Ministry of Agriculture & Rural Development Project Completion Report MS14: PROJECT COMPLETION REPORT 026/05VIE Investigation of rice kernel cracking and its control in the field and during post-harvest processes in the Mekong Delta of Vietnam APPENDIX Influence of harvesting time around grain maturity on rice cracking and head rice yield in the Mekong River Delta of Vietnam April 2010 63 APPENDIX Influence of harvesting time around grain maturity on rice cracking and head rice yield in the Mekong River Delta of Vietnam ABSTRACT Timely harvesting plays an important role in controlling rice cracking Reduced whole rice grain yield due to cracking causes the value loss and reduces the farmers’ income Field experiments were carried out to study the effect of harvesting time around crop maturity on rice cracking and head rice yield for seven common rice varieties (OM1490, OM2718, OM2517, OM4498, AG24, IR50404 and Jasmine) in three different locations during two cropping years (2006-2008) in the Mekong River Delta, Vietnam The results showed that the rice cracking was strongly influenced by both the variety and time of harvesting around maturity There was a general trend of increase in percentage of cracked rice with late harvesting in relation to estimated grain maturity date The head rice yield also followed the same trend in response to delayed harvesting A delay of 46 days reduced the head rice yield by 11.3 % an average and up to 50 % Similar trends were observed in both wet and dry seasons The large varietal difference in percentage of cracked grain (0.9 to 60.5%) on days after maturity date indicated that the level of rice cracking caused by late harvesting time can be minimized by the selection of suitable varieties INTRODUCTION Head rice yield, which is defined as the weight percentage of rough rice that remains as head rice (the kernels that are at least ¾ of the original kernel length) after milling, is considered as the main quality indicator because the broken rice has often half the commercial value of whole grain rice It has been shown that timeliness of harvesting can influence milling yield significantly Harvesting rice at crop maturity can give a maximum head rice yield (Kester et al 1963, Bal and Oiha 1975) Any delay in harvesting time causes reduction of head rice yield (Bal and Oiha 1975, Ntanos et al 1996, Berrio et al 1989) and extended delay in harvesting can lead to significant losses in head rice yield Berrio et al (1989) showed that among 16 investigated rice varieties studied the whole-milled grain was reduced by 18% when harvesting was delayed by weeks However, it was also found that there was no impact of harvesting time on sensory perception of rice (Champagne et al 2005, Chae and Jun 2002) The incidence of rice fissuring in the field has a potentially significant impact on head rice yield Cracking can develop in the field as a result of changes in grain moisture after the rice matures due to hot sunny days followed by humid nights Harvesting time affects proportion of cracked rice and hence head rice yield Large quantities of immature rice kernels can be detected in early harvested rice Immature kernels are usually thinner and defective, and are easily cracked during subsequent milling (Swamy and Bhattacharya 1980) In contrast, late harvested grain is often associated with a grain product that is too dry and more prone to fissuring Investigations by 64 Chau and Kunze (1982) showed that cracking can develop in low-moisture content kernels (13% or 14% wet basis) before harvesting as a consequence of the swings in relative humidity in the atmosphere Furthermore, improper post-harvest practices, such as a delay in threshing when rice stacks are left in the field, can also provide the potential of moisture adsorption due to an uneven moisture content and uneven maturity within the bulk rice (Kunze and Prasad 1978) Reduced whole rice grain yield due to cracking is one of the major issues that directly reduce income and availability of staple food to the farmers in the Mekong River Delta of Vietnam Mekong River Delta is the largest rice production region in Vietnam The cracking or partial fissuring of rice kernels may occur right in the paddy field due to incorrect harvesting time and improper harvesting practices, and occur also due to adverse post-harvest drying conditions and inappropriate milling operations The weather pattern (temperature and humidity) in Mekong River Delta is unique The rice is grown and harvested in both wet and dry seasons Weather conditions at around harvesting period are different between the two seasons and this can impact the rice fissuring and cracking during milling However, there is no experimental data available on the impact of harvesting time on rice cracking and head rice recovery on the rice varieties grown at different seasons in the Mekong River Delta This research work is an attempt to systematically collect the rice cracking and head rice yield data based on field experimentations in four consecutive harvesting seasons between 2006 and 2008 The main factor considered in this study during the collection of data was harvesting time- before and after grain maturity The objective of this experiment was to evaluate the effects of harvesting time of several rice varieties on the level of rice cracking and head rice yield in different seasons This study will assist to determine the optimal harvesting time for various rice varieties grown in the Mekong River Delta MATERIALS AND METHODS Rice samples Experiments were carried out at three locations, namely Seed Centre (An Giang Province), Tan Phat A Cooperative (Kien Giang Province) and Tan Thoi Cooperative (Can Tho City) in four consecutive harvesting seasons during two years (2006-2008) Seven rice varieties commonly cultivated in these cooperatives and seed centre were selected for field experiments as shown in Table The grain maturity date of these rice varieties provided by the local extension centers were in the range of 86-98 days (Table 1) The maturity date is defined here as the harvesting date expressed in days after sowing (DAS) planned by the farmer as recommended by the extension centre based on the predicted physiological maturity of the grain 65 Table Rice varieties and their maturity dates used for this study Experimental MD†† Variety Crop season Recommended MD† Wet OM1490 Dry 92 Wet OM2718 92 Wet 90 85-90 Dry 86 Wet OM4498 92 90-95 Dry OM2517 92 87-92 90 90-95 Dry 91 Jasmine 95-105 98 AG 24 Wet 90-95 90 IR50404 † Wet Wet 90-95 92 Recommended maturity date (days after sowing) given by local extension centre Maturity date (days after sowing) chosen for this study †† Experimental design Each experiment consisted of seven treatments corresponding to the harvesting times prior and after expected maturity date for each of seven varieties These varieties were grown in different paddies at the three locations There were seven harvesting times, two days apart commencing six days prior to maturity date (MD) and ending at six days after MD Experiments were designed in RCBD (Random Complete Block Design) method with five blocks (Table 2) Table Treatments (harvesting days) in relation to the maturity date (MD) 0, +2, +4, +6 and -2, -4, -6 are harvesting days after and before estimated maturity, respectively A, B, C, D, E are the replication blocks Block A B C D E Treatment (Days vs MD) (-6) -6A -6B -6C -6D -6E (-4) -4A -4B -4C -4D -4E (-2) -2A -2B -2C -2D -2E (0) 0A 0B 0C 0D 0E (+2) +2A +2B +2C +2D +2E (+4) +4A +4B +4C +4D +4E (+6) +6A +6B +6C +6D +6E 66 Experimental procedure Some rice fields of selected varieties were chosen and used for the experiment Wet season experiments were sown in March-April and harvested in June-July (in 2006 some varieties were grown in late wet season and harvested in September), while sowing and harvesting for dry season experiments were November-December and February-March, respectively Figure depicts plot layout for harvesting time experiment of each rice variety Grains were harvested from 35 sub-plots of m x m (total harvesting area is 70 m2) at harvesting dates according to the treatments from days before to days after maturity date (MD) with five replications for each treatment (Figure 1) Cutting and threshing operations were done manually using a sickle for cutting the rice stalk Rice was harvested in the morning to avoid intense sun light, aiming to reduce natural cracking due to sudden change of moisture distribution inside the kernel when it goes between wet night and dry day After cutting, rice was transferred into a shaded area for manual threshing and cleaning in which most bulk straw, chaff, immature grain, and very light and fine impurities were separated from the grains The straw and chaff were manually separated and the grain was dropped though a cross-wind to remove the lighter impurities 1.5 m Samples were then transferred to the dryer after undertaking moisture determination Samples were gently dried at 35 oC using a laboratory tray drier developed by Chemical Engineering Department, Nong Lam University Ho Chi Minh City, Vietnam until the moisture content reached 14 % wet basis Then samples were once again cleaned to remove residual immature grains, measured for moisture content using grain moisture tester (Kett Co Ltd., Japan), packed in nylon bags, and transferred into lab for rice cracking and head rice yield analyses 1A 2A 3A 4A 5A 6A 7A 1m 1m 2m 3B 1B 5B 2B 6B 7B 4B 4C 3C 1C 6C 7C 2C 3C 5D 6D 7D 3D 1D 4D 2D 7E 4E 6E 5E 2E 3E 1E Figure Plot layout of harvesting experiments for each rice variety Each plot is m long and m wide and the border around harvesting area is 1.5 m 67 Measurements Cracking of paddy rice This is the direct indicator for effects of harvesting time on cracking Three 150 g paddy samples were taken from each plot to ensure the repetition of each plot Grains were dehulled by hand to avoid cracking developing during this procedure Fifty dehulled grains were randomly checked to count cracking grains under microscope, and cracking fraction calculated Head rice yield Exact 180g of paddy was dehusked and then 100g of brown rice was whitened for 60 seconds using a laboratory milling system Whole kernels were separated by grader from broken kernels, to determine head rice yield which is defined as the ratio of the mass of unbroken kernel to the total mass of paddy rice The head rice is composed of grains which maintain at least 75% of their length after milling Statistical analysis Data were analysed by statistical software Statgraphics® 3.0 (StatPoint, Inc.) using ANOVA (Analysis of Variance) procedure RESULTS AND DISCUSSION Level of rice cracking Percentages of cracked grain before husking obtained from seven varieties in four consecutive crop seasons, i.e., wet season 2006, dry and wet season 2007, and dry season 2008, are shown in Table For each rice variety, level of rice cracking before husking were significantly different among harvesting dates (P0.05) † harvested in ‘late wet season’ which was in September 2006 Increased rice cracking due to delayed harvesting also depended on the variety There was a large amount of cracked grains after maturity date for OM2517 (16.00-60.53%) and AG24 (21.47-53.07%) in dry season 2007 and dry season 2008, respectively In contrast, percentages of cracked grain of IR50404, OM2718, and OM4498 varieties had lower values in both wet and dry seasons (in the range of 0.4-12.27%, 3.20-10.80% and 1.07-10.40%, respectively) after maturity date This implied that there is a varietal difference on rice cracking and hence the selection of variety is important in decreasing cracked grain percentage The cracking behavior of the rice in the field is expected to depend on the season due to the different patterns of temperature fluctuation during day and night, solar radiation intensity, sunshine hours and frequency of rain During the rainy season, the rice grain can develop cracks during the late maturity stage due to rewetting At the same time, during dry season it is likely that the grains over-dry if not harvested by its maturity However, data on Table obtained from four consecutive crop seasons (wet 2006, dry and wet 2007, and dry 2008) showed that crop seasons did not have much impact on level of rice cracking as similar trend was observed in both wet and dry seasons 69 Head rice yield The head rice yield as a function of harvesting time for seven rice varieties is presented in Table The head rice yield was generally less at late harvesting time A delay of 4-6 days reduced the head rice recovery by up to 50% of the head rice yield at the expected maturity The head rice yield followed the opposite trend to rice grain cracking, indicating that the presence of cracks in the original paddy reduced the head rice recovery The overall results as influenced by harvesting time are presented in Table It should be noted that the head rice yield is affected by a laboratory milling system, as it is a function of milling efficiency Therefore, the head rice yield data is presented in relative term in Table 5, with the recovery on the harvesting at maturity (0 day) being assigned a value of 100% In addition, due to the limited number of experiments undertaken, the values are presented as a range for each variety Table Change in head rice yield of seven rice varieties at different harvesting time (days after expected maturity date) Variety Crop Head rice yield (%) before and after maturity day season -6 -4 -2 +2 +4 +6 cd d cd c b a 52.30 50.73 48.08 42.23 36.51 34.53a OM1490 Wet ‘06 51.06 66.21c 66.93c 67.90c 64.57bc 60.25ab 56.35a Dry ‘07 63.13bc a a a a a a 45.10 52.15 45.56 49.81 49.26 49.01a Wet ‘07 50.03 c d bc bc ab a 51.47 43.54 43.91 38.76 36.83 40.72abc 0M2718 Wet ‘06 45.41 67.01b 66.40b 67.48b 66.22b 63.81a 62.41a Dry ‘07 67.93b d b b c c b 41.09 45.19 56.68 53.18 43.74 28.63a OM2517 Dry ‘07 64.58 c bc a ab bc c 44.16 37.88 42.19 44.47 49.24 44.34bc Wet ‘07 48.01 65.36c 64.67c 59.84c 60.55b 55.29a 52.90a Dry ‘08 65.68c a bc bc d cd bc 54.35 54.02 58.33 56.95 53.78 52.55b OM4498 Dry ‘07 43.80 37.77a 35.83a 39.35ab 37.87ab 42.42b 35.35a Wet ’07 36.64a b bc b bcd d a † 42.35 40.76 43.50 46.99 35.90 35.35a AG24 Wet ‘06 40.35 c bc b a a a 55.42 52.38 42.62 43.55 36.48 37.94a Dry ‘08 61.66 56.94b 57.79c 53.27a 56.54bc 55.67abc 54.55ab Wet ‘07 58.08c de cd e c b b 61.75 64.57 60.28 57.40 56.99 51.68a IR50404 Dry ‘08 64.28 a c bc c bc b † 54.65 51.82 55.36 54.59 48.15 49.46bc Jasmine Wet ‘06 41.59 All data represent mean values of five replications The same superscripts in the same row indicates that the values are not significantly different (P>0.05) † harvested in ‘late wet season’ which was in September 2006 In general, the optimum harvesting time presented in Table is similar to the maturity time shown in Table for all varieties used in this investigation Suggested optimum harvesting times in wet season for OM1490 (94 days) and OM2517 (94 days) are 2-4 days longer than recommended maturity day by local extension centre It can be concluded that (1) even if the rice varieties were harvested about the right time, varieties differ considerably in the cracking and 70 hence intervention opportunity of growing low cracking varieties such as OM2718 for farmers and developing such varieties for rice breeders, (2) harvesting at optimum harvest time had rather small cracking problem but delay of days can cause major problem, and hence intervention opportunity here to ensure harvesting at the right time, and (3) varieties differ in their response to time of harvesting hence time of harvesting is more critical for some varieties than others, for example OM2517 was the most sensitive variety, and hence there is an opportunity for intervention to ensure quick harvesting of particular varieties Table Seasonal trend of effect of harvesting time before and after maturity (4-6 days prior and 4-6 days later than the expected day of maturity) on the proportion of cracked grains (prior to milling) and head rice recovery Head rice yield is expressed as relative to the yield on maturity day Crop Rice Optimum Proportion of cracked grain % Relative head rice yield % season variety harvesting Before maturity After maturity Before maturity After maturity date OM1490 0.8-9.6 1.1-23.6 101-109 72-88 94 Wet OM2718 0.4-1.2 4.0-10.8 103-117 84-93 92 OM2517 3.5-15.7 12.1-20.3 90-114 105-117 94 OM4498 2.5-3.9 8.1-10.4 91-93 96-108 94 AG24 0.3-1.5 1.1-4.1 93-97 83-108 94 IR50404 1.1-1.5 0.4-1.3 103-105 99-106 90 Jasmine 4.0-4.5 6.0-7.7 75-99 87-99 98 OM1490 0.5-2.3 5.6-22.4 93-99 83-95 92 Dry OM2718 0.7-6.3 3.2-8.5 98-101 92-98 92 OM2517 0.7-3.6 9.3-60.5 77-106 51-97 86 OM4498 1.1-3.7 1.1-9.3 75-93 90-98 91 AG24 6.5-16.4 21.5-53.1 133-145 86-102 88 IR50404 0.8-2.8 1.7-12.3 105-107 86-95 88 CONCLUSIONS A few days early harvesting (before maturity) is better than late harvesting by to days because late harvesting will make the grain more sensitive to cracking Therefore, any delay or longer harvesting time can cause more losses, as is often the case of harvesting by hand The degree of harvesting time effect is also dependent on the variety REFERENCES Bal, S., & Oiha, T P., 1975 Determination of biological maturity and effect of harvesting and drying conditions on milling quality of paddy Journal Agricultural Engineering Resource, 20, 353-361 Berrio, L E., & Cuevas-Perez, F E., 1989 Cultivar differences in milling yields under delayed harvesting of rice Crop Science, 24, 1510-1512 71 Calderwood, D L., Bollich, C N., & Scott, J E., 1980 Field drying of rough rice: Effect on grain yield, milling quality energy saved Agronomy Journal, 72, 644-653 Chae, J C., & Jun, D K., 2002 Effect of harvesting date on yield and quality of rice Korean J Crop Sci., 47(3), 254-258 Champagne, E T., Bett-Garbet, K L., Thompson, J., Mutters, R., Grimm, C C., & McClung, A M., 2005 Effects of Drain and Harvest Dates on Rice Sensory and Physicochemical Properties Cereal Chemistry, 82(4), 369-274 Chau, N N., & Kunze, O R., 1982 Moisture content variation among harvested rice grains Transactions of the ASAE, 25(4), 1037-1040 Kester, E B., Lukens, H C., Ferrel, R E M., A., & FIinfrock, D C., 1963 Influences of maturity on properties of western rice Cereal Chemistry, 40, 323-326 Kunze, O R., & Prasad, S., 1978 Grain fissuring potentials in harvesting and drying of rice Transactions of the ASAE, 21(2), 361-366 Ntanos, D., Philippou, N., & Hadjisavva-Zinoviadi, S., 1996 Effect of rice harvest on milling yield and grain breakage CIHEAM-Options Mediterraneennes, 15(1), 23-28 Swamy, Y M I., & Bhattacharya, K R., 1980 Breakage of rice during milling- Effect of kernel defects and grain dimension Journal of Food Process Engineering, 3, 29-42 72 Table List of positions of diffraction peaks detected for variety A10 rice flour samples T δ τ Positions of diffraction peaks (2θ values) 2θ o 2θ o 15.17 17.21 15.04 17.09 30 15.17 17.04 40 14.99 17.03 60 15.08 17.12 14.97 17.09 30 15.12 17.14 40 15.12 17.20 60 15.07 17.10 90 2.5 15.08 17.20 30 15.05 17.13 40 15.03 17.22 60 15.14 17.20 15.03 17.21 30 15.21 17.29 40 15.12 16.96 60 14.95 16.93 Reference sample δ: drying temperature; τ: drying time; T: tempering time o C 80 2.5 2θ o 18.09 17.92 18.00 17.88 18.01 17.97 17.97 18.03 18.01 17.99 17.98 18.00 18.06 17.88 18.07 17.92 17.82 2θ o 19.96 19.82 20.00 19.92 19.97 19.87 17.98 19.87 20.02 19.98 20.06 19.95 - 2θ o 23.12 22.93 23.12 22.92 22.95 22.99 23.06 23.01 22.93 22.99 22.98 23.09 23.06 23.15 22.94 22.86 Absolute crystallinity % 34.0 35.4 33.8 29.9 34.2 33.4 35.1 29.4 34.2 32.5 33.4 28.5 35.4 33.7 31.3 28.3 36.6 Microstructure of rice kernels Figure presents the microstructure and cracking of cross-sectional areas of reference rice kernels (35oC thin layer drying for 16 h) and the fissures existed between and inside endosperm cells can be seen in Figure A close-up view (Figure 5b) shows the cracks with starch granules in a polygonal shape Figure depicts the microstructure of rice kernels subjected to the most severe heating conditions used in this study (drying/tempering regime: 90oC for min/86oC for 60 min) at different magnifications It is hypothesized that the gel network created during gelatinization can heal the fissures within the rice kernel by filling the void between adjacent fissure traces Consequently, kernel integrity may be improved through a partial gelatinization process resulting in higher head rice yield (a) (b) Figure (a) Cracks between endosperm cells observed in thin-layer A10 rice kernels; (b) Close-up image of fractured surface 144 A (a) (b) PB (c) (d) (e) Figure The microstructure of cross-sections of fluidized bed dried rice kernels 145 Conclusion As stated in the first part of this study (MS6), the tempering step significantly reduced the level of kernel fissuring and improved the head rice yield The enhancement of kernel integrity with respect to hardness and stiffness during tempering was identified in previous study Resistance to breakage during milling might have been improved by the fusion of starch at outer layers of kernels due to partial gelatinisation at high temperature and over a prolonged heating time Consequently, the head rice yield was even higher than for the reference sample due to partial gelatinization, as confirmed by SEM photographs in this second part The occurrence of partial gelatinization during high temperature drying and tempering, alters some of the physicochemical properties and microstructure of fluidized dried rice Pasting viscosities decreased with increasing temperatures, while pasting/gelatinization temperatures tended to increase with more severe drying and tempering treatments As the rice becomes harder and stiffer, it may require a longer cooking time when compared with conventionally dried rice However, the texture of the rice tempered for a prolonged time, can be softer on account of reduced pasting properties These findings assisted in improving our understanding of the role of tempering Further studies are needed on cooking quality 146 Ministry of Agriculture & Rural Development Project Completion Report MS14: PROJECT COMPLETION REPORT 026/05VIE Investigation of rice kernel cracking and its control in the field and during post-harvest processes in the Mekong Delta of Vietnam Appendix 4B Changes in cracking behaviour and milling quality due to postdrying annealing and subsequent storage April 2010 147 Appendix 4B Changes in cracking behaviour and milling quality due to post-drying annealing and subsequent storage Introduction Some studies have attempted to explain the effects of drying and tempering conditions on the head rice yield, based on the glass transition concept Using glass relaxation concept, it is suggested that there is a possibility of structural relaxation or physical ageing during rice tempering/annealing It is necessary to further investigate this phenomenon with post-drying annealing on freshly dried paddy The investigation of post-drying annealing effect at above and below glass transition temperature of rice on mechanical strength and its association with the level of kernel fissuring and milling quality can provide additional valuable insight to understand the rice cracking behaviour The objectives of this study were (i) to investigate the effect of drying and post-drying annealing at above and below glass transition temperature of rice on the mechanical strength of rice, in relation to level of kernel fissuring and milling quality; (ii) to examine the changes in mechanical properties and milling quality of rice during storage Australian rice varieties were used in the research undertaken Materials and Methods Three Australian grown rice varieties, namely Kyeema, Amaroo and Reiziq were used in this study Rewetted rough rice (24-27% wb) of each rice cultivar was subjected to thin layer drying (1 cm thick) at three drying conditions 40oC-25%RH, 60oC-20%RH, 80oC-16%RH Rice lots were then subdivided and immediately transferred to sealed glass jars, which were subsequently held in an incubator for annealing experiment Rice samples were annealed at the same drying temperatures at which they were dried (40, 60, and 80oC) for 0, 40, 80, and 120 in an incubator After annealing, rice samples were cooled down by placing in the incubator set at 25oC and 65% RH The same final moisture contents of rice samples were also targeted to minimize the effect of moisture content on measurements of mechanical strength of rice afterwards Rice samples then were sealed in plastic bags, kept at room temperature for 2-3 days before determining the amount of fissured kernels, mechanical strength and head rice yield (HRY) The optimum drying and annealing conditions for each of rice varieties which gave the highest head rice yield were selected to further investigate the effects of annealing during subsequent storage To reduce the amount of treatments, only drying temperature at 40oC and 80oC were chosen to investigate the storage effect Rice samples were then subdivided into 150 g for each treatment and kept in sealed plastic bags in an incubator at three levels of annealing/storage temperatures (4, 20, 38oC) up to months of storage Every month, rice samples were removed from incubator, equilibrated at room temperature and subjected to measurements of amount of fissured kernels, mechanical strength, head rice yield and pasting properties 148 Results Effects of drying and post-drying annealing conditions on level of rice kernel fissuring, mechanical strength and milling quality of rice Drying temperature, annealing time and the interaction between them had significant effects on the level of rice kernel fissuring, mechanical strength and milling quality for all three varieties Fissured kernels The amount of fissured kernels of all treatments after each drying temperature and annealing time was investigated Within each rice variety, the level of kernel fissuring tended to increase with increases in the drying temperature (Table 1) Without annealing, the internal stresses created within the rice kernels during drying cannot be fully relaxed and this also results in kernel fissuring in the post-drying conditions The level of fissuring (in terms of percentage of fissured kernels) significantly decreased with increasing annealing time up to h For drying temperature at 40oC, the drying rate was not sufficiently great to have an adverse effect on fissuring levels Annealing at this temperature did not have any additional beneficial effect because the amorphous matrix of rice is already in the glassy state and the relaxation Mechanical strength Annealing duration (ranging from to 120 min) versus mechanical strength parameters (hardness and stiffness) for the three rice cultivars at three drying temperatures is shown in Figure For the three varieties, it is apparent that increasing drying temperature resulted in stronger rice kernels, as both hardness and stiffness values of the kernels increased Mechanical characteristic, particularly stiffness, also improved with annealing duration Table Head rice yield and the amount of fissured kernels of three rice varieties for different drying temperature and annealing durations Fissured kernels, % Head rice yield, % δ τ o C Kyeema Amaroo Reiziq Kyeema Amaroo Reiziq 39.0±1.5ed 24.0±4.0cd 64.3±3.7c 45.8±2.8cd 12.7±2.1c 62.3±1.4b 40 41.7±2.5d 25.0±2.0c 65.2±4.3c 46.6±3.0cd 7.5±0.9cd 64.6±3.3bc 40 cd ed cd bc cd 40.3±0.6 22.7±1.5 67.2±2.8 45.4±1.2cd 10.0±4.0 64.8±1.9 80 cd ed c bc cd 40.0±1.0 24.7±0.6 67.1±2.1 48.3±2.6cd 9.7±1.2 65.0±1.5 120 ab ab b c a 63.0±1.0 30.0±2.0 30.1±0.4 25.3±3.8a 21.0±1.2 40.7±1.3 60 bc b cd b b 58.0±3.5 23.0±2.0 52.2±2.4 33.2±4.6b 17.3±1.2 58.8±3.3 40 c c cd b b 50.3±6.7 21.7±1.2 56.9±1.6 44.1±7.4cd 14.3±2.1 61.6±0.3 80 45.3±1.5cd 21.7±0.6cd 60.8±2.4bc 43.9±3.2cd 8.67±1.2d 63.7±3.1b 120 a a a a a 67.0±2.7 36.0±2.0 22.2±3.0 22.2±1.8a 24.3±0.6 25.0±4.9 80 b b bc b b 58.7±1.5 26.0±2.0 54.6±1.2 32.5±1.6b 18.7±1.2 59.5±0.8 40 bc cd cd b b 45.7±2.5 22.0±2.0 54.9±2.2 40.5±3.4c 15.7±2.9 60.7±1.3 80 16.0±1.0bc 59.2±1.5b 45.5±1.3cd 20.3±0.6cd 60.0±1.2bc 45.8±0.6cd 120 e e d c cd 35.0±2.1 20.0±2.3 65.6±1.7 67.9±2.5 52.0±3.1d Reference 4.0±2.1 δ: drying temperature; τ: annealing time 149 Figure Mechanical strength (hardness and stiffness) of three rice varieties at different drying temperatures and annealing durations up to 120 150 Head rice yield In general, the head rice yield of all three varieties (Kyeema, Amaroo and Reziq) decreased with increased drying temperature The mild drying temperature of 40oC maintained the head rice yield at levels close to the reference samples for all three rice varieties (Table 1) The higher drying temperatures used in this study (60 and 80oC) created a higher external drying rate The head rice yield is reduced due to large moisture gradients creating cracks in the grains As shown in Table 1, the head rice yield tended to increase with the increase in annealing duration The head rice yield increased by between and 22% after undergoing 40 of annealing at drying temperatures of 60oC and 80oC (in comparison with non-annealed samples in all three rice varieties) An extension of the annealing duration to 120 was found to be more beneficial for the medium-grain variety Effects of storage conditions on cracking behaviour and physico-chemical properties of rice Figures and illustrate the changes in the level of kernel fissuring and head rice yield, over months of storage The changes in mechanical strength parameters with storage duration are also illustrated in Figures and Table depicts the changes in pasting properties with reference to peak viscosity, final viscosity and pasting temperature Fissured kernels As shown in Figures and 3, it is interesting to see that the amount of fissured kernels, in fact, increased during investigated storage period The number of fissured kernels in the rice samples dried at 40oC showed a marked increase in first two months of storage, then remained constant To the contrary, for rice samples subjected to the 80oC drying temperature and annealed for at least 80 min, there was no fissuring during the first two months of storage, rather fissuring commenced after this period The level of rice kernel fissuring was also affected by storage temperature The observations on the changes in the levels of kernel fissuring in this study indicate that rice kernels continue fissuring during storage, irrespective of previous drying conditions Mechanical strength The mechanical strength parameters were analysed using the three-point bending test on the samples, as a function of storage temperature and time It is apparent from Figures and that stiffness increased with storage duration for all storage temperatures and for all three rice varieties studied Higher storage temperature (38oC) remarkably accelerated stiffness, while the magnitude of this increase was much lower at the lower temperatures (20oC and 4oC) Hardness of kernels of the varieties Kyeema and Amaroo, previously dried at 40oC, was higher than those dried at 80oC The continuous increase in stiffness of sound rice kernels during storage period may be the result of physical ageing or structural relaxation of the amorphous portion in rice starch The annealing/storage temperature was lower than the glass transition temperature of the stored rice The rice is therefore in the glassy state under storage conditions, and secondary relaxation of the amorphous structure would take place A long annealing period would provide an opportunity for very slow localized mobility in molecular 151 structure to reach the more steady state, resulting in densification of the amorphous matrix This molecular arrangement makes the internal structure of stored rice more rigid Head rice yield The effect of storage temperature and storage duration on the head rice yield of rice dried at two drying temperatures was investigated In general, the head rice yield of rice samples dried at 80oC was lower than for samples dried under more mild conditions (40oC); this applied to all durations of storage Head rice yield showed an increasing trend up to three months of storage for rice samples dried at both 80oC and 40oC at all three storage temperatures (Figures and 3) For storage beyond three months, head rice yield generally remained stable Storage temperature also influenced the head rice yield of Kyeema and Reiziq, with higher yields being associated with lower storage temperatures The remarkable increase in head rice yield during storage might be attributed to the enhancement of rice stiffness which was recorded in this study The increase in the stiffness means fissure resistance is enhanced after milling, leading to an improvement in head rice yield Pasting properties Table describes the pasting properties for all three rice varieties as a function of storage temperature and storage duration Peak and final viscosities generally showed an increasing trend with storage duration for the initial two months of storage With further storage, peak and final viscosities of the Kyeema variety tended to increase then decline to levels close to the original values, but generally remained unchanged in Reiziq from the third month of storage For Amaroo, peak and final viscosities continued to increase over the four month storage period The pasting properties were also affected by the storage temperature Rice samples stored at 38oC had much higher peak and final viscosities than those stored at 20oC and 4oC Peak and final viscosities were also influenced by drying conditions before storage The viscosities of rice dried using a high drying temperature (80oC) had lower values than those dried using a mild drying temperature (40oC) Pasting temperature, however, did not exhibit the same pattern for all three rice varieties The pasting temperature (PT) of Reiziq showed only a slight change over the storage period, while for Kyeema and Amaroo it showed a continual increase with storage duration These changes in pasting temperature were more pronounced in rice samples stored at 38oC 152 nfluence of storage temperatures of 38oC ( ), 20oC ( ), and 4oC ( ) over months of storage period of three rice varieties dried at 40oC on the ercentage of fissured kernels and head rice yield ndard deviations (HRY;fissured kernels) of Kyeema, Amaroo and Reiziq were (1.03; 1.84), (1.58;1.89) and (1.84;2.03), respectively nfluence of storage temperatures of 38oC ( ), 20oC ( ), and 4oC ( ) over months of storage period of three rice varieties dried at 80oC on the ercentage of fissured kernels and head rice yield ndard deviations (HRY;fissured kernels) of Kyeema, Amaroo and Reiziq were (1.03; 1.84), (1.58;1.89) and (1.84;2.03), respectively (a) (b) (c) Changes of mechanical strength parameters of three rice varieties (previously dried at 80oC) over months of storage at 38oC ( ), 20oC ( ), and ndard deviations (hardness;stiffness) of Kyeema, Amaroo and Reiziq were (0.95;1.26), (1.04;1.20) and (1.34;1.09), respectively (a) (b) (c) Changes of mechanical strength parameters of three rice varieties (previously dried at 40oC) over months of storage at 38oC ( ), 20oC ( ), and ndard deviations (hardness;stiffness) of Kyeema, Amaroo and Reiziq were (0.95;1.26), (1.04;1.20) and (1.34;1.09), respectively Table Pasting properties in terms of Pasting temperature (PT), Peak (PV), and Final (FV) viscosities of three rice varieties under various storage temperatures for a month storage period Kyeema Amaroo Reiziq ∆ δ τ PT PV FV PT PV FV PT PV FV o o o o C month C RVU RVU C RVU RVU C RVU RVU 84.6ab 210.5c 214.6b 89.4b 215.5b 252.0d 88.6bc 202.8efg 226.50ab 40 154.1a 220.3a 85.8ab 202.9bc 213.6b 88.6b 211.3b 236.2c 85.9b a f ab ab b b b a 202.8 226.5 85.8 193.6 209.1 89.8 197.8 219.4a 85.4 ab g ab b a a b a 86.6 209.9 235.2 85.8 178.9 197.0 89.3 193.0 228.8b ab ef a b a a b a 200.9 221.3 86.2 175.3 190.3 88.9 191.8 232.5bc 86.2 bc efg ab ab c b b bc 202.8 226.5 84.6 210.5 214.6 89.4 215.5 252.0d 88.6 20 bc ef bc b c b b c 197.8 247.5 85.4 207.9 215.0 89.4 222.7 233.5bc 88.6 b de ab b b a ab cde 87.8 195.3 227.3 85.3 195.4 198.5 87.2 238.8 247.5d 196.1ef 237.3b 86.1bc 215.8cd 216.3b 88.6b 228.1cd 237.0c 87.8b 86.8bc 211.2c 210.3b 89.1b 230.6cd 249.9d 88.2bc 186.1cd 232.9ab bc f ab ab c b b 202.8 226.5 84.6 210.5 214.6 89.4 215.5bc 252.0d 88.6 38 bc g cd a ef de ab 211.8 262.3 83.9 242.3 245.3 88.1 231.7d 247.8d 89.5 285.7d 84.3ab 242.5ef 229.5c 86.1a 278.5g 290.8g 89.5bc 231.9h cd b cd ab fg e ab g 172.2 269.5 85.0 249.8 252.5 87.0 276.9 287.3g 90.9 cd b cd b g fg b g 91.3 171.6 262.3 86.3 258.1 276.3 88.7 278.8 290.0g bc c c ab b b b cd 182.8 256.5 85.1 195.7 208.9 89.0 228.5 241.3d 89.0 80 182.8c 256.5c 86.2b 226.4d 232.0cd 89.3b 234.9d 247.6c 89.7c bc f c b e cd ab de 88.2 201.0 258.1 86.0 237.4 238.4 88.1 238.8 249.8a cd cd c bc ef d b d 186.3 258.3 86.9 243.4 242.4 89.5 235.3 247.8b 90.9 86.2b 235.5e 234.5cd 88.6b 236.8de 247.7bc 90.2cd 186.1cd 256.4c 256.5c 85.1ab 195.7b 208.9b 89.0b 228.5cd 241.3cd 89.0bc 182.8c 20 cd b bc b df e b 171.6 247.7 85.8 244.3 253.1 87.8 237.5de 245.6d 90.5 bc d bc b cd b a 188.8 250.3 85.7 215.4 211.6 86.1 244.6e 245.3d 89.0 263.7cd 86.9bc 250.8fg 247.3de 88.1ab 259.6f 260.8e 90.5cd 189.5d bc b c bc f de b f 89.4 172.0 258.8 87.6 247.5 247.1 88.6 267.1 269.5f bc cd c ab b b b cd 182.8 256.5 85.1 195.7 208.9 89.0 228.5 241.3cd 89.0 38 cd gh e bc fg e ab g 90.9 207.4 298.8 86.6 252.9 253.6 87.7 278.4 303.4hi 85.0ab 267.1h 269.3f 86.2a 302.5i 307.9i 90.9cd 195.1ef 286.0ed 91.6d 194.9e 286.8ed 88.2c 279.9i 282.3g 87.0a 293.7h 299.2h d de d c j h ab g 91.7 190.2 274.7 88.6 298.0 304.0 88.3 281.4 286.8g 2.2 5.4 0.7 3.8 4.0 0.9 2.9 2.9 Pooled STDEV 0.7 ∆: drying temperature; δ: storage temperature; τ: storage duration Conclusions The results in this study demonstrate another important role of annealing process which also has an effect on cracking behaviour, mechanical strength and milling quality of rice kernels In addition to the benefit of moisture redistribution throughout rice kernel driven by tempering, annealing process occurred concurrently enhances the physical integrity of rice kernels The relaxation of the molecular structure within rice starch results in the densification of the internal structure of rice kernels that making the kernels then being strong enough to withstand breakage during subsequent milling The results of the investigations reported in this study have added to a better understanding of rice ageing during storage related to changes in rice fissuring, mechanical properties and pasting properties Although there were small variations in the parameters measured during 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 157 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 ageing, by using X-ray diffraction or solid state NMR, which were not covered in the scope of this study 158 ... 1 -1 -1 -1 -1 -1 -1 -1 -1 0 -1 . 414 2 0 1. 414 2 -1 . 414 2 0 10 -1 1 11 0 1. 414 2 12 -1 -1 -1 13 1. 414 2 0 14 -1 1 15 1 -1 16 0 -1 . 414 2 17 -1 -1 18 0 0 19 0 0 20 0 0 21 0 0 22 0 0 Run ID X4 11 1 Table... 10 1- 1 09 7 2-8 8 94 Wet OM2 718 0. 4 -1 .2 4. 0 -1 0.8 10 3 -1 17 8 4-9 3 92 OM2 517 3. 5 -1 5.7 12 . 1- 2 0.3 9 0 -1 14 10 5 -1 17 94 OM4498 2. 5-3 .9 8. 1- 1 0.4 9 1- 9 3 9 6 -1 08 94 AG24 0. 3 -1 .5 1. 1- 4 .1 9 3-9 7 8 3 -1 08 94 IR50404 1. 1- 1 .5... 24 .1 18 .1 17.9 17 .8 18 .6 18 .4 18 .7 17 .8 17 .8 18 .2 17 .5 19 15 .2 15 .3 15 .9 15 .3 15 .9 15 .8 16 .9 16 .1 16.8 16 .4 16 .5 day after 13 .8 13 .7 13 .9 13 .9 13 .9 14 .9 15 .1 15.4 15 .5 15 .6 15 .3 16 .5 15 .7 16 .5