| Proteins-based composite films with rich nutritional value and good barrier properties, become the first choice of the research on preparation of edible film. Milk casein containing essential amino acids, is a complete protein. Casein, which has coil structure and these function such as solubility, thermal stability and emulsification properties, easily form films due to their ability to from extensive intermolecular hydrogen, hydrophobic bonds. However, the strong hydrophilicity make casein-based films block-water performance, simple composition and mechanical properties become inferior. In order to overcome the disadvantages, Unshaped casein (Ucas) or purified casein (Pcas), which obtained from fresh milk, was first hydrolyzed in order to form sodium caseinate under the condition of 95℃,0.5mol/l NaOH solution for 3h. Using in-situ assembly technology, the edible composite films based on purified sodium caseinate (PNacas) or unshaped sodium caseinate (UPacas), were prepared by means of casting and included a certain amount of glycerin, gelatin and stearic acid.The properties, such as tensile strength, elongation, water vapour permeability and peroxide value of the composite films, emphatically investigated and ratio of sodium caseinate composite films was also determined. The thermal stability of composite films were studied by differential thermal analysis (DTA) and thermogravimetry(TG).it is necessary to elucidate the interaction between modifier and sodium caseinate through the spectral characteristics such as fluorescence spectra, Fourier transform infrared spectroscopy, and ultraviolet spectra. Finally, possible mechanism of film formation has been put forward. These study not only present a new method for preparation of edible packaging materials, but also provide a new way for making use of milk effectively and protein supplement the human body.The main experimental results were as follows:(1) polyhydroxy Glycerol could be full of spatial structure of sodium caseinate macromolecules by intermolecular interaction, thus softening the rigid structure of the films, while has little effect on the mechanical properties. Gelatin with strong gel properties can make sodium caseinate sol system regroup, change the original spatial structure of polymer, and run through the network structure of sodium caseinate better, which improve the flexibility of composite films and enhance mechanical properties. Stearic acid with hydrophobicity and lipophilicity can form a continuous, dense "wax layer" on the surface of the films, thus effectively preventing water vapor. The optimum mass ratio of stearic acid, gelatin, glycerol and Pacas was 0.4:0.8:0.16:1.0, The optimum mass ratio of glycerol, gelatin and Uacas was 0.16:0.6:1.(2) DTA-TG showed that the sodium caseinate composite films which has higher glass transition temperature and better thermal stability than sodium caseinate itself. stearic acid-glycerol-PNcas composite films had maximum weight loss rate of 32.5% at 334℃,the glass transition temperature is 126℃.The weight loss rate of glycerol-gelatin-UNcas composite films was 25.2% at 386℃,the glass transition temperature is 119℃.(3) The results of IR spectroscopy:glycerol make that stretching vibration of C=O and C-N move the high band in the scope of PNacas infrared spectroscopy 1500cm-1～1700cm-1, PNacas secondary structure was changed, its beta folding structure reduced, a spiral,, random curl, beta Angle structure significantly increased, intermolecular hydrogen enhanced. The results of stearic acid PNacas composite film:absorption peak significantly enhanced at 2923 cm-1,absorption peak of O-H shifts not clearly, stearic acid modified PNacas and Hydrogen bonding is little, verify that the stearic acid modified is blending effect. O-H absorption peak electron cloud density was mean, O-H absorption peak move to the low band after gelatin was added. It is account for the acting force of gelatin and PNacas by non covalent bond; Crude fat is contained in the casein, the fat hydrolysis to produce structure of primary alcohol glycerin, cause that C-O absorption peak at 1051 cm-1. the absorption peak of gelatin-glycerin-UNacas composite membrane stronger taper. It is account for that methyl, methylene and C-O bonds enhance (Namely the hydrophobic groups was increased).(4) The results of fluorescence spectroscopy:the fluorescence intensity of sodium caseinate decreases due to the addition of glycerol. The binding constant and the number of binding sites obtained by the double logarithmic regression curve analysis. Gelatin and PNacas fluorescence quenching constant KG is bigger than the maximum dynamic diffusion quenching constant. However, fluorescence intensity of UNacas composite films increased, when glycerol and gelatin were added. It was show that Glycerol and gelatin through hydrogen bonding enhance the π electron conjugate due to electron-donating group such as -OH、-OR、-NH2. Fluorescence quenching constant KG of stearic acid -PNacas composite films is less than zero.(5) The results of evaluation on preventing water vapor of different composite film used to package bread:under 50-5% RH for 24 h, the weight of packed bread gained by 14.5% to 23.7%. |
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