Binding Properties

Many of pectin's unique properties result from its propensity to self-aggregate or to interact with cosolutes (6). A case in point is the pectin gels, which are three-dimensional networks capable of entrapping water. High-methoxyl pectins (HMP) with DM in the range 80 to 57% are induced to gel by adding sugars, which are thought to promote the self-aggregation of pectin by removing bound water, reducing ionic repulsions, and stiffening the polymer chain. These nonspecific interactions with sugars may be responsible for pectin's ability to reduce glucose intolerance in diabetics. For HMP at constant pH, gel strength and rate of gelling increase with DM, presumably through increased interchain hydrophobic interactions. HMPs of decreasing DM can be induced to gel by decreasing the pH, possibly through increased interchain hydrogen bonding. Low-methoxyl pectins (LMP), DM below 50%, gel with the addition of calcium or other divalent cations through interchain cross-links. Because decreasing DM increases the number of potential cross-links, LMP exhibits increased gel strength and higher gelling temperatures with decreasing DM. For HMP and LMP, gel strength increases with increased molecular size, possibly by cooperative self-interactions. It appears that all three major mechanisms exist for interchain interactions, namely divalent cation cross-links, hydrogen bonding, and hydrophobic interactions. Furthermore, for any mechanism to prevail, a critical number of functional groups must be present to pro mote cooperative interactions. Electron spin resonance studies support the concept that divalent cation binding to pectin in apple cell walls results in interchain bridges between adjacent smooth regions of pectin and that binding is a sequential cooperative process. Clinical studies suggest that pectin lowers blood cholesterol. This may be a case where the formation of divalent cation bridges is important in that calcium may form bridges between negatively charged pectin and bile acids. It has been suggested that binding and excretion of bile acids leads to a decrease of cholesterol in the bloodstream.

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