Effects of hydroxypropyl methylcellulose on the properties of dough and the quality of deep-fried dough sticks

Abstract: The effects of hydroxypropyl methylcellulose (HPMC) addition (0-2%) on the properties of dough and the quality of deep-fried dough sticks were studied. The rheological properties and gelatinization properties of the dough were measured by a rheometer, and the thermogravimetric properties of the dough were measured by a thermogravimetric analyzer. At the same time, the oil content, water content, specific volume and texture of the fried dough sticks were examined. The results show that the addition of HPMC can increase the dough's elastic modulus G', viscous modulus G", peak viscosity, valley viscosity and final viscosity of flour gelatinization; with the increase of HPMC addition, the water holding capacity of dough Enhanced, when it reaches 2%, the water holding capacity decreases. Compared with the control group, adding HPMc reduces the oil content and specific volume of fried dough sticks. Adding an appropriate amount of HPMC can reduce the hardness of fried dough sticks, and when the amount reaches 2%, it will Cause the hardness of fritters to increase.

Key words:hydroxypropyl methylcellulose; dough; fried dough sticks; oil content; quality

Fried dough sticks are a kind of food made of flour as the main raw material and processed by high-temperature frying. Deep-fried dough sticks are golden in color and crisp in taste, and have a very long history of processing and consumption in my country. In recent years, as people pay more and more attention to food safety, nutrition and health, deep-fried dough sticks, as a kind of food with high oil content, have had a great impact on their consumer reputation. How to improve the quality of deep-fried dough sticks, especially to reduce the oil content of deep-fried dough sticks has become the key to research on deep-fried dough sticks processing.

Edible gum, also known as hydrophilic colloid or hydrosol, is a kind of solution or gel that can be dissolved or dispersed in water and hydrated with water molecules to form a viscous, slippery solution or gel, so that food can obtain the desired shape or stable structural characteristics and good texture. taste food additives. Edible glue has been widely used in beverage, frozen, candy and other food fields, but the research and application in the field of fried food processing has only begun to receive attention in recent years. How does edible glue affect the processing quality of fried dough sticks? There are only a few reports in the literature, and they mainly focus on several common colloids such as gelatin, konjac powder, xanthan gum, and CMC.

Hydroxypropylmethylcellulose (HPMC) is a non-fermented, semi-synthetic cellulose derivative with physiological properties of soluble dietary fiber, and has been used in the processing of many foods. Studies have shown that HPMC can increase the specific volume of grain-free bread, improve the texture properties of bread, and increase the stability of the gluten network structure. However, the current data research has not found any research on the influence of HPMC on the processing quality of fried dough sticks. Therefore, the experiment applied HPMC to the processing of deep-fried dough sticks. First, its effect on dough properties is studied. On this basis, its influence on the oil content, specific volume, texture and other quality characteristics of deep-fried dough sticks was investigated, in order to find an edible glue suitable for deep-fried dough sticks processing.

1. Materials and methods

1.1 Test material

1.1.1 Materials and reagents

Zhongyu wheat flour, provided by Binzhou Zhongyu Food Co., Ltd.; aluminum-free compound fried dough stick leavening agent, provided by Angel Yeast Co., Ltd.; salt (Nacl mass fraction ≥ 99.1%), provided by Anhui Salt Industry Corporation; Hydroxypropyl methylcellulose (food grade, 2% viscosity 4 000 mPa.s, methoxyl 28%-30%, hydroxypropyl 7.0%-12%), provided by KIMA CHEMICAL CO., LTD; Jinlong Fish soybean oil, provided by Qinhuangdao Jinhai Food Industry Co., Ltd.; petroleum ether (boiling range 30-60°C). The above reagents were all analytically pure.

1.1.2 Main Instruments

SM-986S type household mixer, product of Dongguan Dingchu Electric Technology Co., Ltd.; LHS-150HC-Ⅱ type constant temperature and humidity box, DHG-9030A type electric blast drying oven, product of Shanghai Yiheng Scientific Instrument Co., Ltd.; BSA124S type electronic Balance, product of Sartorius AG, Germany; DHR-3 rheometer, product of American TA company; STA449F3 synchronous thermal analyzer, product of German NETZSCH company; YF-25 electric multifunctional fryer, Ruian Yifang Products of Machinery Manufacturing Co., Ltd.; TA—xTplus physical property tester, product of British stable company; SC—80c automatic color difference meter, product of Beijing Kangguang Co., Ltd.

1.2 Test method

1.2.1 Preparation of flour with different HPMc content

Add HPMC to the flour in proportions of 0.5%, 1.0%, 1.5%, and 2.0% by mass fraction, mix well and set aside for later use. The flour without adding HPMC is used as the blank control.

1.2.2 Production process of fried dough sticks

Weigh 400 g of the flour prepared in 1.2.1, pour it into the kneading bowl of the dough mixer, then add 6 g of salt and 10 g of aluminum-free leavening agent, mix evenly at low speed for 1 min, and then knead along the Slowly add 240 mL of distilled water to the wall of the noodle bowl, adjust the speed to high speed and stir for 4 minutes. After forming a dough, take it out and knead until the surface is smooth, then wrap it with plastic wrap and put it in a constant temperature and humidity box with a temperature of 30°C and a humidity of 60%. . After 20 minutes of fermentation, do the first layering, then wrap it with plastic wrap and ferment for 20 minutes again for the second layering, and brush a layer of vegetable oil on the plastic wrap to wrap the dough and put it in a constant temperature and humidity box for fermentation After 4 h, set aside.

1.2.3 Determination of dynamic rheological properties of dough

The determination of the rheological properties of the dough refers to the method of Wan Jinhu et al., with appropriate modifications. Use scissors to cut an appropriate amount (about 2 g) of the sample from the central part of the dough and put it on the measuring platform of the DHR-3 rheometer, select a flat mold with a diameter of 40 mm, cut off the excess dough after lowering the flat plate, and seal it with vaseline to prevent water evaporation. After equilibrating for 10 min, the linear viscoelastic region of the sample was first determined by the stress sweep program in the dynamic measurement mode. The measurement parameters were 1 mm spacing, 25 °C temperature, and 1.0 Hz frequency. After determining the linear viscoelastic region, the influence of HPMC on the elastic modulus, viscous modulus and loss tangent of the dough was studied by frequency scanning program, and the scanning frequency range was 0.1-10.0 Hz.

1.2.4 Determination of pasting properties

The pasting properties of flour with different HPMC content were determined by rheometer. The specific procedures and parameters refer to Zhou Yujie’s method with slight modifications. Firstly, flour with different HPMC content is made into a 10% suspension, and a plate with a diameter of 40 mm is selected, the distance is set to 1 mm, and the rotation speed is 5 r/s. Keep warm for 1 min under the same conditions, then raise the temperature to 95 °C at 5 °C/min and keep it warm for 2.5 min, then cool it down to 50 °C at 5 °C/min and keep it warm for 5 min. According to this procedure, the gelatinization properties of the sample are measured, and the gelatinization curve is obtained with time as the abscissa and viscosity and temperature as the ordinate. According to GB/T1449-2008, the corresponding gelatinization index is obtained.

1.2.5 Determination of thermogravimetric properties of dough

The thermogravimetric properties of the dough were measured according to Zhang Jun’s method. About 20 mg of the sample was taken from the center of the fermented dough and placed in a crucible. When the point temperature is 150°C, it is kept for 5 minutes, and the information such as the water loss ratio and DTG curve is obtained by the program.

1.2.6 Making fried dough sticks

Stretch the fermented dough into a dough with a thickness of 1 cm and a width of about 10 cm, then use a knife to cut into small dough with a width of 5 cm, take 2 pieces of the same small dough and stack them together, and press them in the middle with chopsticks Tighten to get the fried dough stick embryos, and finally stretch the fried dough stick embryos to about 35 cm and put them in a frying pan heated to 180°C for 2 minutes. During this period, keep turning them to make them heated evenly. After frying, remove and drain the oil for later use.

1.2.7 Determination of oil content and water content of fried dough sticks

The oil content of fried dough sticks was measured according to the "Soxhlet extraction method" in GB 5009.6-2016, and the moisture content of fried dough sticks was measured in accordance with the "direct drying method" in GB 5009.3-2016.

1.2.8 Determination of specific volume of fried dough sticks

The volume y (cm³) of the fried dough sticks was determined by the rapeseed replacement method, and the mass m (g) of the deep-fried dough sticks was measured by an electronic balance.

1.2.9 Determination of texture properties of fried dough sticks

The texture properties of fried dough sticks were determined according to the method of Zhang Kangyi et al. with appropriate modifications. The middle part of fried dough sticks with uniform thickness and good quality was selected, and cut into 4 cm small sections to measure the TPA index. The specific parameters are the probe model P, 100, the pre-test, post-test and test speeds are 5 mm/s, the compression degree is 75%, the dwell time is 0.5 s, and the trigger force is 5 g.

1.2.10 Data processing

All the results are expressed in the form of mean ± standard deviation. OriginPm 8.5 is used for graph making, and SPsS 19.0 is used for significance test, and the significance level is 0.05.

2. Results and Analysis

2.1 Effect of HPMC addition on dynamic rheological properties of dough

From the effect of the amount of HPMC added on the rheological properties of the dough, it can be seen that with the increase of the amount of HPMC added, the elastic modulus and viscous modulus of the dough both showed an increasing trend. The reason may be that on the one hand, HPMC can interact with starch and protein in flour to form a complex, which strengthens the network structure of gluten; The strength becomes larger, which eventually leads to an increase in the viscoelasticity of the dough. When the addition amount is 0.5%, the viscoelasticity of the dough increases slightly; when the addition amount is greater than 0.5%, the viscoelasticity of the dough increases rapidly, which may be due to the water holding capacity of the dough at a lower content of HPMc The network structure of gluten and gluten has no significant effect, so when the addition amount is small, the viscoelasticity of the dough does not increase significantly.

Tanγ(G”/G’) reflects the viscoelastic ratio of the material, and is also used to describe the content and degree of polymerization of high polymers in the dough system. The smaller the value, the higher the content of high polymers and the greater the degree of polymerization. With the increase of HPMC addition, the tanγ surface value showed a decreasing trend, which indicated that the addition of HPMC was beneficial to the increase of polymer content. In addition, when the HPMC addition reached 1% to 2%, tanγ(G”/G ') did not change significantly, which indicated that after the addition of HPMc reached 1%, the network structure of gluten in the dough had been well formed, and excessive addition had no significant effect on the content of high molecular polymers and the degree of polymerization in the dough. This may be due to the fact that the sites in the dough capable of binding to HPMc to form complexes are saturated and unable to interact with more HPMc.

2.2 The effect of HPMC addition on pasting properties of flour

When the amount of HPMC added was 0-2%, the peak viscosity, minimum viscosity and final viscosity of flour all showed an increasing trend. The increase of peak viscosity and minimum viscosity may be due to the interaction between hydrocolloid and amylose and low-molecular-weight amylopectin leached from starch granules to form complexes with the increase of temperature during gelatinization, resulting in high The molecular weight polymer content increases, making the viscosity increase old. The increase of the final viscosity may be due to the fact that the addition of HPMC can promote the rearrangement of the swollen starch molecules during the cooling process, so that the final viscosity increases with the increase of the HPMC content.

The attenuation value showed an increasing trend, indicating that the addition of HPMC was not conducive to the thermal stability of starch. The retrogradation value reflects the degree of recrystallization of starch molecules after wheat flour gelatinization. The retrogradation value also showed an increasing trend, which indicated that the addition of HPMc promoted the short-term retrogradation of starch in flour, which may be due to the formation of non-covalent hydrogen bonds between HPMc and starch during the cooling process, thereby increasing the aggregation of starch molecules degree. Wang Yusheng and others added different hydrocolloids to flour, and also found that the peak viscosity, minimum viscosity, final viscosity, attenuation value and retrogradation value all increased to varying degrees.

2.3 Effect of HPMC addition amount on dough thermogravimetric properties

It can be seen from the TG curve that during the heating process, with the increase of HPMC addition, the proportion of dough moisture loss becomes smaller, which is mainly due to the strong water holding capacity of HPMC, which limits the evaporation of free water in dough.

It can be seen from the DTG curve that the maximum rate of water evaporation in the dough decreases with the increase of HPMC addition, and the temperature required to reach the maximum rate becomes larger. In the middle and low temperature zone (25～120℃), the evaporation of water is mainly free water, so the evaporation rate of the blank sample is higher than that of the sample containing HPMC, and when entering the high temperature zone (>120℃), the moisture content of the blank sample It is already low, and it is mainly bound water, while the moisture content of the sample added with HPMC is relatively high, so when the temperature is higher than 120 ° C, the water loss rate of the sample added with HPMC begins to be higher than that of the blank sample. At the same time, when the temperature exceeds 100°C, it can be seen from the DTG curve of the dough that the water loss rate of the dough with an addition of 2% exceeds that of the dough with an addition of 1.5%, which is due to the high addition of HPMc At the same time, due to its strong water absorption, it will hinder the water absorption, swelling and bonding between gluten proteins, which is not conducive to the formation of gluten network structure, thus making the water holding capacity of the dough worse. Therefore, when the added amount is too high, the water evaporation rate will increase instead.

2.4 Effect of HPMC addition on oil content and water content of fried dough sticks

With the increase of the amount of HPMC added, the oil content of the fried dough sticks gradually decreased, while the water content of the deep-fried dough sticks gradually increased, which indicates that the addition of HPMC can reduce the evaporation of water in the deep-fried dough sticks. The reason may be that HPMC has a strong water holding capacity. One of the main mechanisms for food to absorb oil during frying is "water-oil replacement", that is, during high-temperature frying, the water in the food is evaporated in large quantities, thereby forming many pores, and frying oil enters through these pores. Pour in food. HPMC can reduce water evaporation, which is not conducive to the formation of pores during frying, so the oil entering the fried dough sticks is also reduced accordingly.

On the other hand, as a hydrophilic colloid, HPMC can form a colloidal film on the surface of food during frying, which can not only reduce the evaporation of water in food, but also prevent oil from entering the food. Kim J et al. also found that the addition of HPMC can reduce the oil content and water loss of fried soybean donuts in their research, and they think that it may be due to the thermal gelation of HPMC, and the high temperature makes it work with protein on the food surface Forms a barrier that acts as a barrier to the ingress of frying oil.

2.5 Effect of HPMC addition on specific volume and texture of fried dough sticks

With the increase of HPMC addition, the specific volume of fried dough sticks decreased significantly, from 3.73 cm³/g in the blank to 2.32 cm³/g with 2% HPMC added, a decrease of 37.80%. Mccanhy D F et al. also found that the specific volume of bread gradually decreased with the increase of HPMC content in the study of the effect of HPMC addition on the specific volume of grain-free bread. The reason may be that the colloidal film formed on the surface of the fried dough sticks after adding HPMc reduces the evaporation of water in the dough, so that the fried dough sticks cannot be effectively heated and expanded, and the volume increases.

From the effect of the amount of HPMC added on the texture properties of fried dough sticks, it can be found that the hardness of fried dough sticks does not change significantly when the amount of HPMC is added, but when the amount of HPMC is added, the hardness reaches the maximum. The reason may be that when the amount of HPMC added is too much, the specific volume of fried dough sticks decreases significantly, the internal pores become less, and the structure becomes too dense, resulting in a significant increase in hardness. The elasticity of fried dough sticks showed a trend opposite to that of hardness, and there was no significant difference in the elasticity of fried dough sticks with different HPMc additions compared with the blank group. The relationship between chewiness and the amount of HPMc added was similar to that of hardness; the recovery force did not change significantly with the increase of HPMc content.

3. Conclusion

The effect of adding HPMC on the properties of dough and the quality of deep-fried dough sticks was studied. The results showed that the elastic modulus and viscous modulus of dough gradually increased with the increase of HPMC addition, and the loss tangent value showed a decreasing trend within a certain range. When greater than l%, the loss tangent tends to be stable. The peak viscosity, minimum viscosity and final viscosity of flour gelatinization showed an increasing trend with the increase of HPMc addition. HPMc can significantly increase the water holding capacity of dough, reduce the oil content and specific volume of fried dough sticks, and proper addition can improve the texture characteristics of fried dough sticks.