Temperature ice crystallization

The cloudiness of ordinary ice cubes is caused by thousands of tiny air bubbles. Air dissolves in water, and tap water at 10°C can - and usually does - contain 0.0030 wt% of air. In order to follow what this air does when we make an ice cube, we need to look at the phase diagram for the HjO-air system (Fig. 4.9). As we cool our liquid solution of water -i- air the first change takes place at about -0.002°C when the composition line hits the liquidus line. At this temperature ice crystals will begin to form and, as the temperature is lowered still further, they will grow. By the time we reach the eutectic three-phase horizontal at -0.0024°C we will have 20 wt% ice (called primary ice) in our two-phase mixture, leaving 80 wt% liquid (Fig. 4.9). This liquid will contain the maximum possible amount of dissolved air (0.0038 wt%). As latent heat of freezing is removed at -0.0024°C the three-phase eutectic reaction of... 

Supercooled fog is fog having a temperature less than (1C. It consists of small droplets at temperatures less than freezing which can exist as liquid down to approximately - 40 C. Below this temperature, ice crystals tend to form automatically and the fog changes to ice fog. 

Culture conditions influence the survival of cells submitted to cryopreservation. To be frozen, cells should be in an active growth phase, with a viability greater than 90% and free of contaminants. The optimal cryopreservation conditions are different for each cell line. When a cell is exposed to low temperatures, ice crystals are generally formed and can disrupt the cell membrane, causing death. Therefore, cells should be treated with a cryoprotector to support the freezing and thawing processes. 

Freezing of tissues (vegetable or animal) generally causes some damage. At temperatures above — 10°C, ice crystals only form outside the cells. This causes freeze concentration of the extracellular liquid, hence to an osmotic pressure difference between intra- and extracellular liquid and hence to osmotic dehydration of the cells (plasmolysis). At lower temperatures, ice crystals tend to penetrate the cells, whereby intracellular crystallization occurs. This reduces plasmolysis but tends to increase mechanical damage, possibly leading to a soft texture of the tissue after thawing. 

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. 

Sublimation of ice crystals to water vapor under a very high vacuum, about 67 Pa (0.5 mm Hg) or lower, removes the majority of the moisture from the granulated frozen extract particles. Heat input is controlled to assure a maximum product end point temperature below 49°C. Freeze drying takes significantly longer than spray drying and requires a greater capital investment. 

Fig. 9.2. The excellent crystallographic matching between silver iodide and ice makes silver iodide a very potent nucleating agent for ice crystals. When clouds at sub-zero temperatures are seeded with Agl dust, spectacular rainfall occurs.

Freeze-drying, like all drying processes, is a method to separate liquid water from a wet solid product or from a solution or dispersion of given concentration. However, the main difference is that the liquid water is separated by solidification (i.e., the formation of ice crystals) and subsequent vacuum sublimation instead of evaporation. This allows a drying at subzero temperatures which can be advantageous in case of heat-sensitive products. There are two general applications... 

The freezing point of sea water, defined as the temperature at which ice crystals begin to form, is —2° C. 

To survive freezing, a cell must be cooled in such a way that it contains little or no freezable water by the time it reaches the temperature at which internal ice formation becomes possible. Above that temperature, the plasma membrane is a barrier to the movement of ice crystals into the cytoplasm. The critical factor is the cooling rate. Even in the presence of external ice, most cells remain unfrozen, and hence, supercooled, 10 to 30 degrees below their actual freezing point (-0.5 °C in mammalian cells). Supercooled cell water has a higher chemical potential than that of the water and ice in the external medium, and as a consequence, it tends to flow out of the cells osmotically and freeze externally (Figure 1). 

Figure 8. Photomicrographs of frog erythrocytes in serum during the course of slow freezing from -1.5 °C to -10 °C. Note that the cells are confined to the channels of unfrozen solution between the ice crystals (I), that the channels decrease in diameter with decreasing temperature, and that the cells shrink. (From Rapatz el al., 1966.)...

C12-0097. When an aqueous solution cools to low temperature, part of the water freezes as pure ice. What happens to the freezing point of the remaining solution when this occurs A glass of wine placed in a freezer at -10 C for a very long time forms some ice crystals but does not completely freeze. Compute the molality of ethanol in the remaining liquid phase. 

Leila is given a sealed flask of sugar water at room temperature. She places it over a Bunsen burner for a few minutes and notes condensation on the sides. Then she places the flask in an ice bath for ten minutes, and notices that ice crystals begin to form. Leila knows that the one statement that cannot be true is that —... 

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