Section Study on the flat-bed dryer in the Mekong River Delta of Viet nam 17 Section Study on the flat-bed dryer in the Mekong River Delta of Viet nam ABSTRACT The study, including experiments and survey on the flat-bed dryer, focused on the cracking of paddy grains, and on comparing the air reversal mode Results showed that, in both the 8-ton production-scale dryer and the 20-kg laboratory dryer, the effect of air reversal was very apparent in reducing the final moisture differential; however, its effect on the drying time or the drying rate was not statistically significant Mechanical drying, whether with or without air reversal, was superior to sun drying in terms of reducing rice crack However, compared to shade control drying, drying (with or without air reversal) did decrease the head rice recovery and increase the crack; the causing factor was not apparent, most suspected reason was the drying rate The decrease in head rice recovery was inconsistent, slightly lower or higher in each specific pair of experiments with and without air reversal; this was not expected in line with data on the final moisture differential Testing of a 4-ton dryer at LongAn equipped with the solar collector as supplementary heat source resulted with good grain quality and confirmed the good economic potential Major findings from the survey on the current status on the use of flat-bed dryers in Provinces were: The trend for increased drying capacity, the role of local manufacturers and local extension workers, government support with interest reduction for dryer loans, the drying during the dry-season harvest, and especially the unbalance between drying costs and drying benefits INTRODUCTION Flat-bed dryers have been with the rice agriculture of the Mekong Delta of Viet Nam for a long time From the first flat-bed dryers in the 1980’s to about 6500 units in 2007 is quite a good progress But not all is optimistic Acceptance varies among provinces, even among districts or communes in the same province Finding the interrelated factors affecting the dryer acceptance is quite complex Within the context of the CARD Project 026/VIE-05 with focus on the cracking of paddy grains in the area, the study on the flat-bed dryer from 2006 to 2008 included the following objectives: 18 • Conduct experiments under laboratory controlled drying conditions and under actual production conditions to evaluate the effect of air reversal on the rice crack and other drying outputs • Conduct experiments on the 4-ton flat-bed dryer, using solar energy as supplementary heat source • Conduct a Participatory Rapid Rural Appraisal (PRRA) survey to update on the use of flat-bed dryer in the Mekong Delta REVIEW OF RELEVANT INFORMATION The following information is based on data by various Provinces presented during different seminars, on an integrated assessment study by the Ministry of Agriculture-Rural Development in collaboration with DANIDA in 2004, and on the first author’s working experience with flat-bed dryers in the past 25 years Development of the flat-bed dryer The Mekong Delta in Southern Viet Nam, with 2.7 million hectares of rice land, is producing about 50 % of Viet Nam total rice output With 16 million people or about less than 20 % of the total population, this region has accounted for more than 90 % of Vietnamese rice export in the past decade Average farm size is about per household, although in some newlyreclaimed districts, - 10 per household is not uncommon Rice drying became an issue in Mekong Delta in early 1980’s when a second crop was promoted, of which the harvest fell into the rainy season Different dryer models were tried by various agencies; only one model was accepted by the production sector, namely the flatbed dryer (FBD) The first FBD was installed in Soc-Trang Province in 1982 by the University of Agriculture and Forestry (now renamed Nong-Lam University NLU) Farmers in Soc-Trang copied/ modified/ improved this FBD using cheap local materials In 1990, there were about 300 FBD units in the Mekong Delta, half of which were in Soc-Trang Other Provinces began to adopt these dryers In 1997, a survey conducted by a Danidaassisted Project reported a total of 1500 FBD in all Mekong Delta, with leading Provinces (Kien Giang, Soc-Trang, Can-Tho) accounted for 850 units; all remaining 10 Provinces shared the balance of 650 units The above Danida-assisted Project in Can-Tho and Soc-Trang doubled the FBD in each Province from about 250 to 500 units in the two-year span of 1998-1999, through a credit 19 scheme and extension activities The Project terminated in 2001, and replaced by a Program managed by the Ministry of Agriculture, but still assisted by Danida, then with only extension activities The Program terminated in mid-2007 The number of FBD dryer rose rapidly, about 3000 units in 2002 and 6200 units in 2006 The dryers in the Mekong Delta account for more than 95 % of all dryers in Viet Nam The technical development of the FBD in the past 25 years followed an interesting pattern First, a design was released by a research institution, NLU in this case Next, farmers/ mechanics copied/ modified/ improved the design Next, NLU monitored those modifications and came up with a major design change and improvement The cycle repeats The landmarks for these major design releases by NLU have been: 1982: Conventional FBD with central air inlet to the plenum chamber, using flatgrate rice husk furnace with precipitation chamber (Fig.1) 1994: Conventional FBD with side-duct plenum (Fig.2), rice husk furnace with vortex and central-pipe precipitation chamber (Fig.3) 2001: Reversible FBD (Fig.5 & 6) (expected): 2006: Automatic rice husk furnace (model NLU-IRRI-Hohenheim, Fig.4) 2007: Solar collector for FBD Major modifications /improvements by farmer-mechanics have been: 1987: Rice husk furnace with inclined grate 2004: Drying bin for reversible dryer, with distributed central inlet 2006: Raking mechanism under the rice husk hopper for more uniform husk feeding Figure Conventional FBD with central air inlet to the plenum chamber 20 Figure Conventional FBD with side-duct plenum Figure Rice husk furnace with vortex and central-pipe precipitation chamber Drying Air UP Figure The automatic rice husk furnace for SRA-4 reversible flat-bed dryer Drying Air UP 0.3m Grain Drying Air DOWN Grain 0.6m Grain REVERSIBLE SRA DRYER Floor: 25 sq.m / CONVENTIONAL SHG FLAT-BED DRYER Floor: 50 sq.m / ton ton Figure Principle of reversible-air dryer Figure The SRA-10 reversible air dryer (10 tons per batch) NLU have taken the leading role in releasing efficient dryer fans, both for conventional and reversible dryers, with transfer of design and fabrication technology to 15 manufacturers in the Mekong Delta, among them have built fan test ducts according to JIS Standards Quality of paddy dried by the flat-bed dryer The quality of dried paddy is judged by several criteria: The paddy is not contaminated with black ashes from the furnace The paddy final moisture content is uniform at the desired level for storage For seed grain, the germination is high For commercial grain, the dried grain crack is minimized 21 The first criterion (no ash mixed with grain) has been met after some years, due to gaining of experience in furnace building, and competition among furnace builders, to deal with farmers’ first-visual reactions The second criterion is difficult to meet due to the inherent principle of flat-bed drying A final moisture differential of 1.5 % (between the top and the bottom layer) is considered good, while in continuous-flow mixing-type dryer, 1.0 % is normal For flat-bed dryers, farmers rely on manual mixing Technically, a good high airflow rate at moderate temperature (below 44oC) helps reducing the differential The air-reversal principle introduced since 2002 also reduces the non-uniformity All these technical features should be re-evaluated / confirmed in this current CARD Project The preserved seed germination is well established by Seed Companies in using a safe drying temperature below 42 oC, and most importantly to dry the grain within 12 hours after harvest For commercial grain, the dried grain crack is a big issue A report (Phan Hieu Hien, 1998) based on surveys of a few rice mills in Can-Tho and Long-An showed a reduction of to % of farmers’ profit due to more broken rice due to improper drying This high loss was due to the habit of field drying in the dry-season harvest, and estimated to be about 20 million US$ per harvest in the Mekong Delta However, data and estimates were based on a few interviews, and not on systematic testing Thus, in this CARD Project, the need is to confirm or reject based on solid test data MATERIALS AND METHODS Testing Testing of dryers followed standard procedures described in RNAM (1991) and ASABE (2006) Measurement equipment included different thermometers, moisture meter and drying oven, power meter etc For the 8-ton dryers, the drying temperature was at levels: a) Constant at 43 oC; and b) At 50 oC for the first hour, and afterwards constant at 43 oC In reality, due to the furnace configuration, the temperature rarely exceeded 50 oC, and was about 48 oC at most In all tests, the focus was to compare two drying modes: WITH air reversal, and WITHOUT air 22 reversal Some experiments also compared with sun drying on the cement drying yard with a 7-cm paddy layer, as popularly practiced by local farmers The crack analysis and head rice analysis was first done at the Vinacontrol, an accredited agency in charge of certifying rice quality for export, and later at the Rice Quality Laboratory of the NLU Chemical Technology Department, following procedures adopted by International Rice Research Institute and the University of Queensland Each treatment was analyzed by samples, each consists of 50 grains taken at random; each paddy grain was hand-husked and examined under the magnifying glass for fissure The increase in crack or decrease in head rice of each treatment based were on the control shade drying (or further shade drying to 14 %MC) The biggest problem for testing has been the input paddy We encountered severe difficulties in securing batches of the same quantity or initial moisture content This was apparent with the 8-ton batches But even scale-down to 1-ton batch, the 3-factor experiments could not be run, due to different initial MC Finally, from the “lumpsum” conclusions with 8-ton dryers, we had to concentrate on and be contented with 20-kg batches in paired experiments (block) of Air reversal and No air reversal For experiments on the use of solar heat for paddy drying, a 4-ton popular flat-bed models fabricated by a local mechanical shop was selected, and added with a solar collector designed at the NLU Center for Agricultural Energy and Machinery Survey The objectives of the surveys were: (i) to update the role of flat-bed dryers in reducing postharvest losses and in preserving rice quality; (ii) to identify operating factors of the flat-bed dryer which contribute to the reduction of rice crack; and (iii) to identify problems with the flat-bed dryer that the CARD Project could possibly help The survey used the Participatory Rapid Rural Appraisal (PRRA) method, through interviewing different people class, from farmers to rice millers to governmental officials etc But it also relied heavily on both available data gathered in the past 10 years by various 23 agencies, and on personal experience of the people involved with the dryer at NLU over the past 20 years Four Provinces were selected in 2006, namely Can-Tho City, Kien-Giang, Long-An, and Tien-Giang The first three Provinces have sites which had been selected by the CARD Project for all related experiments, demonstrations, and extension activities The fourth Province is adjacent to Long-An, and also planned as site for rice milling survey, so facts and data on the dryer would be relevant In 2007, we visited more Provinces such as Hau-Giang, An-Giang, Kien Giang, Soc-Trang…, with resulted with additional findings RESULTS AND DISCUSSSION TESTING Experimental results on the 8-ton dryer, the laboratory dryer, the solar-assisted dryer, as well as the survey results are presented in the following sections The 8-ton dryer Two 8-ton dryers were selected for experiments One was a NLU-designed air-reversible dryer installed at Tan-Phat-A Cooperative, Tan-Hiep District, Kien Giang Province in July 2006 (Figs 7&8) The other was an air-reversible dryer made by a local manufacturer installed at Tan-Thoi Cooperative in Can-Tho Province, with the design patterned on the SRA-8 of NLU; the difference was the under-plenum duct inside the drying bin, “ong gio chim” in Vietnamese (Fig.9), in order to distribute the airflow evenly Figure The 8-ton dryer at Tan-Phat-A Cooperative, Kien Giang Figure The 8-ton dryer with the air for downward direction 24 Figure 9: The 8-ton flat-bed dryer at Tan-Thoi Cooperative, Can-Tho Province Experiments from Kien-Giang were under more control thus more results are reported here, while results at Can-Tho are supplementary Refer to Phan Hieu Hien (2006, 2007, 2008) for testing details In Kien-Giang experiments were conducted in two wet seasons (July 2006, and July- August 2007), and two dry-seasons (March 2007, and March 2008) Major findings are as follow: • The drying temperature is stable and can be kept within ± oC, usually from the nominal value of 43 oC • The effect of air reversal was very apparent in reducing the final moisture differential When operated correctly, this differential was less than 2.2 % with air reversal, but over 4.6% without air reversal More MC differential means more rice cracking during milling This explains why dryers installed since 2003 have been more and more of the reversible principle • However the effect of air reversal on the drying time or the drying rate was not clear because of several other factors involved (Fig.10) Dr ying r ate 5 0 0 Air Re ve rsa l No a ir re ve rsa l 20 22 24 26 28 30 Ini t ia l M C , % b w Fig.10: Effect of air reversal on the drying rate 25 Data on the crack of rice upon milling in March 2007, and July 2007 with three pairs of drying batches (With Air reversal, and Without air reversal) showed that: • Mechanical drying, whether with or without air reversal, was superior to sun drying in terms of less crack percentage or more head rice recovery About 3- % less cracking, and about % more head rice recovery were main data obtained from March 2007 experiments • The grain cracking in Air-reversal batches were lower than No-air-reversal batches (Fig.11) This is a basic result • However, the decrease in head rice recovery was inconsistent, slightly lower or higher in each specific pair (Fig.12) This is confirmed by the statistical comparison on the head rice recovery with t-test between batches of Air reversal and No air reversal, which did not show significant difference at 5% level This was not expected in line with the above data on Final MC differential The reason was probably due to the sample milling; the whitening time was only minute, thus some slightly cracked kernels might not be broken during milling • In both cases (Air reversal and No air reversal) drying did decrease the head rice recovery and increase the crack The causing factor was not apparent, due to so many factors involved in a large mass of tons of grain: paddy non-uniformity, drying rate… Most suspected reason was the drying rate (Fig.13), data pointed to an optimum drying rate in the 1.0– 1.2 %/hr range, but this has to be confirmed by further elaborate experiments, or from laboratory scale experiments Head rice, Kien Giang 0 Wet-s eas o n (AR = Air Revers al; NAR = No air revers al B2 = Batch No ) 40 35 30 25 20 15 10 Ave(3batches) Air reversal StDev(AR ) B9 & B6 Batches NAR B6 B1 & B6 NAR B5 B2 & B5 AR B9 70 60 50 40 30 20 10 AR B2 Crack % Crack % INCREASE (Kien Giang 2007 wet-season) No air reversal Head Rice Before drying, % Figure 11 Crack% INCREASE, Kien-Giang, wetseason 2007 26 Head Rice After drying, % Figure 12 Head rice Before and After drying storage, an overall increasing trend was observed in the level of rice kernel fissuring, mechanical properties, head rice yield, and pasting characteristics for all three varieties used in the studies Rice kernels continued to fissure during storage for to months, surprisingly without adversely affecting head rice yield The increase in head rice yield during storage, regardless of an increasing amount of fissured kernels, implies that the physical integrity of the rice kernels was strong enough to resist cracking during milling This suggests the occurrence of physical ageing of the rice kernels when stored below the glass transition temperature, as previously noted in starchy materials by other researchers, making rice internal structure more rigid Further research is needed at molecular level to confirm the existence of physical 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and starch in the aging of non-waxy rice flour Food Chemistry 69, 229-236 Villareal RM, Resurreccion AP, Suzuki LB, Juliano BO (1976) Changes in physicochemical properties of rice during storage Starch 28, 88-94 Yasumatsu K, Moritaka S (1964) Fatty acid composition of rice lipids and their changes during storage Agricultural Biological Chemistry 28, 257 98 Zhang Q, Yang W, Jia, C (2003b) Preservation of head rice yield under high-temperature tempering as explained by the glass transition of rice kernels Cereal Chemistry, 80, 684-688 Zhou Z, Robards K, Helliwell S, Blanchard C, Baxterb G (2003) Rice ageing I Effects of changes in protein on starch behaviour Starch 55, 162-169 99 SECTION Measurement of glass-rubber temperature (Tg-r) of rice by Thermal Mechanical Compression Test (TMCT) 100 SECTION Measurement of glass-rubber temperature (Tg-r) of rice by Thermal Mechanical Compression Test (TMCT) ABSTRACT TMCT (Thermal Mechanical Compression Test) was applied to measure the Tg-r (glass-rubber transition temperature) of rice flour and individual rice kernels at low moisture content (2.4-19.5 % wet basis) As expected, Tg-r decreased with increasing moisture content The Tg-r temperatures measured by TMCT were comparable with those measured by DSC (Differential Scanning Calorimetry), TMA (Thermo-mechanical Analysis), and DMTA (Dynamic Mechanical Thermal Analysis) as reported in the literature These results indicated that the TMCT technique can be applied to measure the glass-rubber transition of a single grain of rice or the rice in powder form The sensitivity of the test is higher with the rice kernel than with rice flour TMCT technique has the advantage of simplicity and cost-effectiveness compared with the other techniques INTRODUCTION Determination of the glass transition temperature (Tg) of rice corresponding to its water content has become very important since the glass transition concept has been applied to explain rice fissure formation during drying [1, 2] Most of the studies involved in estimating Tg of granular starch was carried out on the water-starch system [3-6] rather than native starch or individual kernel It has been pointed out that DSC (Differential Scanning Calorimetry), a common method to determine glass transition temperature, could not detect the Tg of a low moisture, high molecular weight biological polymers such as starch systems This is due to several drawbacks related to the nature of native starch at molecular and micro-structural levels and small change in the heat capacity during the transition from one state to other [3, 6] These two factors result in broadening of the thermal event which makes it difficult to detect the step changes in specific heat For such materials, detection of changes in the mechanical properties has been proved to be more sensitive than the changes in the heat capacity [7] Therefore, TMA (Thermo-mechanical Analysis) and DMTA (Dynamic Mechanical Thermal Analysis) are employed to detect the glass transition event [8, 9] Recently, Rahman et al.[10] has published a comparative study between various methods used to determine the glass transition temperature by considering spaghetti as a model food material TMA has been applied to develop the state diagram for mapping glass 101 transition temperature at different moisture levels in brown rice [8] DMTA has also been employed to measure the glass transition temperature of brown rice kernels at low moisture content range (5-20% wet basis) [9] However, in both methods a complex sample preparation technique is required, for example, covering rice kernel with Al2O3 powder, to prevent moisture loss when heating rice kernels [9] The recent TMCT (Thermal Mechanical Compression Test) device, which was developed by Bhandari and co-workers at The University of Queensland, Australia, has a potential to measure the physical state transition temperature of any solid materials without much sample preparation step [11-13] This technique is based on the force-deformation or stress-strain response of a material under compression during heating The amorphous state of material will be transformed from a glassy to rubbery state under the compression and heating, thereby allowing the phase change to be detected by a sudden displacement of the compression probe The transition is termed as glass-rubber transition (Tg-r) since the probe displacement occurs due to the change in the viscosity of material at the interface of heating surface This is similar to a creep test but under thermal scanning condition The sample cell is designed to be compatible with mechanical analyzers such as Instron or Texture Analyser Any mechanical changes that occur on the particle surface can be sensitively detected, owing to material being in contact with a large surface area under the compression probe Furthermore, moisture loss can be minimized since the contact surface of sample is covered by the compression probe To note, only the change in the mechanical property of the sample surface that is in contact with the heating surface is enough to detect the transition temperature This TMCT has been applied to detect the glassrubber transition temperature (Tg-r) of various dry food materials, i.e skim milk, whey, honey, and apple juice powders, pasta, and starch powders against standard DSC and TMA methods [1113] The objective of this work was to investigate the applicability of TMCT to measure glass- rubber transition temperatures of individual rice kernel and rice flour The values obtained by this method will be compared with the literature values which are obtained by TMA and DMTA techniques 102 METHODOLOGY TMCT System Figure illustrates the TMCT system comprising of a thermally controlled aluminum sample block (50x50x25 cm) which is heated normally at a rate of 30 oC/min through heating elements inserted into the sample block The Texture Analyser TA-XTplus (Stable Microsystems, UK) records the probe position and compression force at specific time, and temperature through 35 mm cylindrical probe and temperature probe, respectively Computer Heater controller Temperature probe Thermocouple for heating control Texture analyser probe Rice sample Sample block Heating elements Figure Illustration of Thermal Mechanical Compression Test System In this test, the probe displacement or movement is detected when there is a transformation of physical state of the sample from the glassy to rubbery state The onset temperature at this point is referred as ‘glass-rubber transition temperature’ (Tg-r) TMCT is designed to detect glassrubber transition by heat scanning only The samples should already be in glass state at ambient condition It is not possible to measure the reversible transition event while cooling Sample preparation The measurements were done using single kernel of rice or the flour from the same rice sample Milled YRM64 rice flour was dried in a vacuum oven at 40 oC for 24 h The sample was then 103 cooled down in a desiccator and stored in tight container at 25 oC for further use Saturated salts at different water activities were prepared at 25 oC as indicated by Bell and Labuza [14] Aluminum pans each containing a thin layer of g dried milled rice flour was transferred into vacuum desiccators Samples were allowed to equilibrate at 25 oC for at least weeks Moisture content of rice flour was determined according to AOAC method (32.1.02) [15] Sample data correction and testing procedure All the tests were carried out under creep mode of TA-TXplus (Stable Microsystems, UK) A 35 mm cylindrical compression probe was used Constant force (stress) 49.033 N was applied to the sample and corresponding deformation (strain) was measured with time Data acquisition rate was pps (point per second) Due to the possible expansion of the block and the probe during heat scanning, there can be a movement of the probe which will underestimate the result Therefore, a blank heat scanning was carried out The blank data were used to correct the data obtained from the sample scanning Pre-dried maltodextrin (DE6) was used as a blank material for data correction [13] Approximately g of maltodextrin was spread thinly on the sample cell and held under the probe at the force of 49.033 N for 300 seconds before the thermal scanning The maltodextrin was then scanned from room temperature to 200 oC at a heating rate of 30 o C/min During this thermal scanning, the displacement distances of the probe were recorded The procedure to conduct TMCT for equilibrated rice flour was similar to maltodextrin, except the scanning temperature up to 150 oC was used because rice flour is likely to decompose at higher temperature Each sample was carried out in triplicate and average value was used as glass-rubber transition temperature Representative curves obtained during thermal scanning are presented in Figure The changes in the temperature and corresponding probe displacement can be seen in this figure Determination of glass-rubber transition temperature The temperature-distance-temperature data was extracted for further analysis The corrected curve for each sample was obtained by subtracting the displacement blank data from the sample data The temperature vs subtracted displacement was plotted to estimate the Tg-r by performing a linear regression as shown in Figure 104 160 Temperature line 120 0.6 80 0.4 TMCT curve 0.2 300 400 500 Temperature, o C Distance, mm 0.8 40 600 T ime, second Probe displacement, mm Figure The TMCT curve and the temperature line are obtained in TA.XTplus at heating rate of 30 oC/ Transition region Tg-r Temperature, 0C Figure Estimation of glass-rubber transition temperature from TMCT curve RESULTS AND DISCUSSION TMCT curves of YRM64 milled rice flour at some selected moisture contents are presented in Figure The slopes were steeper at higher moisture contents indicating that moisture content possibly weakens the intermolecular forces of materials in solid state Our attempt to measure the transition event using a DSC was not successful No change could be detected in the DSC thermograms (results not presented) As stated earlier, the sharp changes in the mechanical 105 property of the material allowed detecting the glass transition regions easier than using the DSC technique which is based on specific heat change of material during this transition 0.30 Displacement, mm 0.25 15.9% 0.20 14.5% 0.15 12.8% 9.8% 0.10 8.3% 0.05 5.1% 0.00 20 40 60 80 100 120 140 160 180 T emperature, o C Figure TMCT curves of rice flour at some selected moisture contents (wet basis) Figure presents the dependence of Tg-r on moisture content It shows clearly that the Tg-r decreases with increasing moisture content This result emphasizes the role of water as strong plasticizer because the presence of water enhances the molecular mobility resulting in lower Tg-r according to the free volume theory The results presented in Figure indicated that the Tg-r value was not decreased sharply at higher moisture content, probably due to the limit of the plasticization effect of water on rice In this case, additional water does not interact strongly with the starch molecules, therefore fails to decrease the Tg-r rapidly The system can behave as phase separated into water and solid To predict glass-rubber transition temperature as a function of solid fraction of water, Gordon-Taylor model was used to fit the data Note that, this model was fitted based on the measured data Tg-r in the range of moisture content 2.4-19.5 % wet basis Due to the possible phase separation system at high moisture content, this equation can not be extrapolated beyond the experimental range used in this investigation The k value calculated from this model is 1.339 A low value of k indicates nearly a linear behavior of the model Tg-r (0K) can be predicted for rice flour by TMCT technique as following equation: 106 Tg − r = 370.175w1 + 184.782 w2 w1 + 1.339w2 Where, w1 and w2 are mass fraction of water and solid respectively 400 o Tg-r , K 350 300 250 200 0.1 0.2 0.3 Water fraction, g/g Figure Tg-r as a function of moisture content (the curve is the Gordon and Taylor equation fit) The predicted Tg-r in this study was compared to the published data on Tg of rice at the same moisture content range as presented in Table At the moisture content range 12-16 % wet basis, predicted Tg-r (41.6-56.7 oC) was comparable to Tg (approximately 50 oC) determined by DSC [16] At 14.4 % wet basis the predicted Tg-r in our study was 47.7 oC, which was quite close to that of measured by TMA at the same moisture content [8] The Tgs defined by DSC and DMTA (around 45 oC) at approximately 16 % wet basis were higher than our measured Tg-r (40.38 oC) [16] This discrepancy may be explained partly by the evaporation loss in test chamber in DMTA In DMTA, the measured temperature is not the actual sample temperature but of the test chamber temperature, and the Tg may be higher than the actual value In addition, the Tg-r measured by TMCT is an onset value, whereas in DSC and DMTA analyse it is generally reported as a midpoint value (between Tg-on and Tg-end) Independent tests with the same procedure to rice flour were applied to determine the Tg-r of individual rice kernels in order to investigate if single rice kernel can be used directly instead of ground flour It was found that glass-rubber transition temperature of single rice kernel could be 107 measured by TMCT as shown in Figure The measured Tg-r values of rice kernels at 17, 14 and 10 % wet basis were 40.9, 48.6 and 54.8 oC, respectively It is interesting to note that the measured Tg-r was almost identical for both individual rice kernel and rice flour at the same moisture content Furthermore, rice kernel showed sharper changes in displacement compared to those changes in rice flour (Figure 7) Table Published data on Tg of rice as compared to Tg-r measured in this study Moisture Samples Tg /Tg-r (oC) Method Reference content 12-16% ≈ 50.0 oC Milled rice DSC Rice flour TMCT 41.6-56.7 oC This study Brown rice kernel 16.3 %wb TMA 45.0 C [8] Rice flour 14.4 %wb o [16] TMCT 47.7 oC This study DMTA 45.0 C [9] DSC Brown rice kernel 45.1oC [17] Brown rice sections* Rice flour o 40.38 oC TMCT This study DSC: differential scanning calorimetry TMA: thermomechanical analysis DMTA: dynamic mechanical thermal analysis *Brown rice sections: brown rice kernel was cross-sectioned into three or four sections Displacement, mm 0.3 17% 0.2 14% 0.1 10% 20 40 60 80 100 120 140 160 180 o T emperature, C Figure TMCT curves of individual rice kernels at different moisture contents 108 Another interesting observation was an occurrence of rice kernel cracking at low water content under compression during thermal scanning above the transition temperature This showed a discontinuous change at 120 oC in TMCT curve as represented in Figure It is possibly a consequence of temperature gradient within the rice kernel resulting in rubbery exterior and glassy interior This translates into a mechanical stress enough to cause cracks in the kernel The same mechanism has been understood to cause rice cracking during rewetting of the dried rice grain At high water content, the temperature gradient might not be that large to cause such cracking under our experimental condition The brittleness of the grain will also be low at higher moisture content 0.5 RK, 7.8% Displacement, mm 0.4 Rice kernel cracking effect 0.3 RK, 12.8% 0.2 RF, 12.8% 0.1 RF, 7.8% 20 60 100 140 180 o T emperature, C Figure Mechanical behavior of both rice kernel and rice flour plotted at the same moisture contents RK: Rice kernel; RF: Rice flour CONCLUSION In brief, thermal mechanical compression test was found to be applicable to determine the glassrubber transition temperature of rice flour at a range of moisture content 2.4-19.5 % wet basis, which is difficult to detect by common DSC method The measured Tg-r values in this study were not 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starch thermal properties Ms Thesis The University of Arkansas, Fayetteville, AR, USA 1997 17 Cao, W.; Nishiyama, Y.; Koide, S Physicochemical, mechanical and thermal properties of brown rice grain with various moisture contents International Journal of Food Science and Technology 2004, 39, 899-906 111 .. .Section Study on the flat-bed dryer in the Mekong River Delta of Viet nam ABSTRACT The study, including experiments and survey on the flat-bed dryer, focused on the cracking of paddy grains,... author’s working experience with flat-bed dryers in the past 25 years Development of the flat-bed dryer The Mekong Delta in Southern Viet Nam, with 2.7 million hectares of rice land, is producing about... to the age-old constraint on the drying cost The theme of the current CARD Project seems to be in line with these requirements b) Extension: In light of the “new” recognition on quality, the