Effect of Cooking on the Content of Dietary Fiber and Method for Determination of Dietary Fiber in Vegetables

Abstract

The effects of cooking on the contents of dietary fiber in 11 kinds of vegetables were studied using Prosky method and modified Prosky method, and the contents of dietary fiber measured by both methods were compared in each other. In the modified Prosky method, dietary fiber values were determined without subtraction of crude protein (nitrogen×6.25) in enzyme digested residue. Boiling or simmering increased the amounts of dietary fiber in carrot, edible burdock, onion and cabbage, but noticeably decreased those in santohsai, Chinese chive and Chinese cabbage. Frying and deep-frying increased the amounts of dietary fiber in carrot, edible burdock, Japanese radish, cabbage, spinach, santohsai, Chinese cabbage and sweet pepper, but decreased those in onion, Chinese chive and komatuna. The content of crude protein in enzyme digested residue was greater in leafy vegetables except cabbage(44-126mg/g) than in root vegetables(24-37mg/g), and was further increased by cooking in most kinds of vegetables. The contents of dietary fiber determined by the modified Prosky method were 10 to 50% larger than those determined by Prosky method in raw leafy vegetables, but in root vegetables the contents of dietary fiber determined by both methods were not so different each other. From above results it was supposed that the composition of nitrogen containing compounds in enzyme digested residue was needed for the accurate determination of the content of dietary fiber especially in leafy and cooked vegetables.