| Drying is necessary for harvested paddy. Quality of post-drying paddy rice has direct impact on paddy rice price and market positioning of processed product. If drying method is unappropriate, not only cost will be increased, but also will lead to edible quality decrease and therefore adversely affect yield rate of deep processing. The traditional method of hot air drying has not radically solved quality problems with post-drying paddy rice. Therefore, it is necessary to seek an efficient and reasonable drying method and technology for quick emergency drying of paddy rice, decreasing power consumption, improving drying efficiency and securing post-drying quality.Vacuum drying will vaporize moisture in low temperature and is favorable for grain color, smell and taste. Reports on researches on vacuum low temperature drying of paddy rice are very few in home and abroad. Aiming at problems on paddy rice drying such as the quality decrease, the paper makes the following researches on vacuum drying principle, parameters selection of drying technology and taste quality of post-drying paddy by experimental method of vacuum drying.(1) Makes mass transfer analysis on paddy rice vacuum drying process and sets up mathematical model. According to heat and mass transfer rule of porous media, analysis on vaporization and dispersion of two-phase moisture on surface and interior of paddy rice and on paddy rice vacuum drying principle are made. Based on water potential, moisture transfer mathematical model for paddy rice vacuum drying is set up. By analysis and test on drying curve, it shows that: paddy rice vacuum drying is similar to hot air drying, moisture on material surface and interior will transfer from interior to outside at the effect of chemical potential difference. Also it indicates that paddy rice vacuum drying rate is related to media temperature, pressure and relative humidity. The lower the absolute pressure, the higher the drying rate. Provides reference for technology design of paddy rice vacuum drying.(2) By quadratic orthogonal rotary regression design, sets up forecast models between paddy rice reduction of moisture content, increase of cracking rate, head rice yields and drying time, vacuum degree and drying temperature respectively. By dimension reduction method, single factor analysis, primary and secondary factor analysis on objective function and drying parameters is done. Reduction of moisture content is positively related to drying time, vacuum degree and drying temperature; increase of cracking rate is positively related to drying time and drying temperature and is negatively related to vacuum degree; Relation of head rice yields, increase of cracking rate to the above factors is just the opposite. Affect significance order of three factors on each objective function is: drying temperature, drying time and vacuum degree. To secure good drying quality and high efficiency, all achieved regression models are optimized and comprehensively analyzed by nonlinear optimization method. The optimal combination of parameters within test range is determined by computation: if paddy rice with original moisture content of 21.7% (w.b) is dried to safe level, when drying temperature is at 36.2℃, vacuum degree is 0.067MPa, drying time is 2.32h, increase of cracking rate will be 2%, head rice yields will up to 77%. This can be used as an instruction for determining drying parameter in designing vacuum drier.(3) By near infrared technique and fuzzy neutral theory, tests taste quality of brown rice at different parameters by taste meter. Effect of original paddy rice moisture content, vacuum degree and drying temperature on taste value is studied by 23 orthogonal rotary tests and taste value regression model is set up. Effect of drying parameters interaction on post-drying quality is studied by response surface analysis in drying condition based on MATLAB. The research indicates that: in condition of original moisture content of 18-26%(w.b), vacuum degree of 0.03-0.07MPa, drying temperature at 35-45°C, effect of 3 factors on taste value is: drying temperature>original moisture content>vacuum degree; effect of drying temperature is notable, drying temperature can not be higher than 45°C, otherwise taste quality will drop dramatically. Vacuum degree has a minor impact on taste value. Therefore, excess vacuum pressure is not appropriate in drier designing. In addition, new model between taste value of post-drying paddy rice and drying parameters is set up based on BP neural network and has a better fitting result than orthogonal regression model with a average relative error 2.88%. Therefore, BP neutral network can serve as a new model to illustrate paddy rice vacuum drying taste value. Instructs technology route design of vacuum drying and reasonable selection of drying parameters.(4) Studying is made on post-drying brown rice rate taste change rule, gelatinization and cooking properties through thin-layer drying test. The test indicates that drying temperature should be controlled lower than 45°C. Otherwise as a result of main elements and interior microstructure change, taste change rate will drop dramatically, gelatinization temperature up, breakdown value down, recooling value up, heating moisture absorption up, rice hardness up, viscidity down, taste will become worse. In addition, drying temperature has great impact on main physicochemical index of brown rice. With the increase of drying temperature, amylose and fatty acid content will rise accordingly, yet protein content is not sensitive to temperature change. Both amylose and fatty acid content is negative related to eating taste. It also indicates that the higher the original moisture content, the lower the drying temperature. Only in this way can post-drying quality be secured, which is relevant to thermal denatured temperature of paddy rice. Consequently, variable temperature drying technology that first low then high should be employed in paddy rice drying process.(5) Sets up a mathematic model between brown rice physicochemical index (amylose content A, protein content P, moisture content W and fatting acid content F) and taste value (TV) by parameter estimation. Zheng's white rice model uses for reference. The method of parameter estimation sets up a forecast model between brown rice taste value and its physicochemical index. Thus through routine laboratory test, taste character of paddy rice can be determined to monitor production process, which will be instructive for drying operation.(6) Evaluates and analyses power consumption, economic benefits and environment benefits during paddy rice low temperature vacuum drying process. It shows that within the range of test parameters, unit power consumption of vacuum drying is low, which will save 10%-20% compared to hot air drying. And during the whole drying process, unit power consumption is step varied, average power consumption changes with vacuum degree and drying temperature. The higher the vacuum degree and the higher the drying temperature, the lower the required average power consumption will be. It will provide reference for energy saving design of vacuum drier. The economic benefits analysis on paddy rice low temperature drying shows: though invest capital and service cost for vacuum drying equipment is higher compared to hot air drying, but quality of vacuum post-drying paddy rice is better and has more profit margin. As a result, for the long-term benefit, it will bring about more economic value and will have a more extensive prospect. The environment impact analysis on paddy rice low temperature drying shows: paddy rice vacuum drying has minor impact on environment, lower powder dust and lower heat dissipation than hot air drying. Therefore, low temperature vacuum drying is favorable for ambient environment protection and workers'bodies and minds.
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