Study on rheological behavior of konjac glucomannan and hydroxypropyl methylcellulose compound system

The compound system of konjac glucomannan (KGM) and hydroxypropyl methylcellulose (HPMC) was taken as the research object, and the steady-state shear, frequency and temperature sweep tests were carried out on the compound system by rotational rheometer. The influence of solution mass fraction and compound ratio on viscosity and rheological properties of KGM/HPMC compound system was analyzed. The results show that the KGM/HPMC compound system is a non-Newtonian fluid, and the increase in the mass fraction and KGM content of the system reduces the fluidity of the compound solution and increases the viscosity. In the sol state, KGM and HPMC molecular chains form a more compact structure through hydrophobic interactions. Increasing the system mass fraction and KGM content is conducive to maintaining the stability of the structure. In the low mass fraction system, increasing the content of KGM is beneficial to the formation of thermotropic gels; while in the high mass fraction system, increasing the content of HPMC is conducive to the formation of thermotropic gels.

Key words: konjac glucomannan; hydroxypropyl methylcellulose; compound; rheological behavior

Natural polysaccharides are widely used in the food industry due to their thickening, emulsifying and gelling properties. Konjac glucomannan (KGM) is a natural plant polysaccharide, composed of β-D-glucose and β-D-mannose in a ratio of 1.6:1, the two are linked by β-1,4 glycosidic bonds, in the C- There is a small amount of acetyl at position 6 (approximately 1 acetyl for every 17 residues). However, the high viscosity and poor fluidity of KGM aqueous solution limit its application in production. Hydroxypropyl methylcellulose (HPMC) is a propylene glycol ether of methylcellulose, which belongs to non-ionic cellulose ether. HPMC is film-forming, water-soluble, and renewable. HPMC has low viscosity and gel strength at low temperatures, and relatively poor processing performance, but can form a relatively viscous solid-like gel at high temperatures, so many production processes must be carried out at high temperatures, resulting in high production energy consumption. Production costs are high. The literature shows that the unsubstituted mannose unit on the KGM molecular chain can form a weakly cross-linked hydrophobic association region with the hydrophobic group on the HPMC molecular chain through hydrophobic interaction. This structure can delay and partially prevent the thermal gelation of HPMC and lower the gel temperature of HPMC. In addition, in view of the low-viscosity properties of HPMC at relatively low temperatures, it is predicted that its compounding with KGM can improve the high-viscosity properties of KGM and improve its processing performance. Therefore, this paper will construct a KGM/HPMC compound system to explore the influence of solution mass fraction and compound ratio on the rheological properties of the KGM/HPMC system, and provide a theoretical reference for the application of the KGM/HPMC compound system in the food industry.

1. Materials and methods

1.1 Materials and reagents

Hydroxypropyl methylcellulose, KIMA CHEMICAL CO.,LTD, mass fraction 2%, viscosity 6 mPa·s; methoxy mass fraction 28%~30%; hydroxypropyl mass fraction 7.0%~12% .

Konjac glucomannan, Wuhan Johnson Konjac Food Co., Ltd., 1 wt% aqueous solution viscosity ≥ 28 000 mPa·s.

1.2 Instruments and equipment

MCR92 rotational rheometer, Anton Paar Co., Ltd., Austria; UPT-II-10T ultrapure water machine, Sichuan Youpu Ultrapure Technology Co., Ltd.; AB-50 electronic analytical balance, Swiss Mette company; LHS-150HC constant temperature water bath, Wuxi Huaze Technology Co., Ltd.; JJ-1 Electric Stirrer, Jintan Medical Instrument Factory, Jiangsu Province.

1.3 Preparation of compound solution

Weigh HPMC and KGM powders with a certain compounding ratio (mass ratio: 0:10, 3:7, 5:5, 7:3, 10:0), slowly add them into deionized water in a 60°C water bath, and stir for 1.5~ 2 h to make it dispersed evenly, and prepare 5 kinds of gradient solutions with total solid mass fractions of 0.50%, 0.75%, 1.00%, 1.25%, and 1.50%, respectively.

1.4 Test of rheological properties of compound solution

Steady-state shear test: The rheological curve of the KGM/HPMC compound solution was measured using a CP50 cone and plate, the gap between the upper and lower plates was fixed at 0.1 mm, the measurement temperature was 25 °C, and the shear rate range was 0.1 to 100 s-1.

Strain scanning (determination of linear viscoelastic region): Use PP50 plate to measure the linear viscoelastic region and modulus change law of KGM/HPMC compound solution, set the spacing to 1.000 mm, fixed frequency to 1Hz, and measurement temperature to 25 °C. The strain range is 0.1%~100%.

Frequency sweep: Use a PP50 plate to measure the modulus change and frequency dependence of the KGM/HPMC compound solution. The spacing is set to 1.000 mm, the strain is 1%, the measurement temperature is 25 °C, and the frequency range is 0.1-100 Hz.

Temperature scanning: The modulus and its temperature dependence of the KGM/HPMC compound solution were measured using a PP50 plate, the spacing was set to 1.000 mm, the fixed frequency was 1 Hz, the deformation was 1%, and the temperature was from 25 to 90 °C.

2. Results and Analysis

2.1 Flow curve analysis of KGM/HPMC compound system

Viscosity versus shear rate curves of KGM/HPMC solutions with different compounding ratios at different mass fractions. Fluids whose viscosity is a linear function of shear rate are called Newtonian fluids, otherwise they are called non-Newtonian fluids. It can be seen from the curve that the viscosity of KGM solution and KGM/HPMC compound solution decreases with the increase of shear rate; the higher the KGM content, the higher the system mass fraction, and the more obvious the shear thinning phenomenon of the solution. This shows that KGM and KGM/HPMC compound system are non-Newtonian fluids, and the fluid type of KGM/HPMC compound system is mainly determined by KGM.

From the flow index and viscosity coefficient of KGM/HPMC solutions with different mass fractions and different compound ratios, it can be seen that the n values of KGM, HPMC and KGM/HPMC compound systems are all less than 1, indicating that the solutions are all pseudoplastic fluids. For the KGM/HPMC compound system, the increase of the mass fraction of the system will cause entanglement and other interactions between the HPMC and KGM molecular chains in the solution, which will reduce the mobility of the molecular chains, thereby reducing the n value of the system. At the same time, with the increase of KGM content, the interaction between KGM molecular chains in the KGM/HPMC system is enhanced, thereby reducing its mobility and resulting in a decrease in n value. On the contrary, the K value of the KGM/HPMC compound solution increases continuously with the increase of the solution mass fraction and KGM content, which is mainly due to the increase of the system mass fraction and KGM content, which both increase the content of hydrophilic groups in the system. , increasing the molecular interaction within the molecular chain and between the chains, thereby increasing the hydrodynamic radius of the molecule, making it less likely to be oriented under the action of external shear force and increasing the viscosity.

The theoretical value of the zero-shear viscosity of the KGM/HPMC compound system can be calculated according to the above logarithmic summation principle, and its experimental value can be obtained by Carren fitting extrapolation of the viscosity-shear rate curve. Comparing the predicted value of the zero-shear viscosity of the KGM/HPMC compound system with different mass fractions and different compounding ratios with the experimental value, it can be seen that the actual value of the zero-shear viscosity of the KGM/HPMC compound solution is smaller than the theoretical value. This indicated that a new assembly with a dense structure was formed in the complex system of KGM and HPMC. Existing studies have shown that the unsubstituted mannose units on the KGM molecular chain can interact with the hydrophobic groups on the HPMC molecular chain to form a weakly cross-linked hydrophobic association region. It is speculated that the new assembly structure with a relatively dense structure is mainly formed through hydrophobic interactions. When the KGM ratio is low (HPMC > 50%), the actual value of the zero-shear viscosity of the KGM/HPMC system is lower than the theoretical value, which indicates that at low KGM content, more molecules participate in the denser new structure. In the formation of , the zero-shear viscosity of the system is further reduced.

2.2 Analysis of strain sweep curves of KGM/HPMC compound system

From the relationship curves of modulus and shear strain of KGM/HPMC solutions with different mass fractions and different compounding ratios, it can be seen that when the shear strain is less than 10%, the G′ and G″ of the compound system basically do not increase with the shear strain. However, it shows that within this shear strain range, the compound system can respond to external stimuli through the change of molecular chain conformation, and the structure of the compound system is not damaged. When the shear strain is >10%, the external Under the action of shear force, the disentanglement speed of molecular chains in the complex system is greater than the entanglement speed, G′ and G″ begin to decrease, and the system enters the nonlinear viscoelastic region. Therefore, in the subsequent dynamic frequency test, the shear strain parameter was selected as 1% for testing.

2.3 Frequency sweep curve analysis of KGM/HPMC compound system

Variation curves of storage modulus and loss modulus with frequency for KGM/HPMC solutions with different compounding ratios under different mass fractions. The storage modulus G’ represents the energy that can be recovered after temporary storage in the test, and the loss modulus G” means the energy required for the initial flow, which is an irreversible loss and is finally transformed into shear heat. It can be seen that, with As the oscillation frequency increases, the loss modulus G″ is always greater than the storage modulus G′, showing liquid behavior. In the test frequency range, the storage modulus G’ and the loss modulus G” increase with the increase of the oscillation frequency. This is mainly due to the fact that with the increase of the oscillation frequency, the molecular chain segments in the system have no time to recover to the deformation in a short time The previous state, thus showing the phenomenon that more energy can be stored (larger G′) or needs to be lost (G″).

With the increase of the oscillation frequency, the storage modulus of the system drops suddenly, and with the increase of the mass fraction and KGM content of the system, the frequency point of the sudden drop gradually increases. The sudden drop may be due to the destruction of the compact structure formed by the hydrophobic association between KGM and HPMC in the system by external shearing. Moreover, the increase of system mass fraction and KGM content is beneficial to maintain the stability of the dense structure, and increases the external frequency value that destroys the structure.

2.4 Temperature scanning curve analysis of KGM/HPMC composite system

From the curves of storage modulus and loss modulus of KGM/HPMC solutions with different mass fractions and different compounding ratios, it can be seen that when the mass fraction of the system is 0.50%, the G′ and G″ of the HPMC solution hardly change with temperature. , and G″>G′, the viscosity of the system dominates; when the mass fraction increases, the G′ of the HPMC solution first remains unchanged and then increases sharply, and G′ and G″ intersect at around 70 °C (The intersection point temperature is the gel point), and the system forms a gel at this time, thus indicating that HPMC is a thermally induced gel. For the KGM solution, when the mass fraction of the system is 0.50% and 0.75%, the G′ and G of the system “shows a decreasing trend; when the mass fraction increases, the G’ and G” of the KGM solution first decrease and then increase significantly, which indicates that the KGM solution exhibits gel-like properties at high mass fractions and high temperatures .

With the increase of temperature, the G′ and G″ of the KGM/HPMC complex system first decreased and then increased significantly, and G′ and G″ appeared intersection points, and the system formed a gel. When HPMC molecules are at low temperature, hydrogen bonding occurs between the hydrophilic groups on the molecular chain and water molecules, and when the temperature rises, the applied heat destroys the hydrogen bonds formed between HPMC and water molecules, resulting in the formation of HPMC macromolecular chains. The hydrophobic groups on the surface are exposed, hydrophobic association occurs, and a thermotropic gel is formed. For the low mass fraction system, more KGM content can form gel; for high mass fraction system, more HPMC content can form gel. In the low mass fraction system (0.50%), the presence of KGM molecules reduces the probability of forming hydrogen bonds between HPMC molecules, thereby increasing the possibility of exposure of hydrophobic groups in HPMC molecules, which is conducive to the formation of thermotropic gels. In the high mass fraction system, if the content of KGM is too high, the viscosity of the system is high, which is not conducive to the hydrophobic association between HPMC and KGM molecules, which is not conducive to the formation of thermogenic gel.

3. Conclusion

In this paper, the rheological behavior of the compound system of KGM and HPMC is studied. The results show that the compound system of KGM/HPMC is a non-Newtonian fluid, and the fluid type of the compound system of KGM/HPMC is mainly determined by KGM. Increasing the system mass fraction and KGM content both decreased the fluidity of the compound solution and increased its viscosity. In the sol state, the molecular chains of KGM and HPMC form a denser structure through hydrophobic interactions. The structure in the system is destroyed by external shearing, resulting in a sudden drop in the storage modulus of the system. The increase of system mass fraction and KGM content is beneficial to maintain the stability of the dense structure and increase the external frequency value that destroys the structure. For the low mass fraction system, more KGM content is conducive to the formation of gel; for the high mass fraction system, more HPMC content is conducive to the formation of gel.