- Email: [email protected]
Effect of storage temperatures on the head rice yield in relation to glass transition temperatures and un-freezable water
Journal of Cereal Science 70 (2016) 164e169
Contents lists available at ScienceDirect
Journal of Cereal Science journal homepage: www.elsevier.com/locate/jcs
Effect of storage temperatures on the head rice yield in relation to glass transition temperatures and un-freezable water Khongsak Srikaeo a, *, Chutamas Boonrod a, Mohammad Shafiur Rahman b a
Faculty of Food and Agricultural Technology, Pibulsongkram Rajabhat University, Muang, Phitsanulok, 65000, Thailand Department of Food Science and Nutrition, College of Agricultural and Marine Sciences, Sultan Qaboos University, P.O. Box-34, Al-Khod, 123, Muscat, Oman b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 October 2015 Received in revised form 11 May 2016 Accepted 10 June 2016 Available online 11 June 2016
Head rice yield (HRY) in relation to glass transition temperatures and un-freezable water of waxy (Sanpah-tawng) and non-waxy (Phitsanulok 2) rough rice were studied. Freezing points and glass transitions were measured by differential scanning calorimetry (DSC) as a function of solid fraction. Both waxy and non-waxy rice samples exhibited similar trends in glass transition when solid fraction was decreased. The transition temperature and enthalpy of ice melting decreased as the solid fraction increased. The onset (i.e. initial) glass transition was increased from 27 to 35 C when solid fraction increased from 0.80 to 0.97. Un-freezable water contents were found to be different for both rice samples. The rate of HRY (i.e. rate constant) increased differently (i.e. higher above glass transition) when rice samples were stored above (i.e. 35 and 45 C) and below glass transition temperatures. It was found that storage of fissured rice (low HRY) at the temperatures especially above glass transition could relax the strains inside a rice kernel and subsequently improved HRY. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Rice milling HRY Glass transition Un-freezable water
1. Introduction Rice in the form of milled rice kernels is consumed as staple food by almost half of the world population. Milling, an important processing step of rough rice, is usually performed to produce white polished grain. A commercial rice milling system is a multi-stage process when the rough rice is first subjected to dehusking and then to remove brownish outer bran layer, known as whitening. Finally, polishing is carried out to remove the bran particles and provide surface gloss to the edible white portion. The most important parameters during milling are head rice yield (HRY). Rice kernel with three quarters or more of their original length after complete milling operation is termed as head rice, and higher HRY indicates high quality yield (Yadav and Jindal, 2008). The ultimate goal of the rice industry is to achieve maximum HRY from the milling process. HRY is the current standard to assess commercial rice milling quality. The HRY is related to the pre- and post-harvest stress cracks (fissures) development in the kernels, and the postharvest drying, handling and storage of the paddy. The rice grain is mechanically strong, but it is susceptible to moisture stress and
* Corresponding author. E-mail address: [email protected] (K. Srikaeo). http://dx.doi.org/10.1016/j.jcs.2016.06.006 0733-5210/© 2016 Elsevier Ltd. All rights reserved.
develops fissures upon rapid hydration or dehydration during harvesting, handling and/or processing (Iguaz et al., 2006). Extensive research has been done for improving milling quality through plant breeding, improved cultural practices and optimization of harvesting and drying conditions (Abud-Archila et al., 2000). It has focused mainly on the pre-milling optimum conditioning especially during drying and tempering within the drying process to maximize the HRY (Sadeghi et al., 2013; Schluterman and Siebenmorgen, 2007). Rice is largely composed of starch and regarded as a hygroscopic biomaterial. Rice kernel undergoes a glass transition as the kernel goes through temperature and water content change during thermal processing i.e. drying and tempering. Recently, a great deal of research has been reported on the utility of glass transition to determine foods’ stability and their ingredients during storage and processing. The glass transition was also related to the molecular mobility and chemical reaction of foods, thus shelf life could be predicted (Rahman, 2004, 2006; 2009, 2012). Glass transition temperatures of rice were previously developed and presented in the literature (Perdon et al., 2000; Sablani et al., 2009). Earlier research has shown that glass transition concept can be used to explain rice fissure formation during drying and
K. Srikaeo et al. / Journal of Cereal Science 70 (2016) 164e169
tempering (Cnossen et al., 2002; Cnossen and Siebenmorgen, 2000; Tajaddodi Talab et al., 2012; Yang and Jia, 2004; Yang et al., 2005; Zhang et al., 2003). Glass transition is detected by observing characteristic changes in various thermal (heat capacity, enthalpy and thermal expansion coefficient), mechanical (modulus, viscosity) and dielectric (dielectric constant) properties. The most widely used technique for studying the glass transition is the differential scanning calorimetry (DSC), though there are other techniques available. As mentioned earlier, much research has been done to improve HRY with regards to pre-milling optimum conditioning. However, in practice, uncontrollable weather conditions usually cause improper harvesting conditions and consequently fissured rough rice. Relatively little information is available in the literature on the handling of fissured rough rice (i.e. defective rough rice due to improper harvesting, drying and tempering) to restore the quality of the rice. As the glass transition is a reversible phenomenon, starch in rice exhibits basic reversible glass transition. The present research aimed to study the effects of glass transition and unfreezable water to manage fissured rough rice so that HRY could be increased. Two Thai rice varieties with different amylose contents (waxy and non-waxy rice) were used in this study. The information obtained from this study might demonstrate that the use of glass transition concept could help improving HRY. The hypothesis is that storage above glass transition could reduce the rate of fissures, which could result in improved HRY during rice milling process. 2. Materials and methods
165
time used in this study was eight weeks. The samples in satchels were kept in temperature control chambers (Memmert GmbH, Germany) which were maintained at 5, 25, 35 and 45 C. The storage temperatures at 35 and 45 C were used to simulate rubbery regions (i.e. above glass line) (as described later). The storage temperatures at 5 and 25 C were used to simulate glassy region (i.e. below glass line). 2.3. Thermal characteristics Thermal characteristics were measured using the DSC (DSC 1, Mettler Toledo, Leicester, UK) equipped with a refrigerated cooler capable to cool down to 90 C. The instrument was calibrated for heat flow and temperature using distilled water and indium. Aluminum pans of 70 mL with lid were used in all experiments with an empty sealed pan as reference, and nitrogen at a flow rate of 50 mL/ min was used as a purged gas. The Stare software (ver. 13.00, Mettler Toledo, Leicester, UK) was used for analyzing the thermograms. For freezing curve analysis, samples (5 mg) were firstly cooled to 90 C at 10 C/min and then scanned from 90 C to 120 C at 10 C/min. Freezing temperatures for each sample were recorded from the total heat flow thermograms considering the maximum slope of the endothermic peaks (Rahman, 2004) and enthalpies were determined from the areas of the endothermic peaks. Unfreezable water content was determined by plotting enthalpy as a function of solid content. For glass transition measurement, samples (5 mg) were scanned from 25 C to 100 C at heating rate 10 C/ min. The glass transition was determined from a shift in the heating thermogram and it was recorded as onset, mid and end temperatures. Experiments were conducted in triplicate.
2.1. Rice samples Waxy (San-pah-tawng, amylose ¼ 28.6 g/100 g dry sample) and non-waxy (Phitsanulok 2, amylose ~1.0 g/100 g dry sample) rice samples were used in this study. They were freshly harvested from Phitsanulok (Thailand) rice fields during the months of November and December 2014. The initial moisture content of the rice samples was about 25 g/100 g wet basis as measured by the grain and seed moisture meter (PM650, Kett, Japan). The total starch and protein content of both rice samples (milled rice) were found to be about 72e75 and 11e12 g/100 g dry sample respectively. 2.2. Sample preparation For measuring thermal transitions, freshly harvested rice samples were dried in an oven at 80 C until the moisture content reached about 10 g/100 g wet basis. The samples (whole rough rice) were ground using a laboratory hammer mill (Lab Mill 120, Perten Instruments, US) to pass 100-mesh screen. Moisture contents of the ground samples were determined using the standard air oven method and adjusted by either drying in an infrared oven (FD-100, Kett, Japan) to reach the predetermined moisture content or adding predetermined amount of distilled water and followed by equilibration at 4 C for 24 h. Samples with moisture contents from 10 to 90 g/100 g rice (wet basis) (i.e. solid fraction from 0.9 to 0.1) were used to determine their freezing points. Samples with moisture contents from 3 to 20 g/100 g rice (i.e. solid fraction from 0.97 to 0.80, or samples containing un-freezable water) were used for measuring glass transitions. For measuring HRY (as explained later), freshly harvested rough rice was dried in an oven at 80 C until the moisture content reached about 10 g/100 g wet basis. In order to avoid moisture gain or loss, samples of 500 g of rough rice were packed using vacuum seal satchels. Satchels stored at different storage temperatures were taken out every week and used for milling test. The storage
2.4. Milling test In order to measure HRY, rice samples in vacuum sealed satchels were milled using the pilot milling unit (NW 100, Natrawee Technology Co., Ltd. Thailand). The rice sample (500 g) was passed through dehusk equipment with rubber rollers to remove the husk and then it was passed over an abrasive cone to produce white rice. Head rice yield was calculated as the ratio of the mass of whole white rice kernels to the total mass of white rice. Head rice or white kernels with 3/4 or more the original kernel length was carefully sorted manually by visual inspection (Yadav and Jindal, 2008). Experiments were conducted in triplicate. HRY kinetics were modeled by first order kinetics. The first order reaction kinetics can be written as:
ln ðHRYÞ ¼ kt þ ln C
(1)
where k is the first order rate constant (week1), t is the storage time (week) and C is the constant. The rate constant could be determined by the regression of the linear plot of ln(HRY) versus time. 2.5. Statistical analysis Analysis of variance (ANOVA) and Tukey’s test for significant difference between means were performed using Minitab® ver. 16 and p < 0.05 was considered as significant. The rate constant was determined using regression procedure of MS-Excel. 3. Results and discussion 3.1. Thermal characteristics Typical DSC thermograms for determining freezing points
166
K. Srikaeo et al. / Journal of Cereal Science 70 (2016) 164e169
showed clear single endothermic peak (refers to Supplement material Fig. S1). Similar thermograms were found for both waxy and non-waxy rice. The endothermic peak during heating of cooled sample indicated freezing process. The onset freezing (Tm), freezing point (Tf, maximum slope) and peak temperature (Tp) for both rice samples containing freezable water (solid fraction from 0.1 to 0.6) are shown in Table 1. Both waxy and non-waxy rice samples showed similar pattern. The freezing transition (i.e. endothermic peak) could not be detected in DSC thermograms for samples with solid fraction from 0.7 or higher, as visualized in the DSC thermograms. The samples with freezable water showed clear endothermic peaks caused by melting of ice around 0 C in the DSC. The plots of freezing points as a function of solid fraction indicated freezing curves of the samples (refers to Supplement material Fig. S2). The plots of transition enthalpy as a function of solid fraction can be used for determining un-freezable water of the samples (Fig. 1). The freezing point and enthalpy of ice melting decreased as the solid fraction increased (i.e. decreased moisture content). The proportion of water that could freeze is frequently named as “free” water in an attempt to differentiate it from water which is unable to form ice (i.e. un-freezable water) (Farroni and del Pilar Buera, 2014). The presence of frozen water was observed on DSC experiments as endothermic peaks centered close to 0 C produced by the melting of ice crystals. It was also noticed from the DSC thermograms that no endothermic peak was observed for rice samples (both waxy and non-waxy) with solid mass fraction from 0.7 or higher, indicating that those samples contained only un-freezable water. The enthalpy of this endothermic peak is proportional to the amount of freezable water present in the sample. A plot of the enthalpy versus solid fraction/water content could determine the amount of unfreezable water by extrapolating it linearly to zero enthalpy. Unfreezable water is associated in some way more closely with the solute molecules although it may not be totally immobilized or bound (Li et al., 1998). In this study, the un-freezable water contents of waxy and non-waxy rice varieties were found to be 12.4 and 27.1 g/100 g sample wet basis, respectively. Earlier, un-freezable water of rice was reported as 36.4 g/100 g sample (Rahman et al., 2005; Sablani et al., 2009), which was higher than the observed values in this study. It has been reported that the values of unfreezable water in starch gels usually ranging from 25.0 to 32.0 g/ 100 g depending upon the botanical source of starch, annealing and gelatinization treatments (Li et al., 1998). In this study, the unfreezable water content of waxy rice was lower than that of nonwaxy rice. This may be caused by the differences of sample composition, such as amylose and amylopectin composition (i.e.
Fig. 1. Enthalpies of a) waxy rice (San-pah-tawng) and b) non-waxy rice (Phitsanulok 2) as a function of solid fraction (Xs).
amylose contents were 28.6 and 1.0 g/100 g for waxy and non-waxy rice, respectively). As less water is incorporated in a threedimensional gel matrix, more water becomes freezable (Chung et al., 2004). Amylopectin rich starch (waxy) has much larger molecule than amylose (non-waxy) with highly branched structure. That may increase the bound or interacted water within the solids matrix (less water becomes freezable). In addition, it has been reported that un-freezable water appeared to be related to the conformational structure of the amylosic molecules. It was affected,
Table 1 Thermal transition measurement data of rough rice containing freezable water. Solid fraction (Xs) Waxy rice (San-pah-tawng) 0.1 0.2 0.3 0.4 0.5 0.6 Non-waxy rice (Phitsanulok 2) 0.1 0.2 0.3 0.4 0.5 0.6
Tm ( C) 1.55 2.71 3.39 4.95 5.47 6.70
± ± ± ± ± ±
Tf ( C)
DH (J/g)
Tp ( C)
0.14a 0.37ab 0.38b 0.23c 0.39cd 0.39d
0.05 0.08 1.62 2.59 4.40 6.51
± ± ± ± ± ±
0.03a 0.04a 0.41ab 0.57b 0.59c 0.56d
3.29 2.26 1.63 1.32 1.38 1.42
6.81 ± 0.23a 7.30 ± 0.23a 7.63 ± 0.54a 7.83 ± 0.05a 9.73 ± 0.22b 10.99 ± 0.42b
1.14 0.55 0.81 1.41 3.80 7.17
± ± ± ± ± ±
0.25a 0.01a 0.29a 0.04a 0.25b 0.28c
0.93 ± 0.09a 1.08 ± 0.23a 1.18 ± 0.22a 0.91 ± 0.04a 0.90 ± 0.10a 0.58 ± 0.23b
Values are means ± standard deviations (triplicate). For each parameter and each rice variety (column), values with the same letters are not significantly different (p > 0.05).
± ± ± ± ± ±
0.23a 0.17b 0.08c 0.05c 0.00c 0.01c
95.84 83.73 74.80 64.21 63.05 31.18
± ± ± ± ± ±
0.66a 0.55b 0.45c 0.06d 0.00d 0.45e
102.97 ± 3.68a 97.22 ± 1.65a 81.81 ± 3.09b 66.78 ± 3.59c 38.53 ± 1.53d 16.84 ± 0.36e
K. Srikaeo et al. / Journal of Cereal Science 70 (2016) 164e169
not only by the degree of polymerization, but also by the linear or ring structure (Suzuki and Kitamura, 2008). The retrogradation (recrystallization) of a gelatinized starch gel increased the unfreezable water (Chung et al., 2002; Tananuwong and Reid, 2004). This could be due to the formation of order structure of starch molecules, thus less possible binding of water with starch molecules. The glass transition temperature was observed by a shift in the DSC thermogram of waxy and non-waxy rice samples. Typical DSC thermograms for glass transition temperature are shown in the Supplement material (Fig. S3). Several previous works suggested that DSC showed low sensitivity to detect thermal transition in rice samples with un-freezable water (Perdon et al., 2000; Sablani et al., 2009; Sun et al., 2002). However, the setting of DSC in this study was found acceptable shift in the thermograms. The onset (Tgi), mid (Tgm), and end-point (Tge) of the glass transitions were determined from the thermograms and the detailed measurement data are shown in Table 2. As discussed earlier, the un-freezable water contents of both rice samples were found to be different. However, the glass transition temperatures were found to be similar in both waxy and non-waxy rice samples. This indicated that un-freezable water did not affect the glass transition temperatures. Freezable water played a major role in altering the glass transition temperatures. In this study, glass transition temperatures of the rice samples increased with the increasing solid fraction. The increasing of glass transition temperatures with increasing solid fraction is due to the plasticization effect of water on the amorphous parts of the rice matrix. The glass transition temperatures in foods and biomaterials can occur within a relatively wide range (DTg ¼ Tge Tgi ~50 C) which has been associated, in some instances, with heterogeneity of the materials, hence it is recommended to report all three, i.e. initial, mid and end-points of the transitions (Sablani et al., 2009). Glass transition temperatures of rice were previously reported. Basmati rice as determined using modulated DSC was found to exhibit Tgm in the range of 32e48 C with moisture content from 7 to 17 g/100 g (Sablani et al., 2009). Glass transition temperatures of MR219 rice variety as measured by DSC were found to be in the
Table 2 Onset (Tgi), mid (Tgm) and end-point (Tge) glass transition temperature of rough rice containing un-freezable water. Solid fraction (Xs)
Tgi ( C)
Waxy rice (San-pah-tawng) 0.800 26.93 ± 0.888 27.07 ± 0.908 27.60 ± 0.910 29.45 ± 0.914 30.06 ± 0.920 30.63 ± 0.921 31.62 ± 0.930 31.52 ± 0.950 32.20 ± 0.970 33.71 ± Non-waxy rice (Phitsanulok 2) 0.800 28.84 ± 0.888 29.53 ± 0.895 29.87 ± 0.900 29.72 ± 0.901 30.36 ± 0.906 31.01 ± 0.910 32.41 ± 0.930 32.74 ± 0.950 33.13 ± 0.970 34.87 ±
Tgm ( C)
167
range of about 10e62 C with moisture content from 7 to 27 g/100 g (Tajaddodi Talab et al., 2012). Individual rice kernel was measured for glass transition temperatures by thermomechanical analysis (TMA) and dynamic mechanical thermal analysis (DMTA) and it was found that the values ranged from 22 to 58 C with moisture content from 3 to 27 g/100 g (Perdon et al., 2000; Siebenmorgen et al., 2004; Sun et al., 2002). The results in this study are in agreement with previously published data. The total solids of rice samples are primarily a mixture of starch (70e79 g/100 g) and about 20 g/100 g of other minor components e.g. protein, lipid, fiber and minerals (Zhou et al., 2002). 3.2. Milling test Based on the glass transition temperatures obtained from the DSC as described earlier, the glassy and rubbery regions can be constructed (Fig. 2). For milling test, two points below glass transition (5 and 25 C) and above glass transition (35 and 45 C) were selected to store rough rice samples. These locations are indicated in Fig. 2. The samples were dried with at 80 C to facilitate fissures on the surface. The moisture content was reduced from 25 (harvesting moisture content) to about 10 g/100 g wet basis, while general recommendation for moisture content of rice during storage is less than 13 g/100 g. The initial HRY was found to be very low; only 24% for waxy rice and 29% for non-waxy rice. The HRY values of waxy and non-waxy rice stored at 5, 25, 35 and 45 C over the period of eight weeks are shown in Fig. 3. In general, both rice samples stored above and below glass transition improved HRY over storage time. This indicated that there was a relaxation process, which could enhance the reversing of the fissures by structure building on the surface. However, the storage above glass transition (35 and 45 C in this study) showed better HRY as compared to those stored below glass transition (5 and 25 C), thus reversing rates of fissures were much higher in the rubbery state as compared to the glassy state. The rates of HRY were further analyzed by first order kinetics. Kinetic rate constant obtained from the plot between ln[HRY] versus storage time for all temperatures for both rice samples are shown in Table 3. Examples of the plots of ln[HRY] versus storage time obtained from waxy rice samples stored at 5 and 45 C are shown in the Supplement material (Fig. S4). From all storage conditions and for both rice varieties, linear equations fit well with the
Tge ( C)
0.07f 0.06f 0.26f 0.39e 0.25de 0.33cd 0.32b 0.17bc 0.06b 0.25a
27.32 29.20 30.24 30.79 31.40 33.27 34.59 34.47 35.52 35.14
± ± ± ± ± ± ± ± ± ±
0.11e 0.15d 0.23cd 0.18c 0.22c 1.00b 0.56ab 0.15ab 0.20a 0.22a
30.60 33.80 34.78 35.18 35.45 36.14 36.46 36.49 37.18 37.85
± ± ± ± ± ± ± ± ± ±
0.05g 0.06f 0.16e 0.09e 0.20de 0.07cd 0.17bc 0.51bc 0.09ab 0.06a
0.06g 0.39f 0.00ef 0.01f 0.10e 0.00d 0.24c 0.06bc 0.01b 0.16a
29.37 30.39 31.18 30.38 32.46 33.35 34.24 34.37 35.07 36.06
± ± ± ± ± ± ± ± ± ±
0.09f 0.01e 0.19e 0.13e 0.52d 0.09c 0.30b 0.20b 0.07b 0.06a
31.08 32.89 33.53 32.77 34.65 35.43 36.82 36.94 37.43 38.11
± ± ± ± ± ± ± ± ± ±
0.08e 0.11d 0.35d 0.04d 0.23c 0.61c 0.15b 0.06b 0.16ab 0.06a
Values are means ± standard deviations (triplicate). For each parameter and each rice variety (column), values with the same letters are not significantly different (p > 0.05).
Fig. 2. Combined glass transition temperatures of both waxy and non-waxy rice showing rubbery and glassy regions. * indicated storage condition (5, 25, 35 and 45 C). Solid fraction of the rice sample for storage test was 0.9 (10 g/100 g moisture wet basis). The linear model is statistically significant (p < 0.01).
168
K. Srikaeo et al. / Journal of Cereal Science 70 (2016) 164e169
Fig. 3. Head rice yield (HRY) of (a) waxy rice and (b) non-waxy rice during the storage for 8 weeks at 5, 25, 35 and 45 C.
experimental data giving r2 for more than 0.94. It could be seen that, above glass transition, slope of the plot increased as compared to the slope of the plot of below glass transition (refers to Supplement material Fig. S5). The values of kinetic rate constant of both rice samples stored above the glass transition were higher than those stored below the glass transition. At the same storage temperature, the kinetic rate constants of waxy rice were generally higher than those of non-waxy rice (Table 3). HRY improved significantly as the storage time increased, especially at the storage temperatures above glass temperature. This could be explained by the fact that above glass transition, the reversing rate of fissures was increased due to a relaxation process above glass transition. The rice kernels before drying usually had uniform moisture and
temperature distribution within it. In the progress of drying, moisture of the outer layers of the kernel was low than the center, resulting a moisture gradient throughout the kernel. The surface could be glassy, rubbery or partially glassy and rubbery depending on the drying temperature and relative humidity of the air. This differential moisture gradient may create stresses within the kernel, and could cause kernel to fissure (Cnossen et al., 2003; Perdon et al., 2000). Tempering within the drying process was proved to preserve HRY after heated air drying. Most reported tempering works were done adiabatically at a temperature below drying temperature for certain period of time, usually not more than a few days. Recently, high-temperature tempering was found to be an effective way to preserve the whole kernel percentage (i.e. HRY) for rice dried at extreme conditions. It was suggested that tempering was needed to maintain high HRY if drying above glass transition and moisture gradient difference existed (Cnossen and Siebenmorgen, 2000). Tempering helped to relax the strains inside a rice kernel induced by internal stresses developed during the drying process. The strains had two components (elastic component and viscous component) due to the viscoelasticity of rice kernels. The reduction of moisture gradients inside a rice kernel during tempering helped to eliminate the elastic component of the strains due to the elasticity of the rice kernel (Zhang et al., 2003). In addition, research has suggested that the internal work or strains inside the rice kernel could only be relaxed or eliminated by tempering at a temperature above the glass transition of the rice. The internal stress would exist (at the termination of drying) inside the rice kernel indefinitely, which would cause internal fissures to initiate or propagate if the sample is not tempered (Yang et al., 2005). Using mechanical spectra, Madeka and Kokini (1996) and Rahman and Al-Saidi (2010) observed an entangled polymer flow region in polymeric biomaterial above glass transition. This was due to cross-linking reaction occurring once the ability of the molecules to interact. In this case, molecules gain enough mobility to come together and caused formation of cross-links (Rahman and Al-Saidi, 2010). All these relaxation processes and cross-linking during storage above glass transition caused the improved HRY of rice. The finding in this study could be useful for rice milling industry in assisting the management of reducing rate of fissured rice. The recommendations for industrial application could be drawn as follows; 1) tempering after drying of rough rice before milling increases the HRY and 2) paddy with improper harvesting conditions should be properly dried and stored at the temperature higher than 35 C (above glass transition temperatures) for several weeks before milling in order to increase the HRY. 4. Conclusions Freezing points, un-freezable water and glass transition temperatures of waxy (San-pah-tawng) and non-waxy (Phitsanulok 2) rice samples were determined using the DSC technique. Solid fraction of the samples affected the freezing points and glass transition temperatures. The components in waxy and non-waxy rice could contribute to the difference of un-freezable water as
Table 3 Rate constants (slope) and r2 values obtained from linear plot of ln[HRY] vs. storage time (week) at different storage temperatures.a
Storage temperature ( C)
Waxy rice (San-pah-tawng) (week1)
Non-waxy rice (Phitsanulok 2) (week1)
5 25 35 45
8.45 102 8.94 102 9.83 102 10.22 102
5.91 6.31 7.12 7.58
a
HRY (%) and storage time (week). All models are statistically significant (p < 0.01). Minimum r2 is 0.94.
102 102 102 102
K. Srikaeo et al. / Journal of Cereal Science 70 (2016) 164e169
observed in the ice melting enthalpies. This study found that glass transition temperatures affected the HRY of stored rice. Prolonged tempering or storage of rough rice especially at the temperatures above glass transition temperatures can improve the HRY. The storage at the temperatures above glass transition temperatures showed better HRY as compared to the storage below the glass transition. This finding could provide useful information for rice industry. However, the application in industrial scale should be validated. A concern on energy demand for manipulating high temperature storage conditions should also be taken into account for industrial application. Acknowledgements This research was financially supported by the Thailand Research Fund (TRF) e Grant No. MSD57I0141. The cooperation from Phitsanulok Rice Cluster is greatly acknowledged. Authors would like to acknowledge the support of Sultan Qaboos University during DSC analysis of the rice samples in their laboratory. Appendix A. Supplementary material Supplementary material related to this article can be found at http://dx.doi.org/10.1016/j.jcs.2016.06.006. References Abud-Archila, M., Courtois, F., Bonazzi, C., Bimbenet, J.J., 2000. Processing quality of rough rice during drying-modelling of head rice yield versus moisture gradients and kernel temperature. J. Food Eng. 45, 161e169. Chung, H.J., Lee, E.J., Lim, S.T., 2002. Comparison in glass transition and enthalpy relaxation between native and gelatinized rice starches. Carbohydr. Polym. 48, 287e298. Chung, H.J., Woo, K.S., Lim, S.T., 2004. Glass transition and enthalpy relaxation of cross-linked corn starches. Carbohydr. Polym. 55, 9e15. Cnossen, A.G., Jimenez, M.J., Siebenmorgen, T.J., 2003. Rice fissuring response to high drying and tempering temperatures. J. Food Eng. 59, 61e69. Cnossen, A.G., Siebenmorgen, T.J., 2000. The glass transition temperature concept in rice drying and tempering: effect on milling quality. Trans. ASAE 43, 1661e1668. Cnossen, A.G., Siebenmorgen, T.J., Yang, W., 2002. The glass transition temperature concept in rice drying and tempering: effect on drying rate. Trans. ASAE 45, 759e766. Farroni, A., del Pilar Buera, M., 2014. Cornflake production process: state diagram and water mobility characteristics. Food Bioprocess Tech. 7, 2902e2911. Iguaz, A., Rodríguez, M., Vírseda, P., 2006. Influence of handling and processing of rough rice on fissures and head rice yields. J. Food Eng. 77, 803e809. Li, S., Dickinson, L., Chinachoti, P., 1998. Mobility of “unfreezable” and “freezable” water in waxy corn starch by 2H and 1H NMR. J. Agric. Food Chem. 46, 62e71.
169
Madeka, H., Kokini, J.L., 1996. Effect of glass transition and cross-linking on rheological properties of zein: development of a preliminary state diagram. Cereal Chem. 73, 433e438. Perdon, A., Siebenmorgen, T., Mauromoustakos, A., 2000. Glassy state transition and rice drying: development of a brown rice state diagram. Cereal Chem. 77, 708e713. Rahman, M.S., 2004. State diagram of date flesh using differential scanning calorimetry (DSC). Int. J. Food Prop. 7, 407e428. Rahman, M.S., 2006. State diagram of foods: its potential use in food processing and product stability. Trends Food Sci. Tech. 17, 129e141. Rahman, M.S., 2009. Food stability beyond water activity and glass transtion: macro-micro region concept in the state diagram. Int. J. Food Prop. 12, 726e740. Rahman, M.S., 2012. Applications of macro-micro region concept in the state diagram and critical temperature concepts in determining the food stability. Food Chem. 132, 1679e1685. Rahman, M.S., Al-Saidi, G., 2010. Thermal relaxation of gelatin and date flesh measured by isothermal condition in differential scanning calorimetry (DSC) and its relation to the structural and mechanical glass transition. Int. J. Food Prop. 13, 931e944. Rahman, M.S., Sablani, S.S., Al-Habsi, N., Al-Maskri, S., Al-Belushi, R., 2005. State diagram of freeze-dried garlic powder by differential scanning calorimetry and cooling curve methods. J. Food Sci. 70, E135eE141. Sablani, S.S., Bruno, L., Kasapis, S., Symaladevi, R.M., 2009. Thermal transitions of rice: development of a state diagram. J. Food Eng. 90, 110e118. Sadeghi, M., Nasrnia, E., Masoumi, A.A., Hemmat, A., 2013. Head rice yield response to low and high drying and tempering conditions. Int. Agrophys. 27, 219e223. Schluterman, D.A., Siebenmorgen, T.J., 2007. Relating rough rice moisture content reduction and tempering duration to head rice yield reduction. Trans. ASAE 50, 137e142. Siebenmorgen, T.J., Yang, W., Sun, Z., 2004. Glass transition temperature of rice kernels determined by dynamic mechanical thermal analysis. Trans. ASAE 47, 835e840. Sun, Z., Yang, W., Siebenmorgen, T., Stelwagen, A., Cnossen, A., 2002. Thermomechanical transitions of rice kernels. Cereal Chem. 79, 349e353. Suzuki, S., Kitamura, S., 2008. Unfrozen water in amylosic molecules is dependent on the molecular structures e A differential scanning calorimetric study. Food Hydrocoll. 22, 862e867. Tajaddodi Talab, K., Ibrahim, M., Spotar, S., Talib, R.A., Muhammad, K., 2012. Glass transition temperature, mechanical properties of rice and their relationships with milling quality. Int. J. Food Eng. 8 http://dx.doi.org/10.1515/15563758.2197. Tananuwong, K., Reid, D.S., 2004. Differential scanning calorimetry study of glass transition in frozen starch gels. J. Agric. Food Chem. 52, 4308e4317. Yadav, B.K., Jindal, V.K., 2008. Changes in head rice yield and whiteness during milling of rough rice (Oryza sativa L.). J. Food Eng. 86, 113e121. Yang, W., Jia, C., 2004. Glass transition mapping inside a rice kernel. Trans. ASAE 47, 2009e2015. Yang, W., Zhang, Q., Jia, C., 2005. Understanding rice breakage through internal work, fracture energy, and glass transition of individual kernels. Trans. ASAE 48, 1157e1164. Zhang, Q., Yang, W., Jia, C., 2003. Preservation of head rice yield under hightemperature tempering as explained by the glass transition of rice kernels. Cereal Chem. 80, 684e688. Zhou, Z., Robards, K., Helliwell, S., Blanchard, C., 2002. Composition and functional properties of rice. Int. J. Food Sci. Tech. 37, 849e868.
Contents lists available at ScienceDirect
Journal of Cereal Science journal homepage: www.elsevier.com/locate/jcs
Effect of storage temperatures on the head rice yield in relation to glass transition temperatures and un-freezable water Khongsak Srikaeo a, *, Chutamas Boonrod a, Mohammad Shafiur Rahman b a
Faculty of Food and Agricultural Technology, Pibulsongkram Rajabhat University, Muang, Phitsanulok, 65000, Thailand Department of Food Science and Nutrition, College of Agricultural and Marine Sciences, Sultan Qaboos University, P.O. Box-34, Al-Khod, 123, Muscat, Oman b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 October 2015 Received in revised form 11 May 2016 Accepted 10 June 2016 Available online 11 June 2016
Head rice yield (HRY) in relation to glass transition temperatures and un-freezable water of waxy (Sanpah-tawng) and non-waxy (Phitsanulok 2) rough rice were studied. Freezing points and glass transitions were measured by differential scanning calorimetry (DSC) as a function of solid fraction. Both waxy and non-waxy rice samples exhibited similar trends in glass transition when solid fraction was decreased. The transition temperature and enthalpy of ice melting decreased as the solid fraction increased. The onset (i.e. initial) glass transition was increased from 27 to 35 C when solid fraction increased from 0.80 to 0.97. Un-freezable water contents were found to be different for both rice samples. The rate of HRY (i.e. rate constant) increased differently (i.e. higher above glass transition) when rice samples were stored above (i.e. 35 and 45 C) and below glass transition temperatures. It was found that storage of fissured rice (low HRY) at the temperatures especially above glass transition could relax the strains inside a rice kernel and subsequently improved HRY. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Rice milling HRY Glass transition Un-freezable water
1. Introduction Rice in the form of milled rice kernels is consumed as staple food by almost half of the world population. Milling, an important processing step of rough rice, is usually performed to produce white polished grain. A commercial rice milling system is a multi-stage process when the rough rice is first subjected to dehusking and then to remove brownish outer bran layer, known as whitening. Finally, polishing is carried out to remove the bran particles and provide surface gloss to the edible white portion. The most important parameters during milling are head rice yield (HRY). Rice kernel with three quarters or more of their original length after complete milling operation is termed as head rice, and higher HRY indicates high quality yield (Yadav and Jindal, 2008). The ultimate goal of the rice industry is to achieve maximum HRY from the milling process. HRY is the current standard to assess commercial rice milling quality. The HRY is related to the pre- and post-harvest stress cracks (fissures) development in the kernels, and the postharvest drying, handling and storage of the paddy. The rice grain is mechanically strong, but it is susceptible to moisture stress and
* Corresponding author. E-mail address: [email protected] (K. Srikaeo). http://dx.doi.org/10.1016/j.jcs.2016.06.006 0733-5210/© 2016 Elsevier Ltd. All rights reserved.
develops fissures upon rapid hydration or dehydration during harvesting, handling and/or processing (Iguaz et al., 2006). Extensive research has been done for improving milling quality through plant breeding, improved cultural practices and optimization of harvesting and drying conditions (Abud-Archila et al., 2000). It has focused mainly on the pre-milling optimum conditioning especially during drying and tempering within the drying process to maximize the HRY (Sadeghi et al., 2013; Schluterman and Siebenmorgen, 2007). Rice is largely composed of starch and regarded as a hygroscopic biomaterial. Rice kernel undergoes a glass transition as the kernel goes through temperature and water content change during thermal processing i.e. drying and tempering. Recently, a great deal of research has been reported on the utility of glass transition to determine foods’ stability and their ingredients during storage and processing. The glass transition was also related to the molecular mobility and chemical reaction of foods, thus shelf life could be predicted (Rahman, 2004, 2006; 2009, 2012). Glass transition temperatures of rice were previously developed and presented in the literature (Perdon et al., 2000; Sablani et al., 2009). Earlier research has shown that glass transition concept can be used to explain rice fissure formation during drying and
K. Srikaeo et al. / Journal of Cereal Science 70 (2016) 164e169
tempering (Cnossen et al., 2002; Cnossen and Siebenmorgen, 2000; Tajaddodi Talab et al., 2012; Yang and Jia, 2004; Yang et al., 2005; Zhang et al., 2003). Glass transition is detected by observing characteristic changes in various thermal (heat capacity, enthalpy and thermal expansion coefficient), mechanical (modulus, viscosity) and dielectric (dielectric constant) properties. The most widely used technique for studying the glass transition is the differential scanning calorimetry (DSC), though there are other techniques available. As mentioned earlier, much research has been done to improve HRY with regards to pre-milling optimum conditioning. However, in practice, uncontrollable weather conditions usually cause improper harvesting conditions and consequently fissured rough rice. Relatively little information is available in the literature on the handling of fissured rough rice (i.e. defective rough rice due to improper harvesting, drying and tempering) to restore the quality of the rice. As the glass transition is a reversible phenomenon, starch in rice exhibits basic reversible glass transition. The present research aimed to study the effects of glass transition and unfreezable water to manage fissured rough rice so that HRY could be increased. Two Thai rice varieties with different amylose contents (waxy and non-waxy rice) were used in this study. The information obtained from this study might demonstrate that the use of glass transition concept could help improving HRY. The hypothesis is that storage above glass transition could reduce the rate of fissures, which could result in improved HRY during rice milling process. 2. Materials and methods
165
time used in this study was eight weeks. The samples in satchels were kept in temperature control chambers (Memmert GmbH, Germany) which were maintained at 5, 25, 35 and 45 C. The storage temperatures at 35 and 45 C were used to simulate rubbery regions (i.e. above glass line) (as described later). The storage temperatures at 5 and 25 C were used to simulate glassy region (i.e. below glass line). 2.3. Thermal characteristics Thermal characteristics were measured using the DSC (DSC 1, Mettler Toledo, Leicester, UK) equipped with a refrigerated cooler capable to cool down to 90 C. The instrument was calibrated for heat flow and temperature using distilled water and indium. Aluminum pans of 70 mL with lid were used in all experiments with an empty sealed pan as reference, and nitrogen at a flow rate of 50 mL/ min was used as a purged gas. The Stare software (ver. 13.00, Mettler Toledo, Leicester, UK) was used for analyzing the thermograms. For freezing curve analysis, samples (5 mg) were firstly cooled to 90 C at 10 C/min and then scanned from 90 C to 120 C at 10 C/min. Freezing temperatures for each sample were recorded from the total heat flow thermograms considering the maximum slope of the endothermic peaks (Rahman, 2004) and enthalpies were determined from the areas of the endothermic peaks. Unfreezable water content was determined by plotting enthalpy as a function of solid content. For glass transition measurement, samples (5 mg) were scanned from 25 C to 100 C at heating rate 10 C/ min. The glass transition was determined from a shift in the heating thermogram and it was recorded as onset, mid and end temperatures. Experiments were conducted in triplicate.
2.1. Rice samples Waxy (San-pah-tawng, amylose ¼ 28.6 g/100 g dry sample) and non-waxy (Phitsanulok 2, amylose ~1.0 g/100 g dry sample) rice samples were used in this study. They were freshly harvested from Phitsanulok (Thailand) rice fields during the months of November and December 2014. The initial moisture content of the rice samples was about 25 g/100 g wet basis as measured by the grain and seed moisture meter (PM650, Kett, Japan). The total starch and protein content of both rice samples (milled rice) were found to be about 72e75 and 11e12 g/100 g dry sample respectively. 2.2. Sample preparation For measuring thermal transitions, freshly harvested rice samples were dried in an oven at 80 C until the moisture content reached about 10 g/100 g wet basis. The samples (whole rough rice) were ground using a laboratory hammer mill (Lab Mill 120, Perten Instruments, US) to pass 100-mesh screen. Moisture contents of the ground samples were determined using the standard air oven method and adjusted by either drying in an infrared oven (FD-100, Kett, Japan) to reach the predetermined moisture content or adding predetermined amount of distilled water and followed by equilibration at 4 C for 24 h. Samples with moisture contents from 10 to 90 g/100 g rice (wet basis) (i.e. solid fraction from 0.9 to 0.1) were used to determine their freezing points. Samples with moisture contents from 3 to 20 g/100 g rice (i.e. solid fraction from 0.97 to 0.80, or samples containing un-freezable water) were used for measuring glass transitions. For measuring HRY (as explained later), freshly harvested rough rice was dried in an oven at 80 C until the moisture content reached about 10 g/100 g wet basis. In order to avoid moisture gain or loss, samples of 500 g of rough rice were packed using vacuum seal satchels. Satchels stored at different storage temperatures were taken out every week and used for milling test. The storage
2.4. Milling test In order to measure HRY, rice samples in vacuum sealed satchels were milled using the pilot milling unit (NW 100, Natrawee Technology Co., Ltd. Thailand). The rice sample (500 g) was passed through dehusk equipment with rubber rollers to remove the husk and then it was passed over an abrasive cone to produce white rice. Head rice yield was calculated as the ratio of the mass of whole white rice kernels to the total mass of white rice. Head rice or white kernels with 3/4 or more the original kernel length was carefully sorted manually by visual inspection (Yadav and Jindal, 2008). Experiments were conducted in triplicate. HRY kinetics were modeled by first order kinetics. The first order reaction kinetics can be written as:
ln ðHRYÞ ¼ kt þ ln C
(1)
where k is the first order rate constant (week1), t is the storage time (week) and C is the constant. The rate constant could be determined by the regression of the linear plot of ln(HRY) versus time. 2.5. Statistical analysis Analysis of variance (ANOVA) and Tukey’s test for significant difference between means were performed using Minitab® ver. 16 and p < 0.05 was considered as significant. The rate constant was determined using regression procedure of MS-Excel. 3. Results and discussion 3.1. Thermal characteristics Typical DSC thermograms for determining freezing points
166
K. Srikaeo et al. / Journal of Cereal Science 70 (2016) 164e169
showed clear single endothermic peak (refers to Supplement material Fig. S1). Similar thermograms were found for both waxy and non-waxy rice. The endothermic peak during heating of cooled sample indicated freezing process. The onset freezing (Tm), freezing point (Tf, maximum slope) and peak temperature (Tp) for both rice samples containing freezable water (solid fraction from 0.1 to 0.6) are shown in Table 1. Both waxy and non-waxy rice samples showed similar pattern. The freezing transition (i.e. endothermic peak) could not be detected in DSC thermograms for samples with solid fraction from 0.7 or higher, as visualized in the DSC thermograms. The samples with freezable water showed clear endothermic peaks caused by melting of ice around 0 C in the DSC. The plots of freezing points as a function of solid fraction indicated freezing curves of the samples (refers to Supplement material Fig. S2). The plots of transition enthalpy as a function of solid fraction can be used for determining un-freezable water of the samples (Fig. 1). The freezing point and enthalpy of ice melting decreased as the solid fraction increased (i.e. decreased moisture content). The proportion of water that could freeze is frequently named as “free” water in an attempt to differentiate it from water which is unable to form ice (i.e. un-freezable water) (Farroni and del Pilar Buera, 2014). The presence of frozen water was observed on DSC experiments as endothermic peaks centered close to 0 C produced by the melting of ice crystals. It was also noticed from the DSC thermograms that no endothermic peak was observed for rice samples (both waxy and non-waxy) with solid mass fraction from 0.7 or higher, indicating that those samples contained only un-freezable water. The enthalpy of this endothermic peak is proportional to the amount of freezable water present in the sample. A plot of the enthalpy versus solid fraction/water content could determine the amount of unfreezable water by extrapolating it linearly to zero enthalpy. Unfreezable water is associated in some way more closely with the solute molecules although it may not be totally immobilized or bound (Li et al., 1998). In this study, the un-freezable water contents of waxy and non-waxy rice varieties were found to be 12.4 and 27.1 g/100 g sample wet basis, respectively. Earlier, un-freezable water of rice was reported as 36.4 g/100 g sample (Rahman et al., 2005; Sablani et al., 2009), which was higher than the observed values in this study. It has been reported that the values of unfreezable water in starch gels usually ranging from 25.0 to 32.0 g/ 100 g depending upon the botanical source of starch, annealing and gelatinization treatments (Li et al., 1998). In this study, the unfreezable water content of waxy rice was lower than that of nonwaxy rice. This may be caused by the differences of sample composition, such as amylose and amylopectin composition (i.e.
Fig. 1. Enthalpies of a) waxy rice (San-pah-tawng) and b) non-waxy rice (Phitsanulok 2) as a function of solid fraction (Xs).
amylose contents were 28.6 and 1.0 g/100 g for waxy and non-waxy rice, respectively). As less water is incorporated in a threedimensional gel matrix, more water becomes freezable (Chung et al., 2004). Amylopectin rich starch (waxy) has much larger molecule than amylose (non-waxy) with highly branched structure. That may increase the bound or interacted water within the solids matrix (less water becomes freezable). In addition, it has been reported that un-freezable water appeared to be related to the conformational structure of the amylosic molecules. It was affected,
Table 1 Thermal transition measurement data of rough rice containing freezable water. Solid fraction (Xs) Waxy rice (San-pah-tawng) 0.1 0.2 0.3 0.4 0.5 0.6 Non-waxy rice (Phitsanulok 2) 0.1 0.2 0.3 0.4 0.5 0.6
Tm ( C) 1.55 2.71 3.39 4.95 5.47 6.70
± ± ± ± ± ±
Tf ( C)
DH (J/g)
Tp ( C)
0.14a 0.37ab 0.38b 0.23c 0.39cd 0.39d
0.05 0.08 1.62 2.59 4.40 6.51
± ± ± ± ± ±
0.03a 0.04a 0.41ab 0.57b 0.59c 0.56d
3.29 2.26 1.63 1.32 1.38 1.42
6.81 ± 0.23a 7.30 ± 0.23a 7.63 ± 0.54a 7.83 ± 0.05a 9.73 ± 0.22b 10.99 ± 0.42b
1.14 0.55 0.81 1.41 3.80 7.17
± ± ± ± ± ±
0.25a 0.01a 0.29a 0.04a 0.25b 0.28c
0.93 ± 0.09a 1.08 ± 0.23a 1.18 ± 0.22a 0.91 ± 0.04a 0.90 ± 0.10a 0.58 ± 0.23b
Values are means ± standard deviations (triplicate). For each parameter and each rice variety (column), values with the same letters are not significantly different (p > 0.05).
± ± ± ± ± ±
0.23a 0.17b 0.08c 0.05c 0.00c 0.01c
95.84 83.73 74.80 64.21 63.05 31.18
± ± ± ± ± ±
0.66a 0.55b 0.45c 0.06d 0.00d 0.45e
102.97 ± 3.68a 97.22 ± 1.65a 81.81 ± 3.09b 66.78 ± 3.59c 38.53 ± 1.53d 16.84 ± 0.36e
K. Srikaeo et al. / Journal of Cereal Science 70 (2016) 164e169
not only by the degree of polymerization, but also by the linear or ring structure (Suzuki and Kitamura, 2008). The retrogradation (recrystallization) of a gelatinized starch gel increased the unfreezable water (Chung et al., 2002; Tananuwong and Reid, 2004). This could be due to the formation of order structure of starch molecules, thus less possible binding of water with starch molecules. The glass transition temperature was observed by a shift in the DSC thermogram of waxy and non-waxy rice samples. Typical DSC thermograms for glass transition temperature are shown in the Supplement material (Fig. S3). Several previous works suggested that DSC showed low sensitivity to detect thermal transition in rice samples with un-freezable water (Perdon et al., 2000; Sablani et al., 2009; Sun et al., 2002). However, the setting of DSC in this study was found acceptable shift in the thermograms. The onset (Tgi), mid (Tgm), and end-point (Tge) of the glass transitions were determined from the thermograms and the detailed measurement data are shown in Table 2. As discussed earlier, the un-freezable water contents of both rice samples were found to be different. However, the glass transition temperatures were found to be similar in both waxy and non-waxy rice samples. This indicated that un-freezable water did not affect the glass transition temperatures. Freezable water played a major role in altering the glass transition temperatures. In this study, glass transition temperatures of the rice samples increased with the increasing solid fraction. The increasing of glass transition temperatures with increasing solid fraction is due to the plasticization effect of water on the amorphous parts of the rice matrix. The glass transition temperatures in foods and biomaterials can occur within a relatively wide range (DTg ¼ Tge Tgi ~50 C) which has been associated, in some instances, with heterogeneity of the materials, hence it is recommended to report all three, i.e. initial, mid and end-points of the transitions (Sablani et al., 2009). Glass transition temperatures of rice were previously reported. Basmati rice as determined using modulated DSC was found to exhibit Tgm in the range of 32e48 C with moisture content from 7 to 17 g/100 g (Sablani et al., 2009). Glass transition temperatures of MR219 rice variety as measured by DSC were found to be in the
Table 2 Onset (Tgi), mid (Tgm) and end-point (Tge) glass transition temperature of rough rice containing un-freezable water. Solid fraction (Xs)
Tgi ( C)
Waxy rice (San-pah-tawng) 0.800 26.93 ± 0.888 27.07 ± 0.908 27.60 ± 0.910 29.45 ± 0.914 30.06 ± 0.920 30.63 ± 0.921 31.62 ± 0.930 31.52 ± 0.950 32.20 ± 0.970 33.71 ± Non-waxy rice (Phitsanulok 2) 0.800 28.84 ± 0.888 29.53 ± 0.895 29.87 ± 0.900 29.72 ± 0.901 30.36 ± 0.906 31.01 ± 0.910 32.41 ± 0.930 32.74 ± 0.950 33.13 ± 0.970 34.87 ±
Tgm ( C)
167
range of about 10e62 C with moisture content from 7 to 27 g/100 g (Tajaddodi Talab et al., 2012). Individual rice kernel was measured for glass transition temperatures by thermomechanical analysis (TMA) and dynamic mechanical thermal analysis (DMTA) and it was found that the values ranged from 22 to 58 C with moisture content from 3 to 27 g/100 g (Perdon et al., 2000; Siebenmorgen et al., 2004; Sun et al., 2002). The results in this study are in agreement with previously published data. The total solids of rice samples are primarily a mixture of starch (70e79 g/100 g) and about 20 g/100 g of other minor components e.g. protein, lipid, fiber and minerals (Zhou et al., 2002). 3.2. Milling test Based on the glass transition temperatures obtained from the DSC as described earlier, the glassy and rubbery regions can be constructed (Fig. 2). For milling test, two points below glass transition (5 and 25 C) and above glass transition (35 and 45 C) were selected to store rough rice samples. These locations are indicated in Fig. 2. The samples were dried with at 80 C to facilitate fissures on the surface. The moisture content was reduced from 25 (harvesting moisture content) to about 10 g/100 g wet basis, while general recommendation for moisture content of rice during storage is less than 13 g/100 g. The initial HRY was found to be very low; only 24% for waxy rice and 29% for non-waxy rice. The HRY values of waxy and non-waxy rice stored at 5, 25, 35 and 45 C over the period of eight weeks are shown in Fig. 3. In general, both rice samples stored above and below glass transition improved HRY over storage time. This indicated that there was a relaxation process, which could enhance the reversing of the fissures by structure building on the surface. However, the storage above glass transition (35 and 45 C in this study) showed better HRY as compared to those stored below glass transition (5 and 25 C), thus reversing rates of fissures were much higher in the rubbery state as compared to the glassy state. The rates of HRY were further analyzed by first order kinetics. Kinetic rate constant obtained from the plot between ln[HRY] versus storage time for all temperatures for both rice samples are shown in Table 3. Examples of the plots of ln[HRY] versus storage time obtained from waxy rice samples stored at 5 and 45 C are shown in the Supplement material (Fig. S4). From all storage conditions and for both rice varieties, linear equations fit well with the
Tge ( C)
0.07f 0.06f 0.26f 0.39e 0.25de 0.33cd 0.32b 0.17bc 0.06b 0.25a
27.32 29.20 30.24 30.79 31.40 33.27 34.59 34.47 35.52 35.14
± ± ± ± ± ± ± ± ± ±
0.11e 0.15d 0.23cd 0.18c 0.22c 1.00b 0.56ab 0.15ab 0.20a 0.22a
30.60 33.80 34.78 35.18 35.45 36.14 36.46 36.49 37.18 37.85
± ± ± ± ± ± ± ± ± ±
0.05g 0.06f 0.16e 0.09e 0.20de 0.07cd 0.17bc 0.51bc 0.09ab 0.06a
0.06g 0.39f 0.00ef 0.01f 0.10e 0.00d 0.24c 0.06bc 0.01b 0.16a
29.37 30.39 31.18 30.38 32.46 33.35 34.24 34.37 35.07 36.06
± ± ± ± ± ± ± ± ± ±
0.09f 0.01e 0.19e 0.13e 0.52d 0.09c 0.30b 0.20b 0.07b 0.06a
31.08 32.89 33.53 32.77 34.65 35.43 36.82 36.94 37.43 38.11
± ± ± ± ± ± ± ± ± ±
0.08e 0.11d 0.35d 0.04d 0.23c 0.61c 0.15b 0.06b 0.16ab 0.06a
Values are means ± standard deviations (triplicate). For each parameter and each rice variety (column), values with the same letters are not significantly different (p > 0.05).
Fig. 2. Combined glass transition temperatures of both waxy and non-waxy rice showing rubbery and glassy regions. * indicated storage condition (5, 25, 35 and 45 C). Solid fraction of the rice sample for storage test was 0.9 (10 g/100 g moisture wet basis). The linear model is statistically significant (p < 0.01).
168
K. Srikaeo et al. / Journal of Cereal Science 70 (2016) 164e169
Fig. 3. Head rice yield (HRY) of (a) waxy rice and (b) non-waxy rice during the storage for 8 weeks at 5, 25, 35 and 45 C.
experimental data giving r2 for more than 0.94. It could be seen that, above glass transition, slope of the plot increased as compared to the slope of the plot of below glass transition (refers to Supplement material Fig. S5). The values of kinetic rate constant of both rice samples stored above the glass transition were higher than those stored below the glass transition. At the same storage temperature, the kinetic rate constants of waxy rice were generally higher than those of non-waxy rice (Table 3). HRY improved significantly as the storage time increased, especially at the storage temperatures above glass temperature. This could be explained by the fact that above glass transition, the reversing rate of fissures was increased due to a relaxation process above glass transition. The rice kernels before drying usually had uniform moisture and
temperature distribution within it. In the progress of drying, moisture of the outer layers of the kernel was low than the center, resulting a moisture gradient throughout the kernel. The surface could be glassy, rubbery or partially glassy and rubbery depending on the drying temperature and relative humidity of the air. This differential moisture gradient may create stresses within the kernel, and could cause kernel to fissure (Cnossen et al., 2003; Perdon et al., 2000). Tempering within the drying process was proved to preserve HRY after heated air drying. Most reported tempering works were done adiabatically at a temperature below drying temperature for certain period of time, usually not more than a few days. Recently, high-temperature tempering was found to be an effective way to preserve the whole kernel percentage (i.e. HRY) for rice dried at extreme conditions. It was suggested that tempering was needed to maintain high HRY if drying above glass transition and moisture gradient difference existed (Cnossen and Siebenmorgen, 2000). Tempering helped to relax the strains inside a rice kernel induced by internal stresses developed during the drying process. The strains had two components (elastic component and viscous component) due to the viscoelasticity of rice kernels. The reduction of moisture gradients inside a rice kernel during tempering helped to eliminate the elastic component of the strains due to the elasticity of the rice kernel (Zhang et al., 2003). In addition, research has suggested that the internal work or strains inside the rice kernel could only be relaxed or eliminated by tempering at a temperature above the glass transition of the rice. The internal stress would exist (at the termination of drying) inside the rice kernel indefinitely, which would cause internal fissures to initiate or propagate if the sample is not tempered (Yang et al., 2005). Using mechanical spectra, Madeka and Kokini (1996) and Rahman and Al-Saidi (2010) observed an entangled polymer flow region in polymeric biomaterial above glass transition. This was due to cross-linking reaction occurring once the ability of the molecules to interact. In this case, molecules gain enough mobility to come together and caused formation of cross-links (Rahman and Al-Saidi, 2010). All these relaxation processes and cross-linking during storage above glass transition caused the improved HRY of rice. The finding in this study could be useful for rice milling industry in assisting the management of reducing rate of fissured rice. The recommendations for industrial application could be drawn as follows; 1) tempering after drying of rough rice before milling increases the HRY and 2) paddy with improper harvesting conditions should be properly dried and stored at the temperature higher than 35 C (above glass transition temperatures) for several weeks before milling in order to increase the HRY. 4. Conclusions Freezing points, un-freezable water and glass transition temperatures of waxy (San-pah-tawng) and non-waxy (Phitsanulok 2) rice samples were determined using the DSC technique. Solid fraction of the samples affected the freezing points and glass transition temperatures. The components in waxy and non-waxy rice could contribute to the difference of un-freezable water as
Table 3 Rate constants (slope) and r2 values obtained from linear plot of ln[HRY] vs. storage time (week) at different storage temperatures.a
Storage temperature ( C)
Waxy rice (San-pah-tawng) (week1)
Non-waxy rice (Phitsanulok 2) (week1)
5 25 35 45
8.45 102 8.94 102 9.83 102 10.22 102
5.91 6.31 7.12 7.58
a
HRY (%) and storage time (week). All models are statistically significant (p < 0.01). Minimum r2 is 0.94.
102 102 102 102
K. Srikaeo et al. / Journal of Cereal Science 70 (2016) 164e169
observed in the ice melting enthalpies. This study found that glass transition temperatures affected the HRY of stored rice. Prolonged tempering or storage of rough rice especially at the temperatures above glass transition temperatures can improve the HRY. The storage at the temperatures above glass transition temperatures showed better HRY as compared to the storage below the glass transition. This finding could provide useful information for rice industry. However, the application in industrial scale should be validated. A concern on energy demand for manipulating high temperature storage conditions should also be taken into account for industrial application. Acknowledgements This research was financially supported by the Thailand Research Fund (TRF) e Grant No. MSD57I0141. The cooperation from Phitsanulok Rice Cluster is greatly acknowledged. Authors would like to acknowledge the support of Sultan Qaboos University during DSC analysis of the rice samples in their laboratory. Appendix A. Supplementary material Supplementary material related to this article can be found at http://dx.doi.org/10.1016/j.jcs.2016.06.006. References Abud-Archila, M., Courtois, F., Bonazzi, C., Bimbenet, J.J., 2000. Processing quality of rough rice during drying-modelling of head rice yield versus moisture gradients and kernel temperature. J. Food Eng. 45, 161e169. Chung, H.J., Lee, E.J., Lim, S.T., 2002. Comparison in glass transition and enthalpy relaxation between native and gelatinized rice starches. Carbohydr. Polym. 48, 287e298. Chung, H.J., Woo, K.S., Lim, S.T., 2004. Glass transition and enthalpy relaxation of cross-linked corn starches. Carbohydr. Polym. 55, 9e15. Cnossen, A.G., Jimenez, M.J., Siebenmorgen, T.J., 2003. Rice fissuring response to high drying and tempering temperatures. J. Food Eng. 59, 61e69. Cnossen, A.G., Siebenmorgen, T.J., 2000. The glass transition temperature concept in rice drying and tempering: effect on milling quality. Trans. ASAE 43, 1661e1668. Cnossen, A.G., Siebenmorgen, T.J., Yang, W., 2002. The glass transition temperature concept in rice drying and tempering: effect on drying rate. Trans. ASAE 45, 759e766. Farroni, A., del Pilar Buera, M., 2014. Cornflake production process: state diagram and water mobility characteristics. Food Bioprocess Tech. 7, 2902e2911. Iguaz, A., Rodríguez, M., Vírseda, P., 2006. Influence of handling and processing of rough rice on fissures and head rice yields. J. Food Eng. 77, 803e809. Li, S., Dickinson, L., Chinachoti, P., 1998. Mobility of “unfreezable” and “freezable” water in waxy corn starch by 2H and 1H NMR. J. Agric. Food Chem. 46, 62e71.
169
Madeka, H., Kokini, J.L., 1996. Effect of glass transition and cross-linking on rheological properties of zein: development of a preliminary state diagram. Cereal Chem. 73, 433e438. Perdon, A., Siebenmorgen, T., Mauromoustakos, A., 2000. Glassy state transition and rice drying: development of a brown rice state diagram. Cereal Chem. 77, 708e713. Rahman, M.S., 2004. State diagram of date flesh using differential scanning calorimetry (DSC). Int. J. Food Prop. 7, 407e428. Rahman, M.S., 2006. State diagram of foods: its potential use in food processing and product stability. Trends Food Sci. Tech. 17, 129e141. Rahman, M.S., 2009. Food stability beyond water activity and glass transtion: macro-micro region concept in the state diagram. Int. J. Food Prop. 12, 726e740. Rahman, M.S., 2012. Applications of macro-micro region concept in the state diagram and critical temperature concepts in determining the food stability. Food Chem. 132, 1679e1685. Rahman, M.S., Al-Saidi, G., 2010. Thermal relaxation of gelatin and date flesh measured by isothermal condition in differential scanning calorimetry (DSC) and its relation to the structural and mechanical glass transition. Int. J. Food Prop. 13, 931e944. Rahman, M.S., Sablani, S.S., Al-Habsi, N., Al-Maskri, S., Al-Belushi, R., 2005. State diagram of freeze-dried garlic powder by differential scanning calorimetry and cooling curve methods. J. Food Sci. 70, E135eE141. Sablani, S.S., Bruno, L., Kasapis, S., Symaladevi, R.M., 2009. Thermal transitions of rice: development of a state diagram. J. Food Eng. 90, 110e118. Sadeghi, M., Nasrnia, E., Masoumi, A.A., Hemmat, A., 2013. Head rice yield response to low and high drying and tempering conditions. Int. Agrophys. 27, 219e223. Schluterman, D.A., Siebenmorgen, T.J., 2007. Relating rough rice moisture content reduction and tempering duration to head rice yield reduction. Trans. ASAE 50, 137e142. Siebenmorgen, T.J., Yang, W., Sun, Z., 2004. Glass transition temperature of rice kernels determined by dynamic mechanical thermal analysis. Trans. ASAE 47, 835e840. Sun, Z., Yang, W., Siebenmorgen, T., Stelwagen, A., Cnossen, A., 2002. Thermomechanical transitions of rice kernels. Cereal Chem. 79, 349e353. Suzuki, S., Kitamura, S., 2008. Unfrozen water in amylosic molecules is dependent on the molecular structures e A differential scanning calorimetric study. Food Hydrocoll. 22, 862e867. Tajaddodi Talab, K., Ibrahim, M., Spotar, S., Talib, R.A., Muhammad, K., 2012. Glass transition temperature, mechanical properties of rice and their relationships with milling quality. Int. J. Food Eng. 8 http://dx.doi.org/10.1515/15563758.2197. Tananuwong, K., Reid, D.S., 2004. Differential scanning calorimetry study of glass transition in frozen starch gels. J. Agric. Food Chem. 52, 4308e4317. Yadav, B.K., Jindal, V.K., 2008. Changes in head rice yield and whiteness during milling of rough rice (Oryza sativa L.). J. Food Eng. 86, 113e121. Yang, W., Jia, C., 2004. Glass transition mapping inside a rice kernel. Trans. ASAE 47, 2009e2015. Yang, W., Zhang, Q., Jia, C., 2005. Understanding rice breakage through internal work, fracture energy, and glass transition of individual kernels. Trans. ASAE 48, 1157e1164. Zhang, Q., Yang, W., Jia, C., 2003. Preservation of head rice yield under hightemperature tempering as explained by the glass transition of rice kernels. Cereal Chem. 80, 684e688. Zhou, Z., Robards, K., Helliwell, S., Blanchard, C., 2002. Composition and functional properties of rice. Int. J. Food Sci. Tech. 37, 849e868.