Grafting Modification Of Zein And Its Application

Zein mainly derived from corn gluten meal (CGM, a corn processing by-products). Its content is more than 60%w/w in CGM. The annual CGM yield up to one million tons, and most of them are used in a low value way. Therefore it is very necessary to carry out deep processing and utilization of the most valuable component in corn gluten meal—Zein. The hydrophobic amino acids of Zein reached more than 50%, it is a typical water insoluble, alcohol soluble plant protein, so it is difficult for the large-scale use of food industry. Zein is often used in the preparation of Pickering emulsion/nutrients or micro capsule/nutrition element carrying system for its unique self assembly and biological compatibility. These carrying systems have good embedding effect and practical application values. In this research, the grafting modification of Zein with various reducing sugars were carried out, and the effects of grafting conditions on the maillard reaction products (MRPs) were studied systematically. The self-assembly properties to form nanoparticles of Zein before and after modification were compared and the stability of Pickering emulsion based on these particles were studied. Finally, the spray granulation process of mixed organic solvent system of Zein and Zein/astaxanthin composite particles were prepared. The influence of the spray process, protein modification and other factors on astaxanthin embedding and protection effection in composite particles were comprehensively studied. The main results of the paper are as follows:1. Considering the preliminary experimental results and economic factors, the optimal conditions of Zein grafting with reducing sugar were determined as:reaction temperature is 115℃, reaction time is 30 min, sugar protein mass ratio is 2:1, reaction pH value is 8, and the protein concentration is 20%w/v.2. The Zein-reducing sugar reaction system in the 80% ethanol system was established, the maillard reactions occurred between Zein and three kinds of monosaccharides (glucose, xylose, Arabia sugar) or a disaccharide (maltose). Compared with the reaction time of dry maillard reaction system which usually requires a few days to a few weeks, and the water wet maillard reaction system in which the Zein concentration is only 0.1%w/v, the 80% ethanol wet reaction system with the Zein concentration up to 20%w/v, this reaction time is only 30 min, and the concentration and time have a lot to improve, had significantly improve the efficiency of the reaction. The percent of glucose、xylose、Arabia sugar、maltose glucose grafted in the Zein were 2.59%、2.59%、1.86%、0.92%w/w respectively, glucose and xylose have the better grafting efficiency. After SDS-PAGE, the photo of the silver staining, coomassie staining and Schiff staining all proved that the maillard grafting reaction happened, glycoprotein and more polymers were formed. Circular dichroism results showed that the secondary structure of proteins was somewhat changed that the alpha-helix structure decreased, beta-strand and beta-turn increased. The contact angle test showed that glucose and xylose grafting products increased hydrophilicity significantly, while the ones grafted by Arabia sugar and maltose were not obvious.3. The nanoparticles of Zein and the grafted products were prepared by the classical anti-solvent method. The particle size of grafted products were significantly smaller than that of native Zein. Zein-glucose particles had the smallest average diameter of 63.07 nm. The isoelectric point (pI) of grafted products were shifted to more acidic, from native pI= 6.2 down to about pI= 5.2-5.3. It was found that the Pickering emulsion prepared by nanoparticles of Zein grafted product had better physical stability, could be stable for at least one month.4. The modified Zein/astaxanthin composite particles were prepared by spray drying method. Considering the astaxanthin load efficiency and the actual application needs, the obtained suitable conditions for spray granulation are as follows:air inlet temperature is 115 ℃, astaxanthin concentration is 0.4%w/v, Zein concentration is 2%w/v, input rate 15 mL/min. The field emission scanning electron microscope showed that the composite particles could be the hollow particles. Differential scanning calorimetry (DSC) studies showed that Zein/astaxanthin composite particles formed a homogeneous composite system, rather than a simple mixed state. Astaxanthin is uniformly distributed in the particle, instead of typically core-shell structure. High performance liquid chromatography (HPLC) method was developed for the determination of astaxanthin encapsulation efficiency. Compared with the untreated Zein, the carrying astaxanthin capacity of graft modified Zein did not change significantly as around 90%, and the astaxanthin embedding rate increased by 10%. The composite particles had a strong controlled release effect of astaxanthin. After stirring in ethanol solution for 12 h, the release rate of pure astaxanthin was 99.8%, while those of Zein/astaxanthin, Zein-glucose/astaxanthin, Zein-xylose/astaxanthin composite particles were 49.7%,31.7% and 30.2%, respectively. The composite particles significantly improved the thermal stability of astaxanthin as well. After the heat treatment, the lost rate of pure astaxanthin was as high as 75.29% while that of the composite particle was only 25%, indicating that only the astaxanthin attached on the particle surface was damaged and the astaxanthin inside the particle was effectively protected.