Michigan State University Extension
Preserving Food Safely - 01600512
Some fruits, such as peaches, apples, pears and apricots darken when exposed to air and during freezing. Directions for such fruits list antidarkening treatment as part of the freezing preparation. Several types of antidarkening treatments are used because all fruits are not protected equally well by all treatments.
Ascorbic acid For most of the fruits that need antidarkening treatment, ascorbic acid (vitamin C) may be used. This is very effective in preserving color and flavor of fruit and adds nutritive value.
Ascorbic acid is available at drug stores in various sized containers from 25 to 1,000 grams. (Ascorbic acid may be obtained also in powdered form.) One teaspoon weighs about 3 grams; thus there are approximately 8 teaspoons of ascorbic acid in a 25-gram container. In the recipes, amounts ascorbic acid are given in teaspoons.
Ascorbic acid tablets can be used but are more expensive and more difficult to dissolve than the crystalline form. Also filler in the tablets may make the syrup cloudy. The amount of ascorbic acid in tablets is usually expressed in milligrams.
To use, dissolve ascorbic acid in a little cold water. If using tablets, crush them so they will dissolve more easily.
In syrup pack, add the dissolved ascorbic acid to the cold syrup shortly before using. Stir it in gently so you won't stir in air. Solutions of ascorbic acid should be made up as needed. Keep syrup in refrigerator until used.
In sugar pack, sprinkle the dissolved ascorbic acid over the fruit just before adding sugar.
In unsweetened pack, sprinkle the dissolved ascorbic acid over the fruit and mix thoroughly just before packing. If fruit is packed in water, dissolve ascorbic acid in the water.
In fruit juices, add ascorbic acid directly to the juice. Stir only enough to dissolve the ascorbic acid.
In crushed fruits and fruit purees, add dissolved ascorbic acid to the fruit preparation and mix.
Ascorbic acid mixtures There are on the market special anti-darkening preparations--usually made of ascorbic acid mixed with sugar or with sugar and citric acid. If you use one of these, follow the manufacturer's directions. In these mixtures ascorbic acid is usually the important active ingredient. Because of its dilution with other materials, ascorbic acid purchased in these forms may be more expensive than the pure ascorbic acid.
Citric acid, lemon juice For a few fruits, citric acid or lemon juice (which contains both citric acid and ascorbic acid) makes a suitable antidarkening agent. However, neither is as effective as pure ascorbic acid. Citric acid or lemon juice in large quantities will mask the natural fruit flavors or make the fruits too sour.
Citric acid is available at drugstores. When using citric acid, dissolve it in a little cold water before adding to the fruit according to directions for that fruit.
Steam For some fruits, steaming for a few minutes before packing is enough to control darkening. Steaming works best for fruits that will be cooked before use.