Published on February 3, 2014
Food Intended use Freezing problem Cooked egg whites Salads, sandwiches, Soft, rubbery, tough Milk based sauces casseroles Curdles or separates Sour cream Toppings, salads Separate, watery Gelatin Salads, desserts weeps Cucumbers, lettuce salads Become limp, lose crispness Fried foods Ready to eat Lose crispness, become soggy Reference: Taken from a handout provided by University of Minnesota. (additions done to the original) http://www1.extension.umn.edu/food/food-safety/preserving/teaching-lessons-and-materials/docs/handout.pdf
Eggs along with their outer shell. Cooked egg white Anomalous behavior of water causes expansion of the liquid inside the egg and causes the egg shell to crack. It becomes tough and rubbery due to loss of elasticity
comprises of denatured proteins held in the form of an elastic gel freezing results in the concentration of proteins than present in the original gel Cooked egg white the gel structure contracts irreversibly resulting in the formation of a tougher structure Increase in the size of ice crystals penetrates this gel structure effectively releasing some elasticity
Formation of ice crystals may lead to rupturing of cells. Inability of the cells to reabsorb all of the frozen water. Reasons for limpness in certain fruits and vegetables upon freezethaw. Destruction of the colloidal structure of the cell.
Physical damage due to ice crystals may lead to loss of cell turgor and dislocation of endogenous water. Excessive leaching of water soluble pigments, water soluble vitamins and minerals in the exudate. Free movement of cellular content may cause mixing of materials that were previously compartmentalized which may give rise to off flavors and odors.
Gelatinization of starch upon addition to gravy and subsequent heating. Retrogradation and syneresis upon freezing. On thawing, the gravy appears as a separated and curdled mixture.
Scanning electron microscope pictures of starch gel after freeze-drying and variable number of freeze-thaw cycles. (The hollow dark pits represent ice crystals. 1C-1 freezethaw cycle,3C-3 cycles, 5C-5 cycles) Reference: Hyo-Young Jeong, Seung-Taik Lim, “Crystallanity and Pasting properties of FreezeThawed high Amylose Maize Starch”, Starch/Staerke 55 (2003), 511-517.
Wheat flour is not a wholly satisfactory stabilizer. If it is used in combination with gelatin or some other colloid, the coagulation may be retarded. White sauces and gravies in which wheat flour is largely or wholly replaced by waxy maize or waxy rice flour are relatively more stable.
Sauces like Mayonnaise and Hollandaise sauce are examples of oil in water emulsions. In cases where the oils freeze before the water, oil crystals are formed which act as bridges. If the interfacial layer between the fat droplets is relatively thin, one oil crystal can penetrate the other oil crystal leading to partial coalescence. Upon thawing, these fat molecules completely coalesce together thus separating from the emulsion.
As the water freezes, its expansion causes the fat droplets to squeeze together which in itself may cause partial coalescence. This occurs when the water freezes before the oil droplets can freeze. The emulsifiers may also be squeezed out of the interface separating the droplets creating some oil to oil contact. Upon increasing the temperatures , the oil droplets completely fuse together effectively separating from the continuous phase.
Microscopic images of oil in water type of emulsion. A2- Freshly made B2- After 5 freezethaw cycles.
Donald K Tressler, The freezing Preservation of foods- Freezing of pre-cooked and prepared foods, vol 2, Pg 16-17. Food freezing-Today and tomorrow. Edited by: W.B. Bald. Pg 117 http://www.dissertationtopic.net/doc/1642375 http://lnu.divaportal.org/smash/get/diva2:4278 21/FULLTEXT01.pdf
Sayali Sanjay Savant M.Tech Institute of Chemical Technology, Mumbai
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