Pure inulin powder is sold for nutritional and medicinal purposes. For nutritional purposes, it is sufficient that any toxic components and pathogenic organisms are removed from the inulin. However, for medical and diagnostic uses, inulin must be extremely pure and have a high degree of polymerization (>20). Inulin from Jerusalem artichoke typically has only half of its inulin above a degree of polymerization of 10, with 12 the most frequently occurring chain length in raw tubers. Therefore, the inulin from Jerusalem artichoke, unless fractionated, is less suited to medicinal applications. A number of methods are available to obtain pure inulin for medical usage, including microwave drying and ultrafiltration (Vukov et al., 1993).
Inulin is used in an important test for renal failure called the inulin clearance method (Gretz et al., 1993; Chiu, 1994). As inulin is neither secreted nor reabsorbed in the kidney, it can be administered by injection to measure glomerular filtration rate. The relative amounts of inulin in the plasma and urine give an indication of renal function.
5.7.2 Inulin Fractionated by Degree of Polymerization 126.96.36.199 Fat Substitutes
The use of low-calorie fat replacers in foods facilitates reductions in the energy density of the diet. However, since fat confers a number of important quality attributes, it is critical that such foods be highly palatable. When all or part of the fat is replaced, the foods must have comparable rheological and sensory-quality attributes to the original high-fat food. Textural properties are particularly important since fat has a pronounced impact on texture, mouthfeel, and hence eating quality. Therefore, in addition to lowering the calorie density, an acceptable fat substitute must have the appropriate functional properties, such as heat stability, emulsification, aeration, lubricity, spreadability, texture, and mouthfeel (Lukacova and Karovicova, 2003; Silva, 1996).
Inulin can be used to replace a significant portion of the fat in certain meats (Archer et al., 2004) and traditional squeezable and spreadable food products. As the fat is reduced, the amount of water increases to the detriment of the product's structure. The water binding capacity and melting and rheological properties of inulin in such products, however, allow reducing the fat content from around 80% to 20-40% (Silva, 1996).
The higher molecular weight fractions of inulin function more like fats than lower-dp fractions. Therefore, when inulin is used as a fat substitute, generally the low molecular weight fraction is removed, leaving a product with an average degree of polymerization of 25 or higher. The higher molecular weight inulin can form a gel that has excellent spreadability (Kasapis, 2000). Unless very high levels of inulin are used (25%), gel-forming proteins and hydrocolloids may need to be added to alter the structural properties of the product.
Low-fat squeezable spreads and soft products (e.g., soft cheese, spreadable margarine) require a ratio of plastic stress to maximum stress of 0.95 to 1.0 (Kasapis, 2000). Typically around 15% of a high-dp fraction (~25 dp) can be used in these products. Interestingly, the physical structure of the material does not develop immediately with formulation but requires 1 to 2 days of storage.
Inulin is soluble in water, though its solubility is strongly modulated by temperature (e.g., ~6% at 10°C and 35% at 90°C) (Silva, 1996). It has a water binding capacity of approximately 2:1 and, when in solution, reduces the freezing point of the water. It is dispersible in water but tends to clump due to its hydroscopic characteristics, a problem that can be partially circumvented by mixing it with sugar or starch. Commercially available inulin has a slightly sweet taste due to the presence of glucose, fructose, and sucrose. The odor is neutral.
The functionality of inulin as a fat replacer is due to its effect on water (Silva, 1996). As the inulin concentration in the solution increases, the viscosity of the solution increases (Table 5.7). Initially at 1 to 10% there is a small but gradual increase in viscosity; between 11 and 30% there is a more pronounced increase, but without gel formation. Above 30% inulin in water, discrete particles form and a gel develops within 30 to 60 minutes of cooling. With increasing inulin concentration, gel formation occurs more rapidly, and at very high levels (i.e., 40 to 45%), gelling occurs very rapidly. Such gels are very creamy and fat-like, and their strength is a function of the concentration of inulin, though other factors can also influence gel strength. Further increases in inulin result in gels of increasing firmness, and as the level of inulin approaches about 50%, the gels become very firm but retain their fatty feel.
Gel formation inhibits hydrolysis of the inulin (Silva, 1996). At low concentrations (i.e., below gelling), inulin may be hydrolyzed at a pH below 3 and at very high temperature due to the presence of "free water." In gel form, inulin is stable in acidic and high temperature conditions due to the lack of available water.
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