Gel, irreversible

With further heating there is an increase in viscosity and the syrup sets to an insoluble and irreversible gel which eventually converts, with the evolution of water and formaldehyde to a hard, colourless, transparent and infusible mass. 

It is this action alone which enables a relatively small number of dyes, the so-called substantive cotton dyes, to be absorbed directly by the unmordanted vegetable fibre. The most important of these dyes are the bis-azo dyes, such as Congo-red and related compounds, which are derived from doubly diazotised benzidine. In aqueous solution they are present as sols and are colloidally adsorbed by the fibres as irreversible gels. 

Identification Heat the sample with 3 parts water to 2° to 5° above its melting point. An irreversible gel forms when the sample is held at this temperature. 

Low methoxyl pectins (< 50% esterified) form thermoreversible gels in the presence of calcium ions and at low pH (3 - 4.5) whereas high methoxyl pectins rapidly form thermally irreversible gels in the presence of sufficient (e.g., 65% by weight) sugars such as sucrose and at low pH (< 3.5) the lower the methoxyl content, the slower the set. 

Curdlan is a bacterial polysaccharide made by Agrobacterium biovar [87,88,89]. It is a linear (1 3)- -glucan (MW 73,000) that forms a triple helix. Curdlan is insoluble in cold water. When aqueous dispersions of curdlan are heated, two types of gels form. First curdlan dissolves. When the solution reaches 55-66 °C, then is cooled, a reversible gel forms. The gel melts when held at about 60 °C. When the thermoreversible gel is heated to a temperature above 80 °C, an irreversible gel forms. Heating to higher temperatures results in stronger irreversible gels. Transition temperatures are a function of concentration. 

It is composed of about 25% amylose (anhydrogluco-pyranose units joined by glucosidic bonds) and 75% amylopectin, a branched-chain structure. Properties White, amorphous, powder or granules tasteless. Various crystalline forms may be obtained, including microcrystalline. Irreversible gel formation occurs in hot water swelling of granules can be induced at room temperature with such compounds as formamide, formic acid, and strong bases and metallic salts. 

The presence of cystine is important for irreversible gel formation, but as the content of cystine is practically the same in the hydrolysate made by I, II and IV as shown in Table Illb, it is concluded that the cystine content is not responsible for the differences seen in whipping properties. 

Locust bean gum was cross-linked with epichlorohydrin in an alkaline aqueous solution. Diluted gum solutions are thus transformed into elastic, irreversible gels. These gels were cut into small pieces, dewatered with alcohol and ether, and dried. When the dried pieces were put into water, they swelled up to the original size and... 

The comparison between the two chain pairs is also consistent with the physical properties. Paramylon and amylose are known for their gel forming properties and their carbon reserve function in p1 ants.(13,69) Although (l- -3)-3-D-glucan also plays a structural rFTe when it occurs in the cell walls of yeast( ), it is probably more of a matrix substance than anything else. The ability to form multiple helices probably accounts for unusual solubility phenomena such as retrogradation( ) and irreversible gel formation on heating. (13)... 

Whey proteins denature at temperatures above 70°C and become insoluble and form thermally irreversible gels. 

The thermally-reversible gel returns to the solution because the droving forces are noncovalent bonds such as hydrogen bonding, hydrophobic interaction, n-n interaction and electrostatic interaction. These noncovalent bonds are broken down by heating, so the gel reverts to the sol. Since the thermally-irreversible gel forms a network structure by firm chemical covalent bonds, the gel formed never changes to the sol. This is why the thermally-irreversible gel is called physical gel and the thermally-irreversible gel is referred to as chemical gel. Moreover, the gelation ability is found in not only biopolymers but also low molecular weight compounds. 

In chemical gels the network connection (crosslink) is usually a covalent bond, which leads to a thermally irreversible gel. When the crosslinking is purely physical in nature, a physical gel is formed which is thermally reversible. There have been at least two excellent works published in the last few years. Reversible Polymeric Gels and Related Systems by Russo [2] and Thermoreversible Gelation of Polymers and... 

Curdlan gum is tasteless and produces retortable freezable food elastic gels. It is insoluble in cold water, but aqueous suspensions plasticize and briefly dissolve before producing irreversible gels i.e. curdling, hence its... 

Starch can be isolated as a white powder which is almost insoluble in cold water, although it will absorb some and swell slightly. In hot water irreversible gel formation occurs (see gelling biopolymers below). 

However, if the can lid is not tight fitting, it may cause the formation of a skin, which seals off the paint below from further reaction with the air. Alternatively, the skin may be permeable to the air and the reaction may spread right through the paint, turning it into a cross-linked polymer swollen with solvent. This is called an irreversible gel and, of course, at this point the paint becomes useless. 

On the basis of Figures 15-17, it may be stated that in an oil-wet system the flow of the polymer solution advancing connate water and distilled water takes place with its initial viscosity and mobility further, that in the porous media no apparent permeability reduction occurs. With knowledge of the close relationship between the polymer adsorbed on the rock surface and the flow characteristics, this phenomenon may be traced to the absence of an irreversible gel layer over the pore surface. On an oil-wet surface the quantity of polymer bound by chemisorption, regardless of the type, is zero, and thus the "sweep effect of the carrier phase is also perfect. 

Gel formation may be reversible or irreversible, a very important factor in membrane cleaning. An irreversible gel is very difficult to remove and precautions have to be taken to avoid this situation as much as possible. However, it is not important whether the gel is... 

---