Gels formed on cooling

Carrageenans are obtained from red seaweeds (Rhodophyceae) [13, 14]. Traditional carrageenan grades are obtained on extraction into solution by treatment of the seaweed with 

Carrageenans find widespread use in the food industry notably in meat and dairy products. They are also used, for example, in air fresheners to control the rate of release of the fragrance. 

Gellan gum is stiU relatively expensive and hence finds limited application at present in food products such as dessert jellies, sugar confectionery and fruit preparations. 

Gelatin is widely used in the food industry notably in confectionery, meats, dairy and dessert products. It has major application in the pharmaceutical area where it is used to prepare capsules and also in photography where it is used to adhere the photosensitive silver halide onto paper or film. It is also used in the production of holograms. 

Xanthan gum is a bacterial polysaccharide obtained from the genus Xanthomonas, notably X campestris by aerobic fermentation [25]. The molecules have a 3(l,4)-linked glucopyra-nose backbone with a trisaccharide side-chain on every other glucose residue linked through the C3 position. The side-chain consists of two mannopyranosyl residues linked on 

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To a 0.2-0.5 M soln of Z-Xaa-OH in EtOAc was added a 20% excess of HONp. The calculated amount of DCC was added to the soln at 0°C, which was stirred for 30 min. After Ih at rt, the DCU was removed by filtration, the precipitate was washed with EtOAc, the filtrate and washings were combined and the solvent was removed. The residue was dissolved in hot EtOH and the crystals that formed on cooling were collected, washed with EtOH, and recrystallized (hot EtOH) 5neld 85-90%. Lower yields were obtained for Z-Asn-ONp and Z-Gln-ONp, which were prepared in DMF and crystallized by the addition of water to the EtOH. Fmoc-Ser(tBu)-ONp and Fmoc-Thr-ONp required purification on a silica gel column.04... 

An acid soap of 2 1 complex ratio was discovered recently between triethanolamine stearate and stearic acid (13). In water the acid soap forms a lamellar liquid crystal phase at high temperatures, above 60°C, and transforms to a lamellar gel phase on cooling. The gel phase, however, is not stable at ambient temperature due to the occurrence of the hydrolysis reaction which converts the soap back to stearic acid which precipitates in the triethanolamine aqueous solution. A polymorphism of C, E, and possibly A forms of stearic acid crystals were found resulting from the hydrolysis reaction (8). 

In 0.03 molL KCI, i-cairageenan (c. 3 gL" ), forms a very tenuous gel network on cooling to 4°C, as can be appreciated from the mechanical spectrum displayed in Figure 13a. There is overall predominance of G over G but a clear dependence of both moduli with . However, in the NPEC with CHI-20 (Z = 0.084) under identical conditions, gelation of i-carrageenan at 4°C was completely suppressed (Fig. 13b). Whether the transition of i-carrageenan is shifted to yet lower temperature or else was inhibited completely, cannot be established conclusively at this stage. 

Acid-modified starches are used in the manufacture of gum candies because they form hot concentrated pastes that form strong gels on cooling. ThermaUzed starches are used in foods to bind and carry flavors and colors. Sweetening agents (com symp, HFCS) are made from starch by enzymatic or acid treatment as previously noted. 

Thiourea will react with neutralised formalin at 20-30°C to form methylol derivatives which are slowly deposited from solution. Heating of methylol thiourea aqueous solutions at about 60°C will cause the formation of resins, the reaction being accelerated by acidic conditions. As the resin average molecular weight increases with further reaction the resin becomes hydrophobic and separates from the aqueous phase on cooling. Further reaction leads to separation at reaction temperatures, in contrast to urea-formaldehyde resins, which can form homogeneous transparent gels in aqueous dispersion. 

The solidity of gel electrolytes results from chain entanglements. At high temperatures they flow like liquids, but on cooling they show a small increase in the shear modulus at temperatures well above T. This is the liquid-to-rubber transition. The values of shear modulus and viscosity for rubbery solids are considerably lower than those for glass forming liquids at an equivalent structural relaxation time. The local or microscopic viscosity relaxation time of the rubbery material, which is reflected in the 7], obeys a VTF equation with a pre-exponential factor equivalent to that for small-molecule liquids. Above the liquid-to-rubber transition, the VTF equation is also obeyed but the pre-exponential term for viscosity is much larger than is typical for small-molecule liquids and is dependent on the polymer molecular weight. 

The oily product solidifies on cooling to about 20°, forming white lumps. After filtration by suction, the lumps are crushed, filtered, and washed four times with 50-ml. portions of ice water. Drying over silica gel in a vacuum gives 62-72 g. (82-88%) of white glistening scales, m.p. 51°. 

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