Effect of moisture, temperature, and sulfur dioxide on color of dried apples Public Deposited

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  • 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 increased. 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 approximately 20%. 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 was 63%. 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 approaches 1°C. 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.
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