Determination of total dietary fiber, soluble dietary fiber, and insoluble dietary fiber in foods using an enzymatic-gravimetric-liquid chromatographic method

Abstract

Dietary fiber has been widely recognized for its beneficial effects on human health. Consequently, fiber is increasingly incorporated into dietary supplements and functional foods to enhance daily fiber intake. However, quality control of these products remains challenging due to the structural diversity of dietary fiber and the need for different analytical approaches for each fiber type. This study aimed to determine soluble dietary fiber (SDF), insoluble dietary fiber (IDF), and total dietary fiber (TDF) in food matrices using a combined enzymatic-gravimetricliquid chromatographic method. The method enables the simultaneous determination of TDF, SDF, and IDF in a single analysis and is particularly suitable for complex food samples with unknown fiber composition. Starch was hydrolyzed using pancreatic α-amylase and amyloglucosidase at 37℃ for 4 h, while proteins were digested with protease. IDF was quantified by gravimetric analysis, whereas SDF was determined by liquid chromatography with a refractive index detector. The method was validated in accordance with AOAC guidelines, demonstrating satisfactory specificity, linearity, repeatability (RSD 3.1 - 4.4%), reproducibility (RSD 4.3 - 7.1%), and recovery (92.1 - 105.8%). The validated method was applied to the analysis of 50 randomly collected commercial food samples. Natural food products such as vegetables, fruits, and cereals exhibited high IDF contents (11.3 - 16.7 g/100g), reflecting their characteristic cellulose- and lignin-rich structures. In cereal products, IDF ranged from 5.8 to 9.9 g/100g. In contrast, functional foods and dietary supplements showed significantly lower IDF levels (0.6 - 3.4 g/100g) and predominantly contained soluble dietary fiber, with SDF ranging from 2.1 to 11.8 g/100g.