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 COLD STORAGE PROBLEMS 159 



Changes taking Place in Meat, Fish and Poultry in Cold 



Storage 



The changes taking place in meat, poultry and fish during cold 

 storage are both of physical and chemical nature. The physical changes 

 are chiefly those which are due to freezing and thawing. More than 

 one half of the muscle substance is water. When the temperature of 

 water is lowered the volume is reduced, but below 39° P. increases 

 again. Therefore, when the meat is chilled below 39° F. the water con- 

 tained in the muscle cells filters through the cell membrane and fills 

 the interstices between the layers of cells. Here it freezes when the 

 meat is placed in freezer storage. Gradually the interstices are filled 

 with crystals of ice and the cells are squeezed out of shape. If the 

 meat is thawed the result will be different according to whether the 

 thawing process is carried on slowly or rapidly. If slowly, the water 

 will filter back into the cells and they will assume, nearly at least, their 

 normal shape, and the meat will, to all outward appearance, look like 

 fresh meat. If the meat is thawed rapidly, considerable juice will ooze 

 out and the cells will never resume their normal condition. It is better, 

 therefore, to thaw meat slowly. 



Some of the water contained in the skin naturally evaporates dur- 

 ing cold storage, so that the skin shrivels. To restore the natural ap- 

 pearance and replace the lost weight, frozen poultry is frequently soaked 

 in hot water. This softens the skin and favors the invasion of bacteria. 

 Poultry treated in this fashion will deteriorate rapidly and the practise 

 should therefore be condemned. The physical changes described do not 

 injure the general appearance, flavor and digestibility of the meat, some 

 investigators claiming that both flavor and digestibility are favored. 



Whatever chemical changes take place in meat, poultry and fish in 

 cold storage are chiefly the result of the activity of microorganisms 

 (bacteria and molds). Since, however, bacteria multiply slowly at the 

 freezing point, and probably not at all in frozen meat, the changes are 

 slight. At 32° to 40° some changes are noticeable after 15 days' 

 storage. These changes are also slight and are restricted not only by 

 the temperature, but also by the layer of dry meat forming on the out- 

 side, which makes it difficult for bacteria to penetrate the interior. 



The fat is probably not attacked by bacteria, but undergoes a slow 

 oxidation under the influence of light and the oxygen of the air. By 

 this process the acidity of the fat increases, but the change is very slow, 

 if the meat is kept frozen, and is not noticeable for months. It has 

 also been claimed that there is some change in meat due to the action of 

 ferments, which may be contained in the muscle cells. This process is 

 called " autodigestion." There is, however, no reliable evidence of this ; 

 in fact, new investigations seem to show that bacteria are the only agen- 



