Comparative Study of Effects of Drying Methods and Storage Conditions on Aroma and Quality Attributes of Thai Jasmine Rice

This article describes the effects of various drying treatments and storage conditions on the main quality attributes of Thai jasmine rice; i.e., aroma, milling, pasting, and cooking properties. The experimental results show that drying treatments using various high temperatures (115-150°C) combined with 30 min tempering time between the pass or ambient air drying affect the composition of volatile compounds as well as the commercially accepted quality characteristics of Thai fragrant rice. High-temperature drying followed by ambient air drying can retain most of the quality attributes of Thai jasmine rice than multi-pass high-temperature drying with a tempering period between passes.     

Rapid Drying of Parboiled Paddy Using Hot Air Impinging Stream Dryer

A method for rapid drying of parboiled paddy via the use of an impinging stream dryer was proposed and assessed. The effects of the drying air temperature, number of drying cycles, as well as time of tempering between each drying cycle on the moisture reduction, head rice yield, and whiteness index of the dried parboiled paddy were studied. The drying experiments were carried out at drying air temperatures of 130, 150, and 170°C; inlet air velocity of 20 m/s; impinging distance of 5 cm; and paddy feed rate of 40 kg_{dry_paddy}/h. Parboiled paddy was dried for up to seven cycles. Between each drying cycle the parboiled paddy was tempered for a period of either 0 (no tempering), 15, 30, 60, or 120 min. After impinging stream drying, paddy was ventilated by ambient air flow until its moisture content reached 16% (db). Moisture reduction of the paddy was noted to depend on both the impinging stream drying temperature and tempering time. Drying at a high temperature along with tempering for a suitable period of time could maintain the head rice yield of the paddy at a level similar to that of the reference parboiled paddy. To avoid discoloration and low head rice yield, parboiled paddy should not be dried at a temperature higher than 150°C and should be tempered for at least 30 min.     

Effect of Fluidized Bed Drying Temperature on Various Quality Attributes of Paddy

Abstract

As reported by many researchers, it was found that fluidized bed paddy drying using high drying air temperatures of over 100°C affected the head rice yield and whiteness of dried rice. However, only a few studies on fluidized bed paddy drying with drying air temperatures below 100°C were so far reported. The main objective of this work was therefore to study the effect of fluidized bed drying air temperature on various quality parameters of Suphanburi 1 and Pathumthani 1 Indica rice. Paddy was dried from the initial moisture contents of 25.0, 28.8, and 32.5% dry basis to 22.5 ± 1.2% dry basis using inlet drying air temperatures between 40 and 150°C at 10°C/step. After fluidized bed drying, paddy was tempered and followed by ambient air aeration until its final moisture content was reduced to 16.3 ± 0.5% dry basis. The results showed that the head rice yield of Suphanburi 1 was significantly related to the inlet drying temperature and initial moisture content whilst there was no significant relationship between the head rice yield, drying temperature and initial moisture content for Pathumthani 1. The whiteness of the two rice varieties was slightly decreased with increase in drying air temperature and initial moisture content. It was also found that the hardness of both cooked rice varieties exhibited insignificant difference (p < 0.05) comparing to rewetted rice, which was gently dried by ambient air aeration in thin layer. The thermal analysis by DSC also showed that partial gelatinization occurred during drying at higher temperatures. Using inlet drying air temperatures in the range of 40–150°C therefore did not affected the quality of cooked rice and paddy. The milling quality of paddy was also well maintained.     

INTERMITTENT DRYING OF ROUGH RICE

Abstract

This work obtains thin-layer drying data for rough rice from 108 treatments. A thin-layer drying equation is also derived using these data with drying air absolute humidity, drying air temperature, tempering time interval and drying time interval as the independent variables. In addition, an intermittent drying equation is developed to predict the drying behavior of rough rice in a re-circulating type rice dryer.     

Effect of tempering approach following cross-flow drying on rice milling yields

Abstract

Three tempering approaches were followed after drying rough rice at 16.3% and 20.5% initial moisture contents (IMCs) using 57?°C/13% RH air at an airflow of 0.56 (m³/s)/m² for 30, 60, and 90?min in an experimentally simulated cross-flow drying column. For the longer drying durations, post-tempering head rice yields were consistently less when the interstitial air from rice from different cross sections of the drying column was allowed to “interact” during tempering than when the rice from these different cross sections was tempered separately; this effect was more prominent at the greater rice IMC. RH of the interstitial air during tempering was measured and used to estimate the minimum tempering durations required for the different tempering approaches.     

THIN LAYER AND DEEP BED DRYING OF LONG GRAIN ROUGH RICE

ABSTRACT

The paper presents new data for thin-layer drying characteristics of Thai long grain rough rice measured under various conditions of drying air temperature (35 to 60?°C), drying air relative humidity (30 to 70 % ) and the initial moisture content of rough rice (20 to 40 % dry basis). Empirical equations were developed using the instantaneous weight, the weight loss and drying time, with temperature, relative humidity and initial moisture content of rough rice as the independent variables. A computer program was developed to simulate the deep-bed drying process. The thin-layer drying equation developed before was used in the computer simulation. Experimental data from the fixed bed dryer were compared with the results from the calculation.     

A Combined Experimental and Theoretical Approach to Study Temperature and Moisture Dynamic Characteristics of Intermittent Paddy Rice Drying

Intermittent drying of paddy rice is fully investigated both theoretically and experimentally. A model is developed to describe simultaneous heat and mass transfer for the drying stages and mass transfer for the tempering ones. The model is considered for both cylindrical and spherical geometries. The model excels in considering non-constant paddy rice and air physical properties as well as surface vaporization and convection. The consequent equations are numerically solved with finite-difference method of line using implicit Runge–Kutta. Furthermore, a set of experiments is conducted in a laboratory-scale fluidized bed dryer to estimate the moisture diffusivity of rice and evaluate the effects of different parameters. Two correlations for moisture diffusivity are derived for each geometry based on the experimental results. It is noteworthy that the geometry choice leads to significantly different moisture diffusivities. As a result, the diffusivity values obtained for spherical presentation is 2.64 times greater than that of cylinder. Moreover, the cylindrical model fits the experimental results more precisely, especially for tempering stage (AARD_cyl = 1.03%; AARD_sph = 1.53%). Model results reveal that thermal equilibrium is quickly reached within the first 2 min. Air velocity shows no influential effect on drying upon establishment of fluidized condition. In addition, drying rate is drastically improved after applying the tempering stage. A definition for tempering stage efficiency is also proposed which shows that 3 h tempering will be 80% efficient for the studied case. Rising temperature significantly improves the drying rate, while it does not contribute much in the tempering efficiency.     

流态化及缓苏干燥技术对泰国香稻的香味和磨粉品质的影响

将高温（〉100。C）流态化干燥和缓苏相结合干燥泰国香稻（KhaoDawkMali105，籼稻），使其湿含量降至13-14％（湿基），以便于安全贮存。研究了干燥与贮存过程对籼米香味和磨粉品质的影响。泰国香稻需要经过两个干燥阶段。第一阶段是在高温流化床中干燥，缓苏30分钟，然后返回高温流化床干燥或阴干，直至湿含量降低到安全贮存水平。实验结果表明，干燥温度对香稻的整精米率（HRY）、白度指数（wI）和2AP含量均有影响。多数干燥温度下整精米率的水平都比较低，但150℃除外，此情况下整精米率（psO．05）显著提高。然而，当一次流态化干燥和缓苏之后，以阴干代替二次流态化干燥，香稻的整精米率显著增加，尤其是在135℃和50℃时（p≤O．05）。在贮存期间，白度指数和2AP舍量显著降低（ps0．05），而整精米率保持不变。对比环境温度（28．30℃）和15℃贮存温度，结果表明保持香米2AP舍量和白度指数的适宜保存温度是15℃。然而贮存条件对整精米率影响不大（p〉0．05）。本研究将有益于泰国香米行业的发展，为泰国香米的干燥起到指导性作用。     

COMPUTER SIMULATION ON INTERMITTENT DRYING OF ROUGH RICE

This study applied a partial differential equation model with newly-developed thin layer equations to simulate batch re-circulating dryers under different drying conditions, which are combinations of four parameters: drying air temperature, drying air absolute humidity, drying period duration, and tempering period duration. The moisture change and the drying rate, which were of particular concern with respect to the simulated data, were investigated. Validation drying tests were carried out in a lab scale re-circulating rice dryer. Two sets of experiment were performed involving different drying parameters to simulate re-circulating rice dryers which are extensively used in Asian countries. Comparing these two experimental data with two simulated drying curves respectively, it revealed they are quite consist with each other under the same drying conditions. Drying air temperature, drying air humidity, drying period duration and tempering period duration significantly influenced the drying rate. Under the same drying condition, the tempering period duration effect was insignificant to the drying rate in drying zone as the drying air humidity or temperature increased. And, a higher initial moisture content obtained higher time and energy efficiency for the re-circulating rice dryers.     

EFFECT OF A TEMPERING PERIOD ON DRYING OF CARROT IN A VffiRO-FLUlDIZED BED

ABSTRACT

Drying of diced carrot in a vibrofluidized bed was studied experimentally for various air temperatures, bed heights and size of the cubes. Effect of a tempering period that mav be implemented into ¦ drying cycle was examined with respect to drying kinetics and energy consumption. Although two stage drying with a tempering period increases the overall drying time, it shortens the drying time in a VFB dryer thus reduces energy consumption     

Effects of High-Temperature Fluidized Bed Drying and Tempering on Kernel Cracking and Milling Quality of Vietnamese Rice Varieties

Studies on the effects of high-temperature fluidized bed drying and tempering on physical properties and milling quality of two long-grain freshly harvested Vietnamese rice varieties, A10 (32±1% wet basis moisture) and OM2717 (24.5±0.5% wet basis moisture), were undertaken. Rice samples were fluidized bed dried at 80 and 90°C for 2.5 and 3.0 min, then tempered at 75 and 86°C for up to 1 h, followed by final drying to below 14% moisture (wet basis) at 35°C by thin-layer drying method. Head rice yield significantly improved with extended tempering time to 40 min. Head rice yield tended to increase with decreasing cracked (fissured) kernels. The hardness and stiffness of sound fluidized bed dried rice kernels (in the range of 30–55 N and 162–168 N/mm, respectively) were higher than that of conventionally dried ones (thin layer dried at 35°C). The color of milled rice was significantly (P < 0.05) affected by high-temperature fluidized bed drying, but the absolute change in the value was very small.     

Evolution of mechanical properties of parboiled brown rice kernels during impinging stream drying

Information on mechanical properties of parboiled brown rice kernels upon impinging stream drying, which is important for effective control of kernel fissure and head rice yield, is reported. Experiments were performed at the drying temperatures of 130, 150, and 170°C; inlet air velocity of 20?m/s; impinging distance of 5?cm and paddy feed rate of 40?kg_{dry_paddy}/h. Parboiled paddy was dried for up to seven cycles. Between each drying cycle, the paddy was tempered for a period of either 0 (without tempering) or 30?min. The moisture evaporation rate was noted to be very high during the first two drying cycles and rapidly dropped in the later drying cycles. When tempering was included after a particular drying cycle, the drying rate in a subsequent cycle was higher than without tempering. At the kernel moisture contents immediately after drying of 25.3–47.5% (d.b.), the drying temperature and existence of tempering did not affect the mechanical properties although microcracks were formed in the kernels. However, both factors played a more important role on the mechanical properties when the kernels were evaluated at 16% (d.b.). The head rice yield correlated well with the tensile strength of the kernels.     

AN APPLICATION OF GLASS TRANSITION TEMPERATURE TO EXPLAIN RICE KERNEL FISSURE OCCURRENCE DURING THE DRYING PROCESS*

Fissure formation during rice drying is a major cause of rice milling quality reduction. This work has applied principles of polymer science in studying thermal and hygroscopic properties of rice kernels, particularly the glass transition temperature (T_g ). This data was used to develop a hypothesis that explains the occurrence of rice kernel fissuring as a result of drying. The drying process was mapped onto a state diagram to illustrate the changes in state that a kernel could incur through drying and tempering operations. An experiment was designed to validate the hypothesis in which the effect of the T_g on rice drying and tempering in terms of milling quality was determined. Results showed that drying air temperatures up to 60°C and high moisture removal rates could be used without reducing the milling quality, as long as sufficient tempering was allowed at a temperature above the T_g of the rice.

     

Rough rice convective drying enhancement by intervention of airborne ultrasound – A response surface strategy for experimental design and optimization

This study has examined the influence of ultrasonic-assisted hot air drying process on the dehydration behavior of in-bin rough rice (Oryza sativa) kernels. To this aim, the experimental drying kinetics of rough rice subjecting to different drying air temperatures (35, 40, 45, 50, and 55?°C) and inlet air velocities (0.2, 0.5, 0.8, 1.1, and 1.4?m/s) were carried out by applying various ultrasound power levels (30, 60, 90, 120, and 150?W) in the frequency of 21?kHz. The effect of ultrasound intervention was investigated on drying kinetics, effective moisture diffusivity, energy consumption, and product quality. Experimental plans were designed by response surface method to study the feasible interactions between research parameters. Based on the key results, high-power ultrasound in conjunction with conventional deep bed drying led in 26.47% decrease in drying time, 30.66% increase in moisture diffusivity, as well as improvement in the grain quality, in terms of acceptable reduction in head rice yield and whiteness losses. In addition, energy consumption reduced approximately by 24.36% when high-power ultrasound was applied at selected drying condition. Ultrasound intervention during hot air drying process is recommended as it generates rice kernels with desirable milling quality within shorter drying time.     

Heat Pump Drying of Green Sweet Pepper

Thin-layer drying experiments under controlled conditions were conducted for green sweet pepper in heat pump dryer at 30, 35, and 40°C and hot air dryer at 45°C with relative humidities ranging from 19 to 55%. The moisture content of sweet pepper slices reduced exponentially with drying time. As the temperature increased, the drying curve exhibited a steeper slope, thus exhibiting an increase in drying rate. Drying of green sweet pepper took place mainly under the falling-rate period. The Page equation was found to be better than the Lewis equation to describe the thin-layer drying of green sweet pepper with higher coefficient of determination and lower root mean square error. Drying in heat pump dryer at 40°C took less time with higher drying rate and specific moisture extraction rate as compared to hot air drying at 45°C due to lower relative humidity of the drying air in a heat pump dryer though the drying air temperature was less. The retention of total chlorophyll content and ascorbic acid content was observed to be more in heat pump–dried samples with higher rehydration ratios and sensory scores. The quality parameters showed a declining trend with increase in drying air temperature from 30 to 45°C. Keeping in view the energy consumption and quality attributes of dehydrated products, it is proposed to dry green sweet pepper at 35°C in heat pump dryer.     

Multi-stage intermittent drying of rough rice in terms of tempering and stress cracking indices and moisture gradients interpretation

In this study, multi-stage intermittent drying (MSID) of rough rice is considered based on stress cracking index (SCI), tempering index (TI), and total drying/tempering duration for Hashemi and Koohsar varieties experimentally and theoretically. The samples were dried at 60°C for 20, 40, and 60?min and tempered at 60°C for 40, 80, 120, 160, 200, and 240?min after each drying stage. Afterward, the completion of the tempering process was assessed using the TI along with analysis of moisture content kinetics by a simplified drying model. For both varieties, the SCI decreased significantly until continuing the tempering operation to certain durations and increased for longer drying durations in each drying stage. Considering the SCI and the total drying/tempering duration, the tempering durations of 200 and 160?min after 40?min drying in each stage were determined as the best performed conditions for MSID of Hashemi and Koohsar varieties, respectively. The results achieved by the TI were in conformity with those obtained by the mathematical model. It was concluded that the TI and simulation of surface moisture content on a kernel could be applied for estimating the time required for supplementation of the tempering process to eliminate moisture content gradients created inside the kernels during the drying process.     

Experiments on In-Store Paddy Drying Under Tropical Climate: Simulation and Product Quality

Abstract

The main objective of this work is to study the rice whiteness and paddy qualities of rice in terms of hardness, stickiness, cohesiveness, and germination of rice. The prediction results of moisture content and whiteness are compared with the experimental results using a near-equilibrium drying model, which is modified by including whiteness kinetics of rice kernel. The long grain rice (Suphanburi 1 high amylose indica variety), which consists of 27% amylose was used for all experiments. The experiments were carried out at the average ambient temperature range of 28.6–30.8°C, average relative humidity of 65.2–80.6% with a fixed bed depth of 1.0 m. Specific air flow rates of 0.65 and 0.93 m³/min-m³ of paddy were forced continuously through the paddy bulk at initial moisture contents of 18.5% and 20.1% wet basis, respectively. The desired final moisture content of paddy is about 13.3 ± 0.6% wet basis. The results show that drying rate and the whiteness predictions are in good agreement with those from the experiments. The in-store drying using ambient air condition did not produce notable effect on the rice whiteness, head rice yield, and the percentage of paddy germination. However, the hardness, stickiness, and cohesiveness of rice were changed.     

AN APPLICATION OF GLASS TRANSITION TEMPERATURE TO EXPLAIN RICE KERNEL FISSURE OCCURRENCE DURING THE DRYING PROCESS *

Fissure formation during rice drying is a major cause of rice milling quality reduction. This work has applied principles of polymer science in studying thermal and hygroscopic properties of rice kernels, particularly the glass transition temperature (T_g). This data was used to develop a hypothesis that explains the occurrence of rice kernel fissuring as a result of drying. The drying process was mapped onto a state diagram to illustrate the changes in state that a kernel could incur through drying and tempering operations. An experiment was designed to validate the hypothesis in which the effect of the T_g on rice drying and tempering in terms of milling quality was determined. Results showed that drying air temperatures up to 60°C and high moisture removal rates could be used without reducing the milling quality, as long as sufficient tempering was allowed at a temperature above the T_g of the rice.     

Influence of Combined Hot Air Impingement and Infrared Drying on Drying Kinetics and Physical Properties of Potato Chips

Air jet impingement combined with infrared drying (IMIRD) was developed as an alternative processing method to produce health-friendly potato chips in place of conventional deep-fat frying. This article investigates the effects of IMIRD compared to air jet impingement drying alone (IMD) and conventional convective drying (CCVD) on potato being processed as potato chips in term of drying characteristics, quality attributes (shrinkage, color, and hardness), and specific energy consumption (SEC) of the dryer. The experiments were carried out at three different air velocities (5, 10, 15 m/s) and infrared intensities (0.16, 0.27, and 0.33 W/cm²) at a fixed air temperature of 85°C. The experimental results show that the drying air velocity and infrared intensity had a significant effect on the moisture removal from potato slices. IMIRD, compared to IMD and CCVD, provided a higher drying rate, less shrinkage, lower hardness, and less color deterioration. An increase in air velocity at each infrared intensity caused a decrease in the total SEC value.     

Effects of Microwave-Assisted Hot Air Drying and Conventional Hot Air Drying on the Drying Kinetics,Color, Rehydration,and Volatiles of Moringa oleifera

In this study, the fresh Moringa oleifera pods (Drumsticks) were dehydrated by microwave-assisted hot air drying (MAHD) and conventional hot air drying methods. The samples were dried at three different temperatures, viz. 50, 60, and 70°C, with and without the application of microwaves. Microwave power density of 1 W/g was used for the MAHD. The final moisture content was targeted as 13% wb. The drying curves and drying rate curves were plotted and compared. The kinetics of drying obtained experimentally were correlated with the Page equation. The constants K and N of the Page equation were determined to predict the drying kinetics for varying conditions. The quality attributes, namely, color, rehydration ratio, and volatile compounds, were analyzed and compared with that of the fresh Moringa pods. The volatile compounds were analyzed using z-Nose (an electronic nose; Electronic Sensor Technology, Newbury Park, CA) and bioactive molecules were analyzed using gas chromatography–mass spectrometry. The results showed that the MAHD method had significantly (P < 0.05) reduced the loss of volatiles during drying. Also, MAHD preserved most of the bioactive molecules when compared to the conventional hot air drying method. The samples dried at 50°C using MAHD were the best in terms of all of the quality attributes tested in this study. Also, the results established that the z-Nose can be used as a quick and inexpensive means to assess the effect of different process parameters on the aromatic quality of the product and quantitatively classify quality based on aroma.