Ice crystals, formation

Ice formation is both beneficial and detrimental. Benefits, which include the strengthening of food stmctures and the removal of free moisture, are often outweighed by deleterious effects that ice crystal formation may have on plant cell walls in fmits and vegetable products preserved by freezing. Ice crystal formation can result in partial dehydration of the tissue surrounding the ice crystal and the freeze concentration of potential reactants. Ice crystals mechanically dismpt cell stmctures and increase the concentration of cell electrolytes which can result in the chemical denaturation of proteins. Other quaHty losses can also occur (12). 

Equipment for food freezing is designed to maximize the rate at which foods are cooled to —18° C to ensure as brief a time as possible in the temperature zone of maximum ice crystal formation (12,13). This rapid cooling favors the formation of small ice crystals which minimize the dismption of ceUs and may reduce the effects of solute concentration damage. Rapid freezing requires equipment that can deHver large temperature differences and/or high heat-transfer rates. 

Cloud Seeding. In 1947, it was demonstrated that silver iodide could initiate ice crystal formation because, in the [ -crystalline form, it is isomorphic with ice crystals. As a result, cloud seeding with silver iodide has been used in weather modifications attempts such as increases and decreases in precipitation (rain or snow) and the dissipation of fog. Optimum conditions for cloud seeding are present when precipitation is possible but the nuclei for the crystalliza tion of water are lacking. 

Corrosion inhibitor/lubricity improvement additives are used panicularly in militai y fuel for the dual puiyiose of passivating metal surfaces and improving the lubricating properties of the fuel in equipment such as fuel pumps. The militai y also specifies the use of a fuel system icing inhibitor as an additive to prevent filter blocking by ice crystal formation, because militai y aircraft tend not to use fuel line filter heaters, which are standard equipment on civil aircraft. 

Ethyleneglycol monomethylether (EGME) and diethyleneglycol mono-methylether (DEGME) are both approved as additives to help prevent ice crystal formation in jet fuel. At a maximum treat rate of 1500 ppm, these compounds have minimal effect on degrading the jet fuel MSEP rating. 

Solid shear. In this technique, a frozen cell paste at -20°C is forced through a narrow orifice under very high pressure. The shear forces exerted by the passage of the extruded paste is aided by ice crystal formation in the frozen paste. On the laboratory scale the Hughes press or X-press are used. 

Canet, 1995). It was found that, after two or three cycles, softness ceased to be much affected by subsequent cycles (Figure 7.6). The authors concluded the following (1) that pre-cooling and a high freezing rate during the phase of maximum ice crystal formation has a positive effect on potato texture and tissue structure, (2) that slow thawing has a positive effect and (3) that it is essential not to subject potatoes to more than one freeze/thaw cycle. 

Just as the presence of a solute disrupts the rate of ice crystal formation, so does it disrupt the rate of ice microcrystal formation. As you should recall, it is the formation of microcrystals in fresh water that results in fresh water s expansion as it is cooled below 4°C. Without the formation of microcrystals, saltwater continues to contract all the way to its freezing temperature of about —18°C. Ocean water, therefore, is most dense just before it freezes, which makes its freezing behavior most different from that of fresh water. To see for yourself, place a cupful of saltwater next to a cupful of fresh water in your kitchen freezer. 

Frost-resistant (hardy) plants are less sensitive than others to damage by low temperatures that is caused by water loss and intracellular, ice-crystal formation. Production of such highly hydrophilic proteins as glycoproteins would constitute a potential mechanism, through the formation of hydrogen... 

Metastability with respect to ice crystal formation was not apparent in this freezer, probably because particulate matter in the salt solution induced nucleation. Nevertheless, more ice may be produced for a given pressure difference and surface when the slurry is recycled. The ice in the flowing slurry presents more surface and precludes the necessity of high driving forces to induce nucleation. Also, the size of the ice crystals is increased by recycle of slurry. 

Cryoinjury to the specimen is caused directly by extra- or intracellular ice crystal formation as well as by ice-induced solution effects during cryopreservation. Ice crystals seriously deform cell components. Another disadvantage of the formation of ice crystals near the specimen surface is slowing the cooling rate in areas below the surface because their thermal conductivity is about half that of solid water in a noncrystalline state. Furthermore, ice crystal formation is accompanied by the generation of latent heat, which also slows down the freezing rate. 

Appropriate safety procedures must always be followed when dealing with LN2 storage. Cells can be stored in liquid phase or vapour phase, or in ultracool freezers. Prevention of ice-crystal formation is essential for successful long-term storage and the vials should be protected from temperature fluctuations wherever possible. Temperature fluctuations can damage cells when frozen, even if they do not result in the contents of the vial thawing out. 

Fig. 18. Diagrams illustrating the differences and difficulties during freezing of cells in suspension (a) and on surfaces (b, c and d). In both cases, large ice crystal formation must be avoided, this means that freezing must be rapid and often involves the use of cryo-protectants. In suspension, the use of hypertonic solutions to shrink cells by osmosis helps to avoid membrane rupture. But with cells fixed to surfaces, shrinkage can lead to rupture of the filopodia or to parts of cytoskeleton or cell membrane (c). Additionally, animal cells under stress (including this kind of osmotic stress) tend to build up into a spherical shape. This means they would lose many of their surface contacts before freezing and disappear into solution after re-thawing. Cryo-con-servation of adhered cells in defined positions requires very precise control of the conditions...

Prevention of coarse ice crystal formation during freezing... 

Vitrification The process of liquid water moving directly into the glassy state without ice crystal formation... 

The requirements that must be met to gain the ability to tolerate freezing include the following (1) The formation of true ice is restricted to the extracellular spaces. (2) The formation of ice in the extracellular fluids is not accompanied by extreme dehydration of the cells. (3) Rates of ice crystal formation and the sizes of ice crystals that are generated in the extracellular fluids are held at nonlethal values. (4) Any solid-state water that forms within cells is vitrified water, not true ice. To meet these requirements, freeze-tolerant organisms employ a variety of mechanisms to control the sites, rates, and sizes of ice crystal formation. 

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