| Extrusion cooking is new processing technology in food industry, it was widely used in processing of texturized plant protein products for its high efficiency, low energy consuming, and cleaning to environment. Peanut seed contained rich oil and protein, and provided plenty of oil and protein for human. China possessed the biggest yield and consumption of peanut seed in the world. The total yield of peanut seed is up to 14,710,000 tons in 2005, and 50%~65% of it was used to extract the edible peanut oil. The byproduct, that is defatted peanut meal, contains protein around 50%~70%. The defatted peanut meal is a good resource of plant protein for its pale color, pleasant smell, and containing fewer antinutrients, about 20% that of in soy bean.In this paper, the extrusion parameters for processing the texturized peanut protein (TPP) products were optimized through analyzing the effects of extrusion parameters on the quality of TPP, at first. Then, the effects of extrusion parameters on the resident time of feed in extruder were investigated. The effects of material characteristics on the qualities and structures of TPP products were studied in detail. The changes of some constituents in feed during the extrusion cooking processing of defatted peanut flour (PF) were also determined and analyzed. About the Maillard reaction and its products formed in extrusion cooking of PF were primarily investigated, too. The mechanisms of the fibrous structures formation during extrusion cooking of PF were analyzed and discussed The main conclusions of above researches as follows:Extrusion temperature and moisture content in feed have the most significant effects on the qualities of TPP products during the extrusion cooking of PF, and the effects of feed rate and screw speed are second. Extrusion temperature is the key parameter for PF can be texturized or not. While the effects of moisture content in feed is important condition for the texturized quality of TPP products. The roles of feed rate and screw speed are mainly about to the resident time of feed in the extruder, and therefore bring effects on the texturized structures and sensory qualities of TPP products. In the test conditions, the optimal extrusion parameters is 147.0℃～148.0℃, 49.3%～50.9%moisture content in feed, feed rate at9.5～10.4g/min and the screw speed at 95～112r/min. The mean residence time (MRT) is in the range of 5.28~10.56 min during the extrusion cooking of PF. the effects of feed rate and moisture content on the MRT is the most significant, and the effects of extrusion temperature and screw speed is the second. Extrusion temperature, feed rate and moisture content have more effects on the curve breadth of residence time distribution (RTD), the effect of screw speed is less. The MRT would be prolonged and get broader curve of RTD when decreasing feed rate and screw speed or increasing the extrusion temperature and moisture content in feed. There are evident interaction effects on MRT between extrusion temperature and moisture content, extrusion temperature and screw speed. The flow mode of the melt in extruder is between plug flow mode and perfect mixing flow mode, and the curve of RTD would be close to the perfect mixing flow mode with increasing the residence time of feed in extruder..The material characteristics of PF, such as protein content, size diameter, pH value, oil content and the nitrogen solubility index (NSI)of protein in feed have significantly effects on the sensory qualities, rheological properties and microstructures of TPP products. The TPP products don't possess the good fibrous structure when protein content lower than 40% in feed. The TPP would keep proper hardness and chewiness, and have good texturized structures when PF size diameters in the range of 0.2~0.3mm. The hardness and chewiness of TPP products would be the highest at the pH 4.5; in the range of pH value 7~10, its would be lower increasing the pH value. Few of oil content (3%~6%) in feed is helpful to improving the structure and sensory qualities of TPP products. It is not proper to extrusion cooking when the NSI lower than 20% in PF.The solubility, structure of submits and thermodynamic property of peanut protein were obviously affected by the extrusion processing. The solubility of peanut protein sharply decreased after extrusion. The results of sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) indicated that peanut protein molecule is composed of 12 submits, and its molecule weight were 63.10, 41.64, 38.56, 36.86, 29.95, 28.94, 24.37, 21.98, 20.95, 19.19, 17.62kD and14.74kD, respectively. Some of low molecule weight submits would be degraded at high extrusion temperature, such as 155℃. But there is no new brand present in the results of SDS-PAGE. The results of differential scanning calorimetry (DSC) of peanut protein before and after extrusion showed that the onset denature temperature of peanut protein is 45.01℃, top denature temperature is 95.70℃, and the results indicated that peanut protein possessed high stability and would be much hydrogen bonds in its molecules. The physicochemical characteristics of the carbohydrates (starch and small molecule sugars), lipids, amino acids and minerals in PF are also affected by the extrusion processing. In the test conditions, some starch molecules were degraded besides gelatinizing during extrusion, so the starch content in PF decreased significantly after extrusion. Some of small molecules sugars started to degrade even at low temperature (120℃), part of it changed into reducing sugars, and other part of it would be decomposed into smaller molecules, such as pigment or flavor substances. Free lipids and combined lipids in PF would be hydrolyzed, oxidized and aggregated, and lead to their content decreased in TPP products at high extrusion temperature. The amino acids losing much in extrusion are the high content amino acids in PF, such as glutamic acid, asparaginic acid, alanine and arginine et al. While the lysine losing is relatively little, only lose 4.93%, may be caused by the high moisture content (50%) in feed during extrusion. Some minerals, such as iron, manganese and copper, their contents increased 11.81%, 2.31% and 5.42%, respectively, after extrusion, which mainly originated from the extruder and screws. And the other minerals contents has no change. Moreover, the minerals content from elements of extruder would increase with the extrusion temperature increasing.Ether extractions from different extrusion samples were detected and analyzed by gas chromatographs-mass spectrometer (GC-MS) system. The results showed that the extractions exhibit 33 peaks, in which there are 23 peaks their match quality above 60, and 23 kinds of compounds were analyzed by GC-MS. The results indicated that the ether extraction are mainly composed of middle and long chain fatty acids or its esters, especially methyl esters. These esters mainly originate from the lipids in PF, and they play a main role in the special fragrant of peanut seed and its products.The 10%~20% addition of defatted soy meal in PF could improve the color, morphology and rheological properties of TPP products during extrusion cooking of PF, as well as increase the nutritional value of peanut protein of TPP products. As a kind of bivalent salts, lower level calcium chloride addition (<1.0%) is benefit to the sensory quality and structures of TPP products, but higher level addition (1.5%～2.0%) is harmful and make the sharply qualities decreased of TPP products. While, in the range of 0.2%~3.0% sodium chloride addition in PF have little effect on the quality of the TPP products. Low concentration potassium bromate (10mg/kg) addition in feed has definite effect on the quality and structure of TPP products. While, the TPP products appeared distortion in morphology, roughness in surface, and poor texturized degree when the potassium bromate addition above 30mg/kg. As well as addition of L-cysine in feed, even very low (0.01%), is harmful to the teturization of PF. The results indicated that the electrovalent and disulfide bonds may be not important to the formation of fibrous structure during PF extrusion cooking.The results of solubility test of peanut protein in different solvents showed that noncovalent bonds (hydrophobic interaction and hydrogen bond) play important roles in the formation of fibrous structures during PF high moisture extrusion cooking, and the effects of disulfide and electrovalent bonds are the second. Some of the disulfide bonds in the peanut protein molecules would be ruptured during extrusion texturization of PF, and the reaction would be accelerated at higher temperature. The total sulfhydryl group content in TPP products shows little change increasing the extrusion temperature. Moreover, it is benefit for enhancing the aggregation between protein molecules that possess certain number of free amino groups in the protein molecules, and this also helpful to the information of fibrous structure in TPP products. The results of SDS-PAGE indicated that it is necessary to the information of fibrous structure in TPP products that contain proper rate of long chain protein molecules in peanut protein material.
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