- When sulfur dioxide is used in commercial drying of fruits in
combination with low storage temperatures, browning and microbial spoilage
can be inhibited. However, the relationship between moisture level,
storage temperature, and sulfur dioxide content on the color of dried
fruit is not known; therefore the purpose of this research was to
determine the influence of temperature, sulfur dioxide content, and
moisture level on the color of dried apples during storage.
Dices (3/4" x 1/2" x 1/4") of Golden Delicious apples were dried
after a 90 sec dip in aqueous solutions of 2500, 5000, or 7500 ppm of
sulfur dioxide obtained from sodium bisulfite. The apples were dried to
five different levels of moisture (13, 18, 22, 24, and 26% wet basis)
and stored in controlled temperature rooms at 1°, 21°, and 38°C.
Periodically the samples were analyzed for color (color index (CI)
defined as Hunter L x a [subscript L] x b [subscript L]), total and free sulfur dioxide, moisture
and water activity (a [subscript w]), to appreciate the changes of quality during 385
days of storage.
Sulfur dioxide level was directly influenced by storage temperature.
As temperature increased, the sulfur dioxide level in the dried
apples decreased following a negative exponential curve. At 1°C nearly
no variation in sulfur dioxide level was observed during the experiment.
Loss of free sulfur dioxide followed the same pattern as total sulfur
dioxide. The concentration of free sulfur dioxide was a larger proportion
of the total as the concentration of total sulfur dioxide was
Moisture content of the dried apples decreased during storage at
38°C, but at 21°C moisture content decreased in the first 40 days
reaching a level that remained constant until the end of the experiment.
The constant level was approximately 85% of the initial moisture level.
No appreciable change of water content occurred at 1°C. Water activities
of the samples ranged between 0.40 and 0.85 and the optimal
levels for color retention at the lower concentrations of total sulfur
dioxide were approximately 0.75 a [subscript w]. This corresponded to a moisture of
Total sulfur dioxide, moisture and CI analysis of the dried apples
were used to derive three equations, one for each temperature of
storage. From these equations, the following relationships were evident.
CI described the changes in color during the 385 days of the
experiment. At 1°C sulfur dioxide and moisture influenced the changes
of color approximately the same extent. Moisture content of 20% at all
levels of sulfur dioxide at 1°C was optimal for maximum color
preservation. Samples stored at 1°C retained an acceptable color longer
than those stored at 21°C or 38°C. The approximation of the equation
derived from regression analysis of the data of changes in color at 1°C
Changes in color at 21°C occurred faster than at 1°C. Acceptable
colors were found until the 188 day samples. The water content of the
samples had more influence on the color changes than the total sulfur
dioxide content as determined by the regression equation. The regression
equation describes 88% of the variations in color. Concentrations
higher than 1500 ppm of total sulfur dioxide were necessary to maintain
samples of acceptable colors for periods up to 188 days with moisture
levels close to 20%. Storage temperature of 21°C was satisfactory for
samples that do not require storage periods longer than 200 days and
contain 1500 ppm of total sulfur dioxide and 20% moisture.
Very rapid changes in the parameters studied were observed at 38°C.
The samples were very dark after 40 days and none had an acceptable
color after 101 days of storage. The regression equation derived from
the data described 87% of the variations of color. The temperature of
38°C is not recommended under any condition for storage of dried apples.
It was concluded that temperatures lower than 21°C and concentrations
of approximately 1500 ppm of sulfur dioxide in the fruit tissue
could preserve acceptable colors in samples of dried apples for periods
of 200 days. Longer storage periods would be possible as temperature
Using the equations obtained in this experiment, an estimation can
be made of the storage life of dried apples. The response surface
diagrams obtained are useful for visual comprehension of the influence
of temperature, sulfur dioxide and moisture on color throughout the
time of storage.