Preservation of food by controlling the acid content can be achieved in
two ways. One is to naturally ferment the food--turning cabbage into
sauerkraut. The other is to add an organic acid to the food to reduce
the pH--adding vinegar to cucumbers to make pickles. Some foods such as
berries and fruits naturally contain enough organic acids so their pH is
below 4.6, and preservation of these foods requires only a boiling water
bath heat treatment or freezing.
In a natural fermentation, lactic acid bacteria convert fermentable
carbohydrates in the food to lactic acid. In this way the pH is reduced
and most bacterial growth inhibited. When cabbage is fermented to
sauerkraut, the cabbage's pH is reduced from pH 6.8 during the
fermentation to less than 3.5. Cucumbers can also be fermented to
pickles by a similar process; however, most pickles are made by direct
acidification of the cucumbers.
Direct acidification, that is, adding vinegar which contains 4 to 5
percent acetic acid, is the most common method of making cucumber
pickles in the United States. It is easier, quicker, and foolproof.
Often the natural fermentation will go astray. Other undesirable
microbes may grow, bringing unwanted changes in the food: spoiling
rather than preserving. Other foods made by fermentation include wine.
Regardless of the method used to control the pH of food, to successfully
preserve food by this method it is absolutely essential to heat-process
or freeze to prevent spoilage by yeasts and molds. An example of
spoilage in cucumber pickles not heat-treated after acidification is the
development of cloudiness and bubbling. This common spoilage is caused
by microbes that would be destroyed by heat processing.