without MTGase. Thus, gel with improved properties could be obtained from protein isolate from Indian mackerel with added MTGase.     

PREPARATION OF TOFU GEL BY HIGH-PRESSURE PROCESSING

The potential of high-pressure processing to produce high-quality foods that are microbiologically safe and with extended shelf life has received much attention in the food industry. Tofu is a nutritious and digestible food that is widely consumed in the world. Differential scanning calorimetry analysis indicated that high-pressure processing could denature soymilk protein completely. Soymilk mixed with glucono-delta-lactone, when processed at 500 MPa for 20 min, could form tofu gel with strength. These gels had smooth appearance, homogeneous texture, light-yellow color and less off-flavors. Scanning electron microscopy revealed it had a cross-linked network structure. This provided a novel method for the soymilk and tofu manufacturers to process soymilk for making tofu gels with new properties.

PRACTICAL APPLICATIONS

High-pressure processing is now getting an increasing interest in the food industry. It is a possible alternative to heat treatment. Soy proteins play an important role in food consumption worldwide. Tofu represents one of the major categories of soy foods. The objectives of this study were to investigate the novel method to make tofu with new properties by high-pressure processing. The results will help soymilk and tofu manufacturers to widen the method to make tofu gels.     

Comparison of Two Small-Scale Processing Methods for Testing Silken Tofu Quality

In developing a simple, reliable, small-scale method to assess silken tofu quality in our soybean improvement program, we examined two processing methods and two coagulants, glucono-δ-lactone (GDL) or nigari (magnesium chloride) in two experiments. Silken tofu was prepared from a commercial soybean variety (expt 1) or seven soybean varieties (V1–V7) which were grown and harvested together (expt 2). The soybeans were soaked overnight (the soak method, with 55 g soybeans) or ground dry first (the dry method, with 60 g soybeans) before processing. The quality of the silken tofu was evaluated and compared among varieties and coagulant-processing methods and their interactions. Moisture and protein content in soymilk and soybean seeds, soymilk yield and protein and solid recovery in soymilk were determined. Compared with the dry method, the soak method allowed faster soymilk extraction, produced soymilk with lower solid and higher protein content and firmer silken tofu with either GDL or nigari as coagulant. Depending on whether nigari or GDL was used as coagulant, the soak method also produced silken tofu with the highest or the lowest water loss which correlated strongly and negatively with tofu hardness (r?=??0.93***). Differences were detected among varieties for the key quality attributes. Taken together, the soak method with GDL as coagulant would be the preferred combination to use to assess tofu quality.     

点浆条件对酸浆豆腐品质的影响

为探究点浆时豆乳温度、点浆后蹲脑温度和蹲脑时间对酸浆豆腐品质的影响,提高酸浆豆腐品质,本研究利用质构分析、SDS-PAGE和低场核磁等技术分析了豆腐品质、豆乳中蛋白凝固过程、大豆蛋白各亚基含量和水分分布的变化规律。结果表明,点浆时豆乳温度对豆乳沉淀中蛋白和大豆蛋白各亚基含量无显著影响,但对豆腐品质和水分分布有较大影响。当点浆时豆乳温度为75℃时,豆腐的硬度最小,而其弹性、得率、含水量、保水性、感官得分和T₂₁弛豫时间最大。蹲脑温度和蹲脑时间对豆腐品质、豆乳沉淀中蛋白含量和大豆蛋白各亚基含量影响显著,当蹲脑温度75℃和蹲脑时间20 min时,豆腐硬度最小为15.80 N,而弹性、得率、含水量、保水性和T₂₁弛豫时间最佳,分别为5.15 mm、241.3%、76.36%、65.68%、43.29 ms,且豆腐品质豆乳沉淀中蛋白含量最佳。当点浆时豆乳温度为75℃、蹲脑温度为75℃和蹲脑时间20 min时,豆腐品质和感官得分最优。     

大豆油体对豆腐凝胶性质的影响研究

通过向脱脂豆浆中添加大豆油体,研究其对豆腐凝胶性质的影响。运用质构仪和离心机分别测定了豆腐凝胶的硬度和失水率。结果表明:非油体成分含量一定时,油体的加热时间延长(0~4min)对豆腐凝胶有利,体现在硬度增加和失水率降低;体系固形物含量一定时,油体与非油体成分含量之间存在一个最佳干基比值(0.143),此比值下豆腐凝胶性质最好,过高或过低凝胶性质均会减弱;室温下,随生豆浆放置时间的延长(0~60 min)制得的豆腐硬度略有下降,失水率没有明显变化。此外,与大豆油相比,油体因其独特的结构对豆腐凝胶有利。     

Effect of formaldehyde on protein cross-linking and gel forming ability of surimi from lizardfish induced by microbial transglutaminase

Impact of formaldehyde (FA) at various levels (0–9 μmol/g surimi) on gel properties of surimi from lizardfish added with microbial transglutaminase (MTGase) was studied. During iced storage of 10 days, total and free FA in lizardfish flesh increased continuously (P < 0.05). In the presence of FA, breaking force of gels slightly increased, whilst the deformation decreased (P < 0.05). The addition of MTGase (0.4 units/g surimi) was able to increase gel strength and water holding capacity of resulting gel. Nevertheless, gel strengthening effect of MTGase was lowered when FA at higher level was present. Myosin heavy chain (MHC) dominantly underwent polymerisation to a higher extent when either MTGase or FA was added. The higher reduction in ε-amino group content was observed in natural actomyosin (NAM) when FA at higher levels (0–30 μmol/g protein) was incorporated. Acyl transfer reaction mediated by MTGase was impeded in NAM containing FA, especially at higher levels. Generally, FA had an adverse effect on cross-linking ability towards surimi proteins induced by MTGase. Therefore, cross-linking and gel-forming ability of lizardfish surimi could be maximised by MTGase when surimi contained no FA.     

Improvement of microbial transglutaminase‐induced gelation of β‐conglycinin by conjugation with dextran

This work focused on the effect of glycosylation on the gelation ability of β‐conglycinin induced by microbial transglutaminase (MTGase). Rheological results indicated that the gels of β‐conglycinin‐dextran conjugate products exhibited higher G′ value (172.2 ± 8.6 Pa) compared with those of dry‐heated β‐conglycinin (75.2 ± 5.1 Pa), β‐conglycinin (53.3 ± 4.0 Pa) and β‐conglycinin‐dextran mixture (38.6 ± 2.6 Pa) after 4 h incubation with MTGase. The gels prepared from β‐conglycinin‐dextran conjugate products had higher hardness, fracturability, springiness and cohesiveness values determined by textural profile analysis. The turbidity of β‐conglycinin‐dextran conjugate products solution incubated with MTGase increased faster than those of the other three protein samples. The conjugated dextran in β‐conglycinin‐dextran conjugate products could inhibit extensive protein–protein interactions which might result in the formation of more ordered and stronger gel network structures during MTGase cross‐linking process. A compact and homogeneous gel networks in β‐conglycinin‐dextran conjugate products gels were also observed by scanning electron microscopy.     

Coagulation of soymilk and quality of tofu as affected by freeze treatment of soybeans

The present study was carried out to investigate the effects of freezing of soybeans on the coagulation of soymilk and quality of tofu. Soymilk, prepared from frozen soybeans, was found to coagulate faster in the presence of coagulant and produced a more uniform-structured gel than that from unfrozen soybeans. Tofu, prepared from frozen soybeans, showed a more orderly and denser network structure than that from unfrozen soybeans, thereby inducing an increase in some textural parameters such as hardness, springiness, gumminess and chewiness as well as syneresis. Freezing also brought about some changes in tofu quality: lower yield, lower fat and higher protein contents. Results of sensory evaluation showed that tofu from frozen soybeans had better sensory properties in terms of flavour and mouthfeel. From these results, it was concluded that freezing promoted the coagulation process of soymilk and changed the quality of tofu in a positive way.     

Effect of Soybean Varieties and Growing Locations on the Physical and Chemical Properties of Soymilk and Tofu

ABSTRACT: Soybean varieties and growing location greatly affected the protein content and color of soymilk and the protein content and yield of tofu ( P < 0.05). Protein content of soybeans was the most important affecting factor for the qualities of soymilk and tofu. There were significant correlations between the protein contents of soybeans and soymilks ( P < 0.05). There were also significant correlation between the soybean protein and the total solid content of soymilk (P < 0.05). Tofu moisture content had significant effect on the hardness and yield of tofu (P < 0.05). The correlation between soybean protein and tofu yield was significant at P < 0.05. The protein content and yield of tofu can be predicted by analyzing soybean protein.     

Effect of different coagulants on yield and quality of tofu from soymilk

Soymilk tofu coagulated with four indigenous coagulants was compared in terms of chemical, textural, colour, and sensory attributes with calcium sulphate (CS) coagulated tofu. Coagulants used are Epsom salt (ES), lemon juice (LJ), alum and top water of fermented maize (TWFM). Protein and magnesium content significantly (P < 0.05) increased from 44.5 to 51.7 g/100 g and 252 to 324.6 mg/100 g, respectively, in soybean grain to soymilk. Calcium and magnesium contents increased and decreased significantly (P < 0.05) in tofu coagulated with CS and ES, respectively. Lightness (L) values were 86.3, 86.2, 77.8, 72.4 and 84.6, redness (a) values are 0.34, 0.21, 0.87, 1.05 and 0.32, and yellowness (b) values were 24.0, 23.9, 27.3, 20.3 and 23.4 for CS, ES, LJ, alum and TWFM. The hardness, chewiness and brittleness of textural properties of tofu were significantly (P < 0.05) affected by different sources of coagulation. Sensory evaluation data shows that LJ impacts a significantly acceptable sensory attribute to tofu. This study has demonstrated that tofu quality is affected by the type of coagulant used in curding the soymilk.     

Micronization Effects on Composition and Properties of Tofu

The effect of infra-red heating (micronization) was investigated on the composition and textural, biochemical and nutritional properties of soymilk and tofu. The yield, protein content and textural properties of tofu made from micronized beans using the common procedures (70°C as coagulating temperature and CaSO₄2H ₂O as coagulating agent) were lower than those of tofu from unprocessed beans; tofu prepared from micronized beans and coagulated at 90°C using a mixture of citric acid (0.01M) and calcium sulfate (0.03M) showed improved characteristics. The microstructure of tofu prepared from micronized beans lacked the regularity of the honeycomb-like structure shown by tofu from unprocessed beans. The heat treatment by micronization had little effect on the protein components of soymilk and tofu as determined by polyacrylamide disc gel electrophoresis.     

The localization of phytate in tofu curd formation and effects of phytate on tofu texture

ABSTRACT:  The localization of phytate on tofu making and its effects on tofu texture were investigated. Thirty-eight percent and 3% of phytate in soymilk were bound to soluble protein and particulate protein, respectively, and the others were in free form. In the early stage of curd formation, phytate bound to particulate proteins and then a large part of phytate was taken into the tofu curd. Increase of phytate contents in soymilk resulted in the increase of coagulant requirement to make firm tofu. Optimal coagulant concentration (OCC) for making tofu was influenced by phytate contents. The increase of phytate in tofu caused decrease of hardness at OCC and resulted in the increase of brittleness and viscosity of tofu. We concluded that the phytate content is one important factor that contributes to tofu texture.     

高浓度煮浆对豆腐品质提升的作用及其机制

为明确高浓度煮浆对豆腐品质的影响，本研究通过采取高浓度煮浆（固形物质量分数9.2%～12.2%）低浓度稀释（统一至8.0%）法制备豆腐，对豆腐的品质进行比较，并在此基础上对豆乳和豆乳凝固状态进行比较，明确引起豆腐品质差异的原因。结果发现，与固形物质量分数为8.0%的豆乳组相比，随着煮浆浓度的上升，豆腐的硬度和咀嚼性逐渐降低，其中9.2%时最大，而弹性、得率、水分质量分数和保水性差异较小。进一步研究发现，当固形物质量分数为9.2%时，豆乳蛋白粒子比例最小（26.2%）、蛋白表面疏水性最大，而平均粒径、Zeta电位绝对值和B亚基比例随煮浆浓度增加而呈上升趋势，α与α’比例呈下降趋势。分析凝乳速度变化趋势发现，煮浆浓度越高，凝固起始时间越早，凝固速率越快。结果表明，高浓度煮浆可促使豆乳蛋白聚集程度增加，能够优化豆乳及其凝胶的加工特性，这一结果为实际生产提供了一定的理论基础。