稻谷热风干燥缓苏工艺参数优化与试验

为提高稻谷干燥特性与营养品质,该研究探究了缓苏温度、缓苏起始时刻、缓苏时长、缓苏循环次数等缓苏工艺参数对稻谷爆腰增率、整精米率、蛋白质质量分数与脂肪酸值等干燥品质指标的影响。首先,通过单因素试验分析了稻谷干燥品质随缓苏工艺参数的变化趋势,得出爆腰增率、整精米率、蛋白质质量分数与脂肪酸值的权重均大于20%,为稻谷缓苏干燥的关键性指标;其次,通过隶属函数模型确定影响稻谷干燥品质的主要因素为：缓苏温度、缓苏起始含水率与缓苏时长;最后,以缓苏温度、缓苏起始含水率、缓苏时长为试验因子,采用Central-Composite试验,通过建立回归模型分析了各试验因素与品质指标之间的相互关系并阐释结果产生的原因。结果表明：优化参数组合为缓苏温度45 ℃、缓苏起始含水率21%、缓苏时长1.61 h,此参数组合下稻谷干燥后的爆腰增率6.63%、蛋白质质量分数5.39%、脂肪酸值11.68%,验证试验结果与优化结果间相对误差为2.97%。研究表明,优化后的缓苏干燥工艺明显改善了稻谷干燥品质,该结果可为生产实践及深入探究稻谷品质变化机理提供理论基础。     

基于玻璃化转变的稻谷爆腰产生机理分析

稻谷爆腰是造成碎米的主要原因之一,为此,人们一直在探讨产生稻谷爆腰的机理.介绍了一种新的稻谷爆腰机理--基于玻璃化转变的稻谷爆腰机理.根据该机理分析了稻谷干燥和缓苏过程中爆腰的产生原因.基于玻璃化转变的稻谷爆腰机理,干燥过程中,当稻谷颗粒由外表向里形成玻璃态时,颗粒内部形成玻璃态和橡胶态两个部分.由于玻璃态和橡胶态的弹性模量和膨胀系数有很大的差别,因此在一定水分梯度条件下,银纹(微裂纹)从两者交界处产生,并可能进一步扩展成爆腰.稻谷缓苏过程中,颗粒内部如果存在橡胶态区,则其不同含水率部分就会在不同水分梯度下分别进入玻璃态,如果此时水分梯度足够,就会引起各部分不均匀收缩和各不相同的应力、应变,这样使玻璃态层产生大量银纹,银纹持续生长即产生爆腰.最后,介绍了两种控制稻谷爆腰的干燥工艺--变温干燥工艺和高温干燥工艺.     

磁场对稻谷干燥特性的影响

针对稻谷的传统干燥过程存在能量消耗大,发芽率低和爆腰率高等缺点,把磁场引入稻谷干燥过程中,能快速而经济地降低稻谷的水分并保持其良好品质。采用永磁场强化稻谷的干燥过程,比较干燥时间、发芽率和爆腰率的变化。结果表明：在相同功率下稻谷在磁场中干燥时间比无磁场时缩短5％,平均失水速率加大,稻谷的发芽率有所升高、爆腰率下降;随着干燥功率的增加,磁场对稻谷的发芽率和爆腰率的影响加大。在试验基础上探讨了磁场对稻谷干燥过程的影响机理。     

高水分稻谷干燥工艺试验研究

针对中国南方地区稻谷收获季节需及时干燥高水分稻谷的市场要求,采用试验方法,在分批循环式稻谷干燥机上试验了低恒温干燥、变温干燥和变温干燥过程中增加缓苏时间的三种干燥工艺。依据试验结果,分析稻谷含水率、干燥介质温度、稻谷温度、缓苏烘干时间比等参数之间的联系与相互作用。试验表明：稻谷含水率高于21%时,降水速率可大于每小时1%,可采用60～70℃的介质。当稻谷含水率小于18%时,介质温度应小于60℃,降水速率小于每小时1%。当高水分稻谷进行了3～4次烘干缓苏后,利用中间缓苏仓增加缓苏时间,使稻谷内部与表层的温度、水分趋于平衡,有利于改善烘后品质和后续工艺的干燥降水。该结论对高水分稻谷干燥工艺设计和设备研制具有实用参考价值。     

水稻混流干燥工艺的试验研究

混流式粮食干燥机是当前中国应用最广泛的粮食干燥机机型,但由于水稻干燥的特殊性,横流循环干燥机基本上是水稻干燥的唯一机型,这造成国内粮食干燥机的利用率普遍不高。为了充分提高现有混流式干燥机的利用率,利用移动式混流粮食干燥机进行了稻谷的干燥试验,研究了不同的风温、不同的干燥缓苏比对稻谷爆腰率及降水速率的影响规律。研究结果表明：稻谷混流干燥过程中应有必要的缓苏。当干燥缓苏比一定时,烘干温度与爆腰率增值存在线性关系;当烘干温度一定时,干燥缓苏比与爆腰率增值存在线性关系;采用混流干燥机干燥稻谷时热风温度可以高于横流干燥机5～10℃,降水速率最高可达1.3%/h。     

增湿加热稻谷干燥工艺的试验研究

研究了增湿加热的稻谷干燥工艺减少爆腰产生的机理。采用增湿的热空气干燥稻谷时,可以减少稻谷内部水分梯度,降低了内部应力。通过试验分析了稻谷干燥爆腰率增值与空气温、湿度关系,提出了合理的增湿加热干燥工艺参数。     

基于ANFIS的稻谷深床干燥爆腰率增值预测模型的研究

为提高稻谷干燥爆腰率增值预测的精度,采用自适应神经模糊推理系统(ANFIS)建立了稻谷深床干燥爆腰率增值预测模型.经试验数据检验,爆腰率增值预测值的最大误差为14.57%,最小误差为1.68%,平均误差为5.68%,预测精度达到了94.32%.结果分析表明,该模型泛化能力强,预测精度高且可简便预测干燥参数对稻谷干燥爆腰率增值的影响,有助于准确认识爆腰率增值随干燥参数的变化规律,为干燥参数的优选和稻谷干燥品质的控制提供了依据.     

高湿稻谷逐步升温干燥工艺试验研究

针对南方双季稻区高湿稻谷,对薄层谷粒的水分迁移过程进行动态跟踪,发现降速干燥过程具有明显的二段性。通过测定深层干燥中气流状态的变化过程,由热质衡算得出深层干燥特性曲线,研究了高湿稻谷深层干燥中的热质运动规律,结合稻谷爆腰率的变化规律,给出了合理的干燥工艺     

微波干燥稻谷和糙米的能量效率

对厚层及均匀平铺载荷条件下稻谷和糙米的微波干燥过程中的能量效率问题进行了研究。根据本研究的实验数据,如果干燥方法和工艺安排合理,微波干燥稻谷和糙米可达到较高的能量效率,甚至可达到高于传统干燥方法。在微波干燥过程中安排一定的间歇时间以发挥照射后期作用、间歇照射作用或缓苏作用,可以有效地提高微波干燥过程中的能量效率。有2h间歇的干燥工艺的能量效率是没有间歇的干燥工艺的能量效率的5倍。载荷形式对微波干燥过程中的能量效率影响也较大。微波干燥中建议采用薄层载荷形式。     

增湿加热稻保干燥工艺的试验研究

研究了增湿加热的稻谷干燥工艺减少爆腰产生的机理，采用增湿的热空气干燥稻谷时，可以减少稻谷内部水分梯度，降低了内部应力。通过试验分析了稻谷干燥爆腰率增值与空气温、湿度关系，提出了合理的增湿加热干燥工艺参数。     

Preservation of Head Rice Yield Under High‐Temperature Tempering as Explained by the Glass Transition of Rice Kernels

Tempering has been shown in literature to preserve head rice yield after heated air drying. Most reported tempering work was done adiabatically at a temperature below that for rice drying. In this study, the effect of a tempering temperature above that for rice drying on the whole kernel percentage was investigated. High‐temperature tempering is an effective way to preserve the whole kernel percentage for rice dried at a raised temperature (e.g., 60°C) at which head rice yield would otherwise incur a pronounced reduction without tempering. Tempering helped relax the strains inside a rice kernel induced by internal stresses developed during the drying process. The strains had two components (elastic component and viscous component) due to the viscoelasticity of rice kernels. The reduction of moisture content gradients inside a rice kernel during tempering helped eliminate the elastic component of the strains due to the elasticity of the rice kernel. Results showed that to effectively eliminate the viscous component of the strains due to the viscosity of the rice kernel, tempering temperatures must be kept well above the glass transition temperature of the rice kernel. A tempering temperature below the glass transition temperature failed to preserve the whole kernel percentage. For example, with a tempering temperature of 80°C and a tempering duration of 80 min, the whole kernel percentage for the rice with an initial moisture content of 20.4% wb dried at 60°C and 17% rh for 120 min down to 10.2% wb (10.2 percentage points of moisture content removal in one drying pass) was preserved to a level close to that of the control sample.     

水稻顺流干燥工艺缓苏过程的研究

系统地分析了水稻顺流干燥工艺中,谷物温度、降水幅度、初始水分对缓苏过程的影响,通过计算机求解扩散模型,揭示出缓苏与提高干燥能力之间的关系。该文求解出的谷物内部传热传质模型的模拟值与实验结果相符。     

Thin-Layer Models for Intermittent Drying of Rough Rice

Thin-layer drying data was collected for rough rice from 108 treatments. Four thin-layer equations were derived by regression analysis to predict the drying behavior for deep-bed drying of rough rice. The proposed equations consider the effects of drying air temperature, drying air absolute humidity, drying time interval, and tempering time interval. Drying air temperature range was 35–65°C, drying air absolute humidity range was 10–26 g/kg of dry air, drying time interval was 5–15 min, and tempering time interval was 40–120 min. In addition, the variations of moisture content with tempering time intervals and drying time intervals were considered. Within the tempering time interval range of 40–120 min and drying time interval of 5–15 min, a shorter drying time interval and a longer tempering time interval are preferable in a recirculating type rice dryer.     

Drying Enhancement of Rough Rice by an Electric Field

Corona discharge produced by a multiple point-to-plate high-voltage electric field (HVEF) was used to investigate the enhancement of rough rice drying and its effect on rice fissuring and germination. The HVEF consisted of a 16 needle point cathode with a direct current power supply, and a grounded stainless steel plate anode. The drying rate of the treated rough rice was notably greater than that of the control, and the drying rate was described by an exponential model. There was a good agreement between the predicted and experimental results (coefficient of determination R²=0·98). The electric field treatment significantly enhanced drying but had no effect on rice fissuring at a lower temperature. The average drying rate of the treated rice increased 2·83, 1·59 and 1·63 times at 25, 40 and 50°C, respectively, compared to the corresponding control. The drying rate also increased with increasing electric field strength. The electric field treatment did not have significant effects on the percentage of kernels having heavy fissures or the germination rate of rough rice (probability P>0·05). The total number of fissured kernels in the treated sample was increased compared to the control.     

Effect of Infrared and Conventional Drying Methods on Physicochemical Characteristics of Stored White Rice

The objective of this study was to investigate the effect of infrared (IR) drying followed by tempering and natural cooling on the change of physicochemical characteristics of white rice during up to 10 months of storage. The physicochemical characteristics of IR‐dried rice were also compared with those of conventionally dried rice. It took only 58 s to heat the rough rice from room temperature to 60°C with IR, and 2.17 percentage points of moisture was removed. After four months of storage, the increases in yellowness index, water uptake ratio, and volume expansion ratio of the rice dried with IR were 73.8, 63.9, and 55.3% those of rice dried with an ambient air drying method, respectively. IR drying slightly decreased the gelatinization temperature, enthalpy, and viscosities, reduced the changes in microstructure, and maintained cooking characteristics during storage. Therefore, the IR drying process is recommended to maintain the quality of white rice during storage.     

Relationship of Kernel Moisture Content Gradients and Glass Transition Temperatures to Head Rice Yield

The relationship of glass transition temperature T_g and moisture content (MC) gradient of rice kernels to head rice yield (HRY) variation was investigated. Mathematical models describing heat and moisture transfer inside rice kernels during drying were developed and solved using the finite element method. Moisture distributions inside a kernel were simulated and verified using thin-layer drying experiments, and the intra-kernel MC gradients during drying were accordingly determined and analysed. Results showed that in the glassy region, rice did not incur measurable HRY reduction after drying. However, when rice was dried in the rubbery region and then cooled down immediately without being tempered following drying, HRY decreased markedly after MC gradients exceeded certain levels. It was found in this study that the time when the percentage point of moisture removal reached a maximally allowable level before HRY decreased dramatically coincided with the time at which the curve of kernel MC gradients versus drying duration reached its peak. Such a relation was verified with the HRY data of two varieties (Cypress and M202) as measured in this study and cited from literature. The HRY trends for these two varieties were well explained through the behaviour of glass transition and MC gradients of rice.     

Infrared Drying Characteristics of Long‐Grain Hybrid,Long‐Grain Pureline,and Medium‐Grain Rice Cultivars

The objective for this study was to investigate the effectiveness of scaled‐up infrared (IR) heating followed by tempering steps to dry freshly harvested rough rice. An industrial‐type, pilot‐scale, IR heating system designed to dry rough rice was used in this study. The heating zone of the equipment had catalytic IR emitters that provided heat energy to the sample as it was conveyed on a vibrating belt. The sample comprised freshly harvested rough rice of long‐grain pureline (Cheniere), long‐grain hybrid (6XP 756), and medium‐grain (CL 271) cultivars at initial moisture contents of 23, 23.5, and 24% wb, respectively. Samples at a loading rate of 1.61 kg/m² were heated with IR of radiation intensity 5.55 kW/m² for 30, 50, 90, and 180 s followed by tempering at 60°C for 4 h, at a product‐to‐emitter‐gap size of 450 mm, in one‐ and two‐pass drying operations. Control samples were gently natural air dried in an equilibrium moisture content chamber set at relative humidity of 65% and temperature of 26°C to moisture content of 12.5% wb. The effects of IR treatments followed by tempering on percentage points of moisture removed, head rice yield, energy use, rice color, and pasting characteristics were evaluated. For all cultivars, percentage point moisture removed increased with increase in IR drying duration. For all rice cultivars, one‐pass IR treatments for 180 s resulted in head rice yield significantly lower than that of rice dried with natural air in the controlled‐environment conditions (P < 0.05). Energy required to dry rice increased with increase in drying duration. Viscosity values of all the experimental samples were significantly greater (P value < 0.05) than that of the control samples for all the cultivars, except those treated with IR for 180 s. There was a significant difference (P < 0.05) in the color index (ΔE ) of treated milled samples and the controls. In conclusion, the study provided information crucial to understanding the effects of scaled‐up radiant heating and tempering of rough rice on drying rates and rice quality for long‐grain pureline, long‐grain hybrid, and medium‐grain rice cultivars.     

稻谷颗粒内部水分迁移过程的有限元分析

研究稻谷颗粒内部水分的迁移规律,以求达到较好的烘干效果,是一个重要的问题。该文研制了可用于空间轴对称湿扩散问题的有限元计算软件,并用其对描述圆粒、长粒稻谷的３层圆球和椭球两种模型进行计算,得到了两种模型在干燥、缓苏状态下的湿度及其梯度场随时间变化规律。经分析发现,烘干温度较低时,椭球脱湿速度低于圆球,烘干温度较高时,两者差异变小。两模型的含水率（干基）及其梯度分布有明显差别。     

热风干燥联合真空降温缓苏提升黄秋葵干制品品质

为提升黄秋葵热风干燥产品品质,试验将真空降温缓苏技术应用于黄秋葵热风干燥过程中。研究了不同缓苏时长下黄秋葵干燥特性和品质指标的变化规律;利用Weibull分布函数分析缓苏处理对黄秋葵热风干燥过程中水分扩散机制的影响;采用一元非线性回归分析构建适用于黄秋葵真空降温缓苏-热风联合干燥过程中干燥特性和品质指标随缓苏时长变化的学数模型;以总干燥耗时、总干燥能耗、复水比、色相角以及总营养物质保存率为指标,对不同缓苏时长下的黄秋葵热风干燥进行加权综合评价。结果表明：缓苏处理能够提升黄秋葵热风干燥速率,且随着缓苏时长的延长其促进作用会增强;Weibull分布函数能够准确描述（R2>0.99且离差平方和χ2处于10-4数量级）黄秋葵真空降温缓苏-热风联合干燥过程中水分比随干燥时间的变化规律;常用函数一元非线性回归分析能够构建出黄秋葵真空降温缓苏-热风联合干燥过程中各干燥特性和品质指标随缓苏时长的变化规律的动力学模型;联合干燥过程中,缓苏60 min处理的综合评分值最高为0.55,在干燥温度和风速分别为60℃、1.5 m/s条件下,该缓苏时长较适合应用于黄秋葵热风干燥。研究表明,真空降温缓苏处理能够提升黄秋葵热风干燥的干燥速率和干燥品质,该文可为真空降温缓苏技术在高品质黄秋葵干制品工业生产上的应用提供理论依据。