RICE QUALITY AND PROCESSING Effects of Drying and Tempering Rice Using a Continuous Drying Procedure 1 J.W. Fendley and T.J. Siebenmorgen ABSTRACT The objective of this research was to determine the effects of various drying and tempering strategies on rice quality using a continuous drying procedure. The drying procedure used was designed to implement a hypothesis involving the glass transition temperature of rice as applied to moisture content (MC) removal rates and tempering durations. The drying procedure consisted of four stages. First, rice was dried in a chamber for various durations aimed at removing 1 to 6 percentage points of moisture content (PPMC). The rice was then sealed in plastic bags and tempered inside the drying chamber for certain durations, as determined in previous research, to reduce MC gradients within the kernels. The rice was then dried further to approximately 12 to 13% MC. After this second drying stage, the rice was again tempered. There was little to no reduction in HRY when less than 4 to 5 PPMC were removed during the first drying stage. Conversely, if drying exceeded a 4 to 5 PPMC removal, there was a significant reduction in HRY. If too much moisture was removed during a drying pass, tempering did not prevent HRY reduction. Also, if less than 4 to 5 PPMC were removed during the first drying stage, then drying could continue after sufficient tempering without HRY reduction. INTRODUCTION Commercial rice drying operations typically use cross-flow column driers to remove moisture from rice during the harvest season. Rice arriving from the field is 1 This is a completed study. 382
B.R. Wells Rice Research Studies 2002 typically passed through cross-flow driers two to three times to reduce the harvest MC levels to approximately 13% wet basis (w.b.). Cnossen and Siebenmorgen (2000) showed that certain drying procedures, in which kernel temperatures exceeded the kernel s glass transition temperature (T g ), could lead to fissuring. Cnossen also showed that tempering rice at temperatures above the T g could minimize HRY reduction. The purpose of this study was to determine if a continuous multi-step drying process, incorporating tempering, would be able to dry rice to approximately 13% without reducing HRY. MATERIALS AND METHODS Four rice cultivars (Table 1), Bengal, Cocodrie, Francis, and Wells, were harvested from the Rice Research and Extension Center near Stuttgart, AR, in the fall of 2002. An additional lot of Bengal (Table 1) was harvested at the Northeast Research and Extension Center near Keiser, AR, in the fall of 2002. Cocodrie, Francis, and Wells are long-grain rice cultivars while Bengal is a medium-grain cultivar. Immediately after harvest, the rice was cleaned with a Carter-Day Dockage tester (Carter-Day Co., Minneapolis, MN). Before drying, bulk sample MCs were determined by drying 15 g of rough rice in a convection oven for 24 h at 130 C (Jindal and Siebenmorgen, 1987). The initial MCs of the rice are listed in Table 1. All experiments were conducted within two days of rice harvest. The experimental design consisted of drying conditions in which the drying-air and tempering temperatures were above the T g. Drying tests were conducted using two drying chambers with each chamber set to a specific drying condition. The first chamber was supplied with air at 61 C and 17% RH while the second chamber was supplied with air at 52 C, 25% RH. The air in the first chamber corresponded to an equilibrium moisture content (EMC) for rice of 5.8%, and that in the second chamber to an EMC of 7.7%, based on the Chung Equation (ASAE, 2001). Air conditions were maintained using two temperature and RH control units (Parameter Generation and Control Inc., Black Mountain, NC) and were monitored by a Hygro-MZ dew-point monitor (General Eastern, Woburn, MA). After each chamber reached equilibrium, 110 g of rough rice from a given rice lot (Table 1) were placed in each of sixteen trays inside the chamber. Each tray was 15 cm wide and 25 cm long. This resulted in a rice bed two to three kernels deep, which is within the thickness considered as a thin layer (ASAE, 2001). Combining two trays produced enough rice for a milling sample while allowing each tray to be dried in a thin layer. After the drying and tempering treatments, paired trays from each side of the drier were combined. Eight trays were used for replication one while the remaining eight trays were used for replication two. This allowed for two simultaneous drying replications. Weight loss of the rice was monitored after each drying duration to determine MC removal from each sample. For each drying condition, samples were dried for four different durations aimed at removing 3, 4, 5, and 6 PPMC. After this first drying duration, the samples were placed in sealed plastic bags and returned to the drying trays to temper. All tempering 383
AAES Research Series 504 durations were determined using guidelines established by Cnossen and Siebenmorgen, 2000. After the samples were tempered, the rice was removed from the plastic bags and returned to the drying trays to continue drying. Drying was then completed (approximately 12 to 13% MC) by removing 3, 3, 2, and 1 PPMC, respectively. The first set of drying durations were paired with the second set of drying durations to result in 6 to 7 PPMC removed. After the second drying pass, the samples were placed in sealed plastic bags and returned to the drying trays to temper for a second time. After final tempering, samples were taken from the drying chamber and placed immediately inside a large chamber controlled at 21 C and 48% RH. Once inside this chamber, the samples were removed from the plastic bags and placed in trays to cool and allow all samples to equilibrate to 12.5% MC. Samples were then stored in a walk-in cooler at 6 C until milling analyses were conducted. One hundred fifty grams of dried rough rice from each sample was hulled with a Satake Rice Machine (Satake Engineering Co., Ltd., Japan). The resultant brown rice was milled with a McGill No. 2 mill (Rapsco, Brookshire, TX) for 30 s with a 1.5 kg weight placed 15 cm from the centerline of the milling chamber. Head rice mass was measured using a Graincheck 2312 Analyzer (Foss Tecator, Sweden). HRY was calculated as the mass percentage of rough rice that remained as head rice. RESULTS Cocodrie that was dried at 52 C and 25%RH (Fig. 1A) showed only a 1.3 percentagepoint drop in HRY when it was dried as much as 6.9 PPMC and then tempered for 160 minutes, compared to the HRY measured when it was dried 2.0 PPMC for its first drying pass. Also, Cocodrie showed less than a 1 percentage point drop in HRY when it was dried 6.1 PPMC at 61 C and 17% RH (Fig. 1B). However, Cocodrie did experience a 3.8 percentage point drop in HRY when it was dried 8.0 PPMC in the first pass at 61 C and 17% RH. Wells that was dried at 52 C and 25% RH (Fig. 2A) showed a 2.1 percentage point drop in HRY when it was dried as much as 5.5 PPMC and then tempered for 120 minutes compared to harvest samples that were gently equilibrated to 12.5% MC. Wells showed a 1 percentage point drop in HRY when it was dried 4.1 PPMC at 61 C and 17% RH (Fig. 2B). In addition, Wells experienced a 5.6 percentage point drop in HRY when it was dried 5.2 PPMC at 61 C and 17% RH. This reduction in HRY increased to 15 percentage points when Wells was dried 5.7 PPMC at 61 C and 17% RH. The results from the remaining long-grain cultivar (Francis) showed similar trends. HRY reductions were always more pronounced when the cultivars were dried at 61 C and 17% RH compared to being dried at 52 C and 25% RH (Figs. 1 and 2). It is postulated that this was due to the larger MC gradients formed in the kernel by the higher temperature air stream. When an HRY reduction did occur, the amount of the reduction increased as the PPMC removed during the first drying duration increased. Bengal at a harvest MC of 20.2% showed more than 1 percentage point of HRY reduction with only 3.7 PPMC removed in the first pass when it was dried with 61 C and 384
B.R. Wells Rice Research Studies 2002 17% RH air (Fig. 3A). When the PPMC removed was increased to 5.9 percentage points, the HRY dropped 13.2 percentage points. However, Bengal dried under the same conditions with a harvest MC of 24.8% (Fig. 3B) showed less than 1 percentage point drop in HRY even when the PPMC removed was as much as 6.5 percentage points. Comparing the high and low harvest MC Bengal results showed that higher harvest MC allowed more moisture to be removed during a drying pass without experiencing a reduction in HRY (Figs. 3A and 3B). This trend held true for Bengal dried at both drying-air conditions. There was little to no reduction in HRYs in all cultivars when there were less than 4 to 5 PPMC removed during the first drying stage. However, if rice was dried long enough to remove more than 4 to 5 PPMC, samples experienced a reduction in HRY unless the harvest MC was high (Fig. 3B). According to the T g hypothesis (Cnossen and Siebenmorgen, 2000), if rice is dried for too long a duration at a temperature above the kernel T g (thus creating sufficiently large MC gradients within kernels) then the dried kernel surface will transition into the glassy region while the center is still in the rubbery region. This condition will result in kernel fissuring and a reduction in HRY. The results in this research supported the T g hypothesis. If the rice was dried for too long of a duration, particularly with the more severe drying-air condition, it would cross the T g line with a kernel MC gradient. This resulted in an HRY reduction. However, if tempering occurred before the rice crossed the T g line, drying could continue without a reduction in HRY. SIGNIFICANCE OF FINDINGS Several conclusions can be drawn from the data gathered for these tests. First of all, the higher the MC of the rice as it entered the drier, the more moisture that could be removed without causing a reduction in HRYs. Also, drying rice using air corresponding to a lower EMC caused greater HRY reduction than drying with higher EMC air. It was also found that there is a maximum amount of moisture that can be removed during the first drying period without causing a reduction in HRY. ACKNOWLEDGMENTS The authors wish to thank the Arkansas Rice Research and Promotion Board and the corporate sponsors of the University of Arkansas Rice Processing Program for the financial support of this project. LITERATURE CITED ASAE. 2001. ASAE Standards, Standard S448.1. ASAE. St. Joseph, MI. ASAE 2001. ASAE Standards, Standard D245.4. ASAE. St. Joseph, MI. 385
AAES Research Series 504 Cnossen, A.G. and T.J. Siebenmorgen. 2000. The glass transition temperature concept in rice drying and tempering: Effect on milling quality. Transactions of the ASAE 43(6):1661-1667. Jindal, V.K. and T.J. Siebenmorgen. 1987. Effects of oven drying temperature and drying time on rough rice moisture content determination. Transactions of the ASAE 30(4):1185-1192. Table 1. Harvest data for rice cultivars used in drying procedures. Cultivar Harvest date Harvest moisture content Harvest location (%) Bengal 10 Sept 2002 20.2 Stuttgart, AR Bengal 2 Oct 2002 24.8 Keiser, AR Cocodrie 10 Sept 2002 20.1 Stuttgart, AR Francis 31 Aug 2002 22.1 Stuttgart, AR Wells 31 Aug 2002 18.7 Stuttgart, AR 386
B.R. Wells Rice Research Studies 2002 (A) 2.0 3.0 6.1 6.9 3.8 4.4 2.0 1.3 20-80-30-80 35-120-30-120 55-120-18-120 70-160-5-120 (B) 3.5 4.7 6.1 8.0 3.5 3.3 1.9 1.8 10-80-15-80 20-120-15-120 30-120-10-120 45-160-5-120 Fig. 1. Head rice yields obtained from drying Cocodrie rice (harvest moisture content = 20.1%) at 52 C and 25% RH (A) and 61 C and 17% RH (B) for different durations in a multi-step drying procedure. Rice samples were dried, tempered, dried, and tempered. The numbers above and below the line in each graph are the percentage points of moisture content removed during the first and second drying steps, respectively. 387
AAES Research Series 504 3.4 4.5 3.0 2.3 (A) 5.5 6.3 1.6 0.9 20-80-30-80 35-120-30-120 55-120-18-120 70-160-5-120 (B) 2.4 4.1 3.0 2.4 5.2 1.9 5.7 1.5 10-80-15-80 20-120-15-120 30-120-10-120 45-160-5-120 Fig. 2. Head rice yields obtained from drying Wells rice (harvest moisture content = 18.7%) at 52 C and 25% RH (A) and 61 C and 17% RH (B) for different durations in a multi-step drying procedure. Rice samples were dried, tempered, dried, and tempered. The numbers above and below the line in each graph are the percentage points of moisture content removed during the first and second drying steps, respectively. 388
B.R. Wells Rice Research Studies 2002 (A) 2.4 3.7 4.7 2.7 2.6 1.9 5.9 1.3 10-80-15-80 20-120-15-120 30-120-10-120 45-160-5-120 (B) Head Rice Yield (% 3.0 4.1 5.0 6.5 3.2 2.8 2.4 1.5 10-80-15-80 20-120-15-120 30-120-10-120 45-160-5-120 Drying - Tempering - Drying - Tempering Durations (mintues) Fig. 3. Head rice yields obtained from drying 20.2% moisture content Bengal (A) rice and 24.8% moisture content Bengal (B) rice at 61 C and 17% RH for different durations in a multi-step drying procedure. Rice samples were dried, tempered, dried, and tempered. The numbers above and below the line in the graph are the percentage points of moisture content removed during the first and second drying steps, respectively. 389