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Professor Zhu Xiuqing, Harbin University of Commerce, et al.: Effect of TG enzyme-MgCl2 synergistic induction on cold-pressed soybean flour gel

 Last Update: 2022-11-26
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As a by-product of traditional soybean cold-pressed oil processing, cold-pressed soybean flour has the advantages of low protein denaturation, higher total protein content than soybean (about 40%~45%), low fat and rich in dietary fiber, and can be processed into soy products as protein raw materials, but has not been widely developed and utilized
in the food field.
As one of the traditional soybean products on the national table, tofu is characterized by its mellow mouth, good digestion and rich protein
.
The mechanism of forming tofu gel lies in thermal denaturation of soy protein by heat treatment, exposing intermolecular hydrophobic groups, enhancing intermolecular interaction forces (such as disulfide bonds, hydrophobic bonds, etc.
), aggregating soy protein, and forming a protein gel network
containing a large amount of water through the bridge action of coagulant.
The solidification mechanism of salt brine (MgCl2) is recognized by most scholars as a "salt ion bridge", which is widely used in the production
of tofu.

Wang Fengqiu, Yang Xinxin, Zhu Xiuqing*, School of Food Engineering, Harbin University of Commerce, Heilongjiang Provincial Key Laboratory of Comprehensive Processing of Grain Food and Grain Resources, etc.
, using cold-pressed soybean flour as raw materials, added MgCl2 and TG enzyme as protein gelling agents, and took ordinary brine tofu as the control, further explored the effects of synergistic induction on the formation of tofu gel from the perspectives of raw material differences, finished product apparent indicators and microscopic indicators, so as to diversify and industrialize the application of cold-pressed soybean flour.
Improve the controllability of the production process of cold-pressed soybean flour tofu, provide a new idea and approach for China's tofu processing industry, and have certain theoretical value and practical significance
for the deep processing of soybean products.

1.
Composition index of raw materials and samples

It can be seen from Table 2 that the moisture content of MT tofu and MTG tofu is significantly lower than that of OT tofu, mainly because MT tofu and MTG tofu are made of cold-pressed soybean flour raw materials, cold-pressed soybean flour contains a large amount of insoluble fiber, which partially blocks the stability of protein gel formation, resulting in weakened effect of protein gel on water wrapping, and water is easier to flow out during the pressing process of tofu making; And MT tofu, MTG tofu processing raw materials are different from OT tofu, OT tofu with ordinary soybeans as raw materials, ordinary soybeans protein dry base mass fraction is (40.
58±0.
18), similar to cold-pressed soybean flour protein dry base mass fraction (41.
96±0.
03), soybean raw materials after refining and dumping process to remove a large amount of insoluble fiber, compared to the same concentration of cold-pressed soybean flour slurry protein content, retain more soluble protein, so cold-pressed soybean flour after preparation into tofu, Its protein content is slightly lower than that of OT tofu, which is also one of the reasons for the decrease in the total content of
protein-coated water.
The fat content of MTG tofu is about 1/3 that of OT tofu, reflecting the advantage of
the low fat content of cold-pressed soybean flour.
The fat content of MTG tofu and MT tofu is slightly higher than that of raw cold-pressed soybean flour, mainly because some macromolecular insoluble fiber components in cold-pressed soybean flour slurry settle in the process of slow brain pointing, and do not participate in the formation of tofu gel, and the oil in it is combined with protein after adding MgCl2 coagulant and wrapped in protein gel, enhancing the aggregation behavior of oil, producing macromolecular substances that are difficult to remove in the form of yellow pulp water during the pressing process, and finally remain in tofu
.
The dietary fiber content of MTG tofu is about 3 times that of OT tofu, which meets the definition of
tofu products rich in dietary fiber.
The dietary fiber content of MTG tofu and MT tofu is significantly lower than that of raw cold-pressed soybean flour, which is caused by the cold-pressed tofu production process, the material-to-water ratio is 1:9, a large amount of water is inhaled and a certain amount of crude fiber is lost without protein
crosslinking.

2.
Analysis of water holding capacity and yield rate

As shown in Figure 1, the water holding capacity of MTG and MT tofu is significantly higher than that of OT tofu, because cold-pressed soybean flour is rich in dietary fiber, which can better retain water and reduce water loss.
The water holding capacity of MTG tofu (51.
01%) was significantly higher than that of MTG tofu (46.
31%), which may be that the crosslinking of TG enzymes increased the strength of protein gels and its water-holding capacity, which was consistent with
the results of Zhang Mingkai et al.
The yield rate of MTG tofu and MT tofu is significantly higher than that of OT tofu, and in terms of the physical composition of dietary fiber, many hydrophilic groups can significantly improve the water absorption capacity
of finished tofu gel.
The yield rate of MTG tofu (244%) is significantly higher than that of MTG tofu (217%), and from the analysis of the characteristics that fat is not easily soluble in water, it is more likely that MTG tofu contains more protein and dietary fiber to improve its production rate
.

3.
Chromaticity analysis

The color determination results of the prepared tofu samples are shown in Table 3, and the visual diagram of the sample is shown
in Figure 2.
Due to the difference in raw materials, MT tofu and MTG tofu have significant differences in chromaticity compared with OT tofu
.
The brightness values of MT tofu and MTG tofu were significantly lower than OT tofu, which was due to the yellowish-brown color of the dietary fiber contained in
cold-pressed soybean flour.
Compared with MT tofu, MTG significantly increased brightness value, increased red-green value, decreased yellow-blueness value, and the overall performance was reddish and yellowish, which was closer to OT tofu
.

4.
Texture analysis

As shown in Table 4, the hardness of OT tofu is significantly higher than that of MT tofu and MTG tofu, which is related to
the high soluble protein content and low dietary fiber content in soy milk.
Among them, the maximum hardness of MT tofu is 1 046.
67 g, which is due to the fact that the appropriate concentration of magnesium ions can stabilize the protein conformation, so that the protein molecules are cross-linked with each other, and the phenomenon of "salt dissolution" occurs, so that the protein condenses into a uniform and stable gel form
.
The hardness of MTG tofu is significantly higher than that of MT tofu, because TG enzymes can induce lysine groups between protein molecules and promote the formation of aggregates
between proteins in a covalent binding manner.
The adhesiveness of MTG tofu was significantly higher than that of MTG tofu, indicating that after the composite improvement of the sample, the ability to resist external forces to maintain the original state was significantly improved, and the taste of tofu was enhanced
.
The overall texture of MTG tofu after synergistic induction by MgCl2 and TG enzyme was the best, and its elasticity and cohesion were not significantly different from OT tofu, but the hardness and adhesiveness were significantly different from OT tofu, which was also reflected in the sensory evaluation that there was a certain gap between the taste of MTG tofu and OT tofu, which was due to the negative correlation between dietary fiber and gel formation, and high content of dietary fiber would hinder the formation of protein gel network, thereby significantly reducing the texture
of tofu.

5.
Sensory assessment analysis

As shown in Table 5, the sensory score of MT tofu was the lowest, while the sensory score of MTG tofu reached 88.
00±1.
00, which was the closest to OT tofu, indicating that the addition of TG enzyme significantly improved the apparent texture of the product with MgCl2 as a single coagulant, making the structure dense and delicate, which was consistent with
the texture evaluation results of section 2.
4.
All things considered, the products produced by MTG tofu meet expectations and significantly improve the appearance of the product
.

6.
Low-field NMR analysis

As shown in Figure 3, OT tofu has a longer relaxation time, strong water fluidity, and the most free water than MT tofu, which explains that the water holding capacity of tofu prepared from soybean is significantly lower than that of the cold-pressed soybean flour experimental group
.
Xing Gangliang et al.
have shown that the combination of TG enzyme and lactic acid bacteria can obtain a protein gel with a higher degree of polymerization, reduce protein porosity, and thus improve the water-holding capacity
of product tofu.

7.
Protein secondary structure

As shown in Figure 5, the relative content relationship of the protein secondary structure of the prepared tofu sample can be clarified, and the gel characteristics of tofu are determined
by the relative content of the protein secondary structure.
As shown in Table 6, the relative content of the secondary structure of the three tofu samples was mainly β-folded, which was the main reason why the gel was quite rigid, among which OT tofu had the highest relative content of β-fold, which also explained the reason why the hardness of MT tofu and MTG tofu was lower than that of ordinary tofu
.
The relative content of α-helix in OT tofu was higher than that in the MT tofu group and MTG tofu, which indicated that under the influence of salts and enzyme coagulants, part of the basic structure α-helix began to dissociate, and the disordered structure began to increase
.
The relative content β of OT tofu and MT tofu was significantly different, but with the addition of TG enzyme, the relative content of β-turn angle of MTG tofu showed a decreasing trend, close to OT tofu.
β
。 With the addition of MgCl2 and TG enzyme, the relative content of random curl gradually increased, among which the relative content of random curl in the MTG tofu group increased significantly, reaching 21.
45%, and the degree of protein subunit aggregation increased, which further elucidated the effect of TG enzyme crosslinking protein gel
.

8.
Determination results of free sulfhydryl group, total sulfhydryl group and disulfide bond content

As shown in Table 7, the free sulfhydryl groups of OT tofu after gel formation were significantly lower than those prepared with cold-pressed soybean flour, and the total sulfhydryl and disulfide bond content was higher than that of cold-pressed soybean flour prepared from raw materials, which may be due to the high dietary fiber content of cold-pressed soybean flour hindering the formation and stability of gel, which is consistent
with the previous conclusion.
After the addition of TG enzyme, more free sulfhydryl groups were converted into disulfide bonds, which indicated that TG enzyme formed macromolecular aggregates through the cross-linked protein network structure during the formation of tofu and enhanced its gellability
.

9.
Rheological characteristic analysis

As shown in Figure 6, the frequency scanning results of three samples, where G' is expressed as the elasticity of the gel, the larger its value, the stronger the resilience of the gel after stress; the loss modulus G'' is usually related to the viscosity of the gel, the smaller the value, the easier it is to flow between the gels; Tanδ represents the loss value, which is the ratio of G'' to G', reflecting the formation of gels by proteins, that is, the transformation
of soy milk to tofu gel.
The G′ of the three samples was greater than G'', that is, tanδ<1, indicating that the sample was a gel dominated by elasticity<b11>.
Among them, the G', G'' of OT tofu were higher than those of the two experimental groups of cold-pressed soybean flour, which indicated that the higher concentration of protein in soy milk was conducive to the formation of gel, while the presence of dietary fiber hindered the formation of protein gel, but with the extension of squatting time, cold-pressed soybean flour gradually changed from free state to solid, forming strong covalent bonds between protein molecules, such as disulfide bonds, or "salt bridge" action to form more ionic bonds, and its gel structure was gradually enhanced
.
Using cold-pressed soybean flour as raw material, MTG tofu has higher G' and lower tanδ
than MT tofu.
This is because after induction by TG enzyme, MTG gel G' and G'' rose relatively significantly, eventually reaching 10 498 Pa, which was much higher than the MT gel maximum of 4 810.
7 Pa, and the overall trend showed rapid rise and then flattened, indicating the formation of a stronger gel network structure
.

10.
Scanning electron microscopy analysis

As can be seen from Figure 7, OT tofu has the most uniform internal porosity and compact overall structure.
MT tofu has a loose structure and the most loose protein network pores, which is due to the presence of cold-pressed soybean flour fiber that will break the microenvironment of the gel, increase the size of the internal pores, and form an uneven state; Compared with MT tofu, MTG tofu significantly improves the gel network structure due to the addition of TG enzyme, wraps more cold-pressed soybean flour fiber particles, and transforms the randomly aggregated gel network into a continuous protein network structure, making it denser.
This microscopic indication shows that MTG tofu is closest to OT tofu, which is consistent
with the evaluation results of texture characteristics and sensory analysis.

conclusion

The results showed that the dietary fiber content of tofu prepared with cold-pressed soybean flour as raw material was about 3 times that of brine tofu, and the fat content was about 1/3 of that of brine tofu; the addition of TG enzyme would significantly improve the internal moisture of cold-pressed soybean flour gel, making it more uniform, and the tofu gel had a higher storage modulus G' (10 498 Pa), indicating that a stronger gel network structure was formed, resulting in the largest water holding capacity (51.
01%) and the highest yield (244%) of MgCl2-TG enzyme tofu.
The relative content of random curl in the secondary structure was the highest (21.
45%).
The disulfide bond content (5.
46 μmol/g) maintained in the gel network was close to that of brine tofu (7.
02 μmol/g).
The results of scanning electron microscopy showed that TG enzyme and MgCl2 made the gel network of cold-pressed tofu more detailed and compact, and the whole was close to that of brine tofu
.
This shows that the synergistic effect of TG enzyme and MgCl2 coagulant can improve the quality of cold-pressed soybean flour gel-forming gel, and this study provides a theoretical basis
for the multi-component development of cold-pressed soybean flour.

About the corresponding author

Professor Zhu Xiuqing

School of Food Engineering, Harbin University of Commerce

Professor Zhu Xiuqing, doctoral supervisor, head of plant protein chemistry and deep processing technology, School of Food Engineering, Harbin University of
Commerce.
He used to be the deputy director and chief engineer of the National Soybean Engineering Technology Research Center, the director of the Soybean Processing Technology Research Center of Heilongjiang Green Food Science Research Institute, the current professor of the School of Food Engineering of Harbin University of Commerce, the person in charge of plant protein chemistry and deep processing technology, and the executive director of the soybean industry technology innovation strategic alliance, the standing director of the Heilongjiang Food Science and Technology Society, the national and provincial project evaluation experts, and the post experts
of the modern agricultural industry technology collaborative innovation system in Heilongjiang Province 。 Reviewer of international journals Journal of Food Science and Nutrition Therapy, Journal of Food Processing and Preservation, editorial board member of domestic journals "Soybean Science and Technology", reviewer of "Food Science" and "Transactions of Agricultural Engineering", etc
.

Over the years
, he has been committed to the research and application development of soybean protein chemistry and deep processing technology.
It has certain advantages in the analysis of soybean nutrients, the influence and characterization of processing technology on the structure and characterization of soybean protein and the comprehensive utilization of soybean processing by-products, and has achieved certain results
in the physical, enzymatic, chemical modification, soybean protein extrusion and recombination technology research and traditional soybean food processing and application.

So far, he has presided over and participated in the completion of 29 science and technology projects such as the "Twelfth Five-Year Plan" national science and technology plan in the rural field, the national 863, the science and technology plan project of Heilongjiang Province, and the major science and technology special project of Heilongjiang Province; He has won 1 second prize of the National Science and Technology Progress Award, 1 special award of the Governor, 1 first prize of the Provincial Science and Technology Progress Award, and 2 second prizes of the Provincial Science and Technology Progress Award; Obtained 15 invention patents; Published 3 monographs and more than 100 papers; It has trained 27 master's
degree students.

This article "Effect of TG enzyme-MgCl2 synergistic induction on cold-pressed soybean flour gel" is from Food Science, Vol.
43, No.
16, 2022, pages 169-176, authors: Wang Fengqiu, Yang Xinxin, Gu Xuelian, Luan Binyu, Huang Yuyang, Zhu Ying, Zhu Xiuqing
.
DOI:10.
7506/spkx1002-6630-20210806-081
。 Click to view information about
the article.

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