Section IV. PRESERVATION OF FRUITS AND VEGETABLES
3-23. PRESERVATION OF FRUITS
a. Sugar as a Preservative. Fruits may be preserved by many of the methods
employed for preserving foods of animal origin. Sugar is commonly used to preserve
many fruits. The minimum concentration of sugar that will effectively serve as a
preservative is 25 percent. The heat treatment, the fruit is subjected to and the acidity
of the product will both have a direct bearing on the amount of sugar required. In the
case of jams, jellies, and preserves, 45 percent sugar and an acidity of 0.8 percent will
not be adequate as a preservative. With the combination of 45 percent sugar and 1.2
percent acidity, jam, jellies, and preserves will be adequately preserved.
b. Drying of Fruits. Drying has been used for many centuries to preserve
fruits. Bacteria, yeasts, and molds are commonly found on dried fruits. It is uncommon
to find sterile dried fruits. The total microbial count will usually be less than 10,000
organisms per gram, and this count will gradually decrease if the moisture content
remains low. Souring in dried fruits results from the growth of yeasts. Dried fruits will
generally have microbial counts much lower than the count in dried vegetables.
c. Pasteurization. Preservation by pasteurization is a relatively new process
and is used more on dates than on any other kind of fruit. Following pasteurization, the
number of microorganisms will be reduced by as much as 93 to 99 percent. Large,
commercial, continuous pasteurizers are used and are capable of processing several
thousand cases daily. The pasteurization temperatures will vary from 160 to 180F
(71 to 82C). Dates are held in this temperature range from 30 to 90 minutes, and the
humidity will vary from 70 to 100 percent.
d. Sulfur Dioxide. Sulfur dioxide has been used for many years in the
preservation of dried and fresh fruits. Sulfur dioxide is of definite value in preserving the
color of dried fruits. Sulfur dioxide has a minor bacteriostatic action against bacteria,
yeasts, and molds. Yeasts are more sensitive to sulfur dioxide than bacteria and molds.
Sulfur dioxide will delay the rate of deterioration in storage grapes by as much as
50 percent.
e. Freezing. Fruits are frozen to equalize the supply available during all
seasons of the year. The preservation principles are the same as with foods of animal
origin. Freezing preserves fruits by lowering the temperature below the growth range of
microorganisms and by converting available moisture to ice. A temperature of 0F (-
18C) or lower is desirable for freezing and storing frozen fruits. Freezing will have little
immediate effect on the microbial content, but there will be a gradual reduction in
numbers during storage. The treatments given to fruits in preparation for freezing, such
as blanching and the addition of sugar, are usually for the purpose of preventing
oxidation or enzyme activity. Enzyme activity is one of the important causes of
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