Horizontal gas pressure and temperature distribution responses to OSB flake alignment during hot-pressing

This study examines the effect of different flake alignments and densities on horizontal heat and gas flow from the mat centre to the edges. Robot-formed 660×660 mm single layer oriented-strandboard (OSB) mats, with various flake alignment distributions, were hot-pressed at 180°C to 10 mm thickness. Fine gauge thermocouple wire and steel capillary tubes were built into the mat to monitor core temperatures and gas pressures at ten locations across the board-plane. Results confirm the existence of lateral temperature and gas pressure gradients across the mat, with its edges having lower temperatures and pressures than its interior. Flake alignment was seen to skew the lateral gradients in the alignment directions, the extent being strongly influenced by interactions with mat density and the proximity to the edge. In highly aligned mats, lower temperatures and pressures were measured along the alignment direction than perpendicular to it, thus indicating greater heat and mass transfer along the flake alignment axis. These horizontal temperature and pressure distributions have practical implications on panel uniformity and quality as mat consolidation is dependent on internal conditions during hot-pressing.     

Internal temperature and pressure responses to flake alignment during hot-pressing

and 446 kg/m³, were pressed at 180 °C to a 10 mm thickness. Steel capillary tubes and thermocouple wires were inserted during mat forming at various thickness positions to monitor gas pressure and temperature. Panel density was found to positively affect thermal conduction, and negatively affect lateral and transverse permeability and transverse thermal convection. Flake-alignment positively affected lateral permeability, and negatively affected transverse thermal convection. These results indicate that lower density mats will heat faster and have lower internal gas pressures. Less aligned mats will initially heat faster and have higher internal gas pressures, but towards the end of pressing they will take slightly longer to heat.     

Internal temperature and pressure responses to flake alignment during hot-pressing

3 and 446 kg/m³, were pressed at 180 °C to a 10 mm thickness. Steel capillary tubes and thermocouple wires were inserted during mat forming at various thickness positions to monitor gas pressure and temperature. Panel density was found to positively affect thermal conduction, and negatively affect lateral and transverse permeability and transverse thermal convection. Flake-alignment positively affected lateral permeability, and negatively affected transverse thermal convection. These results indicate that lower density mats will heat faster and have lower internal gas pressures. Less aligned mats will initially heat faster and have higher internal gas pressures, but towards the end of pressing they will take slightly longer to heat.

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3 wurden zu 10 mm dicken Platten gepre?t. Stahl-Kapillaren und Me?fühler wurden w?hrend des Formens in verschiedenen Plattentiefen eingefügt, um Gasdruck und Temperatur zu messen. Die Plattendichte erh?ht die thermische Leitf?higkeit und erniedrigt die Gasdurchl?ssigkeit und Konvektion parallel und senkrecht zur Oberfl?che, d.h. Platten geringerer Dichte werden schneller erw?rmt und haben einen geringeren inneren Gasdruck. Weniger gut ausgerichtete Platten werden zun?chst schneller erw?rmt und zeigen einen h?heren inneren Gasdruck; gegen Ende der Pre?zeit brauchen sie jedoch l?ngere Zeit zum Aufheizen.

     

Regression modelling of two-dimensional internal temperature and gas pressure distributions during panel hot-pressing

Fitting a suitable regression model to often scattered experimental temperature and pressure data can aid in visualising and detecting underlying trends and behaviour. This paper presents the derivation of an empirical two-dimensional regression model for fitting to lateral temperature and gas pressure gradients during wood-composite panel hot-pressing. The regression model yielded a good fit of the spread of experimental measurements of internal temperature and gas pressure distributions during single layer oriented-strandboard hot-pressing, and provided an accurate fit to theoretical temperature and gas pressure distributions from a hot-pressing heat and mass transfer model. The regression model’s form and shape thus reflect a valid approximation of theoretical two-dimensional temperature and pressure distributions during hot-pressing, and give the regression an advantage over other general non-linear regressions. In addition to wood-composite hot-pressing, the regression model may have further applications in other similar processes of heating, drying or hot-pressing of rectangular materials.     

棉仁生坯低温压榨和浸出过程中组分含量的变化

为评估一种脱酚棉籽粕新工艺技术的可行性,对工艺过程中各阶段产品的关键成分(粗蛋白质、游离棉酚、赖氨酸和蛋氨酸等)进行了测定。以脱壳棉仁为原料,经低温压榨和己烷浸出制得脱脂粕,再经甲醇浸出制得脱酚棉籽粕。结果表明,脱酚棉籽粕保留了大部分赖氨酸和蛋氨酸,而粗蛋白质和游离棉酚含量分别为50.15%和0.008 2%;棉仁低温压榨结合己烷及甲醇浸出工艺是一种很有前景的优质脱酚棉籽粕生产技术。     

Changes in available methionine and tryptophan contents during cereal flake production

The changes in available methionine and tryptophan contents during industrial production of wheat, rye, barley and oat flakes have been investigated. The contents of available methionine and tryptophan were lowered significantly (p = 0.05) under conditions of technological processing. The total decrease in available methionine ranged from 13 to 26% and tryptophan from 10 to 20%. Hydrothermal treatment (120 °C, 60 min, 1.99 × 10⁵ Pa) of whole cereal grain had a considerable impact on the damage to available methionine, but the highest decrease in tryptophan was determined after the flaking process (70 °C, 0.3 mm gap). The total decrease in these amino acids correlates significantly (p = 0.001) and positively with values for lipid oxidation products, expressed on a total lipid basis, in cereal flakes as well as in untreated cereal grains. Available methionine and tryptophan contents in proteins of cereal flakes equalled or exceeded requirements for adults recommended by the FAO 1985 reference pattern. © 2000 Society of Chemical Industry     

Use of aluminum oxide nanoparticles in wood composites to enhance the heat transfer during hot-pressing

In this study, the effect of Al₂O₃ nanoparticles on the heat-transfer properties of medium density fiberboard (MDF) was investigated. Al₂O₃ nanoparticles were added at two levels (0.5 and 1.0 %) by percentage weight fraction of dry wood fibers to urea formaldehyde (UF) resin. The core temperature profile and thermal conductivity tests showed higher rate of heat transfer after addition of nanoparticles, subsequently it has improved the bonding strength of MDF. Differential scanning calorimetry was used to estimate the heat evolved during the exothermic reaction of UF resin curing. Scanning electron microscopy shows good dispersion of nanoparticles. Thermal conductivity of UF resin improved after nanoparticles addition proved by KD2 pro results. Modulus of rupture, modulus of elasticity and thickness swelling also improved.     

高温高压作用后叶蜡石腔体密度分布及流动规律探讨

文章对高温高压合成金刚石后叶蜡石块不同部位进行了密度测试,通过对叶蜡石不同方向密度的对比分析,讨论和分析了叶蜡石在高温高压下的塑性流动规律。结果表明,叶蜡石外部密度大于内部密度,有从外部向内部流动的趋势;上部密度大于下部密度,有从上向下流动的趋势;叶蜡石密封边中部密度大于顶角处密度,有从密封边中部向顶角处流动的趋势;叶蜡石外表面顶角密度大于中心密度,与三顶锤接触的叶蜡石块顶角位置附近区域是叶蜡石块中密度最高部位,这是在顶锤之间的叶蜡石密封边已建立起高压密封情况下造成的。     

跑鞋类型对走跑过程中足底压力分布的影响

采用鞋垫式足底压力测试系统对15名女性受试者进行动态足底压力测试。针对行走和慢跑2种状态下受试者穿着极简跑鞋和普通慢跑鞋时前掌、中足和后跟区域足底峰值压强、压强时间积分、接触面积、支撑时间进行对比。结果显示：穿着极简跑鞋和普通慢跑鞋相比,行走和慢跑2种状态下,尽管前者后跟峰值压强均小于后者（P〈0．05）,但前者中足峰值压强、中足压强时间积分、前掌接触面积等均大于后者（P〈0．05）。与穿着普通慢跑鞋相比,穿着极简跑鞋时,会增大中足区域峰值压强,长期穿着还容易导致足部的不适,因此,跑步者应该避免长时间穿着极简跑鞋。     

Numerical simulation of stress distribution in heterogeneous solids during high pressure processing

It is generally assumed that the internal stress distribution in foods under high pressure processing (HPP) is uniform, which may not be true for solid foods with hard inclusions, like meats with bones or for particulate foods. Our objective was to simulate the internal stress distribution during HPP of a model heterogeneous solid food made with a gel and a wood inclusion and determine how mechanical properties affect the internal stress distribution. It was determined that hydrostatic pressure decreased and shear stress was generated at the interface between the soft solid and the hard inclusion. The differences in pressure and shear stresses increased as the shear modulus of the soft solid increased. Our studies suggest that a better understanding of the mechanical properties that affect the development of the internal stress field is needed, since they could affect the achieved microbial inactivation levels at different locations in HPP solid foods.     

Prediction of pathogen growth on iceberg lettuce under real temperature history during distribution from farm to table

The growth of pathogenic bacteria Escherichia coli O157:H7, Salmonella spp., and Listeria monocytogenes on iceberg lettuce under constant and fluctuating temperatures was modelled in order to estimate the microbial safety of this vegetable during distribution from the farm to the table. Firstly, we examined pathogen growth on lettuce at constant temperatures, ranging from 5 to 25 degrees C, and then we obtained the growth kinetic parameters (lag time, maximum growth rate (micro(max)), and maximum population density (MPD)) using the Baranyi primary growth model. The parameters were similar to those predicted by the pathogen modelling program (PMP), with the exception of MPD. The MPD of each pathogen on lettuce was 2-4 log(10) CFU/g lower than that predicted by PMP. Furthermore, the MPD of pathogens decreased with decreasing temperature. The relationship between mu(max) and temperature was linear in accordance with Ratkowsky secondary model as was the relationship between the MPD and temperature. Predictions of pathogen growth under fluctuating temperature used the Baranyi primary microbial growth model along with the Ratkowsky secondary model and MPD equation. The fluctuating temperature profile used in this study was the real temperature history measured during distribution from the field at harvesting to the retail store. Overall predictions for each pathogen agreed well with observed viable counts in most cases. The bias and root mean square error (RMSE) of the prediction were small. The prediction in which mu(max) was based on PMP showed a trend of overestimation relative to prediction based on lettuce. However, the prediction concerning E. coli O157:H7 and Salmonella spp. on lettuce greatly overestimated growth in the case of a temperature history starting relatively high, such as 25 degrees C for 5 h. In contrast, the overall prediction of L. monocytogenes under the same circumstances agreed with the observed data.     

微波加工过程中食品温度分布规律及其均匀性研究

微波加热因其加热速度快、操作简便等特点,已成为食品加工业的研究热点之一。虽然微波加热技术在过去几十年里发展快速,但加热不均匀仍是微波加工过程中存在的主要问题,许多学者对其温度分布规律、加热均匀性改善等方面做了大量研究。该文首先介绍了微波加热技术在食品行业中的发展过程及应用优势,并阐述了微波加热的基本原理、介电特性及穿透深度的含义,在此基础上按照食品种类,对微波加工过程中发现的温度分布不均的现象进行归纳,并对现有改善均匀性的方法进行总结,改善方法包括提高电场分布均匀性、改善食品组分及几何尺寸、优化环境媒介及包装形式,最后对改善食品微波加热的均匀性研究提出几点展望。     

Proteolysis,protein distribution and stability of UHT milk during storage at room temperature

Proteolytic degradation and distribution of caseins and whey proteins between the soluble and colloidal phases were studied in six batches of commercial UHT milk (three skim and three whole milks) during storage at 25 ± 2 °C. For that purpose, at 30 day intervals, milk samples were ultracentrifuged and the pellets and supernatants analysed by capillary electrophoresis and SDS‐PAGE. Samples were also visually examined for signs of gelation. Extensive proteolytic degradation of the micellar fractions and severe changes in the electrophoretic pattern of the proteins present in the serum fractions were observed in all the batches. A higher proportion of denatured whey proteins not attached to the micelle surface was found in the skim milk samples as compared with the whole milk samples that could provide less resistance against gelation. In addition to β‐Lg, para‐κ‐casein was also found in the serum fraction. A high proteolytic activity against κ‐casein could be responsible for the hydrolysis of serum‐liberated κ‐casein or could have enhanced the liberation of β‐Lg–para‐κ‐casein complexes through proteolysis of micellar κ‐casein. © 1999 Society of Chemical Industry     

超薄气膜润滑压力分布的计算方法

根据超薄气体润滑方程的特点,采用有限差分法离散方程,改变迭代主元,建立了计算超薄气膜润滑压力分布的计算方程和误差估计方程,并利用VC 编程技术求解方程,实现了磁头飞行高度在10 nm以下的压力分布计算.     

不同形状包装食品在微波加热过程中的三维温度分布

为了考察矩形、六边柱形、圆柱形三种几何形状样品在微波加热过程中的温度分布情况,以含水量为99%的凝胶琼脂为研究对象,依据Maxwell方程和传热方程,用有限元方法建立了三维温度分布模型。并通过考察各几何形状包装食品的表面、切面和内部三维等温面等情况,确定了各部位温度差异情况。结果表明:相同体积、不同几何形状的样品,由于各表面的形状不一样,进入样品的微波能量在内部形成不同的复杂聚焦区,温度分布有明显区别。微波加热60s后,矩形、六边柱形和圆柱形样品内部各出现4个、7个和1个热点区域,高低温差分别为23.87、21.92、25.10℃。实验值和计算值呈现良好的一致性。温度分布特性与样品的介电特性有关,对于含水量为99%的凝胶琼脂,在10~45℃范围内,频率为2450MHz的微波穿透深度在10~25mm之间。三维模型可较为清晰地描述不同几何形状包装食品的内、外部温度分度,为改善微波食品加热过程中的温度不均匀和优化包装食品质量提供基础数据和理论基础。      

颗粒状态对米饭微波复热过程温度分布的影响

为了研究和开发不同颗粒状态的方便米饭,运用微波工作站和光纤探针在线监测的方法,以碎米、完整大米加工的不同厚度/半径比(L/R)圆柱型尺寸的米饭作为研究对象,综合考察物料颗粒状态对米饭微波复热过程中温度分布规律以及各位置点微波吸收功率规律的影响。结果表明,两类米饭的温度变化规律基本一致:随样品L/R的增加,高温区域从米饭内部逐渐向样品上下表面转移。大米颗粒状态可以较为明显地影响米饭温度分布的对称性。完整的颗粒状态在L/R<1.5时,会使微波功率在米饭内部产生更强的中心聚集吸收效果;在L/R≥1.5时,其会引起米饭复热时边角温度过高。