Crystallization behavior of glucomannan

Five different glucomannan samples were recrystallized from dilute solution. Depending on the experimental conditions, the crystals obtained could be identified as corresponding to the mannan I (anhydrous precipitate of more or less regular lozenge‐shaped crystals) or mannan II (hydrated gel‐forming pseudo‐fibrillar precipitate). High‐molecular‐weight material, low temperature of crystallization, or a polar crystallization medium favored the mannan II polymorph, whereas a low‐molecular weight, a high temperature of crystallization, and a crystallization medium of low polarity yielded the mannan I polymorph. Since the base‐plane unit‐cell dimensions are fairly constant with respect to variation of glucose, it is likely that isomorphous replacement of mannose by glucose occurs in glucomannan crystallization; the data also indicate that perfection of the glucomannan crystals was reduced in specimens having a high glucose:mannose ratio. The oriented crystallization of glucomannan on cellulose microfibrils was also studied under conditions where the mannan I polymorph was obtained. This gave shish‐kebab structures that were characterized.