Packaging

The shelf life of a dehydrated fruit product is influenced to a large extent by its packaging, which must protect the dehydrated product against moisture, light, air, dust, microflora, foreign odor, insects, and rodents. To avoid caking, air-conditioned and dehumidified rooms (below 30% RH) are required to package low-moisture fruits with high sugar content.

Freeze-dried fruits must be packed in inert gas to ensure storage stability. Nitrogen gas is commonly used to extend the storage stability of oxygen-sensitive products. In inert gas packaging, a headspace oxygen level of 1 to 2% is targeted.

PROCEDURES FOR SELECTED COMMODITIES Apples

Apples are either dried immediately after harvesting or after being stored in cold or controlled-atmosphere (CA) storage until a convenient processing time occurs. The best varieties for drying are Gravenstein, Pippin, and Golden or Red Delicious apples. Two types of dried apple products are recognized under U.S. standards: (1) evaporated apples and (2) dehydrated (low-moisture) apples.

Evaporated apples, also called regular moisture or dried, are cut to desired size and dried to average, not more than 24% moisture by weight. Unsulfured apples however, must average not more than 20% moisture. Only artificial dryers such as kiln, tunnel, or continuous belt dryers are commonly used to produce dried apples. The process involves sizing, coring, and slicing the fruit pieces to 0.95 to 1.3 cm (3/8-1.5 in.) in thickness. The fruit is usually dipped into a sodium sulfite solution, then dried in a kiln, tunnel, or continuous belt dryer to approximately 16 to 25% moisture. The fruit slices during drying are exposed to the fumes of burning sulfur. Drying in the kiln requires 14 to 18 h at 65 to 74°C (150-165°F).

If a tunnel dryer is used, the air entering the tunnel is at 74°C (165°F) with a relative humidity of 35%. At the outlet, the air is at about 54°C (130°F) with a relative humidity of 35%.

Evaporated apples can be stored for short periods of time, less than three months, at ambient room temperatures in a dry atmosphere. For prolonged storage, 7°C (45°F) or less is required. Unsulfured evaporated apples require 4 to 5°C (39-40°F) cold storage. Evaporated apples will generally reconstitute with one part apples in five parts water by weight.

If the product is processed to a low-moisture state (less than 5% moisture), the dried apples are cut to the desired size, usually 0.64 and 0.85 cm (1/4 and 3/8 in.), dice. Frequently the fruit pieces are instantized by compression or perforation prior to dehydration. Dehydrated (low-moisture) apples are processed in forced-air dryers, such as the continuous belt dryer, using evaporated fruit as raw material. Some apples are vacuum dried for snack or other specialty product applications.

Only 300 ppm S02 is necessary to prevent browning. To prevent caking, 0.5% maximum calcium stearate may be added. Packaging is generally in fiberboard boxes with a net weight of 15 to 40 lb (6.8-18 kg), depending on the product destiny.

Approximately 100 kg of fresh unpeeled fruit will yield about 10 to 11 kg of dehydrated apples. Dehydrated apples will generally fully reconstitute with one part of apple in six parts water by weight. The maximum allowable S02 level in dried apple products in the United States is 1000 ppm and a maximum of 500 ppm in the European Community.

Apricots

Apricots are picked for drying from mid-June to early July when they are fully tree-ripened. The best varieties for drying are Royal, Blenheim, and Moorparks. California produces more than 90% of the U.S. crop and 10% of the world crop.

Apricots are either sun-dried or dried in tunnel driers. The fruit is halved, the pit is removed, and the fruit is placed cup up on a flat wooden tray. The filled trays are exposed to sulfur dioxide fumes for about 12 h. After sul-furing, the trays are transferred to a field, exposing the fruit to full sun. Apricots are allowed to dry in this manner for one day, then the individual trays are transferred to a shady area and stacked 3 to 4 ft high. They are allowed to dry in the stack for approximately one week, then they are removed from the trays, placed into boxes or bins, and ultimately delivered to a packing plant.

Berries

Berry flavor and color degrades with exposure to prolonged elevated temperatures. Also, the juices of both strawberries and raspberries tend to drip during heating. Therefore, conventional air drying techniques, such as continuous belt driers are not used for drying berries.

Freeze drying has been the typical method for drying strawberry and raspberry pieces. The berries are individually quick-frozen and then spread onto drying trays. The freeze-dry chamber is filled with the trays, and the shelf temperatures during the cycle may be varied to maintain an optimal product temperature.

The packaging must be a high-moisture barrier film, such as foil-paper laminate or foil polyethylene laminate, because with exposure to air freeze-dried berries absorb moisture very quickly. Cereal and confection products often contain freeze-dried berries.

Drum dryers are also used to dry strawberry and raspberry purees. Because of high temperatures (>220°F, >105°C) for a time period of 1 to 5 min, the color is often browner and flavor is more caramelized than those of freeze-dried products.

Cranberries are being dehydrated and sold as a snack food and are being used in mixtures of other dehydrated fruit as a snack. The Ocean Spray Company has developed Craisin, which is a sugar-induced cranberry that is dried and used in baked and cereal products or sold as a snack in its own package.

Blueberries are dried by tunnel dryers and freeze-dry-ers. For both processes individually frozen berries are used as the raw material. The use of dried blueberries is gaining acceptance in cereals and baked products. The drying of currants and gooseberries is mainly a home industry due to lack of volume production of these fruits.

Dates

Dates are almost exclusively sun-dried, as a favorable dry climate exists in the areas where date palms are grown. Such areas as Southern Iran, Iraq, and Egypt have a very high daily temperature during the growing and harvesting seasons and thus are ideal for natural sun drying of dates. In Arizona and Southern California, where dates are also grown, they are seldom dehydrated except in the preparation of low-moisture date products. In such cases, the dates are cut to small pieces or extruded to paste and dried in vacuum dryers.

Figs

The principal varieties grown for drying are Calimyrna, Mission, Adriatic, and Kadota. Figs are usually allowed to dry partially on the trees. In some cases, the trees are lightly shaken at intervals. Figs are usually mechanically gathered from the ground and are typically dry enough to be loosely packed in boxes or bins. Sometimes they are further dried on trays in the sun to a moisture content of approximately 17 to 18%.

After being transported to the plant in sweat boxes or bins, the figs are normally screened, graded for size, and then inspected to remove insect-damaged fruits. The screening is necessary to divide the figs into the required finished product styles. After the first sorting and grading operation, the figs to be stored for later processing are packed in boxes and placed in an airtight chamber and fumigated. This operation is repeated several times over a two-week holding period.

The figs to be processed are conveyed through a cold-water reel washer to remove dust and foreign material. They are then directed to a processing unit where they are immersed in hot water for 5 to 10 min. Soak time depends on the size and variety of fruit being processed. The figs at this point have adsorbed some water and, because of increased susceptibility to mold, are sprayed with potassium sorbate. They are conveyed, typically, over a dewatering belt where they may either be put into small plastic tubs to equilibrate or placed into retorts directly. The retorting process includes exposure to live steam for 2 or 3 min, which further softens the figs. The fruit is air-cooled and packaged.

Peaches and Nectarines

The primary peach for drying is the Lovell, a highly colored freestone variety. Faye Elberta is also dried. Sungrade and Le Grande nectarines are preferred for drying; both are freestone fruits.

Freestone peaches and nectarines are harvested and processed similarly to apricots. After sulfuring, the fruits are placed in full sun for two to three days or longer, depending on weather conditions, at which time they are transferred to shady stack storage and dried for several more weeks. Finally, they are removed from the trays, transferred to boxes or bins, and delivered to the packing plant.

Dehydrated (low-moisture) apricots and peaches are processed from the sun-dried (evaporated) fruit to a limited extent. Because of the high sugar content and the sensitivity of the yellow pigment to heat, a vacuum process is necessary to dehydrate these fruits to less than 5% moisture. Vacuum shelf dryers are used for the process. The evaporated fruit halves are sliced or diced before loading on the drying trays.

Pears

Pears that are to be dried are allowed to ripen on the tree. The summer Bartlett variety is used for drying in California. The fruits are handpicked and transported to cutting sheds where they are cored and halved. Placed cup up on wooden racks, they are stored overnight in sulfur houses where they are exposed to burning sulfur to prevent browning. The pear halves are removed from the sulfur house, dried in the sun for four to eight days, and then transferred to stacked storage for an additional two to three weeks.

Once dried, the fruit is delivered to the packing plant, where it is usually processed to fill orders. The dried fruit from the field may sometimes be stored as long as several years before being repacked. Pears are little known as a dried fruit and most commonly appear in packages of mixed dry fruits.

Prunes

Although plums are one of the most widely distributed fruits in the United States, only one type of plum, the French type, is designated a prune. The La Petite d'Agen variety brought to the Santa Clara Valley of California by a French nurseryman in 1856, known as the California French prune, is used for drying and is grown almost entirely in California. Prunes are the second most important dried fruit crops. The California production accounts for 100% of the U.S. production and 70% of the world's supply.

Most of California's plums for prune production are harvested mostly in late August by machine. The soluble solids content of juice must reach 22% prior to harvest. Immediately after harvesting, the orchard-ripened fruit is taken to the dehydrator yard where it is washed, placed on large wooden trays, and dehydrated to about 18% moisture in forced-draft tunnel dehydrators. The drying process requires 24 to 36 h, depending on the size and solids content of the fruit. Three pounds of plum will yield 1 lb of prune.

Dried prunes are processed through a series of screening, grading, and washing steps. Grading involves separation according to size, ranging from 23 to 150 prunes per pound. Hand sorting for cull removal follows, after which the prunes are conveyed to a blancher where they are held from 8 to 20 min to inactivate enzymes and preserve color and flavor. Potassium sorbate and fresh water are then sprayed onto the prunes to maintain proper moisture content and to add a preservative. Fruit to be pitted is sent through automatic pitting machines that either squeeze the pit out with mechanical fingers or punch it out. The pitted or unpitted prunes are again handsorted for rejects, automatically weighed into boxes or sacks, sprayed with potassium sorbate preservative, and sealed. Other popular prune products include prune juice (a water leachate of the prune) and prune paste, used in baking and confections.

Raisins

The United States and Turkey produce over two-thirds of the world's supply of raisins. Approximately 25% of the U.S. grape production is made into raisins, processed almost entirely in the San Joaquin Valley of California. Raisins are the most important dried fruit crops. Cultivation for raisin grapes is little different than for fresh market grapes.

Thompson Seedless, Muscat, Black Corinth, and Sultan grapes are the principal varieties processed for raisins. The fruit is handpicked in August from the vine in bunches and set on paper to be dried in the sun for 7 to 10 days. At this point, fruit dehydration has not been uniform. Thus, turning of the berries is done to speed the drying process and allow raisins to dry more uniformly. Without turning, some raisins will be too dry, while others will be too moist, requiring a long curing period later on. Within approximately five to seven days after turning, the raisin trays are ready to be retrieved and loaded into sweat boxes to equalize and then to be shipped to the processing plant. The maximum moisture allowed at the packinghouse for incoming raisins is 16%, but usually they are in the 9 to 12% moisture range.

Golden Bleach raisins are dipped in 0.25% hot NaOH (lye) solution after harvest for 3 to 6 s, sulfured for 4 to 6 h, and dried in the sun or in forced-air dryers. Soda-dipped raisins are dipped for 30 to 60 s in a 4% solution of soda ash and Na2C03 at 35 to 38°C and then processed in the same manner as Golden Bleach, but they are not sulfured. From this point, the processing is the same as for natural-dried raisins.

Raisins are processed by a series of screening, destemming, and air separations. These processes are repeated until lightweight particles are removed. Small raisins may then be utilized in distilled alcohol products or in cereal or bakery products.

The raisins are next washed and sent through a de-watering operation, which removes excess surface water. They go through another destemming operation, and their moisture content is adjusted to 18% by water sprays. The fruits are sorted by a mechanical sorter and packaged for storage or shipment. The packages used for raisins are 14-g (0.5-oz) miniboxes, 500-g (1.1-lb) canisters, and 1-kg (2.2-lb) bags, and bulk size packages that contain from 13 to 500 kg (28.6 to 1100 lb) each. Raisins are also sold to industrial users as raisin paste and raisin juice concentrate (70° Brix).

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