J. Cent. South Univ. Technol. (2008) 15(s1): 516-519
DOI: 10.1007/s11771-008-412-1
Rheology and influence factor of low-concentration Konjac gum solutions
WANG Yuan-lan(王元兰), LI Zhong-hai(李忠海), WEI Yu(魏 玉)
(School of Science, Central South University of Forestry and Technology, Changsha 410004, China)
Abstract: Rheological properties of low-concentration Konjac gum and impacts of concentration, shearing rate, pH value and stirring time on its viscosity were studied. The results show that the viscosity of low-concentration Konjac gum increases with the increase of its concentration. The solution presents pseudo-plasticity fluid, which shows the characters of shear thinning as follows: first, its viscosity decreases gradually as the temperature increases, and then increases with the increase of temperature. The viscosity decreases sharply at both acid and alkaline conditions, indicating that it is influenced apparently by pH value. And the viscosity of low-concentration Konjac gum is also impacted by the stirring time.
Key words: Konjac gum; low-concentration; rheology; viscosity
1 Introduction
Amorphophallus rerieri is a perennial herbaceous plant of amorphophallus blume, araceae, and is native to China and Vietnam, mainly in Hubei Province, Hunan Province and some other provinces in the southwest of China. Konjac gum is a carbohydrate gum processed and refined from amorphophallus rerieri stem tuber. Its main chemical ingredient is Konjac Glucomannan (KGM), which is a heteropolysaccharide conjugated by D-glucose and D-Mannose with the mass ratio of β-1, 4 glucosidic bond 1?1.6 [1-2]. Its molecular fraction is up to 1×106 and its viscosity of hydrosol is great and it has typical pseudoplasticity[3]. It is applied widely in the area of food, package, drilling, coating, bio-medicine and make-up because of its favorable properties of thickening, blending, setting, gelling, film-forming, lubricating and biodegradable proprieties and its medical and health function of decreasing serum cholesterol, reducing blood lipid, reducing glycemia, preventing constipation, preventing obesity and preventing digestive tract cancer[4-9]. In order to get a better understanding of the behavior of Konjac gum solution and the influence factor, the viscosity of low-concentration Konjac gum solution was mainly tested with different concentrations, shearing rates, pH values, and stirring time.
2 Experimental
2.1 Material and apparatus
Material are Konjac power (food-grade), was supplied by Hunan Bureau of Food Quality and Technical Supervision.
Main equipment and apparatus were as follows: NDJ-5S style digit viscosimeter, DSY-2-8 electric-heated thermostatic water bath, JJ-1 precision timing agitator, pHS-3C micrometric pH, magnetic stirring agitator and micrometric electronic balance.
2.2 Solutions preparation
Only fresh Konjac gum solutions were used. Konjac gum solutions were prepared in glass beaker: first, the powder was dispersed in distilled deionized water, under magnetic stirring at ambient temperature(25 ℃) and then the dispersion was heated at 60 ℃ during 30 min in a water bath, till it was dissolved and dispersed fully and placed to reserve at room temperature. Konjac gum concentration varied from 0.05% to 0.4%.
2.3 Rheological measurements
Viscosity was monitored by using NDJ-5S style digit viscosimeter at No.2 rotor at room temperature[10-12].
2.3.1 Influence of concentration on the viscosity of Konjac gum solutions
The Konjac gum solutions were prepared with mass fraction of 0.05%, 0.1%, 0.2%, 0.3% and 0.4%, and the viscosity was monitored.
2.3.2 Influence of change of shearing rate on viscosity of Konjac gum solutions
The influence on the viscosity of Konjac gum solutions (0.05%, 0.1%, 0.3% and 0.4%) was monitored at the shearing rate of 6, 12, 30 and 60 r/min.
2.3.3 Influence of temperature on viscosity of Konjac gum solutions
The Konjac gum solutions were separately confected in the mass fraction of 0.1% and 0.3%, then was heated at 30, 40, 50, 60, 70, 80 and 90 ℃ separately for 30 min, and the change of viscosity was monitored at the shearing rate of 6 r/min.
2.3.4 Influence of pH value on viscosity of Konjac gum solutions
The Konjac gum solutions were separately confected in the mass fraction of 0.1% and 0.3%. The acide solution and alkali solution were used to change its pH value(monitored by micrometric pH), and the influence on Konjac gum of change of pH values was monitored under the shearing rate of 6 r/min.
2.3.5 Influence of stirring time on viscosity of Konjac gum solution
The Konjac gum solutions were separately confected in the mass fraction of 0.1% and 0.3%. Magnetic stirring agitator was used to stir 15, 30, 60, 90 and 120 min at low rotate speed separately, and then the influence of different stirring time on the viscosity of Konjac gum solutions was monitored under the shearing rate of 6 r/min.
3 Results and discussion
3.1 Influence of concentration on viscosity of Konjac gum solutions
Fig.1 shows the influence of concentration on the viscosity of Konjac gum solutions. It can be seen that there is different interactions between molecules in different concentrations of Konjac gum solution, and this reflects viscosity of Konjac gum solution directly. The
Fig.1 Effects of concentration on Konjac gum solution viscosity
concentration of about 0.30% is the turning point of effect, that is, when the concentration is lower than 0.30%, the change of the viscosity with concentration is slower, however, its viscosity increases rapidly when the concentration is higher than 0.30%.
3.2 Influence of change of shearing rate on viscosity of Konjac gum solutions
When it was used as a food additive, Konjac gum should be processed by the shearing action of transportation, stirring and homogenization. Therefore, the influence of the change of shearing rate on the viscosity of Konjac gum solutions was monitored at room temperature (25 ℃). From Fig.2 it can be seen that different concentrations of Konjac gum solutions clearly demonstrates the shear-thinning phenomenon. Its viscosity distinctly reduced as the shear-rate increases, which reflects that Konjac gum solutions is non-Newtonian fluid and it has typical pseudo-plasticity fluid character. And the pseudo-plasticity fluid character is more remarkable with the concentration increases. When the viscosity of Konjac gum solutions is higher than 0.3%, the influence of the change of shearing rate on the viscosity is gradually reduced.
Fig.2 Effects of different shearing rates on Konjac gum solution viscosity with different concentrations of Konjac gum solutions
3.3 Influence of temperature on viscosity of Konjac gum solutions
The heating sterilization is a common processing technology in the process of producing food. Therefore, influence of temperature on viscosity of 0.1% and 0.3% Konjac gum solutions is shown in Fig.3.
From Fig.3, it can be seen that the viscosity of Konjac gum solutions with mass fraction of 0.1% and 0.3% reduces gradually when the temperature increases. It reflects that the temperature has obvious effects on it. The point of inflexion on a curve is at about 70 ℃, and the change rate of the viscosity of Konjac gum solutions with temperature is the greatest at this moment. It is reduced gradually when the temperature is higher or lower than 70℃. This is due to the action of molecules quickened with the temperature increases and the molecules brokes loose the hydrogen bond binding of Konjac gum. And the change of reciprocal actions caused the change of the viscosity of Konjac gum solutions.
Fig.3 Effects of temperature on Konjac gum solution viscosity with different mass fractions
3.4 Influence of pH value on viscosity of Konjac gum solution
From Fig.4, it can be seen that different pH has different influences on the viscosity of Konjac gum solutions. The inherent pH value of Konjac gum solutions is 5.5. When the pH value is 5.5, its viscosity reaches the maximum, but its viscosity has a great reduction, when pH＞5.5 or pH＜5.5, and the decreasing extent of its viscosity reaches to the greatest in alkalescence solution.
Fig.4 Effects of pH on Konjac gum solution viscosity
3.5 Influence of stirring time on viscosity of Konjac gum solution
Fig.5 shows the viscosity of Konjac gum solutions with stirring time and its stirring rate is certain. If the stirring time is less than 60 min, the viscosity of Konjac gum solutions reduces steadily but not distinctly when the stirring time increases. However, when the stirring time is more than 90 min, the viscosity reduces clearly and the tolerance of mechanical destruction decreases.
Fig.5 Effects of stirring time on Konjac gum solution viscosity
4 Conclusions
The low-concentration Konjac gum solutions is non-Newtonian fluid and it has typical pseudo-plasticity fluid character. The viscosity of low-concentration Konjac gum increases with concentration. When the concentration is higher than 0.3%, the change of viscosity with concentration increases rapidly. It reflects typical pseudo-plasticity fluid character and its viscosity is distinctly reduced when the shearing rate is increased. The temperature has obvious effects on viscosity of Konjac gum solution. And the change rate of the viscosity with temperature is the greatest at about 70 ℃. Different pH values have different influences on the viscosity of Konjac gum, and the viscosity reaches to the maximum when the pH value is 5.5. At a certain stirring rate, the viscosity of Konjac gum solution changes with the change of stirring time. In a word, Konjac gum can be made to reach the best useful effect according to its rheological property through suitable processing technology in concrete productive practice.
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(Edited by CHEN Can-hua)
Foundation item: Project(08RM05) supported by Institute of Rheological Mechanics and Material Engineering of Central South University of Forestry and Technology
Received date: 2008-06-25; Accepted date: 2008-08-05
Corresponding author: WANG Yuan-lan, Doctoral candidate; Tel: +86-731-5623648; E-mail: csfuyl@163.com
