Experimental study on drying of pear slices in a convective dryer

The experiments were conducted on pear slices with thickness of 5 mm at temperatures of 50, 57, 64 and 71 °C with an air velocity of 2.0 m s^?1. Prior to drying, pear slices were pretreated with citric acid solution (0.5% w/w, 1 min, 20 °C) or blanched in hot water (1 min, 85 °C). Also, the untreated samples were dried as control. The shortest drying time of pear slices was obtained with pretreatment with citric acid solution. It was observed that whole drying process of pear slices took place in a falling rate period. Four mathematical models were tested to fit drying data of pear slices. According to the statistical criteria (R², χ² and RMSE), the Midilli et al. model was found to be the best model to describe the drying behaviour of pear slices. The effective diffusivity of moisture transfer during drying process varied between 8.56 × 10^?11 and 2.25 × 10^?10 m² s^?1, while the activation energy of moisture diffusion in pear slices was found to be 34.95–41.00 kJ mol^?1.     

Mathematical modelling of hot air drying of sweet potato slices

The effect of air dry bulb temperature, air relative humidity, air velocity and sample thickness on the thin-layer air drying of sweet potato slices was investigated. The drying rate curves consisted of two approximately linear falling rate periods and contained no constant rate period. Several mathematical models were fitted to the drying rates of sweet potato slices under a range of drying conditions. It was found that the modified Page equation best described the thin-layer air drying of sweet potato slices down to a moisture content of 10% dry basis. Correlations were also determined for the slope and intercept of the modified Page equation in terms of the experimental variables.     

The glass transition approach to determination of drying protocols for colour stability in dehydrated pear slices

Colour stability in dehydrated fruit is often attained by use of sulphite treatment to inhibit browning during processing and storage. However, colour stability may be able to be conferred on products by ensuring that the molecular mobility is restricted in dried fruit by drying to sufficiently low moisture to raise the glass transition temperature (under mild conditions). Pear slices were dehydrated at a low temperature (40 °C) to a low final moisture content (below 5%) after it was shown that, for a moisture content of 22% (which is the moisture content of commercial dehydrated pears) pretreatment with sulphite would be necessary to stabilise the colour. The use of longer drying regimes resulted in a moisture content where the food matrix would be closer to the glassy state and conferred colour stability on the dehydrated product compared to a product containing more moisture. For these low‐moisture products, pretreatment with sulphite would not be necessary to preserve colour stability.     

梨片酶褐变控制方法的研究

探讨了抗坏血酸在不同温度、时间和浓度条件下,对梨片多酚氧化酶的抑制效果。实验表明,该酶活性在1%～1.5%的抗坏血酸溶液中显著降低。1%抗坏血酸溶液在pH1或pH2的条件下,能显著抑制酶活性;在60～70℃范围内15min能明显降低酶褐变,延长浸泡时间也能控制酶褐变。     

Experimental investigation and theoretical modelling of absolute convective drying of starch

Starch is a granular, hygroscopic, capillary-porous material with an intricate structure. It is known that drying starch till 0% moisture is a physical treatment that modifies starch properties. However, depending upon the heat and mass transfer mechanisms prevalent in the drying system, starch properties are modified in a unique manner. Therefore, the drying operation itself needs to be thoroughly understood. Potato starch was treated in an oven at three high-temperature levels (110, 130 and 140 °C) and for three sample thicknesses (2.8, 5.6 and 11.2 mm). It was observed that drying of starch with moisture content in the hygroscopic range occurred only in the 2nd falling rate period. Two existing theoretical, drying models, one based upon the Krischer Theory and the other based upon the Percolation Approach, were considered apt for modelling the oven drying. Both models describe the experimentally measured moisture profiles well. During drying, transition from one water sub-population to another was smooth. The effective diffusion coefficient decreased with moisture content and increased with temperature, but did not follow Arrhenius’ Law above 130 °C.     

Kinetics of drying apple slices

The process of drying apple tissue under most conditions may be divided into two main periods viz primary and secondary falling-rate periods. For most drying conditions the secondary falling-rate period is longer and has the greater influence on the overall rate. When the drying capacity of the air is very low an initial constant-rate period may occur. The drying rate constant varies with temperature with an Arrhenius-type relationship and the available evidence suggests that temperature-induced changes in the rate may be caused by vapor pressure changes Air velocity and relative humidity have their greatest influence in the early part of the drying process Towards the end of drying, internal resistance to water movement becomes rate-controlling, and external factors have little influence. An explanation of the observed drying characteristics of apple tissue in terms of the diffusion of water vapour through a ‘skin’ of dried cells is suggested.     

红外辐射干燥胡萝卜的试验研究

采用红外辐射干燥的方法,对胡萝卜片进行了不同条件下的干燥试验,研究红外辐射干燥法对不同干燥参数条件下胡萝卜片的含水率及颜色变化,结果表明物料的厚度、加热温度对干燥含水率变化影响较大,而物料的厚度、加热温度和辐照距离对胡萝卜片的颜色变化均很大。在胡萝卜片厚度为4.5mm,辐照距离为12cm,物料温度55℃时,在保证干燥品质较好的条件下,效率最高。     

Infrared drying of button mushroom slices

The effect of different infrared power levels on the drying kinetics of button mushrooms was investigated. Mushroom slices were dried at infrared power levels of 83, 125, 167, and 209 W. The power level affected the drying and rehydration characteristics of mushroom slices. Drying time was reduced from 300 min to 40 min as the infrared power level increased from 83 to 209W. Mathematical models frequently used to represent drying of agricultural products were fitted to experimental data of mushroom drying. The parabolic model was the best for representation of mushroom drying. Effective moisture diffusivity varied from 3.81×10^?10 to 4.20×10^?9 m²/s over the infrared power levels used. The activation energy was estimated using a modified Arrhenius-type equation and calculated to be 7.55 kW/kg.     

热风干燥蘑菇片的工艺研究

目前脱水蘑菇片脱水工艺中的产品质量指标衡量多为感官的实验所确定,且侧重于某种工艺条件对产品的质量影响。本文以复水率、水分含量、色泽、蛋白质含量、灰分含量作为衡量指标,采用正交实验对蘑菇片热风干燥的各项工艺条件(护色剂的选择,预煮时间的选择,前期干燥时间,后期干燥时间)分别进行优化,得到综合最优工艺为0.3％NaSO3与柠檬酸混合作为护色剂,不经预煮,前期干燥(35℃)2h,后期干燥(55℃)11h。     

枣片真空干燥方式的比较研究

以木枣为原料,采用真空冷冻和真空低温2种干燥方式加工枣片,探讨比较了2种方式加工的枣片的Vc含量、复水率、褐变度,并且对枣片进行了综合的感官评价。研究表明,真空冷冻干燥的枣片在Vc、褐变度、复水率方面优于真空低温干燥,但冷冻干燥的时间长,成本高。两种真空干燥方式得到的枣片综合评价得分相同,冷冻干燥的枣片色泽、外观、气味均好于低温干燥,但酥脆度较差,口感不如真空低温干燥的枣片。     

Experimental investigation of jet drying of birch and spruce veneers and modelling with a simplified approach

The paper presents experimental results of jet drying tests of birch ( Betula pendula) and spruce ( Picea abies) veneers at temperatures of 140°C and 190°C. Results include drying rates for 1.5 mm thick birch veneers as well as 1.5 mm and 2.6 mm thick spruce veneers of both heartwood and sapwood. Based on the test results, material parameter values for a simplified drying model are assessed. The model is based on the use of an effective diffusion coefficient and an effective surface emission coefficient, which values are calibrated to fit to the experimental results. It is observed, that separate model parameter sets are needed for the two different species but also for occurrence of heartwood or sapwood (spruce) and different thickness values of veneers. Zusammenfassung Das Manuskript stellt experimentelle Ergebnisse von Schnelltrocknungstests bei Birken- und Fichtenfurnier vor, bei Temperaturen zwischen 140°C und 190°C. Die Ergebnisse beinhalten Trocknungsraten für 1,5 mm dickes Birken- sowie 1,5 bis 2,6 mm dickes Fichtenfurnier sowohl von Kern- als auch von Splintholz. Basierend auf den Testergebnissen wurden materielle Parameterwerte für ein vereinfachtes Trocknungsmodell geschätzt. Das Modell basiert auf der Verwendung eines effektiven Diffusionskoeffizienten sowie auf einem effektiven Oberflächen-Emissionskoeffizienten, dessen Werte jeweils kalibriert werden, um sie den experimentellen Werten anzupassen. Es wurde beobachtet, dass getrennte Modell-Parameter-Sets für die beiden verschiedenen Holzarten benötigt werden und ebenso für das Vorkommen von Kern- und Splintholz (Fichte) und die verschiedenen Dichtewerte von Furnierholz.     

Experimental investigation of jet drying of birch and spruce veneers and modelling with a simplified approach

The paper presents experimental results of jet drying tests of birch ( Betula pendula) and spruce ( Picea abies) veneers at temperatures of 140°C and 190°C. Results include drying rates for 1.5 mm thick birch veneers as well as 1.5 mm and 2.6 mm thick spruce veneers of both heartwood and sapwood. Based on the test results, material parameter values for a simplified drying model are assessed. The model is based on the use of an effective diffusion coefficient and an effective surface emission coefficient, which values are calibrated to fit to the experimental results. It is observed, that separate model parameter sets are needed for the two different species but also for occurrence of heartwood or sapwood (spruce) and different thickness values of veneers.     

热泵干燥温度对柿子片干燥特性及品质的影响

目的:探明热泵干燥温度对柿子片干燥特性和品质的影响。方法:以柿子为原料,研究干燥温度对柿子片干燥时间、干燥速率,以及干制品色泽、硬度、复水率、单宁、总糖、维生素C和总酸的影响。结果:当热泵干燥温度为65℃时,柿子片干燥速度快,干燥时间短,此时干燥产品色泽鲜艳,与鲜果色泽相近,复水性能好,硬度最低,口感最好,总酸和维生素C含量最高,总糖等营养成分含量适中。结论:柿子片的最适热泵干燥温度为65℃。     

微波气流预干燥和微波真空组合干燥生产甜瓜片

研究了微波气流预干燥(MAD)和微波真空(MVD)组合干燥甜瓜片,表明这是一种有潜力的干燥方法。样品先通过微波气流干燥脱去一些自由水,然后通过微波真空干燥到最终水分含量低于7%(湿基)。甜瓜片利用微波气流(MAD)和微波真空(MVD)组合干燥后测得Vc保留率和物性(收缩率,色差,质地和复水率)然后同冷冻干燥(FD)、微波气流干燥(MAD)、气流干燥(AD)和常规热风干燥(HAD)分别进行比较,表明,MA+MVD方法干燥的甜瓜片的Vc保留率接近冻干的甜瓜片,大大优于传统的热风干燥。当前方法干燥的样品表现出的复水率、色差、质构与冻干的产品非常相似,但是出现了明显的膨化现象,然而,这正是生产果蔬脆片所期望得到的结果。     

Convective hot air drying of blanched yam slices

In this study, a laboratory convective hot air dryer was used for the thin layer drying of blanched yam slices and experimental moisture ratio was compared with Newton, Logarithmic, Henderson and Pabis, modified Henderson and Pabis, approximation of diffusion, modified page 1, two-term exponential, Verma et al. and Wang and Singh models. Among all the models, the approximation of diffusion model was found to satisfactorily describe the kinetics of air-drying of blanched yam slices. The increase in air temperature significantly reduced the drying time with no constant rate period but drying occurs in falling rate period. The effective diffusivity values varied between 7.62 × 10⁻⁸ to 9.06 × 10⁻⁸ m² s⁻¹ and increased with increase in temperature. An Arrhenius relation with an activation energy value of 8.831 kJ mol⁻¹ showed the effect of temperature on moisture diffusivity.     

大蒜片真空微波干燥模型的建立

为探索出不同真空微波条件对大蒜片干燥速率的影响,选取微波功率Q、负载量m、真空度Pw等因素,范围分别在50～150W、80～140 g、0.07～0.09MPa,以干燥速率为指标在切片厚度3 mm条件下进行试验.试验结果表明:大蒜片的真空微波干燥符合薄层干燥模型,可用Page模型描述本试验干燥过程,通过SPSS分析得出Page模型参数k、n值与微波功率Q、负载量m、真空度Pw之间的回归方程:k=-0.03+3.4× 10-4Q-3.205×10-4m+0.6Pw、n=1.467-1.61×10-3Q+3.63×10-3m-7.2Pw,得出Page模型表达式.     

Novel microwave–freeze drying of onion slices

The present study aimed to introduce the capability of a novel dehydration technique. To do so, slices with various thicknesses (3.5 and 7 mm) of white onions were dried using commercial freeze drier (abs. pressure 0.005 mbar, temp. 45 °C), our own designed microwave-vacuum drier (abs. pressure down to 300 mbar) under various microwave powers (120–1200 W) and microwave-vacuum–freeze drier (onion slices kept at −20 °C for 2 h). Then, their dehydration rates and some quality parameters, such as rehydration ratio, colour ( L*, a* and b* ) and micro-structure were investigated. Our findings showed that microwave-vacuum–freeze drier is practically a rapid, simple, efficient, economic and novel dehydration technique which can be used for dehydration of mainly foodstuffs. The quality properties of slices produced by this novel method were also completely comparable and competitive with commercial freeze drier with over 96% saving on processing time and enormous amount on energy and capital investments.     

枣片的预处理及干燥方式研究

以木枣为实验材料,探讨不同的脱水预处理和干燥方式对枣片品质的影响。采用真空渗糖的方式对枣片进行脱水预处理,改变糖的种类和配比,对经预处理和干燥后的枣片进行综合评定,选择出一种较佳的糖液配方和干燥方式。研究结果表明,糖液的最佳配比为25%蔗糖、35%葡萄糖,产品的综合得分高达96.87。采用真空低温方法干燥枣片,所得产品品质较佳、加工时间短。     

Colour change and weight loss of apple slices during drying

Drying of apple slices was studied in a conventional tray drier (initial slice thickness=0.3–1.0 cm, drier temperature=60–100 °C) and in a computer-controlled tunnel drier (initial slice thickness=0.8 cm, air tempera ture=50–100 °C, air flow rate=2.4 m/s). In most of the experiments the drying behaviour was characterized by a constant rate period, followed by a falling rate period. The constant rate drying period did not occur, and the colour of the dried slices was improved substantially with tunnel drying under computer control, when a linearly decreasing temperature profile from 100 °C to 70 °C was used. The quality improvement may be attributed partly ot the elimination of the continuous water film covering the surface in the constant rate drying period, which may serve as a medium for the reaction and transport of the chemical species.

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Farbveränderungen und Gewichtsverluste von Apfelstücken während des Trocknens

Zusammenfassung Apfelscheiben (0,3–1,0 cm) wurden in einem konventionellen Hordentrockner (60 bis 100 °C) und einem rechnerkontrollierten Tunneltrockner (Scheiben 0,8 cm, Lufttemperatur 50 bis 100 °C, Luftfluß 2,4 m · s^–1) getrocknet. Bei den meisten Experimenten wurde eine zweiphasige Trocknung beobachtet. Eine regelmäßige Trocknung fand dagegen bei einem gesteuerten, linearen Temperaturgradienten von 100 zu 70 °C statt. Dies verbesserte die Qualität der Produkte, offensichtlich wegen des Vermeidens einer Wasserfilm-Bildung auf den Oberflächen der Apfelstücke während des Trocknens. Ein derartiger Film kann ein Medium für Reaktionen und den Transport von Reaktanden Scin.

     

猕猴桃片的热风干燥特性

以猕猴桃片为原料,采用热风法对猕猴桃进行薄层干燥试验。通过对不同热风温度的探讨获得了猕猴桃片在热风干燥条件下温度和水分变化的基本规律。结果表明:猕猴桃片热风干燥失水速率前期比后期要快,干燥过程中没有恒速干燥阶段,只存在降速干燥;热风干燥下(温度100℃时)猕猴桃的有效水分扩散系数和干燥活化能分别是10.421×10-8m2/s和26.60 k J/mol;同时建立的猕猴桃片薄层干燥数学模型方程为MR=exp[-(0.097 62-0.002 888 t+0.000 021 23 t2)t(0.201 8-0.054 8 t-0.000 298 9 t2)],模型符合Page方程MR=exp(-ktn),且模型预测值和试验值具有很好的拟合度。