Xanthan gum, also known as yellow gum, is white or light yellow powder. In 1952, it was isolated and extracted from the pathogenic strain of Xanthomonas campestris by the northern laboratory of the United States. After in-depth research, it was found that xanthan gum has excellent rheological properties and can partially or completely replace other gloea. Since then, xanthan gum has been extensively studied.
Xanthan gum was discovered in 1954, and it has been industrialized for less than 70 years since 1964. It has become one of the most widely used biological gums in the world, and has been used in food, petroleum, and pharmaceutical fields.
Xanthan Gum Structure
Xanthan gum can be regarded as a linear connection of pentasaccharide repeating units, which are D-glucose, D-mannose, D-glucuronic acid, acetic acid and pyruvic acid, and their average relative molecular mass is about 2×106 ~2×107 Da; It can also be regarded as composed of D-glucose on the main chain of cellulose and three sugar units on the side chain, these three sugar units are two molecules of D-mannose and one molecule of D-glucuronic acid, they are alternately connected to form a side chain structure, in which the mannose linked to the main glucose chain and the mannose in the side chain have O-acetylation modification and pyruvate ketalization modification. Depending on the strain used and the fermentation conditions, the ratio of pyruvate modification will also vary. Studies have shown that xanthan gum has multiple high-level structures in solution: the side chains in the molecule are reversely wound around the main chain to form a helical structure, the molecules form a rod-shaped double helix structure through secondary forces, and then rely on weak forces to form a double helix network three-dimensional structure. Based on the above-mentioned network three-dimensional structure, xanthan gum has a very strong ability to withstand acid, alkali, salt and temperature. There may be two conformations in aqueous solution: a relatively regular double helix structure and a disordered coil structure. Low temperature and salt ions are beneficial to maintain the double helix structure of xanthan gum; long-term heating will cause xanthan gum to lose its advanced structure and form random coils; after cooling, double helixes and random coils exist in the solution in different proportions, The conformational change in solution gives xanthan gum its unique rheology.
Properties of Xanthan Gum
Xanthan gum is light yellow to white water-soluble biopolysaccharide powder. Compared with organic solvents such as ethanol, it is easier to dissolve in cold water and hot water to form a viscous and stable colloid. It is the most superior performance among many biopolysaccharides. Xanthan gum molecules are acidic with glucuronic acid residues, which are anionic acidic polysaccharides. However, commercial xanthan gum mostly exists in the form of neutral salts, so the solution is generally neutral.
Rheology of Xanthan Gum
Xanthan gum solution has good rheological properties. Under different conditions, the aqueous solution of xanthan gum not only shows obvious pseudoplasticity, but also has typical viscoelasticity and shear thinning characteristics. Xanthan gum can achieve higher viscosity in aqueous solution with lower concentration, and its polysaccharide molecular chains are in the state of rigid helix in aqueous solution, and most of them are connected by non-covalent bonds. When the xanthan gum aqueous solution is placed in a static or low-speed stirring state, its main chain and side chains will be intertwined and cross-linked together, thereby further forming a disordered network structure. This solution state gives xanthan gum Raw gum with excellent viscosity value.
Xanthan Gum Stability
Effect of inorganic salts on the viscosity of xanthan gum aqueous solution. Studies have found that when monovalent salts such as NaCl are added to xanthan gum aqueous solution (concentration less than 0.25%), the viscosity of the solution will decrease to a certain extent. small, the molecular chain shrinks, and the solution viscosity decreases. On the contrary, the viscosity of xanthan gum aqueous solution increases with the increase of salt concentration. When the salt concentration increases to 0.1%, the viscosity of xanthan gum aqueous solution reaches the maximum value. When the concentration of salt continues to increase, the viscosity of xanthan gum aqueous solution no longer change. Salts containing divalent ions (such as MgCl2, etc.) are similar to monovalent salts (such as NaCl, KCl, etc.), that is, the viscosity of xanthan gum changes due to the influence of salt concentration. However, when the salt concentration increases to 1%-2%, the hydration rate of xanthan gum will become lower. Therefore, if it is necessary to obtain an aqueous solution of xanthan gum, it is recommended to do it under the condition of no salt. Once the xanthan gum aqueous solution is hydrated, even if the salt concentration continues to increase, when the concentration increases to 20%-30%, the hydration process of the xanthan gum aqueous solution will not be reversed. In fact, studies have found that xanthan gum can maintain a good viscosity for a long time in 5% NaCl and 10% KCl and CaCl2 salt solutions with a mass fraction of 25°C.
The effect of pH on the stability of xanthan gum. Xanthan gum solution can maintain a good viscosity in a wide pH range (4-10), and only under extreme pH conditions (1-3 or 11-12) and at a concentration of 0.3%, its viscosity becomes will change. In addition, the acid-base stability of xanthan gum aqueous solution will continue to increase with the increase of its concentration. At present, most organic acids, such as acetic acid, phosphoric acid, lactic acid, citric acid and tartaric acid, can coexist with xanthan gum aqueous solution. Xanthan gum can be directly hydrated in acidic solution, but it is recommended to prepare the xanthan gum aqueous solution first, and then gradually add the acid solution.
Temperature has a great influence on the viscosity of xanthan gum aqueous solution. In the temperature range of 10-90°C, there is a critical value in the viscosity range, that is, the temperature reaches 40°C, and the xanthan gum is dissolved in deionized water with a concentration of 0.3%. When the temperature exceeds the critical point, the viscosity of the solution drops rapidly, but immediately after the temperature is cooled, the viscosity of the xanthan gum aqueous solution can recover to more than 90% of its original value.
Xanthan gum has good resistance to a variety of hydrolytic enzymes, and is hardly hydrolyzed by common enzymes such as amylase, protease, cellulase, and pectinase. This ability to resist the hydrolysis of various enzymes is due to the double helix structure of the intramolecular temperature: its side chains surround the main chain and wind up in reverse, which can prevent the interaction between various enzymes and molecular chains.
Application of Xanthan Gum in the Field of Medicine
Because xanthan gum contains strong polar groups such as -COO- and -OH, the intermolecular force is relatively large, so the stability of the aqueous solution is relatively high, and it has good properties such as viscosity increasing, rheology, salt resistance and shear resistance. It also has good stability to enzymatic hydrolysis, and many enzymes such as protease, amylase, cellulase and hemicellulase cannot degrade xanthan gum. The above properties of xanthan gum make its application in pharmaceuticals have broad prospects. Studies have found that xanthan gum is an excellent new excipient for pharmaceutical preparations, and can also participate in the formation of drug carriers. Researchers have prepared starch-xanthan gum cross-linked films by using potato starch and xanthan gum as monomers, and found that they have good drug-loading properties. In addition, it can also be used as a protective material for active pharmaceutical ingredients to participate in the synthesis of drug delivery carriers.
