Advantages and Disadvantages of the Gravimetric Method vs. the Component Analysis of Dietary Fiber in Food Products
Gravimetric Method AOAC 985.25 (AACC 32.05)
• Officially approved
• Internationally validated
• Not requiring sophisticated equipment Disadvantages:
• Limited to total dietary fiber
• Applicable to separate soluble or insoluble fibers (precipitation in 80% ethanol)
made available and validated for any specific substance that is proposed or accepted to be a part of the dietary fiber concept.
All dietary fiber components have different physicochemical properties. In general, these properties are determined largely by the way in which the components are linked together. It is important to note that these physical properties have often only been demonstrated in vitro, thus ignoring that the structure of each dietary fiber component can undergo complex modifications during passage through the gastrointestinal tract. Therefore, it is not certain that the in vitro data relate to the physiological properties in vivo. The most often cited physicochemical properties are:
• The water-holding capacity. This refers to the ability to retain water in the matrix. It can be measured in vitro by saturating the fiber with water and then removing the nonadsorbed water by either filtration or centrifu-gation of osmotic suction.3031 The shape of the molecules, their ability to pack closely together, and their solubility (dispersibility) in water are the key features controlling their water-holding capacity. Closely packed, insoluble (nondispersible) compounds (e.g., cellulose) are resistant to hydration and swelling, whereas those with disordered structures and more solubility (dispersibility) are hydrated more easily. These properties are also influenced by the particle size. In general, dietary fiber from fruit and vegetables tends to bind much more water than cereal dietary fiber. Originally, it has been suggested that components with a large waterholding capacity will have a high stool-bulking effect.32 Unfortunately,
• Quantification of individual monomeric sugars
• Separation of neutral and acid sugars
• Separate analysis of lignin
• Can be modified to include new fibers
• Requires more expertise
• Requires complex equipments the water-holding capacity of a dietary fiber determined in vitro does not predict its stool-bulking ability, mostly because it does not take fermentation and subsequent increase in bacterial biomass into account.33 The so-called "potential water-holding capacity" (i.e., the water-holding capacity of the dietary fiber residue and the bacterial biomass after in vitro fermentation) that was developed to take these factors into account is physiologically a more meaningful parameter.34 But it is difficult to measure.26
• The cation exchange capacity. This is determined mainly by the content of carboxyl (COO), hydroxyl (OH), and amino (NH3+) groups in the sugar moieties of the oligo- and polysaccharides. In vitro, many different dietary fibers (e.g., pectins, because of the presence of uronic acid monomers) have been shown to bind minerals, and it has been hypothesized that these might also behave similarly in vivo in the gastrointestinal tract, thus impairing the absorption of some important minerals (like calcium, copper, and zinc).34-36 The in vivo relevance of these data has, however, never really been demonstrated. Moreover, because of the presence of phytates in plants or crude dietary fiber preparations, the interpretation of the data, when using these sources, is difficult. But some dietary fiber components (e.g., cellulose, D-glucans, inulin) have no ionic charge, they do not bind any minerals and, as discussed later (Chapter 10), some of these nonionic dietary fiber components, particularly inulin, have even been shown to increase mineral (especially Ca and Mg) absorption.
• Viscosity. It is determined by the molecular weight of the polysaccharides, their capacity to interact in solution, their volume, and the presence of solid insoluble particles. When in solution, the viscosity of pectins, mixed-linkage D-glucans, and algal polysaccharides (agar and carrageenan) increase. But the importance of this effect strongly depends on the chemical structure of the dietary fiber. Insoluble dietary fiber components have practically no effect on viscosity. Once again, the actual effect of dietary fiber on the viscosity of the intestinal content is difficult to evaluate.
• The binding capacity. Many substances, especially bile acids, may become bound to dietary fiber components. The binding capacity depends on the shape of the molecules, the chemical nature of the surface, and the total area accessible for binding that varies with particle size. Different dietary fiber components can have strikingly different binding capacity (e.g., pectins seem to have the greatest ability to bind bile acids, whereas wheat bran has a moderate capacity and cellulose practically none). Data concerning binding ability of dietary fiber components remain, however, very controversial.26
In conclusion, there is not a single physicochemical property that is common to all dietary fiber components. Most of the information available comes from in vitro experiments, and the in vivo significance of the findings often remains unclear. Thus, based on the present knowledge, viscosity, water holding, binding, and cation exchange capacities cannot be used as an exclusion or selection criterion for classification as dietary fiber. Each component in the dietary fiber concept has its own pattern of physicochemical properties. (Table 6.2)
6.1.6 Physiological Properties of Dietary Fiber: Their Effects on Upper Gastrointestinal Tract
Nondigestibility is, undoubtedly, the most common and the most essential basic property of dietary fiber. Almost all definitions include this as the basic characteristic of dietary fiber. This is primarily because, with the exception of resistant starch, all dietary fiber components are non-D-oligo- and polysaccharides that cannot be hydro-lyzed by the small intestinal D-glycosidase in mammals. The methodologies that can be used to demonstrate resistance to digestion are described previously (see Chapter 4, Section 4.1.3). When applied to the main dietary fiber components, they have demonstrated their nondigestibility, including in vivo in humans, especially in ileostomy volunteers.37 38
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