Mechanism of retrogradation enhancement of maize amylose and amylopectin by phycocyanin and its hydrolysate

Abstract: Phycocyanin is a kind of dark blue powder isolated from spirulina and has a variety of health functions of anti-cancer, blood cell regeneration, etc. In order to widen its application in food, phycocyanin and its hydrolysis are added into maize amylose/amylopectin paste during retrogradation, it has been found that they can promote the retrogradation of amylose/amylopectin. In order to further explore its mechanism of promoting the retrogradation of maize starch, the effects of contents of phycocyanin or its hydrolysate in maize amylose/amylopectin on retrogradation rate were determined, and the method of X-ray diffraction, differential scanning calorimetry, infrared and solid nuclear magnetic analysis were used. The results showed that phycocyanin hydrolyzate promoted maize amylose retrogradation more obviously than that of phycocyanin. 10% addition of phycocyanin hydrolyzate could make maize amylose retrogradation rate increase from 27% to 76.9%, 184.8% higher. Phycocyanin promoted the retrogradation of maize amylopectin more pronouned than its hydrolysate, 10% addition of phycocyanin let maize amylopectin retrogradation rate increase from 26.7% to 45.3%, which increased by 69.6%. According to the UV-visible scanning, phycocyanin hydrolysis co cause the phycocyanin pigment in the center of phycocyanin to be exposed. X-ray results indicated that addition of phycocyanin to maize amylopectin induced a sharper peak at 2θ is 16.60o, as well as that mixture of phycocyanin hydrolysate and maize amylose caused a same sharper one at 2θ is 16.44o. Compared to diffraction angles of retrograded maize amylose/amylopectin, the decrease of sharp diffraction angle in those mixtures supported that interplanar distance at this angle became larger. DSC results demonstrated that mixture of phycocyanin and maize amylopectin, or mixture of phycocyanin hydrolysate and maize amylose, which both promoted retrogradation greatly, both caused the presence of re-crystallization peaks and loss of crystallization peaks. In other words, more phycocyanin/hydrolysate chains and maize amylose/amylopectin with similar length involved in retrogradation. Such process should deal with unwinding of maize amylose/amylopectin. The results of infrared and 13C solid NMR showed that phycocyanin might be hydrolyzed into arginine-rich polypeptide with hydrophilic, polysaccharides and polypeptide with hydrophobic. The mechanism for phycocyanin to promote retrogradation of maize amylopectin was that the formation of hydrogen bond between the arginine amino acid of phycocyanin and aldehyde of reduction end in maize amylopectin, and water molecule of maize amylopectin being driving away from the side chain end by phycocyanin hydrophobic amino acid, promoted the end of the chain-branched starch double helix spin, accelerate the formation of hydrogen bond between the end chains of amylopectin, which lead to improve retrogradation rate of maize amylopectin. The mechanism for phycocyanin hydrolysate to increase retrogradation rate of maize amylose was that the hydrogen bond formed between the sulfydryl of cysteine in phycocyanin hydrolyzate and aldehyde of reduction end in maize amylose during retrogradation of mixture, then the double helix of amylose was unlocked by molecular swinging, which greatly promoted the formation of hydrogen bond between amylose and improved the retrogradation rate of amylose. This study provides a new technique to improve the retrogradation rate of maize starch.