Sodium carboxymethyl cellulose (CMC) is a kind of high polymerization degree fiber ether prepared by chemical modification of natural cellulose. Its structure is mainly composed of D- glucose units linked by β( 1→4) glycosidic bonds. The main reaction is that natural cellulose first reacts with NaOH, and with the addition of chloroacetic acid, the hydrogen on the hydroxyl group of its glucose unit reacts with carboxymethyl group in chloroacetic acid.

As can be seen from the structural formula, each glucose unit has three hydroxyl groups, namely C2, C3 and C6 hydroxyl groups, and the number of hydrogen substituted by carboxymethyl groups on the hydroxyl groups of each glucose unit is expressed by the degree of substitution. If all the hydrogens on the three hydroxyl groups in each unit are replaced by carboxymethyl groups, it is defined as a degree of substitution of 3, and the degree of substitution of CMC directly affects the solubility, emulsification, thickening, stability, acid resistance and salt resistance of CMC. It is generally believed that the emulsification performance is better when the degree of substitution is about 0.6 ~ 0.7, and other properties are also improved with the increase of the degree of substitution. When the degree of substitution is greater than 0.8, its acid resistance and salt resistance are obviously enhanced. In addition, it is mentioned that there are three hydroxyl groups in each unit, namely the secondary hydroxyl groups of C2 and C3 and the primary hydroxyl group of C6. Theoretically, the activity of primary hydroxyl group is greater than that of secondary hydroxyl group, but according to the equilibrium effect of C, the -OH group on C2 is more acidic, especially in strong alkali environment, which is more prone to substitution reaction, followed by C6 and C3. In fact, the performance of CMC is not only related to the degree of substitution, but also to the uniformity of the distribution of carboxymethyl groups in the whole cellulose molecule and the uniformity of the substitution of hydroxymethyl groups with C2, C3 and C6 in each unit of each molecule. Because CMC is a highly polymerized linear compound, its carboxymethyl group has substitution inhomogeneity in the molecule, and when the solution is left standing, the molecules have different orientations. When there is shear force in the solution, the long axis of its linear molecules tends to turn to the flow direction, and this trend becomes stronger with the increase of shear rate until it is completely oriented. This property of CMC is called pseudoplasticity. The pseudoplasticity of CMC is beneficial to reduce the energy consumption of liquid milk production, homogenization and pipeline transportation, and it is not too greasy in liquid milk, which is beneficial to the release of milk flavor.