Tokuya Harada

Cyclic (1→2)-β-d-glucan and the octasaccharide repeating-units of extracellular acidic polysaccharides produced by Rhizobium

Abstract Ten strains of Rhizobium tested produced extracellular, cyclic (1→2)-β- d -glucan, and five of ten strains produced the linear octasaccharide repeating-units of the extracellular, acidic polysaccharides.

Thermal properties of curdlan in aqueous suspension and curdlan gel

Abstract The thermal properties of curdlan in aqueous suspension and curdlan gel were studied by differential scanning calorimetry (DSC). The DSC curve of curdlan in aqueous suspension showed two sharp endothermic peaks, at 50–64 and 140–160°C, and a broad endothermic peak between the two peaks. An exothermic peak at ~140°C was also observed. The two endothermic peaks are caused by the swelling of curdlan and the melting of gel formed. The broad peak may be caused by the formation of firm gel stabilized by hydrophobic interaction. The exothermic peak may be due to the formation of microfibrils. The swelling temperature and enthalpy of curdlan in aqueous suspension were 49.7°C and 10 J/g, respectively. The swelling temperature of curdlan in the presence of sodium chloride or urea shifted to a higher or a lower temperature range, respectively.

Difference of molecular association in two types of curdlan gel

Abstract The molecular association in a curdlan gel formed by neutralizing an alkaline solution of curdlan with carbon dioxide was compared with those in gels obtained by heating aqueous suspensions of curdlan at various temperatures. The neutralized and 60°C-set preparations were soluble in 0·01 m sodium hydroxide, whereas preparations set at above 90°C were soluble only in concentrations of sodium hydroxide above 1 m . The absorption of Aniline blue or Congo red to the preparations decreased with an increase in the temperature of heat treatment and the adsorption to a gel heated at 120°C for 4 h was about 30% of that for the unheated neutralized gel. Seventy-three per cent of the heated preparation was resistant to treatment with 32% sulfuric acid at 32°C for 30 days, whereas none of the neutralized gel was resistant. An electron micrograph of the resistant part of the curdlan showed that it had a pseudocrystalline form. X-ray studies showed a much higher crystalline structure in the resistant part than in the preparation without heat treatment. The X-ray patterns were almost the same for preparations treated with 32% sulfuric acid or (1 → 3)-β-glucanase.

Effect of heating on formation of curdlan gels

Abstract The characteristics of curdlan preparations that were dehydrated after heating at various temperatures in water were examined by X-ray diffraction, differential scanning calorimetry and electron microscopy. X-Ray diffraction patterns showed that crystallinity was higher in preparations heated to above 80°C, with heated preparations at 170°C having the highest crystallinity. Differential scanning calorimetry showed that the temperature of the endothermic peak caused by swelling decreased and that the peak area became smaller with increasing heating temperatures. Moreover, preparations of (1 → 3)-β- d -glucans of low molecular mass, DPn 131 and 49, with and without heating at 120°C gave similar X-ray diffraction patterns and differential scanning calorimetry curves to those of the original curdlan. The crystallinity was particularly high without heating in the preparation of DP n 49 though the endotherm associated with the breakage of hydrogen bonds was lower and the enthalpy smaller than found for the original curdlan. Electron microscopic studies showed that longer and wider microfibrils were formed in curdlan gels, while shorter microfibrils or crystals were formed with the lower molecular mass samples of (1 → 3)-β- d -glucan, which are incapable of forming gels.

Molecular Association and Dissociation In Formation of Curdlan Gels

We examined the formation of curdlan gels by differential scanning calorimetry and electron microscopy. When curdlan heated in water to various temperatures was cooled, an exo-thermic peak appeared at about 40 °C, with formation of hydrogen bonds involving water to give gel. When the cooled preparations were heated again, endo-thermic peaks reappeared at about 60°C and exo-thermic peaks also appeared at higher temperatures. However, no exo-thermic peaks were observed on cooling and then heating above 145 °C. Thus, the gel formation caused by heating to 60-120 °C and cooling is reversible, but that caused by heating to above 145 °C and cooling is irreversible as shown by the heat analysis. In neutralized gel and the gel formed by heating at 55 or 60 °C and cooling, microfibrils of endless length consisting of fibrils of about 100 nm length and 10-25 nm width appeared. In gels obtained by heating at 70, 80, and 100 °C and cooling, microfibrils with released fine stubs appeared. Gels cooled after heating at 120 and 145 °C contained spindle shaped microfibrils of about 100 nm length and 30 nm width. Preparations obtained by heating to 170 °C and cooling gave microfibrils like those formed by unheated preparations.

Structure of curdlan that is resistant to (1 → 3) β-d-glucanase

Abstract A structure that is resistant to (1 → 3)β- d -glucanase is formed when an aqueous suspension of curdlan ( DP n = 455 ) is heated to high temperatures. The resistance increases with increasing temperature and time of heat treatment. On similar treatment, (1 → 3)β- d -glucan ( DP n = 131 ) also becomes resistant to (1 → 3)β- d -glucanase. Electron microscopic examination showed it is a pseudocrystalline form with an electrondense structure that is resistant to (1 → 3)β- d -glucanase, but that fine fibrils are disrupted by the enzyme. The resistant structure may be formed by a hydrophobic reaction.

CHAPTER 15 – CURDLAN

Publisher Summary This chapter describes current information on the production, properties, uses, and potential food and nonfood uses of curdlan. Curdlan can be used to solidify liquids and fine powders in foods and industrial materials. Parent strains of microorganisms capable of producing curdlan also produce an acidic extracellular polysaccharide effectively. The original strains forming curdlan seem always to produce succinoglycan-type polysaccharide as identified by methylation analysis and fragmentation with two specific β-glucanases. The viscosity of an alkaline solution of curdlan decreases on addition of small amounts of salts of alkaline metal salts—such as, calcium or magnesium chlorides—thus facilitating centrifugation. Curdlan is produced as an envelope around the cells in solid culture medium in yield of about 1.5 g per 100 mL. Dried curdlan can be prepared by the process of pressing and drying the gels obtained by neutralization of its alkaline solution. Curdlan can be stained with Aniline blue, Brilliant blue, Trypan blue, and Congo red but not with Toluidine blue or Methylene blue.