Michigan State University Extension
Preserving Food Safely - 01600525
Sort and discard defective fruits. Wash, pit and halve when necessary (as with stone fruits). Most fruit must be pretreated immediately before drying to maintain an appetizing appearance--and to prevent darkening, loss of flavor and vitamin C.
Pretreat fruit by dipping it in a sulfite, ascorbic acid or salt water solution, by syrup blanching or by exposing the fruit to sulfur fumes. The sulfur treatment is preferred as a pre-drying treatment. Sulfuring fruit (exposing it to sulfur fumes) effectively preserves color maintains the quality and decreases loss of vitamins A and C in the fruit during drying and storage.
PRE-TREATING FRUIT WITH SULFUR
Materials: Slatted, wooden trays. DO NOT USE ALUMINUM OR GALVANIZED SCREENING MATERIAL, as sulfur fumes corrode most metals. If wooden, slatted trays are not available, wooden lids from lug boxes may be used.
Thread spools, wooden or plastic (but not styrofoam) or small, wooden blocks. Place at corners of trays to stack them 1 1/2 inches apart.
A heavy cardboard or wooden (no cracks or openings) box. Must be large enough to place over stacked trays with 1 to 1 1/2 inches to spare between the trays and the inside of the box. Box should also be large enough to accommodate the container of burning sulfur under the stacked trays.
Fire bricks to raise the stack of trays high enough off the ground to accommodate the container of burning sulfur.
Sulfur. Use elemental sulfur also called Sulfur Flowers (U.S.P. standard) or flowers of sulfur. It is free of impurities, burns readily and may be purchased at most pharmacies.
Clean, metal container to hold the sulfur. For small amounts of fruit, a flat tuna can or an aluminum pie tin will be large enough.
Procedure ALWAYS USE SULFUR OUTDOORS AWAY FROM CLOSE CONTACT WITH PLANTS, SHRUBS, AND TREES.
1. Spread fruits in a single layer, pit cavity side or cut surfaces up, on trays. Pieces should not touch each other.
2. Stack trays 1 1/2 inches apart, separated by spools placed at the corners.
3. Cover the stacked trays with the box. Make a slash at the bottom of the box and another slash at the upper edge of the opposite side. Open slashes when necessary to permit circulation of sulfur fumes.
4. Measure the sulfur and place it in the container. The amount used varies with the length of time the fruit is to be sulfured, weight of the fruit, and the dimensions of the box. Generally, if you are using a cardboard box to cover the trays, you will need to use 1 to 2 teaspoon of sulfur per pound of fruit (weight before drying). If you have constructed a more air-tight sulfuring box from wood, you only need to use 1 teaspoon of sulfur per pound of fruit. Sulfur fumes do the work, not the burning so it is important that the box be tight. Sulfuring is complete when fruit appears bright and glistening, and a small amount of juice appears in the pit cavity.
The burning time of sulfur will vary with the ventilation, shape of container, and weather conditions.
5. Place the can of sulfur under the box near the lower opening and light the sulfur. It melts before it ignites but soon burns with a clear blue flame that produces the acrid sulfur dioxide fumes. Because of the heat resulting from the burning sulfur, space is necessary between the sulfur and the sides of the box, and between the sulfur and the first tray. Do not leave burned matches in the container since they will impede the burning of the sulfur.
6. Immediately lower the box over the stack, and seal the bottom edges with dirt leaving the flap open.
7. When sulfur is burning well, close openings in box and start timing.
SULFITING FRUIT BEFORE DRYING
Soaking fruits in a solution of sodium bisulfite has an effect similar to sulfuring. Mixing sodium bisulfite with water releases sulfur dioxide which penetrates the surface of the fruit, retarding oxidation and enzymatic browning. Sodium bisulfite looks similar to table salt. Food grade (U.S.P.) sodium bisulfite can be purchased at wine-making supply stores or pharmacies. DO NOT use sodium bisulfate. While the two products may seem similar in name and appearance, their chemical properties are not the same. Due to its chemical structure, bisulfate is unable to inhibit the oxidation and enzymatic browning reactions that cause a fruit to ripen.
Sodium bisulfite is preferred for sulfiting because of its strength, but sodium sulfite or sodium metabisulfite may also be used. The strength ratio is: 1 tablespoon bisulfite=2 tablespoons sodium sulfite=4 tablespoons metabisulfite. You will need to use 2 times as much sodium sulfite or 4 times as much metabisulfite to achieve the same results as one part bisulfite. Sodium sulfite and metabisulfite can be purchased at wine-making supply stores or pharmacies.
Prepare a solution of 1 to 2 tablespoons bisulfite per gallon of water. Soak fruit slices for 5 minutes and halved fruit for 15 minutes. When soaking is completed, remove the fruit and rinse it lightly under cold tap water. Pat dry with paper towels and proceed with drying.
NOTE: Sodium bisulfite is an antidarkening agent that may be used to pretreat fruits before drying. While use of this preparation presents no problem to most people, recent evidence suggests that sulfites may cause adverse reactions in some asthmatic individuals. Thus, these individuals may choose to use another type of pretreatment.
ASCORBIC ACID DRYING PRETREATMENT
Pure ascorbic acid (vitamin C) is an antioxidant that helps to keep fruit from darkening as it is being prepared for drying. Dissolve 2 tablespoons of ascorbic acid crystals, 2 tablespoons of ascorbic acid powder, or 5 crushed one-gram vitamin C tablets in one quart of lukewarm water. Slice or chop fruits directly into the solution. Drain fruit well before loading onto drying trays. Ascorbic acid preparations can be purchased at pharmacies or wherever vitamin supplements are sold. Commercial antioxidant mixtures containing ascorbic acid and citric acid are not as effective as pure vitamin C, although they are often easier to come by. Follow package directions for cut fruits when using these mixtures.
SYRUP BLANCHING PRETREATMENT
Syrup blanching will hold natural fruit color fairly well during drying and storage, but it will produce softer textured and sweeter flavored fruit than other methods.
Prepare a sugar syrup by mixing 1 cup sugar, 1 cup light corn syrup and 2 cups water. Bring the mixture to a boil. Add the prepared fruit and simmer for 10 minutes. Remove fruit from heat and let stand in the hot syrup for an additional 15 minutes. Drain fruit well and proceed with drying.
SALT WATER DIP PRETREATMENT
Dissolve 4 tablespoons of table salt in 1 gallon of luke warm water. Slice or chop the fruit directly into the water. Allow it to soak no more than 10 minutes or fruit will absorb too much water and acquire a salty taste.