烧结混合料干燥过程的解析研究

建立了烧结混合料的一维错流干燥解析模型,并通过穿流干燥实验进行了验证,在此基础上,分析了烧结混合料干燥过程的规律以及主要影响因素。研究结果表明：烧结混合料的干燥过程由较短的恒速干燥段和较长的降速干燥段组成,且降速干燥段的干燥速率近似为直线;热风温度和风速对烧结混合料干燥过程的影响明显大于孔隙率和颗粒平均直径的影响;热风温度控制在200～250℃之间,风速控制在1.5m/s左右利于烧结混合料获得适宜的含水率。     

True Density and Apparent Density During the Drying Process for Vegetables and Fruits: A Review

Abstract: This review presents the concepts involved in determining the density of foodstuffs, and summarizes the volumetric determination techniques used to calculate true density and apparent density in foodstuffs exposed to the drying process. The behavior of density with respect to moisture content (X) and drying temperature (T) is presented and explained with a basis in changes in structure, conformation, chemical composition, and second‐order phase changes that occur in the processes of mass and heat transport, as reported to date in the literature. A review of the empirical and theoretical equations that represent density is presented, and their application in foodstuffs is discussed. This review also addresses cases with nonideal density behavior, including variations in ρ_s and ρ_w as a function of the inside temperature of the material, depending on drying conditions (X, T). A compilation of studies regarding the density of dehydrated foodstuffs is also presented.     

Experimental study and energy analysis on microwave-assisted lignite drying

For value-added utilization of high-humidity lignite, an investigation on its drying behavior was carried out, using a method of Microwave Thermogravimetric Analysis (MTGA). The characteristic of heat and mass transfer during drying process was studied. The effects of initial lignite mass on its drying process and energy consumption were also addressed. In addition, changes of lignite pore structure, sulfur content, and calorific value were tested after drying experiment. The results indicated that lignite microwave drying was divided into three stages: incubation stage, high-efficient dehydration stage, and deep dehydration stage. It was in agreement with the variation of temperature inside the sample. High-efficient dehydration stage and deep dehydration stage were separated at a critical moisture content. Microwave power and initial mass could be properly matched, which was effective to increase dehydration rate and decrease energy consumption. It was further noted that the best power for achieving the least energy consumption was in advance to that for achieving the greatest dehydration rate, especially for initial mass of 20?g and 25?g. The optimum power for initial mass of 20?g and 25?g was found to be 385?W and 539?W, respectively. Pore structure of dried lignite was basically deteriorated. Meanwhile, lignite microwave drying at 231?W for 6?min could lead to an increase of calorific value by 8.1% and a decrease of sulfur content by 68.6%.     

The effect of alkali treatment on rice husk moisture content and drying kinetics

The utilization of rice husks for energy production is often problematic due to their high ash content. A simple method for silica ash removal is the treatment of the husks with sodium hydroxide solutions. The alkali treatment can induce other changes to the husks which need to be investigated. Here, the effects of alkali treatment on the moisture content and drying kinetics of the samples were investigated. The alkali treatment is shown to result in an increase in moisture retention by the as-prepared biomass. Response surface methodology (RSM) using a Box–Behnken design (BBD) is employed to investigate the effects of various input parameters, namely reaction temperature, duration, and alkali mass ratio on moisture uptake of the rice husk. In addition, the drying kinetics of the as-prepared samples are modeled and compared to raw husk prior to treatment. The results show a small increase in the effective diffusivity and a significant decrease in the activation energy required for drying the treated samples.     

Experimental study on drying characteristics of wheat by low-field nuclear magnetic resonance

The characteristics of transverse relaxation time (T₂) of water in wheat were studied by measuring the relaxation time of low-field nuclear magnetic resonance. Analysis of the exponential distribution of T₂ revealed that wheat contains five water components. The T₂ relaxation time and distribution significantly changed during drying. The dynamic characteristics of five water components during wheat drying were determined using the signal quantity of their characteristic peaks, which showed different features. Weakly chemically bound water (T₂₂) and water ascribed to cell wall (T₂₃) were the main source of water loss. Moreover, most T₂₃ and extracellular water (T₂₄) were removed during drying. Water migration between strongly chemically bound water (T₂₁) and the other water components was bidirectional. This process was not only affected by temperature but also by wheat moisture content and proportion of the five water components. The start time of water migration advanced and growth rate of T₂₁ at the end of drying to that before drying increased at 60, 70, and 80°C. Drying at varied temperatures should be applied according to the characteristics of five water components during the drying process. In addition, high initial temperature was found to be necessary to achieve high drying rate of T₂₃, T₂₄, and free water (T₂₅). The use of drying temperature of 80°C at the early stage and then changing to 70°C reduced the heat consumption by 4.81% and increased the drying time by 9.61%.     

Effects of drying temperature and relative humidity on spaghetti characteristics

This study aimed at investigating the effect of drying conditions on spaghetti properties, i.e., its color, surface structure, rupture strength, rehydration characteristics, texture, and sauce retention capacity. The effects of temperature and humidity were independently examined under constant drying conditions, which were compared to those applied industrially, where the temperature and relative humidity are changed stepwise with time. The knowledge obtained in this study is considered useful for reasonably determining the drying conditions for producing spaghetti with desired properties.     

Temperature and pretreatment effects on the drying of Hass avocado seeds

The objective of the present experimental work was to determine the influence of five different drying air temperatures (313, 323, 333, 343, and 353 K) on the drying kinetics and the degree of moisture evaporation from Hass avocado seeds. The drying experiments of the non-pretreated and pretreated (sliced and crushed) Hass avocado seeds were performed in a heating furnace, where the pretreatment process was found to accelerate the drying process. The obtained results suggested that increase in the operating air temperature stimulated the rate of moisture evaporation, but resulted in the charring of the seed surface. The drying air temperature of 313 K was concluded to be suitable for the reasonable drying of Hass avocado seeds. The slicing pretreatment process was found to be better indicative of the total moisture amount present in Hass avocado seeds. The drying process removed a maximum of 58% of the initial water mass of Hass avocado seeds. An additional investigation was performed where the physical appearance of Hass avocado seeds immersed in water at different temperatures (303, 318, 325.5, 333, and 348 K) was examined. The observed study suggested that the surrounding temperature higher than 313 K could damage the physical appearance and reduce the quality of Hass avocado seeds.     

Experimental investigation of the drying characteristics of a mixed mode natural convection solar crop dryer with back up heater

A mixed mode natural convection solar crop dryer with a backup heater was designed and constructed from locally available materials and used to dry freshly prepared pineapples under four drying Scenarios for drying to correspond to specified drying periods for four typical seasons in Ghana. The experiments were devised for the material moisture content to be monitored continuously till the desired moisture content of between +106% and 184% (d.b) was achieved. In solar heating mode of operation, results show that the thermal mass was capable of storing part of the absorbed solar energy but the quantities involved are insufficient to sustain night drying. It was possible to dry a batch of pineapples in each mode of operation. The dryer reduced the moisture content of pineapple slices from about; 924% to 106% in 19 h; 1049% to 184% in 10 h; 912% to 155% in 7 h; and 1049% to 144% (d.b) in 23 h, for drying in Scenarios 1, 2, 3 and 4, respectively. The average moisture pickup efficiency values obtained were 27%, 24%, 11%, and 32% for drying in Scenarios 1, 2, 3, and 4, respectively.     

Effect of clove extract pretreatment and drying conditions on lipid oxidation and sensory discrimination of dried omena (Rastrineobola argentea) fish

Sun‐drying is a low‐cost, low‐technology fish preservation method frequently employed in developing areas. However, the process promotes lipid oxidation and its associated undesired flavours and odours. This study investigated low‐technology solutions for impact on lipid oxidation and sensory attributes of oven‐dried omena fish (Rastrineobola argentea). Two oven‐drying conditions and four doses of clove water extract ‘dip’ pretreatments were studied in a complete factorial design. Lipid oxidation in dried fish was assessed by TBARS, peroxide value and fatty acid analysis by GC‐FID. Results showed that soaking in 10 g L^?1 clove water extract for 1 h and oven‐drying at 150 °C × 30 min significantly reduced TBARS and peroxide values in omena fish by 77% and 79%, respectively, and polyene index showed improved retention of long‐chain polyunsaturated fats, compared to original drying condition. Lastly, panellists of a triangle test were able to discriminate between biscuits made with the modified and original dried fish.     

A feasibility study of Lactobacillus plantarum in fruit powders after processing and storage

The aim of this study was to develop fruit powders (apple, banana and strawberry) enriched with a probiotic strain (Lactobacillus plantarum 299v). Two methodologies were proposed: (i) drying of the fruit with probiotic culture incorporated (by convection) or (ii) drying of fruit (by convection) and addition of spray‐dried probiotic culture. In the first methodology, processing caused a notable reduction in probiotic viable counts in apple, but this reduction was lower during drying of banana and strawberry. A large reduction in viable cells was also recorded during storage. In the second methodology, the survival of L. plantarum 299v was considerably higher during spray‐drying, and fruit powders with a microbial content suitable for a probiotic food (10⁸–10⁹ cfu g^?1) were obtained. The fruit powders incorporating L. plantarum 299v can be stored at 4 °C or at room temperature, for at least 3 months. This preliminary study demonstrated that fruit powders are good carriers of probiotic cultures, but the techniques used to produce them should be carefully considered.