Acid and alkali help in preserving food by surrounding the food completely in a medium that prevents contamination by microorganisms, which cannot survive in acid or alkali. The solution also prevents contact between food and air.
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Acid solutions (low pH) have a high concentration of H+ ions suspended in solution. These H+ ions will readily diffuse into any bacterial cells that appear in the solution and the high concentration of H+ ions will interfere with the many hydrogen bonds in their enzymes. The hydrogen bonds in these enzymes break leaving the enzymes denatured (shape changed) and no longer functioning. The bacteria can no longer survive without its enzymes which are essential for all reactions of metabolism and binary fission (bacterial replication).
By lowering the pH of food, either through the addition of organic acids or through natural fermentation, to 4.6 or below, the environment will be much less hospitable to many spoilage organisms and pathogens. This does not eliminate spoilage by yeasts and molds.
Vinegar (used for pickling) contains Ethanoic acid which prevents growth of bacteria and so, therefore, preserves the food.
The most common use of acids in preserving foods is in pickled foods.
However bacterially produced lactic acid preserves foods like sour cream, sausage, certain cheeses, yogurt, etc.