The Oldest Way To Store Fruit

William Rabak.

Drying, the oldest forth of fruit preservation, is a well-established industry. It dates from ancient times, when man depended on this easy way to store food between crops.

The term is general. It indicates drying by any means, although once it was restricted to sun drying. The words "evaporated" and "dehydrated" generally imply drying in specially constructed dehydrators and under controlled temperature, humidity, and air flow.

The drying of fruits, which reduces the amount of water they contain and concentrates the juices, establishes unfavorable conditions for the yeasts, molds, and other micro-organisms that cause spoilage.

Subjecting light-colored tree fruits apples, apricots, figs, peaches, pears to the fumes of burning sulfur (sulfur dioxide) has long been a predrying treatment to prevent enzymatic oxidation, or darkening. All those fruits contain tanninlike compounds, which are readily oxidized by atmospheric oxygen through the catalytic effect of the fruit enzymes when the cut 'or bruised surface of the fruit is exposed to the air or the fruit cells are killed by the drying process. Such oxidation results in discoloration, commonly called browning, and causes an undesirable change in flavor.

Sulfur dioxide is a reducing agent and an enzyme inactivator or retard-ant, depending on its concentration in the fruit. It prevents enzymatic oxidation and tends to restore the fruit to its natural color by its reducing action when slight discoloration occurs during the preparation of the fruit for drying. Certain dried fruits are subject to nonenzymatic discoloration upon prolonged storage if insufficient sulfur dioxide is present in the tissues of the fruit. The growers and packers of dried fruit learned from long experience and research just how much sulfur dioxide each of the fruits must contain to prevent both enzymatic and nonenzymatic discoloration over the normal period of storage between harvest and consumption.

Because accurate control of temperature and humidity during the drying process has been found to be generally advantageous, the designs of some driers are elaborate and complicated.

In recent years, many innovations have been made in equipment for dehydration. For example, vacuum dehydrators have come into use. The process removes water from commodities under vacuum at relatively low temperatures. It has certain advantages over conventional drying systems in that the product is not exposed to high temperatures or large volumes of air, which cause oxidation of certain food constituents. The resulting products have better color and reconstitute more readily because there is no surface crust. Products of this type are utilized in dry mixtures, which generally require only the addition of sugar and water. Ready-mixed apple pie is an example. It will keep for some time at ordinary temperatures.

Many other dried-fruit mixes are in the offing; it has been found that certain dried fruits improve the texture and flavor of bread and other baked goods. The possibility of dried-fruit ingredients for jams and jellies has been experimentally proved simply add water, bring to a boil, and cool. The product is made.

Yet another innovation freeze-drying subjects fruit juices to a vacuum at below-freezing temperatures. Under those conditions the water evaporates readily.

Dried fruits undergo chemical and physical changes that affect quality, depending on the moisture content of the commodity and the humidity of the climate. They tend to absorb water vapor from humid air and to lose moisture to dry air. Among the unfavorable changes are crystallization of natural sugars, surface hardening or toughening, loss or alteration of flavors, and impairment of colors. To control such impairments and to guard against bacterial spoilage and insect infestation, more effective packaging is needed.

Tight packages made from sheet materials, like waxed paper, regenerated cellulose, rubber hydrochloride, and polyethylene, provide satisfactory protection in most instances. Certain sensitive products require airtight containers, such as tin cans or glass jars.

DEHYDROFREEZING, a new method of food preservation, combines two well-established processing methods. The name was coined at the Western Regional Research Laboratory to designate a process that has been under investigation since 1945.

The preparation of the fruit for dehydrofreezing is similar to that for freezing preservation or dehydration. To avoid enzymatic discoloration during or after processing, the products are treated with sulfur dioxide before the partial dehydration process, which removes approximately two-thirds of the moisture.

Experiments at the Laboratory indicated that during an ordinary dehydration process the major processing impairments discoloration, the surface hardening, off-flavor developments, and losses in nutritive values--occur near the end of the dehydration period. Halting the dehydration before removing the last third of "removable" moisture materially reduced these impairments. Furthermore, freezing the product at that point resulted in less damage to the plant structure and juices than is encountered in ordinary freezing preservation. This was indicated by a marked reduction in the loss of juices after thawing.

After freezing, the product is protectively packaged and kept in freezing storage until used. The dehydrofrozen fruit is quickly and easily restored to a condition closely approximating that of the fresh fruit by covering the pieces with cold water and bringing to the boiling point. Pies made from dehydrofrozen apples and apricots were as acceptable to tasting panels as those made from fresh fruit.

THE EXPERIMENTERS listed the following advantages of the new method of preservation :

The products are readily reconstituted for use.

Savings in transportation and storage costs are considerable.

Substantial economies are effected in the packaging materials.

Partial dehydration reduces the load on the refrigeration system.

The knowledge gained from the age-old preservation by drying and from the infant food-freezing industry combined to bring dehydrofreezing into existence. Like all things new, its value, acceptability, and permanence will be decided by the united efforts of the research worker, the processing industry, bakers, and housewives.

WILLIAM RABAK, a specialist in food technology in the Western Regional Research Laboratory, has been a chemist in the American Medical Association, the United States Food and Drug Administration, the Biscuit and Cracker Manufacturers Association, and the United States Frozen Pack Laboratory of the Bureau of Agricultural and Industrial Chemistry.