Michigan
State University Extension
Preserving
Food Safely - 01600525
10/13/97
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.