【作者】 熊秀芳；

【导师】 郭康权；

【作者基本信息】 西北农林科技大学 ， 农产品加工及贮藏工程， 2012， 博士

【摘要】 大豆食品风靡全球，推动了豆制品加工业尤其是豆腐制造业的迅速发展。但是，作为豆腐的发源地和消费大国，我国的豆腐产业依然采用传统落后的生产方式。我国豆腐生产存在的主要问题有：缺少包装、卫生质量差、保质期短、产品质量不稳定、加工设备简陋、自动化水平低、工艺参数模糊、难于实现规格化和标准化生产。通电加热技术有利于提高豆腐生产过程的自动化水平，改善产品品质，因此本论文对豆浆通电加热凝胶过程中的电导率、动态流变特性、电阻率与动态流变特性之间的关系进行了研究，探索了通过测量电阻率来自动判断豆浆凝固终点，通过测量电阻率来实现动态流变特性在线自动检测的可行性。用有限元方法对豆浆通电加热过程中的温度场分布进行了模拟。在此基础上，完成了豆浆通电加热和豆浆凝固检测系统的设计，为研发通电加热家用豆腐机和商用大型豆腐机奠定了基础。通过上述研究得到的主要结论如下：（1）整个豆浆通电加热系统可用电容和电阻串联组成的等效电路表示。（2）豆浆在通电加热时的升温速率随着电场强度和频率的增加而增加；电场强度为6、12、18V/cm时，升温到90oC所需时间分别大约为：1400s，360s，170s；当电场强度为12、18V/cm时，升温曲线近似为指数曲线。（3）豆浆在通电加热凝固过程中的电导率随电场频率的增加而增加；豆浆的电导率与温度呈线性关系。（4）在通电加热时豆浆中间部位的温度分布比较均匀，靠近加热槽壁的温度较低，加热时间从50s增加到300s，豆浆的最大温差从3℃增大到20.2℃。加热槽吸热和散热是造成豆浆温度分布不均的主要原因。（5）豆浆在凝固过程中的弹性模量G’和黏性模量G"随时间的变化趋势相同：在凝胶反应初期凝胶速率很快，后期变慢。凝胶反应的均匀性与凝固温度有关，凝固温度为80℃时，凝胶反应速率变化比较均匀，制得的豆腐质构均匀。（6）可以将豆腐凝胶过程分为三个阶段：第一阶段是诱导阶段，温度从70~85℃，G’和G"变化不大；第二阶段是加速阶段，温度从85~95℃， G’和G"急剧增大；第三阶段是稳定阶段，当温度大于95℃，凝胶反应结束，豆腐凝胶形成，G’和G"趋于稳定。当温度小于70℃时，豆浆不能形成豆腐凝胶；当温度在87℃时，G’和G"相交，豆腐的凝胶温度为87℃。（7）豆浆在加热凝固过程中，电阻率、G’和G"随时间的变化曲线可以用连续一级反应模型模拟。可以用测量电阻率变化率（或电阻率）判断凝固终点，当电阻率变化率（或电阻率）基本不变时，凝固达到终点。（8）豆浆在加热凝固过程中，G、G与电阻率呈线性相关，凝固温度在75℃时，线性相关系数为0.981和0.980；80℃时，线性相关系数为0.996和0.979；85℃时，线性相关系数为：0.805、0.900。（9）设计了通电加热豆腐加工系统，主要对热电偶温度采集模块和阻抗测量模块进行了设计，实现了豆腐通电加热凝固在线检测。更多还原

【Abstract】 Soybean food is a very popular and healthy product all over the world. Soybeanprocessing industry, especially the tofu manufacturing industry has been developed rapidly.There are many problems in tofu making in China: no packaging, poor sanitation, short shelflife, changeable quality, simple equipment and lack of process standard. To improveautomation of tofu process and tofu’s quality, the paper mainly studied the conductivity ofsoymilk, dynamic rheological properties and the relationship between them during thecoagulation process by ohmic heating, and explored the feasibility of online automaticdetection of dynamic rheological properties by measuring the electrical conductivity. A finiteelement model was established and temperature distribution inside the heating tank wassimulated and validated. On the basis of those findings, tofu process system that can performohmic heating of soymilk and detect the coagulation the coagulation process of tofu wasdesigned, which laid a foundation for development of tofu manufacturing equipment usingohmic heating. In this paper, the following conclusions were drawn.(1) An equivalent circuit model was used to describe the soymilk-electrode systemconsisting of resistance and capacitance in series.(2) Heating rate during ohmic heating process of the soymilk was increased with theincrease of the frequency and electric field strength. The required times for heating soymilksample to90℃were1400s,360s and170s, respectively, when electric field strengths of6,12and18V/cm were applied.(3) Electrical conductivity of soymilk during ohmic heating process increased with theincrease of frequency. Electrical conductivity of soymilk was correlated linearly withtemperature.(4) During ohmic heating process, the temperature distribution in the center part ofsoymilk was more uniform than the other locations and the temperature close to the heatingtank walls was low. For heating time increasing from50s to300s, temperature differenceinside the soymilk increased from3℃to20.2℃. The main reason was heat loss from theheating tank to the ambient. (5) During the coagulation process of soymilk, elastic modulusG’and viscous modulusG"showed the same trends. In the initial stage, coagulation process was fast and thenbecame low. When the coagulation temperature was about80℃, better texture of tofu gelwas easily obtained.(6) The dynamic temperature scanning curve of soymilk during the coagulation processwas divided into three stages. The first stage was induction stage from70to85℃,G’and G"increased gradually. The second stage was acceleration stage from85to95℃,G’and G"increased rapidly. The third stage was stabilization stage over95℃,G’and G"changed minimally and coagulation reaction finished. In temperature below70℃,tofu gel was not formed. Especially at87℃, the intersection point ofG’andG"wasobserved indicating that phase transformation of tofu gel took place.(7) During the coagulation process of soymilk, resistivity-time,G’-time andG"-timecurves were fitted by a consecutive first-order reaction kinetics. It could be concluded thatcoagulation process finished when the volume resistivity remained unchanged. So electricalmeasurement provided an indirect method to judge the end point of coagulation process.(8) Good agreements betweenG’and resistivity were obtained during soymilkcoagulation process, the same as betweenG"and resistivity. High correlation coefficientswere obtained at different temperatures.(9) On the basis of the above findings, tofu process system by ohmic heating waspreliminarily developed, Thermocouple temperature measurement circuit and impedancemeasurement circuit were mainly designed and online detection of soymilk coagulationprocess was realized.更多还原