Study On Purification And Photostability Evaluation Of Monascus Pigments

The use of monascus pigments as food addictive has a long history, their decoloring under light has limited the use of monascus pigments in the food industry. Therefore, the systemic study about the components, structure and the possible photobleaching mechanism of monascus pigments has a great significance both in theory and praxis.This paper includes the following contents:To compare the effect of color characteristics of monascus pigments under different light conditions. Sutdy the light stability of monascus pigments, the result indicates the descending order of favorable light conditions to main color values of monascus pigment is, dark condition>red light>blue light>sun shine condition.Two red pigments were extracted and purified by silica gel column chromatography from monascus fermentation broth. Then structurally characterized by UV-visible spectrum, infrared spectrum, mass spectrometry and ¹H nuclear magnetic resonance spectroscopy. They were confirmed as rubropunctamine and monascorubramine.Photobleaching tests showed that the degradation of pigment accorded with first-order kinetic reaction and the bleaching kinetics have no obvious difference in the presence or absence of oxygen. Rate constant value for the decoloring reaction of rubropunctamine and monascorubramine is 0.3136 h^-1 and 0.2720 h^-1, respectively, which is lower than that of the crude pigment (0.4019 h^-1 ).Their half-lives increased with increasing initial pigment concentration.The color characteristics of monascus pigments over time were analyzed by colorimeter. L^*, a^*, and b^* values of the CIELAB color system for the two pure red pigments were measured. Values of chroma decreased and hue is not changed during the test. The results indicated that rubropunctamine and monascorubramine were sensitive to light. After exposure to sunlight chromophores of two pigments disappeared, this causes bleaching of monascus pigments. And the new compounds were colorless. The chroma values were not significantly affected by new products.The degradation patterns of those pure pigments over time were analyzed by HPLC with photodiode array detector (PDA) method. Rubropunctamine and monascorubramine were sensitive to light, the same compound was found after the light illumination due to decomposition reaction. The possible photobleaching mechanism of the red pigment is deduced according to photochemical theory: firstly, the acyclic side-chain in monascus pigment is cut at a-carbon, radicals are formed. And then a,β-unsaturated ketone, conjugated double-bond chromophores of two pure pigments are broken and pigments degraded to colorless compound, which causes bleaching of monascus pigments.