Changes in content and composition of dietary fiber in yellow onions and red delicious apples during commercial storage

Abstract

Changes in pectin composition and solubility are part of the softening process in apples during ripening and postharvest storage. Lignification may also occur with long-term storage. In the United States, apples and onions are harvested once yearly and then stored and marketed for the next 12 months. The changes that occur in the dietary fiber content and composition in Red Delicious apples and yellow Spanish onions during storage were studied, and the loss of fiber in peeled apples was determined. Dietary fiber was extracted by the enzymatic-chemical method of Theander and Westerlund. Storage had no effect on total or insoluble fiber content of apples; Klason lignin concentration was greater in samples stored for 12 months than in those stored for 0, 4, or 8 months. Peeling reduced apple fiber concentration about 25% by decreasing neutral and acidic sugars and Klason lignin in the insoluble fraction. The total fiber content of onions increased with storage, primarily by increasing the insoluble fiber content of uronic acids. The results suggest that the standardized, environmentally controlled storage of apples, as used in Washington State, has little effect on dietary fiber content. In contrast, the less rigorously controlled storage conditions for yellow Spanish onions increases the insoluble fiber fraction and uronic acid content.