Methylcelluloses And Hydroxypropylmethylcelluloses

Methylcelluloses (MCs) contain methoxyl groups in place of some of the hydroxyl groups along the cellulose molecule; hydroxypropylmethylcelluloses (HPMCs) contain, in addition to methyl ether groups, hydroxypropyl ether groups along the cellulose chain (2,4—6). The properties of MCs and HPMCs are primarily a function of the amount of each type of substituent group and the molecular-weight distribution.

MCs are made by reacting cellulose with methyl chloride until the DS (see "Carboxymethylcelluloses") reaches 1.1 to 2.2. HPMCs are made by using propylene oxide in addition to methyl chloride in the reaction; hydroxypropyl group MS (see "Hydroxypropylcelluloses") levels in commercial products are 0.02 to 0.3.

Members of this family of gums are cold-water soluble. Conversion of some of the hydroxyl groups of cellulose molecules into methyl ether groups increases the water solubility of the polymer and reduces its ability to aggregate (ie, reduces intermolecular interactions). Solubility and solution stability is increased even more when hydroxypropyl groups are added to MC.

The most interesting property of these nonionic products is thermal gelation. Solutions of members of this family of gums decrease in viscosity when heated, as do solutions of most other polysaccharides. However, unlike solutions of other gums, when a certain temperature is reached (depending on the specific product), the viscosity will increase rapidly and the solution will gel. Gelation can occur at various temperatures from 45 to 90°C (115 to 195°F), depending on the viscosity type, DS/MS, and proportions of methyl and hydroxypropyl substituent groups. The thermal gelation is reversible; that is, the gel will revert to a fluid upon cooling.

These gums reduce surface and interfacial tensions and can, therefore, be used to stabilize emulsions and make foams. They will form high-strength films that are clear, water-soluble, and oil- and grease-resistant and have low oxygen and moisture vapor transmission rates. They are used in the preparation of dietetic foods, in baked goods for their glutenlike properties, in dipping batters (where film formation, low oil migration, moisture retention, thermal gelation, and tackiness are important properties), in whipped toppings, in frozen desserts and novelties, in canned fruit juice and fruit drink mixes (as a bodying agent, drying aid, emulsion stabilizer, and/or cloud agent), and as a binder and lubricator in extrusion processes. Their solutions exhibit pseudoplastic rheology similar to that of other cellulosics (see "Carboxymethylcellulose").

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