Fast freezing

Nicolas, M.-T., Morse, D., Bassot, J. M., and Hastings, J. W. (1991). Colocalization of luciferin-binding protein and luciferase to the scintillons of Gonyaulax polyedra revealed by double immunolabeling after fast-freeze fixation. Protoplasma 160 159-166. 

Fig 1. Electron micrograph of a platinum/carbon replica prepared by the fast-freeze, deep-etch, rotary-shadow replica technique printed in reverse contrast. Cell walls of onion parenchyma have an elaborate structure with many thin fibres bridging between thicker cellulosic microfibrils. Scale bar represents 200nm. 

More than 50% water is converted into ice crystals in ice cream at -5°C to -6°C which is the common drawing temperature for correctly operated continuous freezers. This portion of the water freezes very rapidly, often in less than one minute. Fast freezing induces the formation of small ice crystals, a critical prerequisite for smooth ice cream. At slightly higher temperatures (such as -4°C which is the common drawing temperature for batch freezers), less than 40% water is frozen and the freezing time will be longer. This is one of the reasons why ice cream frozen continuously is smoother in texture than batch-frozen products. 

The drying at a lower Tsh did not change the desorption behavior. In the product there exists a combination of two or more structures. Slow freezing produces in a 10% mannitol solution a mixture of a- and p-polymorphs, and fast freezing the 8 form (Kim et al. [1.173] see also [1.149]). If the mannitol solution is frozen in LN2 at a rate... 

Figure 2.6 Electron micrograph of a water-in-oil emulsion by fast freezing and direct observation on a cryogenic stage. From Mikula [66]. Copyright 1992, American Chemical Society.

We use liquid nitrogen to make ammonia for fertilizers, to keep frozen foods cold during transport, and to fast-freeze these foods. 

In cryosurgery, liquid nitrogen is used to fast-freeze cancerous tissues and warts. This proven new technique kills the cancerous area and allows surgeons to safely remove the dead tissue. 

The foam structure can be fixed by fast freezing of low expansion ratio foams. The equation of Stephan [117,118] is employed to estimate the rate of foam freezing when there is a heat exchange at its surface... 

When samples are unstable, or intermediates need to be studied, fast freeze techniques have been developed to enable the study of reactive and unstable systems (see Freeze-Quench Kinetics). 

In general, it appears that fixation by fast freezing, (for example, by clamping between aluminum blocks chilled in liquid nitrogen), followed by pulverisation in impaction mortars chilled to the temperature of liquid nitrogen, and finally rapid homogenisation of the frozen, powdered tissue in 0.1 N HCl, containing radioactively labelled cyclic nucleotides to determine recovery, is the most reliable method of fixation. 

The two main techniques commonly discussed in the literature are known as direct observation (or frozen hydrated observation) and the observation of replicas. Both techniques involve the fast freezing of the sample in a cryogen such as liquid nitrogen, propane, or freon. The frozen sample is then fractured to reveal the interior features. This fractured surface can be coated with a metal film or observed directly. Often, the metal film is removed from the sample and observed as a replica. This type of procedure allows the creation of a permanent archive of the samples prepared, and the observation is the same as with any other electron microscope sample with no concern about contamination of the microscope or beam damage to the sample. 

P-HDL-cholesterol EDTA, 2 g/L Collected after 12-14 hr fast, freeze... 

Stability. As long as the temperature remains below Tg, the composition of the system is virtually fixed. This implies physical stability crystallization, for instance, will not occur. As mentioned, some chemical reactions may still proceed, albeit very slowly because of the high viscosity and the low temperature. The parameters Tg and i//s are, however, not invariable they are not thermodynamic quantities. Their values will depend to some extent on the history of the system, such as the initial solute concentration and the cooling rate. The curve in Figure 16.6 denoted rff (for fast freezing) shows what the relation may become if the system is cooled very fast. The Tg curve is now reached at a lower ice content, so the apparent Tg and i// values are smaller. However, the system now is physically not fully stable water can freeze very slowly until the true i// s is reached. 

---