Gelling temperature

MethylceUulose with a methyl DS less than about 0.6 is alkali-soluble. Erom about 1.6 to 2.4, it is water-soluble (most commercial grades) above 2.4, it is soluble in a wide variety of organic solvents. MethylceUulose solutions in water start to gel at 55° C, independent of molecular weight. The gelation is a function of the DS, rate of heating, and type and amounts of additives such as salts. As the temperature increases, the viscosity initially decreases (typical behavior). When the gelling temperature is reached, the viscosity sharply rises until the flocculation temperature is reached. Above this temperature, the viscosity coUapses. This process is reversible with temperature (75). 

Different characteristics of solvents seriously affect the sol-gel reaction in solution. This in turn influences the physico-mechanical properties of the resultant rubber-silica hybrid composites. Bandyopadhyay et al. [34,35] have carried out extensive research on stmcture-property correlation in sol-gel-derived rubber-sihca hybrid nanocomposites in different solvents with both chemically interactive (ENR) and noninteractive (ACM) mbber matrices. Figure 3.12 demonstrates the morphology of representative ACM-sihca and ENR-sihca hybrid composites prepared from various solvents. In all the instances, the concentration of TEOS (45 wt%), TEOS/H2O mole ratio (1 2), pH (1.5), and the gelling temperature (ambient condition) were kept unchanged. 

The gelling temperature is an important factor for the characterization and application of pectins. The pectin consumer wants a pectin fulfilling his special requirements, this can mean either working with or without pregelation. Pregelation, the weakening of gel structure, occurs when pectin preparations are stressed below their gelation temperature so that the mechanical treatment leads to an irreversible destruction of the three-dimensional network. 

An increase in pH of the hemicellulose solution thus clearly decreases the gelling temperature of the hemicellulose suspension. The gel curves of the... 

K. B. Guiseley, The relationship between methoxyl content and gelling temperature of agarose, Carbohydr. Res., 13 (1970) 247-256. 

Mild thixotropy has been observed in gelatiiized wheat starch dispersions characterized at low temperatures such as 25°C but not at 60°C and above (Doublier, 1981 Harrod, 1989 Bagley and Christianson, 1982). The time dependent rheological behavior shown by starch is attributed to structure formation by amylose leached into the suspending matrix on cooling to low gelling temperatures (Ellis et al., 1989). 

Casting. Castings of liquid epoxy resins cured with hexahydrophthalic anhydride (HHPA) were prepared by heating the liquid resin in a stirred 3-necked flask to which the calculated amount of premelted HHPA was added. A vacuum was drawn to partially degas the mixture then 2 phr benzyldimethylamine (BDMA) was added as accelerator. After further degassing, the mixture was poured into casting molds prepared as described above. The filled molds were put into an oven at the desired gelling temperature subsequently, either the oven was reset to the final cure temperature or the molds were transferred to a second oven, as convenient. 

Entrapment of enzymes and cells, especially in polysaccharide gels is very popular, because it has the advantage of high enzyme stability and ease of preparation. Firefly and bacterial luciferases are very labile, and hence agarose is mostly used for its low gelling temperature (26-30 °C). 

LM-pectin will gel only in the presence of divalent cations. Increasing the concentration of divalent cations (only calcium ion is used in food applications) increases the gelling temperature and gel strength. Divalent cations are unnecessary for the formation of a HM-pectin gel because of the low number of carboxylate groups that need to be bridged and because of the formation of hydrophobic areas parallel to the helix axes by a columnar stacking of methyl ester groups (15). 

Among the commercial LM-pectins, those with the lowest DE values have the highest gelling temperatures and the greatest requirement for divalent cations (for crossbridging). 

Amidatlon results In a higher gelling temperature and a decreased need for a divalent cation. 

Gelatin is a multifunctional ingredient used in foods, pharmaceuticals, cosmetics, and photographic films as a gelling agent, stabilizer, thickener, emulsifier, and film former. As a thermoreversible hydrocolloid with a narrower gap between its melting and gelling temperatures, both of which are below human body temperature. 

The rheological test measured the stress/strain as a function of time and temperature. Again, an arbitrary heating rate is selected. The midpoint of the transition is usually considered to be the melting or gelling temperature. The amount of cooperativity of the system can be determined by the width of the curve (in degrees) usually measured from the 25% point of transition to the 75% point of transition. The greater the cooperativity (or uniformity of the material), the smaller the width observed. 

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