Cellulose, the raw material for the manufacture of MC and HPMC, is insoluble in most solvents due to its high level of intramolecular hydrogen bonding and degree of crystal-linity. To make it soluble, its crystallinity and intramolecular hydrogen bonding need to be reduced. In the case of MC and HPMC products, this is accomplished by producing alkali cellulose by the addition of sodium hydroxide. This step swells the cellulose to facilitate substitution, particularly in regions of high crystallinity. Alkali cellulose is then allowed to react with methyl chloride to form MC. If the production of HPMC is desired, propylene oxide is added to the mixture.
The relative amounts of methoxyl and hydroxypropoxyl substitution are controlled by the weight ratio and concentration of sodium hydroxide and the weight ratios of methyl chloride and propylene oxide per unit weight of cellulose. Any change in the amount of methyl chloride and in the reaction profile will affect the properties of the final product. The by-products formed in this process are removed by slurrying the crude product in water heated to above 90°C and then filtering. As MC and HPMC are insoluble in hot water, the unique thermal gelation properties of MC and HPMC are thus used to simplify the purification process. The resulting product, a moist porous cake, is dried using hot air, followed by grinding and packing.
One of the advantages of MC and HPMC is that they can be produced within narrow specification limits. This is achieved by strict control over the manufacturing process, allowing for consistent production from batch to batch. This permits the manufacturer to formulate his products with a maximum of confidence, avoiding unpredictable behaviors of other less consistent gums in foodstuffs with an extreme pH (e.g., the acidic salad dressings) or that have to undergo stringent heat treatments (e.g., UHT and HTST processes).
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