Sodium carboxymethyl cellulose (CMC) is a kind of highly polymerized cellulose ether prepared by chemical modification of natural cellulose. Its structure is mainly composed of D-glucose unit connected by β (1→4) glucoside bond.
CMC is white or milky white fibrous powder or particle, density 0.5-0.7 g/cm ³, almost odorless, tasteless, hygroscopic. Easy to disperse in water into a transparent colloidal solution, insoluble in organic solvents such as ethanol. The pH of 1% aqueous solution is 6.5 ~ 8.5, when pH>10 or <5, the viscosity of glue is significantly reduced, and the performance is best when pH=7. Stable to heat, the viscosity rises rapidly below 20 ° C, changes slowly at 45 ° C, and long-term heating above 80 ° C can denature its colloid and significantly decrease its viscosity and performance. Easily soluble in water, the solution is transparent; In alkaline solution is very stable, in the case of acid is easy to hydrolysis, PH value of 2-3 will appear precipitation, in the case of polyvalent metal salts will also react sedimentation.
Structural formula: C6H7 (OH) 2OCH2COONa Molecular formula: C8H11O5Na
The main reaction is as follows: the natural cellulose first underwent the alkalization reaction with NaOH, and with the addition of chloroacetic acid, the hydrogen on the hydroxyl group on the glucose unit underwent the substitution reaction with the carboxymethyl group in chloroacetic acid. From the structural formula, it can be seen that there are three hydroxyl groups on each glucose unit, that is, C2, C3 and C6 hydroxyl groups. The number of hydrogen on the hydroxyl group of glucose unit replaced by carboxymethyl group is expressed by the degree of substitution. If the hydrogen on the three hydroxyl groups on each unit is replaced by carboxymethyl group, it is defined as the degree of substitution is 3. The degree of CMC substitution directly affects the solubility, emulsification, thickening, stability, acid resistance and salt resistance of CMC.
It is generally believed that when the degree of substitution is about 0.6 ~ 0.7, the emulsification performance is better, and with the increase of the degree of substitution, other properties are correspondingly improved, when the degree of substitution is greater than 0.8, its acid resistance and salt resistance are significantly enhanced.
In addition, it is also mentioned above that there are three hydroxyl groups on each unit, that is, the secondary hydroxyl group of C2 and C3 and the primary hydroxyl group of C6. In theory, the activity of the primary hydroxyl group is greater than that of the secondary hydroxyl group, but according to the isotonic effect of C, the -OH group on C2 is more acidic, especially in the environment of a strong base, its activity is stronger than C3 and C6, so it is more prone to substitution reactions, C6 is second, and C3 is the weakest.
In fact, the performance of CMC is not only related to the size of the degree of substitution, but also to the uniformity of the distribution of carboxymethyl groups throughout the cellulose molecule and the uniformity of the substitution of hydroxymethyl groups with C2, C3 and C6 in each unit of each molecule. As CMC is a highly polymerized linear compound, and carboxymethyl has an inhomogeneity of substitution in the molecule, the molecules have different orientations when the solution is standing. When there is a shear force in the solution, the long axis of the linear molecules has a tendency to turn to the direction of flow, and this trend becomes stronger with the increase of the shear rate until the final orientation is completely oriented. This property of CMC is called pseudoplasticity. The false plasticity of CMC is conducive to homogenization and pipeline transportation, which will not taste too greasy in liquid milk and is conducive to the release of milk aroma.
The use of CMC products is mainly that we should have a sufficient understanding of the main indicators such as stability, viscosity, acid resistance, viscosity and so on. Know how we choose the right product.
Low viscosity CMC products, fresh taste, low viscosity, almost no thick feeling, mainly used in special sauces and beverages, health oral liquid is also a good choice.
CMC products are mainly used in solid drinks, ordinary protein drinks and fruit juices, the specific choice is based on the personal habits of engineers. In the stability of dairy drinks, CMC has contributed a lot.
High viscosity CMC products, the application space is relatively large, compared with starch, melon gum, xanthan gum and other products, the stability of CMC is still more obvious, especially in meat products, CMC water retention advantages are more obvious! Among the stabilizers such as ice cream, CMC is also a good choice.
The main indicators to measure the quality of CMC are the degree of substitution (DS) and purity. Generally, the nature of CMC is different when DS is different; The greater the degree of substitution, the greater the solubility, the better the transparency and stability of the solution. It is reported that the transparency of CMC is better when the degree of substitution is 0.7 ~ 1.2, and the viscosity of its aqueous solution is maximum when the pH value is 6 ~ 9.
In order to ensure its quality, in addition to the selection of etherifying agents, it is necessary to consider some factors affecting the degree of substitution and purity, such as the relationship between the amount of alkali and etherifying agents, etherification time, system water content, temperature, DH value, solution concentration and salts.
The quality of CMC products is good or bad, mainly look at the solution of the product, if the solution of the product is clear, less gel particles, less free fibers, less impurities, basically determine the quality of cmc is good, if the solution is placed for a few days, the solution does not appear white or cloudy, or very clear, it is a better product!
02. Dissolution and dispersion of CMC products
CMC is directly mixed with water, prepared into a paste glue liquid, set aside. When configuring the CMC paste, first add a certain amount of clean water into the mixing tank with the mixing device. When the mixing device is turned on, spread the CMC slowly and evenly into the mixing tank and stir continuously, so that the CMC and the water are completely integrated and the CMC can be fully dissolved.
When the CMC is dissolved, the reason why it should be evenly dispersed and constantly stirred is to "prevent the occurrence of clumping and caking when the CMC meets the water, reduce the problem of the dissolution amount of the CMC", and improve the dissolution rate of the CMC. The time of stirring and the time of complete melting of CMC is not consistent, it is two concepts, in general, the time of stirring is much shorter than the time required for complete melting of CMC, and the time required for both depends on the specific situation.
The basis for determining the mixing time is: when the CMC is evenly dispersed in water and there is no obvious large clumps, the mixing can be stopped, so that the CMC and the water penetrate and fuse with each other in a static state. The speed of mixing is generally between 600 and 1300 revolutions, and the mixing time is generally controlled in about 1 hour.
The basis for determining the time required for complete melting of CMC is as follows:
(1) CMC and water are completely bonded, and there is no solid-liquid separation between the two;
(2) The mixed paste is uniform and the surface is smooth;
(3) The color of the mixed paste is close to colorless and transparent, and there is no granular object in the paste. From the time the CMC is put into the batching tank and mixed with water, until the CMC is completely dissolved
