Abstract: Adzuki bean starch was complexed with four fatty acids (palmitic, stearic, oleic and linoleic acid), separately. After microwave-assisted pretreatment of the starch, starch-lipid complexes were prepared by water bath heating. The structures of the starch-lipid complexes were investigated by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), Fourier transform infrared (FT-IR) spectroscopy and X-ray diffraction (XRD). Their digestion characteristics in vitro were determined as well. The results showed that the volume of the four complexes, with?uneven?surfaces, increased when compared with the control starch under SEM. DSC analysis indicated that there was only one single peak in rice bean starch but three in each of these complexes, suggesting that the complexes were composed of type I and type II complexes. Gelatinization onset temperature, peak temperature, and conclusion temperature increased in the four complexes, which confirmed that the addition of fatty acids inhibited the gelatinization of the starch. Thermal enthalpy increased with the increase in the carbon chain length or the degree of unsaturation of fatty acids. The increase in enthalpy value was possibly related to the formation of resistant starch. FT-IR spectroscopy demonstrated the formation of complexes. Increased number of hydrogen bonds may increase the content of resistant starch. The complexes generated a diffraction angle of the V-type crystal structure accompanied by changes in crystal characteristics, which proved the existence of the complexes. The characteristic peak intensity of the complexes with palmitic, stearic or oleic acid was significantly higher than that of the complex with linoleic acid. As the intensity of the V-shaped characteristic peak increased, the resistant starch content increased. The relative content of rapidly digested starch in all complexes decreased, the relative contents of slowly digested starch and resistant starch increased, and the anti-digestibility of the starch decreased with the increase in the carbon chain length or unsaturation degree of fatty acids. The four fatty acids had different effects on the structure and in vitro digestion of adzuki bean starch. Among them, palmitic acid had the most significant effect. This study could provide useful information for further studies on the structural properties of starch-lipid complexes and their applications in resistant starch.

Key words: starch-lipid complexes, structure, Fourier transform infrared spectroscopy, X-ray diffraction, in vitro digestion