EFFECT OF FREEZE-DRYING AND HEATING DURING ANALYSIS ON DIETARY FIBER IN COOKED AND RAW CARROTS

Soluble and insoluble dietary fiber contents in raw and cooked carrots were measured by a modified Association of Official Analytical Chemists (AOAC) enzymatic-gravimetric procedure. Analyses were done on cooked and raw carrots, with and without freeze-drying and with and without heating during analysis. Heating for 1.25 h at 121-degrees-C caused a 3-5-fold increase in soluble fiber and a comparable decrease in insoluble fiber, suggesting a conversion of the latter to the former. Cooked carrots generally had more (3-9 g/100 g) soluble fiber than the raw product, but when samples were both freeze-dried and analyzed with gelatinization, soluble fiber was equivalent (ca. 10 g/100 g). Hence, since standard dietary fiber methods generally include freeze-drying and gelatinization, differences in the amount of fiber in cooked and raw foods may be obscured. Sugar and uronic acid analyses on soluble fiber fractions suggested that cooking and/or gelatinization resulted in increased solubilization of pectin (polygalacturonan) and arabinogalactan. Results of the present study indicate that current dietary fiber values may not accurately represent the level of this component in some foods as they are eaten.