On the two forms of amylose-lipid complexes

Abstract: This thesis is about the formation of amylose-lipid complexes, and their properties. Two forms of superstructures can be identified. Complex form I has more randomly distributed helices, while the structure of form II is crystalline and therefore exhibits the typical V-pattern in X-ray diffraction analyses. Form II has higher heat stability, and is produced by heating at a temperature at least sufficiently high to partly melt form I complexes. Complex form II was created in a gel of ordinary starch and in a real food product: "pumpernickel" bread (baked 20 h at 120°C), i.e. not only in model systems. Thus, crystallization into form II is neither prevented by the amylopectin nor the bread structure. Monoglycerides and fatty acids can create both complex forms, but a charged surfactant created only form I, as seen from differential scanning calorimetry. Short monoglycerides (with acyl chain length 10-12) create complex form II more easily than longer monoglycerides. This might be because the complexes are smaller and therefore more mobile for ordering into form II crystals. On the other hand, short fatty acids have lower tendency to create form II compared to longer fatty acids. For fatty acids, the cooperativity of complex dissociation is decreased with shorter chains. The heat stability of each complex form increases with chain length and decreases with unsaturation, both in the case of monoglycerides and fatty acids. Form I complexes with fatty acids are more heat stable than with the corresponding monoglycerides. The heat stability of form II is less dependent on the properties of the included lipid (i.e. chain length and polar head) compared to form I. Form II is somewhat more heat stable in the case of monoglyceride compared to corresponding fatty acid. Monoglyceride mixtures may yield complex form II with higher heat stability than expected from their components. Furthermore, both complex forms can be affected by annealing so that their heat stabilities are increased. Digestibility of starch is influenced by complex formation, amylose content and heat treatments, which was seen by using in vitro models of the human digestive tracts to measure the amount of resistant starch (RS) and the rate of enzymatic starch hydrolysis. Higher amylose content reduces digestibility. Amylose molecules may show four levels of digestibility, increasing in the following order: crystalline RS, complex form II, complex form I, non-crystalline amylose. Heat treatment may enhance amylose retrogradation (i.e. crystallization). Complex-forming polar lipids may decrease digestion rate of amylose, but they are also able to reduce the amylose retrogradation and thereby increase digestibility by forming complexes.