Page:Popular Science Monthly Volume 81.djvu/165

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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 contained 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 during cold storage, so that the skin shrivels. To restore the natural appearance 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 outside, 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