Effect of hydroxypropyl methylcellulose on dough moisture and deep-fried dough quality

Abstract: The effect of hydroxypropyl methylcellulose (HPMC) on dough moisture state, protein and starch crystallization properties, and gluten structure was studied, and the effect of HPMC on improving the eating quality of fried dough sticks was analyzed by TPA combined with sensory evaluation. The results showed that the proper addition of HPMC could improve the gluten network structure and enhance the stability of water state; effectively inhibit starch retrogradation, slow down the disintegration of starch crystal structure, and reduce the relative crystallinity of starch by 0.60%, 0.69%, and 1.94%, respectively. and 1.94%; the specific volume of fried dough sticks was reduced by 46.50%, 37.32%, 21.19% and 19.88% respectively; the aroma, crispness and palatability of fried dough sticks were improved.

Key words:hydroxypropyl methylcellulose; deep-fried dough sticks; moisture state; crystallization properties; gluten structure

As a complex system, dough will form a dense and uniform protein matrix, thereby forming a three-dimensional network structure in which starch and other substances are wrapped. Water molecules are good plasticizers in the dough system. The existence of water molecules in the process of product processing plays a vital role in maintaining the balance of various components, maintaining the stability of dough and ensuring good eating quality of products. Fried dough sticks are favored by consumers because of their crispy, salty taste and convenience. Due to the instability of the frying process, there are mutual influences among the components of the deep-fried dough stick system. Hydrocolloids have been widely studied as a substance that can not only change the properties of dough but also improve the quality of products. Hydroxypropyl methylcellulose (HPMC) is a kind of hydrocolloid formed by chemically linking hydroxypropyl and methyl groups to the β-1,4-D glucan backbone. The substituents of the side chain of HPMC make it have good hydrophilicity, and can quickly form a stable thick colloidal dispersion under low temperature conditions, so it is used as a surfactant, thickener, emulsifier and stabilizer in food middle. In this study, 1M flour was used as the experimental raw material, aiming to initially explore the effect of HPMC in the complex dough system, and then provide a reference for the impact mechanism of HPMC in the product.

1. Materials and methods

1.1 Materials and reagents

1M1 system powder (A), 1M2 system powder (B), Henan Tianxiang Flour Factory; 1M1 system powder (C), 1M2 system powder (D), Henan Hongqiang Flour Factory; hydroxypropyl methylcellulose, KIMA CHEMICAL CO ., LTD; urea, tris(hydroxymethyl)aminomethane (Tris), hydrochloric acid, sodium lauryl sulfate, ethylenediaminetetraacetic acid (EDTA), 5,5 dithiobis(2-nitrobenzoic acid ) (DTNB), analytically pure; distilled water, third grade.

1.2 Instruments and equipment

JHMz-200 needle type noodle mixer, Beijing Dongfu Jiuheng Instrument Technology Co., Ltd.; MFF-13 commercial fermentation box, Guangdong Shengheng Household Appliances Co., Ltd.; GS-25 electric multi-function fryer, Ruian Chenghuang Machinery Co., Ltd. Company; Tz—xT Plus texture analyzer, American StableMicro System Instrument Company; VAMR20—010V—T nuclear magnetic resonance variable temperature analysis system, Shanghai New York Electronic Technology Co., Ltd.; Freezone6 plus freeze dryer, American Labconco Co., Ltd.; D8- Advance X-ray diffractometer, BⅢkerAxs Instrument Company, Germany; uV2150/2150 UV/Vis Spectrophotometer, Ronneco (Shanghai) Instrument Co., Ltd.; QuantaFEG Scanning Electron Microscope, American FEI Company.

1.3 Test method

1.3.1 Preparation of mixed powder

According to the preliminary test, based on the quality of wheat flour, HPMC was added to wheat flour at 0, 0.5%, 1%, 1.5%, and 2%, respectively, and it was stable at room temperature for 24 hours.

1.3.2 Determination of basic indicators of mixed powder

Moisture determination is carried out according to GB5009.3-2016 "Determination of Moisture in Food Safety National Standards"; ash content determination is carried out according to GB5009.4-2016 "Determination of Ash Content in Food Safety National Standards"; crude protein content is determined according to GB 5009. 5—2016 "National Food Safety Standard for the Determination of Protein in Food"; the determination of wet gluten content is carried out according to GB/T 5506.2_2008 "Wheat and Wheat Flour Gluten Content Part 2: Determination of Wet Gluten by Instrument Method".

1.3.3 Preparation of fried dough sticks

The making of fried dough sticks is carried out with reference to the method described by Gao Jie.

1.3.4 Determination of moisture distribution

1 g of dough was placed at the bottom of the nuclear magnetic resonance tube, and the transverse relaxation time was measured by nuclear magnetic resonance. Parameter setting: sampling number 60000, number of echoes 3000, echo time 0.1 ms; repeat scanning 32 times.

1.3.5 Crystalline properties of starch

The continuous scanning diffraction method is adopted, and the characteristic ray Cu-K instrument is used for testing. The scanning range is 4-40°, and the scanning speed is 2°/min. The relative crystallinity of the sample was calculated by the ratio of the area of the crystalline region to the total area by Jade 5.0 software fitting process.

1.3.6 Determination of free thiol content

The content of free sulfhydryl groups was slightly modified with reference to the method of GU0 et al. Put 240 mg of freeze-dried dough samples (m1) in 9 mL of 0.2 mol/L Tris buffer (K), shake at room temperature for 1 h, then add 0.9 mL of 10 mmol/L DTNB (V2) to continue shaking After 1 h, centrifuge at a centrifugal force of 13 600 × g for 20 min, take the supernatant and read the absorbance A1 at a wavelength of 412 nm, and the blank control is the absorbance A2 without the sample.

1.3.7 Dough Microstructure Observation

The freeze-dried sample was glued on a metal disc with the natural fracture facing up, and was plated with gold under vacuum conditions, and then observed with a scanning electron microscope (SEM).

1.3.8 Determination of the quality of fried dough sticks

1.3.8.1 Expansion

The expansibility of deep-fried dough sticks is usually characterized by specific volume.

1.3.8.2 Textural properties

Modify appropriately according to Yang Nian's method. Choose fried dough sticks with relatively uniform thickness and cut the middle part into 2cm width for measurement. Parameter setting: P36R probe, the speed before and after the test is 1 mm/s, the speed during the test is 0.8 mm/s, the compression ratio is 75%, and the dwell time is 5 s.

1.3.8.3 Sensory evaluation

Ten professional sensory evaluators evaluated the aroma and color of fried dough sticks with different HPMC additions.

1.3.9 Data Analysis

Significance analysis (P<0.05) was carried out using IBM SPSS Statistics 25, drawing was performed in Origin 2018, and all experiments were repeated 3 times.

2. Results and Analysis

2.1 The effect of HPMC addition on the basic indicators of wheat flour

From the influence of HPMC on the basic indexes of wheat flour, it can be seen that the initial moisture content and ash content of A flour and B flour are close, the initial moisture and ash content of C flour are higher, and the D flour is the lowest. With the increase of HPMC addition, the moisture content of wheat flour showed a decreasing trend, and the change of ash content did not show regularity. The protein content of powder C and powder D of the blank control group was relatively high. With the increase of HPMC addition, the protein content of wheat flour showed an overall increasing trend, and when the addition amount was 2.0%, the protein content of A, B, C, and D flour increased by 1.53% and 1.68% respectively , 0.32% and 0.31%. Therefore, HPMC has a greater effect on wheat flour with low protein content. Compared with the blank control group, when the amount of HPMC added was 0.5%, the wet gluten content increased significantly; when the added amount exceeded 1.0%, the wet gluten content began to decrease significantly. This shows that an appropriate amount of HPMC can form a polymer with gluten, thereby increasing the wet gluten content, while adding too much HPMC will have a bad effect on the formation of gluten structure. Gluten is a high-molecular polymer formed by cross-linking gliadin and glutenin through disulfide bonds, and its development depends on the joint action of the two proteins. When HPMC is added in excess, its water absorption may be stronger than that of gluten protein and form a state of water competition with it, thus destroying the formation of gluten structure, making protein molecules unable to cross-link and resulting in a decrease in wet gluten content.

2.2 Effect of HPMC on moisture distribution of dough

The peak of bound water representing poor mobility is the bound water tightly bound to macromolecules such as gluten protein and starch in the dough system; t22 represents the weakly bound water peak whose mobility is between bound water and free water, namely Weakly bound water, which is not too closely combined with macromolecular substances, is the main form of water in the dough system; eg. It symbolizes the free water peak with strong fluidity, which is the "channel" for water migration between the dough system and macromolecular substances such as starch and gluten. The relative percentage of water in each component is represented by A21, A, A22, A23, that is, the ratio of each peak area to the total peak area.

From the effect of HPMC on the moisture migration of different doughs, it can be known that HPMC mainly combines with weakly bound water and free water in dough A. When the addition amount is 0.5% and 1.0%, the water molecules in the two states are active. At the same time, it can be observed that the fluidity of weakly bound water and free water in B dough with 0.5% HPMC added increases, and with the increase of HPMC added, bound water and weakly bound water are repeatedly transformed into each other. The change trend of ￡, of dough c and dough D is similar, but the bound water and weakly bound water of dough D are not significantly affected by HPMC, which may be due to the retention of some water when bound water and weakly bound water migrate through the "channel", Make it form free water, so bound water and weakly bound water in the range of this added amount finally keep relatively stable. The amount added in HPMc is O. In the wheat dough system with 5% low protein content (A flour and B flour), it is mainly combined with the water molecules in two states of weakly bound water and free water, while in the dough system with higher protein content (c flour and D flour Flour) only reacted with water molecules in the free water state, which could explain to some extent the phenomenon that 0.5% HPMC added changed the wet gluten content of the four wheat flours. In the complex system of dough, although the gluten network structure is the result of the joint action of many factors, it has a great correlation with the state of moisture. Therefore, an appropriate amount of HPMc can maintain a good shape of the gluten network structure.

2.3 Effect of HPMC on relative crystallinity of starch in dough

From the effect of HPMC on the relative crystallinity of starch, it can be seen that the relative crystallinity of starch in different doughs increases in different degrees when the amount of HPMC reaches 0.5%, but begins to decrease with the increase of the amount of HPMC. The reason is that amylose molecules can form a stable double helix structure through hydrogen separation. When HPMC was added, the water molecules were rapidly migrated and combined with it, maintaining the stability of the double helix structure to a certain extent; when the amount of HPMC continued to increase, the water molecules in the system gradually stabilized, and the double helix structure began to be stabilized. Destruction, eventually leading to a decrease in relative crystallinity, to achieve the effect of inhibiting starch retrogradation.

2.4 Effect of HPMC on free sulfhydryl groups in dough

From the effect of HPMC on the content of free sulfhydryl groups, it can be seen that with the increase of the amount of HPMC added, the contents of free sulfhydryl groups in the four powders showed a trend of first decreasing and then increasing. Compared with the blank control group, the free sulfhydryl content of powders A, B, C, and D decreased significantly when the amount of HPMC added was 0.5%, and the gluten network structure was tight, which indicated that adding HPMC in an appropriate amount was beneficial to the formation of gluten network structure; However, as the addition of HPMC continues to increase and the content of free sulfhydryl groups increases, this compactness is broken, which may be affected by the migration of water, and the water molecules are gradually stabilized. Studies by Qi Baokun and others have shown that when the content of free sulfhydryl groups is high, the degree of folding of the protein molecule is greater, which will lead to more hydrophobic residues being exposed on the surface of the protein molecule, thereby causing an increase in surface hydrophobicity.

2.5 Dough microstructure with different HPMC additions

From the microstructure of A, B, C, and D doughs 6, it can be seen that compared with A and B doughs, C and D doughs have slightly larger holes, and the continuity of the fascia is relatively lacking, so that the starch granules cannot be well wrapped in a three-dimensional network structure. When the amount of HPMC in each group was 0.5%, the compactness of the network structure was strengthened, and almost no air cells were observed. This may be due to the fact that a small amount of HPMc affects the activity of water molecules and interacts with the components in the system to form a fine matrix to block the air chamber. This change in microstructure is consistent with the measurement results of free thiols, so When the amount of HPMC added was 0.5%, the sudden decrease of free sulfhydryl content was related to it. When the amount of HPMC added in each group increased to 1.0%, the previous tight network structure was gradually broken, and the appearance of some air cells could be observed, and as the amount continued to increase to 2.0%, the dough network structure appeared more There are many holes, among which A dough and B dough are more obvious; but compared with the blank group, the network structure with 2.0% HPMC addition is more compact.

2.6 Quality analysis of deep-fried dough sticks

From the effect of HPMC on the quality of different deep-fried dough sticks, it can be known that the specific volume of deep-fried deep-fried dough sticks fried by the four kinds of flour gradually decreased with the increase of HPMc addition, which was consistent with the observation results of microstructure. This is caused by HPMc forming a colloidal film on the surface of the fried dough sticks to keep the moisture inside the fried dough sticks from evaporating. The hardness of the fried dough sticks made from the four kinds of powders reached the maximum when the amount of HPMC was 2%, which may be related to the decrease of the specific volume of the fried dough sticks. The change trend of the elasticity of the fried dough sticks with the addition of HPMC is just opposite to the change trend of the hardness. The change of chewiness is similar to the change of hardness. The stronger the chewiness, the worse the elasticity and lack of flexibility of the fried dough sticks. The recovery of A, B, and C fried dough sticks was not significantly affected by the amount of HPMC added. The sensory score of fried dough sticks decreased with the increase of HPMC addition, and the sensory score was better when the addition of HPMC was 0.5%. Therefore, appropriate addition of HPMc is helpful to improve the eating quality of fried dough sticks.

3. Conclusion

In this study, the comparative analysis of dough with different HPMC additions and its main component properties and fried dough sticks quality proved that the dough protein and starch affected by different HPMC additions were different. The mutual transformation among the three kinds of water existence states is an important reason for the change of the internal structure of the dough, and the weakly bound water, as the main form of water in the dough system, plays a vital role in the processing process. Due to the good hydrophilicity of HPMC, although the addition of HPMC does not change the crystal type of starch, it affects the state of water existence by acting as a medium like a "medium", slowing down the disintegration of the crystal structure and achieving the effect of inhibiting starch retrogradation. The results of scanning electron microscopy proved that the content of free sulfhydryl groups decreased suddenly when the amount of HPMC added reached 0.5%, and then increased in the later period. , the study of the influence of HPMc on the main components of dough - protein and starch, has a positive role in promoting the development, application and quality improvement of products using it as an additive. In the future, on this basis, the mechanism of the purified gluten and starch affected by HPMC can be studied in depth.