SECTION HIGH TEMPERATURE FLUIDISED BED DRYING (Experimental study on high moisture paddy- Vietnamese rice varieties) 75 Effect of high temperature fluidized drying and tempering on head rice yield and mechanical strength of Vietnamese rice varieties Introduction Being as a major exporter of rice all over the world, the quality of rice has become a central issue for Vietnamese farmers, particularly in rainy cropping season Therefore, it is important to dry rice as quickly as possible to prevent spoilage but maintain quality The conventional drying technique such as flat bed drying can take up to hrs (or even more) to dry the paddy to safe moisture level High temperature drying can allow to dry the paddy faster, therefore, the space and time taken to dry the paddy can be much shorter This can serve as a compact drier High temperature fluidized bed drying technique has been investigated to be an effective method for drying high moisture rice grain which is easily deteriorated in humid tropic climate (Taechapairoj et al 2003) In this method, the rice grain are suspended by the upward-moving drying air with high velocity 2-3 m/s, thus paddy and air are mixed vigorously (Kunze and Calderwood 2004) Fluidized bed (or fluid bed) drying is applicable in the first stage of drying when paddy is required to decrease high moisture content down to 18% (wet basis) Paddy then is continued to be dried at low temperature or by ambient air in storage bin (Proctor 1994; Taechapairoj et al 2003) Alternately, a multi-pass fluidized bed drying can be applied as a compact drying process High temperature (over 1000C) fluidized bed drying of paddy has been reported by a few researchers But, normally it is recommended that hot air drying temperature should not exceed 1500C to avoid the adverse effect on the whiteness of rice The lower range of drying temperature 40-1500C has been also investigated by Tirawanichakul et al (2004) They reported an improvement on the head rice yield when the drying temperature over 800C was used for the high moisture content (32.5% db) paddy This was probably contributed by the occurrence of partial gelatinization of starch at high temperature Some researchers suggested the need for tempering the grain for 25-30 minutes if high temperature drying is used (Poomsa-ad et al 2005; Prachayawarakorn et al 2005) Although there are a few research works being reported on high temperature fluidized bed drying, no real application has been reported Therefore, more understanding on this subject is required 76 This study aims to enhance the knowledge on the effect of high temperature drying and tempering on head rice yield, fissured kernels, mechanical strength and quality changes of Vietnamese rice varieties The specific objectives of this study are: To study the feasibility of high temperature fluidized bed drying of high moisture content Vietnamese rice varieties (two rice varieties were considered in this study) To study the optimum drying temperature and tempering duration as reflected by the lowest number of fissured kernel after the completion of drying followed by tempering To study the mechanical strength of the rice kernels under different drying and tempering regimes To study the effect of high temperature drying and tempering on the whiteness, pasting properties and crystallinity of rice starch These properties can reflect the cooking quality of rice Materials and Methods Fluidized bed drier: A batch lab-scale drier (HPFD150) developed at Chemical Engineering Department, Nong Lam University, Vietnam was used in this experiment (Figure 1) This dryer consists of three main components, namely (i) a cylindrical shaped drying chamber (40 cm in height x 15 cm diameter) (ii) kW electric heating units and (iii) a centrifugal fan driven by a 0.75 kW motor Inlet drying temperature ranging from 20-1000C was monitored by a Hanyoung DX7 temperature controller Outlet air temperature was monitored by a Daewon thermometer (Korea) Paddy Samples: Long-grain rice cultivars A10 and OM2717 were collected in fields of local farmers at Tien Giang Province and Hochiminh City in May 2007 Freshly harvest paddy (24-33% moisture, wb) was immediately transported to the laboratory and kept in cold storage maintained at 50C The A10 variety of paddy had 31-33% initial moisture (wb), whereas OM2717 had 25-26% moisture (wb) content Rice samples were allowed to equilibrate at room temperature before subjecting to drying 77 Approximately 200 gram of paddy (thickness of bed cm) was dried in fluidized bed dryer at 80 and 90oC for the drying periods of 2.5 and 3.0 The experimental procedure is illustrated in Figure Samples were transferred to a sealed glass jar immediately after drying and then tempered in an incubator set at 75 and 860C, which were equivalent to the grain temperatures after fluidized bed drying at 80 and 900C respectively The tempering duration used was 0, 30, 40 and 60 mins A sealed glass jar was warmed up to set temperature and stored in a foam box while taking out from the incubator to prevent the loss of heat After tempering, the samples were thin layer dried at 350C to safe storage level of moisture content (below 14% wb) The dried rice was sealed in plastic bags and kept at room temperature for days before determining the head rice yield (HRY), cracked grain ratio and mechanical strength of rice kernels 200 gram of paddy was also dried in the thin layer dryer at 350C for 16 h down to 14% wb and used as control sample All treatments were undertaken in triplicate Moisture content determination The moisture contents before and after tempering and final moisture content after thin layer drying of each drying run were determined by drying 5-10 gram of rough rice (in duplicate) in an oven at 1300C for 16h Head rice yield 100 gram of paddy was dehusked/dehulled and milled by laboratory milling system and whole kernel was separated from broken kernel to determine the head rice yield, which is defined as the ratio of the mass of unbroken kernel to the mass of paddy Cracked grain ratio Fissure enumeration was carried out in duplicate on 50 manually dehulled brown rice kernels per each measurement by visual observation with the assistance of a light box 78 Mechanical strength of kernels The breaking force of each individual brown rice grain was measured by three-point bending test with the test device (developed at the University of Queensland, Australia) attached to Universal Texture Analyser (Micro Stable Systems, UK) 50 rice grains of each lot were randomly selected and dehulled by hand Chalky and fissured kernels were discarded The pre-test, test and post test speeds of the probe used were 1mm/s, mm/s, and 10 mm/s respectively From the force-distance curve obtained by Texture Analyzer software, the peak force at which rice kernel failure occurred was regarded as breaking force of the rice kernel Colour determination The objective of the color measurement was to evaluate the effect of high temperature drying and tempering on the whiteness of the rice It was undertaken on only A10 rice variety Milled rice sample of each treatment was placed in a clear Petri dish and the colour parameters were measured by a Minolta Chroma Meter CR-200 (Japan) in CIE 1976 L*, a*, b* colour space Parameter L*, +a*, -a*, +b* and -b* describe the brightness, red, green, yellow and blue colour, respectively The total colour difference, ∆E* was also calculated Statistical analysis A multilevel factorial design, consisting of two levels of drying temperature (80 and 900C), two levels of drying time (2.5 and 3.0 min) and four levels of tempering time (0, 30, 40, and 60 min), was chosen in this experiment The data was analyzed by using statistical package MINITAB® Release 14 (Minitab Co., USA) and GLM (General Linear Model) procedure 79 Fluidized bed dryer (developed at Chemical Engineering Dept., Nong Lam University, Vietnam) Tempering of rice in sealed glass jars Thin layer dryer used in this study Dried rice of replicate (27 treatments) Figure 1: Equipment used in drying and tempering of the rice samples 80 Fresh paddies (A10 30-33%wb, OM2717 24-26%wb) Moisture determination Fluidized bed drying 800C 2.5 & 3.0 90 0C 2.5 & 3.0 Moisture determination Tempering in incubator 750C 86 0C Tempering time 0, 30, 40 and 60 Thin layer drying at 350C Moisture determination Moisture determination Head rice yield (%) Fissured kernels (%) Breaking force (N) Figure Schematic diagram of experimental and measured parameters 81 Results and Discussion The evolution of moisture content during fluidized bed drying and subsequent tempering for both rice varieties is presented in Figure The percentage of moisture content removal of the paddy was found to be in the range of 7.7 to 12.0% As can be seen in Figure 3, increasing drying temperature led to larger percentage of moisture removal Tempering period not only helped moisture redistribution inside the rice kernel but also caused slight loss of moisture during prolong tempering duration (40 as shown in Figure 3) Within 2.5 of fluidized bed drying, the moisture content for A10 variety dropped by 8.7-9.4% at 800C and 11.0-12.0% at 900C, while it dropped by 7.7-8.6% at 800C and 9.7-11% at 900C for OM2717 variety Extending drying time to 3.0 did remove further 1% moisture content in both drying temperatures for A10 rice However, there was only 0.1-0.3% moisture content drop for OM2717 rice It is suggested that the amount of moisture that can be removed during fluidized bed drying depends on the initial moisture content of paddy Note that, the initial moisture content of A10 (31-33% wb) was higher than that of OM2717 (25-26% wb) An extension of drying time to 3.0 did not remove further moisture because the diffusion of moisture inside the kernel is time dependent If the drying proceeds further moisture removal will continue from outer surface of the grain which will result in to physical stress due to increased differential moisture between the interior and exterior layers of rice kernels Therefore, the tempering step is necessary to be employed to allow moisture to diffuse from interior to exterior prior to further drying 82 30 A10 Drying duration 20 OM2717 M oisture content, %wb 40 10 Tempering duration 0 10 20 30 40 50 60 70 Ope ration tim e, m in 80C, 2.5 80C, 3.0 90C, 2.5 90C, 3.0 80C, 2.5 80C, 3.0 90C, 2.5 90C, 3.0 Figure The reduction of moisture contents during fluidized bed drying and tempering at high temperatures for two Vietnamese rice varieties (A10 and OM2717) Head rice yield Drying time effect: Increased drying time reduced the head rice yield significantly (Figure 4) In our other experiments we found that a longer than minutes drying time caused a remarkable reduction in moisture content below 17.5% wb but caused higher fissures in the grain (results not presented here) As mentioned earlier, a longer exposure of grain at higher temperature causes strain on the grain due to the rapid removal of moisture from the surface while it takes time to diffuse water from the interior The large differential moisture will cause more fissures in the kernels According to this result, not more than 2.5 minutes should be the drying time for the rice at these high temperature conditions, although no experiment was done for less than 2.5 assuming that we will not achieve enough moisture removal for shorter time than this (Figure 3) 83 Tempering time effect: The experimental result in this study showed that at both drying temperatures with drying time of 2.5 min, extending tempering duration to 40 can improve the head rice yield (Figure 4) The same trend was observed for both rice varieties There was a clear trend of increasing head rice yield with tempering time up to 40 minutes It indicates that 30-40 minutes can be an optimal tempering time for both varieties It should be noted that the tempering temperatures used in this work are above the glass transition temperature of rice This means that the rice were in rubbery state during the duration of tempering This will be discussed more in further work in future It was found that the drying temperature, tempering time and interaction between drying temperature and tempering time had significant effect on HRY (P