Effect of freezing rate on crystalline structure of ice

Rates freezing determine the type, size and distribution of ice formation. This can be extracellular or intracellular ice, dendritic or characteristic (in the fast-frozen aqueous solutions; Hey et al., 1997), and may be partially limited matrix. With the help of a very high cooling rate (up to 10 000C/min), you can avoid ice formation at all and instead achieve vitrification, leading to the glassy state.

Angell (1982), francs (1982), Garside (1987) and Blanshard and Franks (1987), among others, were considered crystallization in foods. Due to the difficulty in interpreting the results of ice formation in complex food matrices, most accurate research began with simple systems based on water solutions of (Bald, 1991). A number of studies of ice formation and preventing cryoprotectants or anti-freeze proteins were also held in the context of medical use, preservation of viability of biological tissues, in particular Mazur (1970,1984). This clearly shows a significant " community", between researchers in the field of food and medical Sciences.

Slow freezing produces fewer larger ice crystals, quick frozen produces a greater number of smaller crystals. Whether large or small sizes of crystals depends on the purpose of freezing. In ice cream, ice crystals, should be as small as possible, so as to make the product as creamy and smooth. However, to focus on liquid food products, large crystals easier to separate from freezing concentrate. The freeze drying it is usually desirable to produce a small number of large crystals in order to speed up the subsequent process of sublimation.

When the freeze starts, water that is present in the food moves to join the growing crystals of ice. When a plant or animal tissues are frozen quickly (in laboratory conditions, for sufficiently small or thin samples), the water does not move through the cell membrane and small, evenly distributed ice crystals are formed inside cells.

In commercial food freezing, rates freezing is usually too slow intracellular the form of ice. Food products, frozen slowly, large ice crystals form and ice fills the intercellular space, which leads to dehydration of cells. Ice crystals power of cells or tissue fibers apart. Although the products that quickly (flash) frozen foods small ice crystals, these ice crystals can increase over time through a process known as recrystallization or Ostwald ripening (Smith and Schwarzberg, 1985). Occurs the recrystallization in the frozen foods because of larger crystals are thermodynamically more stable, they have a relatively small surface energy). Recrystallization contributes to the temperature gradients in the products during freezing or thawing, or temperature fluctuations during the extended frozen storage, distribution (transportation of goods) or storage in the home (house of frost-free freezer temperature may rise to almost 0C during defrost)...