All food products, on exposure to air, undergo certain changes which unfit them for use as food. It was once thought that these changes were due to oxidation, but they are now known to be caused by minute living organisms present in the air, in the water, in the ground, and in the food itself. To preserve food two things are necessary; first, to either kill or render harmless those organisms already present; and second, to exclude others from entering. The first we usually accomplish by cooking, and the second by sealing.
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