Adsorption kinetics of monascus red pigments with hydroxyls by retrograded corn starch

Abstract: Retrograded corn amylose is a kind of resistant starch which is formed by the interaction of amyloses through hydrogen bonds. Those bonds hinder the combination of starch and water/amylase, which makes retrograded amylose insoluble and resists amylase hydrolysis. Some of retrograded corn amyloses can form regular crystal with identical characteristic named B-type structure. Such crystal structure provides possibility to absorb organic compounds containing hydroxyls. There are residual carbohydrates, amino acids, water soluble protein and peptides and so on in fermentation broth in the production of monascus red pigments by fermentation liquid method. It is difficult to isolate those substances from pigments in the following process, so the pigments are mixed with impurities in the product. These impurities reduce the degree of pigment purity and bring uncertainties to the stability of pigments in use. The retrograded starch is insoluble in water and contains a lot of hydroxyls, so hydrogen bond can form between starch and hydroxyl-containing pigments or other organic compounds containing a large number of hydroxyl groups. Then the substance containing a lot of hydroxyls is absorbed by retrograded starch, but those ones having no or fewer hydroxyls will stay in solutions. Thus, the component containing hydroxyl groups will be separated from solutions. In this paper, the corn amyloses with different molecular weight distributions were made by retrogradation method and those amyloses were used to absorb monascus red pigments. Absorption capacity and speed of monascus red pigments by those amyloses were determined. The results showed that the maximum absorption capacities of monascus red pigments by amyloses retrograded for 1-4 times were 0.56, 0.84, 1.04, and 1.10 mg/g respectively, when the absorption temperature was 80℃ and the absorption time was 140 h. The narrower the molecular weight distribution of amylose and the higher the absorption temperature, the higher the absorption speed. The average absorption free energies of all samples, which were calculated by Dubinin-Radushkevich adsorption isotherm equation, were less than 8 J/g. Such calculated results suggested that the pattern for retrograded corn amylose to absorb monascus red pigment belonged to physical absorption. The results of the scanning electron microscope (SEM) indicated after drying those retrograded corn amyloses presented some kinds of more bouffant structure when they were combined with monascus red pigments. In order to increase the pigment absorption content of retrograded corn amylose, the regular crystal should be cultured from amylose with the narrowest distribution of molecular weight. So the method of preparing those materials became vital. Only identical amylose with special chain length could involve in retrograded amylose, so amylose with narrow distribution of molecular weight could be obtained by repeated retrogradation. The results of the paper provide a reference for isolating and purifying monascus red pigments with hydroxyls.