cFood Science and Technology Department, Tokyo University of Marine Science & Technology, P.O. Box 108-8477, Tokyo, Japan
dMaterials & Life Sciences Department, Sophia University, P.O. Box 102-8554, Tokyo, Japan
Received 10 April 2016. Accepted 6 June 2016. Available online 16 November 2016. Managing Editor: Fang Hongmin
Abstract Paddy rice samples were parboiled by soaking at 65 °C for 180 min and steaming at 96 °C for 2–10 min, and then dried to achieve the final moisture content of 11% ± 1%. The degree of starch gelatinization (DSG) and several quality attributes (head rice yield (HRY), color value and hardness) of parboiled rice were measured. Results showed that DSG (46.8%–77.9%), color value (18.08–19.04) and hardness (118.6–219.2 N) all increased following steaming. In contrast, the HRY increased (64.8%–67.1%) for steaming times between 2–4 min but decreased (67.1%–65.0%) for steaming times between 4–10 min. Linear relations between DSG and color value (R2= 0.87), and DSG and hardness (R2= 0.88) were observed. The suitable DSG of parboiled rice leading to the highest HRY was found to be 62.5%, obtained following 4 min of steaming. Keywords
parboiling process
rice
head rice yield
color value
hardness
degree of starch gelatinization
Parboiling is a hydrothermal process in which the crystalline form of starch present in paddy rice is changed into an amorphous form due to the irreversible swelling and fusion of starch. This change is accomplished by soaking in hot water and steaming at low pressure, before drying and milling the rice (Himmelsbach et al., 2008 and Ayamdoo et al., 2013b). Parboiling of rice is employed to affect starch gelatinization, which improves milling yield, nutritional value and head rice yield (HRY) (Wu et al., 2002). During gelatinization, starch granules close the cracks present in the endosperm leading to consolidation of the grains (Bauer and Knorr, 2004 and Ituen and Ukpakha, 2011). Level of gelatinization was significantly affected by the soaking/steaming combination treatments (Ayamdoo et al., 2013a). When rice variety Jasmine 85 was soaked for 20 h and steamed for 60 or 90 min, it resulted in 91.5% gelatinization (Ayamdoo et al., 2013a).
Differential scanning calorimetry (DSC) is the most common thermal method for assessing the gelatinization of starch (Himmelsbach et al., 2008). Heat-mediated starch gelatinization is accompanied by loss of birefringence. Gelatinization is generally defined as the hydration and irreversible swelling of the granule, which is concomitant with the destruction of molecular order, melting of starch crystals and starch solubilization (Bauer and Knorr, 2004 and Zhang et al., 2015). Gelatinization is affected by heating starch granules in water, leading to granule swelling, eventually yielding a viscous paste which can be used in food products (Rohaya et al., 2013).
Rice can be gelatinized by steaming. This treatment does not involve water addition to the granules (Ituen and Ukpakha, 2011). Rice moisture content during soaking and extent of heating during steaming increase the degree of starch gelatinization (DSG) (Manful et al., 2008). Successful parboiling should increase the HRY (Delcour and Hoseney, 2010 and Buggenhout et al., 2013). HRY is clearly of great importance for the rice industry as it has twice or three times the economic value of broken rice (Buggenhout et al., 2014 and Min et al., 2014). DSG can be related to the mechanical integrity of the rice, and steaming conditions affect DSG (Buggenhout et al., 2014). However, application of temperatures above gelatinization temperature incurs increased energy costs and affects the quality of parboiled rice in terms of hydration and cooking characteristics. Tenderness, color and yellowness may all suffer (Bello et al., 2012). Marshall et al (1993) measured DSG of parboiled rice and raw rice. The results suggest that DSG may be responsible for many of the attributes of parboiled rice. Maximum HRY was achieved when the starch was about 40% gelatinized. The average hardness of individual parboiled kernels increased from 13.1 to 25.3 N with increasing steaming time from 45 to 75 min (Bello et al., 2006). Saif et al (2004) report that the average ultimate tensile strength depends on steaming time. Also, Islam et al (2004) show that average hardness increases with steaming time up to 60 min at 90 °C and up to 10 min at 100 °C.
The influence of vapor pressure parboiling on milled rice properties has been widely studied (Champagne et al., 1998 and Mohapatra and Bal, 2006). However, information related to the effects of hydrothermal conditions on cooked rice properties is lacking (Miah et al., 2002). In this study, a paddy was parboiled using different steaming conditions. DSG and certain important quality indicators of parboiled rice such as hardness, head rice yield, and color value were determined for different times of steaming. The aims of this study were to determine the effect of steaming time on DSG and quality indicators, the relationship between DSG and the quality indicators, and the best DSG according to the best quality indicators of parboiled rice.
MATERIALS AND METHODS
Samples
A local variety of long rice paddy (Fajr) was provided by the Rice Research Institute of Iran, Deputy of Mazandaran. Moisture content and amylose content of this variety were 11% ± 1% and 22.9%, respectively. The samples were stored in plastic bags at 5 °C ± 1 °C before use (Bello et al., 2012). A gelatinization temperature of 68 °C was determined for the milled rice grain by DSC (Micro DSC 7, Setaram, France).
Parboiling conditions
The samples were parboiled on a laboratory-grade soaking and steaming conditions. The soaking setup (Fig. 1) consisted of a stainless steel cylindrical container, a 2 kW electrical heater, a basket for soaking the paddy in water, and a temperature control system. The thermocouple is connected to the temperature control system.
Fig. 1. Setup of the paddy soaking system.
The samples were soaked in a water bath set at 65 °C ± 0.5 °C for 180 min to achieve the best quality of parboiled rice (Taghinezhad et al., 2015a, b). After soaking, the paddy samples were taken out of the water and left in plastic bag for 6 h to equilibrate naturally to ambient temperature (27 °C ± 1 °C), which allowed water to enter the core part of rice grains (Soponronnarit et al., 2006). For steaming, 200 g paddy was then put into a parboiling vat which was placed on top of a pot containing about 10 L of boiling water (96.3 °C). The water in this pot did not touch the bottom of the vat. The sample was steamed for 2–10 min (at 2 min increments) at 96 °C (Spencer, 1983, Kar et al., 1999 and Islam et al., 2002). The paddy was finally dried in the shade (at 27 °C ± 1 °C and 60% ± 5% relative humidity) to a final moisture content of 11% ± 1% for 3 d (Fofana et al., 2011).
Degree of starch gelatinization (DSG)
DSG (%) of parboiled rice was evaluated during steaming according to Patindol et al (2008):
where, ΔH is the enthalpy change due to gelatinization of parboiled rice and ΔH* is the enthalpy change of raw rice. The enthalpies were measured with a differential scanning calorimeter instrument (Micro DSC 7, Setaram, France) as described by Taghinezhad et al (2015b). Parboiled rice (10 g) was ground into flour in a grinder and sieved (160 microns). Rice flour samples (30% flour + 70% distilled water) were selected randomly from treatments with different steaming times (2, 4, 6, 8 and 10 min). Rice flour samples were put in a pan. The pan was sealed and left to equilibrate at room temperature for 2 h (Himmelsbach et al., 2008). Samples were then heated from 24 °C to 98 °C at a scanning rate of 1 °C/min (Taghinezhad et al., 2015a). Experiments were performed in triplicate.
Head rice yield (HRY)
Rice milling consists of two steps, husking and polishing. The paddy samples were husked using a Satake rubber roll huller (THU-35A, Satake Ltd, Tokyo, Japan). After husking, the brown rice was polished by an abrasive type polisher (Grain Testing Mill, TM, Satake Ltd, Tokyo, Japan) for 90 s (Latifi and Alizadeh, 2014). Subsequently, whole and broken grains were separated automatically using a cylinder-type test rice grader (FQS-13X20, Sensewealth, China). Milled rice grains longer than three quarters of the whole kernel were classed as whole kernel, and the remainders were considered as broken grains (Fofana et al., 2011). HRY was calculated as a percentage ratio of the weight of whole kernels to the weight of the paddy (Bualuang et al., 2011).
Color value
A color meter (CX0738, Reston Company, USA) was used to measure the lightness and color value of whole kernel milled rice utilizing a* and b* uniform color space procedure. The values of a* and b* are the red/green and yellow/blue coordinates, respectively. The instrument was calibrated with a standard white plate having a* = 0.04 and b* = 2.23. Each measurement was replicated ten times and the average value was used for reporting. The color value (B) of parboiled rice was determined using the following equation (Luangmalawat et al., 2008):
Hardness
The hardness of parboiled rice (after milling) was measured using a material testing machine (H50 K-S, Hounsfield, England). The samples were placed on a flat plate and pressed with a flat probe of 12 mm diameter with a 500 N load cell fixed parallel to the base, at a cross-head speed of 1 mm/min. The force at rupture (first breaking) was recorded as the hardness for each run. Six replications were performed for each treatment and the average of the measured forces were calculated (Miah et al., 2002and Islam et al., 2004).
Statistical analysis
A full-factorial design was used for experimental analysis. The obtained data were analyzed by SPSS software (Version 15, SPSS Inc., Chicago, USA) and Excel 2011. Duncan's multiple range tests were used to determine the differences of the treatments at a confidence level of 95%. All experiments were performed in triplicate and mean values are reported with standard deviations.
RESULTS AND DISCUSSION
Degree of starch gelatinization
DSG is one of the most important thermal properties of rice. Fig. 2 and Fig. 3 show the DSC thermograms and the effect of steaming time on starch gelatinization in parboiled rice, respectively. DSG values increased with increasing steaming time (from 46.8% to 77.9%) while gelatinization enthalpy (size of the peaks) gradually decreased (from 1.35 to 0.56 J/g) during steaming. A gelatinization enthalpy of 0.56 J/g was obtained for the highest steaming time. The highest DSG value of 77.9% was found after 10 min of steaming, indicating that 22.1% of the starch had remained ungelatinized. Therefore, the lowest enthalpy values in rice were found in samples exposed to the longest steaming times. These results are in agreement with the reported observations that the amount of DSG increases during parboiling (Islam et al., 2002, Lamberts et al., 2006 and Manful et al., 2008). The experimental data of DSG was fitted versus steaming time using a third order equation DSG = 0.1127 × T3– 2.1331 × T2 + 15.18 × T + 26.315, R2 = 0.99, where DSG and T are degree of starch gelatinization (%) and soaking time (min), respectively. These findings are in agreement with the data reported by Taghinezhad et al (2015a). They suggested a third order polynomial to fit DSG versus soaking time for long grain rice variety (Fajr). During parboiling, as long as heat treatment was high, melting of the starch granules proceeds, sharply decreasing the ΔH (Islam et al., 2002). In addition, higher penetration of moisture occurs during steaming, leading to a reduction in ΔH, or increase in DSG (Saif et al., 2004). The amount of starch gelatinization is an extent indicator of parboiling process and it may depend on the time and temperature applied during process (Takahashi et al., 2005 and Patindol et al., 2008).
Fig. 2. Differential scanning calorimetry (DSC)thermograms of parboiled rice.
Steaming times in minareindicated in the figure.
Fig. 3. Effect of steaming time on gelatinization of parboiled rice.
Different letters indicate a significant difference (P < 0.05).
Head rice yield
Soaking and steaming at the suitable conditions increased the HRY of parboiled rice. While DSG increased monotonically with increasing steaming time, HRY was the maximum at a steaming time of 4 min (Fig. 4). This implies that there is no a linear relationship between DSG and HRY, and that DSC measurement cannot be used to predict HRY. Parboiling of paddy at longer steaming times would result in severe deformation of grain, along with release of endosperm following the absorption of excessive moisture and husk splitting. Deformed grains are more likely to be lost during milling along with the exuded part of the endosperm. In addition, absorbing excessive moisture leads to reduced HRY (Islam et al., 2004 and Bello et al., 2006). As a result, paddy parboiling at longer steaming periods is not advised. We note that the HRY reached a maximum at approximately 60% gelatinization. One reason for parboiling rice is to minimize kernel breakage during milling. Achieving maximum gelatinization during parboiling is not necessary for achieving minimum kernel breakage. According to Marshall et al (1993), 40% starch gelatinization is suggested for commercially produced parboiled rice to achieve maximum HRY. Also, HRY can range from 20% to 70% (Schluterman and Siebenmorgen, 2007).
Fig. 4. Relationships between degree of starch gelatinization (DSG)and the related traits of parboiled rice.
Different letters indicate a significant difference (P < 0.05). Steaming times in min are indicated in the figures.
Color value
Discoloration of rice due to parboiling treatment is another important quality indicator. It is directly related to the market value, because consumers do not accept dark colored parboiled rice. Fig. 4 shows a positive linear relationship (R2 = 0.86) between color value and DSG of parboiled rice. The results show that color value of parboiled rice increased significantly (P < 0.05) in a linear fashion with increasing DSG (CV = 0.0317 × DSG + 16.499, where CV is color value (Chroma). The linear model was considered the best model fit for color value level versus DSG. Color value increased from 18.08 to 19.04 after 10 min of steaming. These findings are in agreement with Islam et al (2002), who reported color values between 17.0 and 21.0 following parboiling at temperatures between 80 °C and 100 °C. The effects of parboiling conditions on color value and DSG have been reported by various researchers (Elbert et al., 2001, Miah et al., 2002 and Sareepuang et al., 2008). The color changes during parboiling were caused by non-enzymatic browning of the Maillard type and the processing conditions determine the intensity of color during parboiling. Sugar can also be released during parboiling because the husks on top of the paddy had split open. The subsequent reaction of sugar with the amino acids of the grain causes discoloration of milled rice (Miah et al., 2002). The husk pigment contributes to the color value by diffusing into the endosperm during soaking (Lamberts et al., 2006, Lamberts et al., 2008 and Sareepuang et al., 2008). The color value of parboiled rice decreased while the lightness increased. The decreasing tendency of lightness was severe at higher steaming times (Taghinezhad et al., 2015a). Therefore, high steaming times can lead to poor quality of parboiled rice.
Hardness
Hardness is the most important of all the physical properties of parboiled rice, as it reduces breakage during milling. As is shown in Fig. 4, the mean hardness of parboiled rice ranged from 118.6 to 219.2 N. Also, a linear regression relationship (R2= 0.88) was fitted between hardness (N) and DSG (%) of parboiled rice during steaming (RF = 3.1158 × DSG – 39.145, where RF is hardness (N). We considered the linear relationship sufficiently robust to describe the best model. The most likely explanation for the increase in hardness with steaming time is that more grains are gelatinized, leading to increased fracture resistance. It has been previously reported that the severity of parboiling increases the hardness of rice (Nasirahmadi et al., 2014).
CONCLUSIONS
DSG and gelatinization enthalpy (ΔH) of parboiled rice were found to be dependent on steaming times. When the steaming time was increased from 2 to 10 min, ΔH decreased gradually (from 1.35 to 0.56 J/g), whereas, DSG, the color value and hardness increased gradually. The highest HRY (67.1%) was obtained at a steaming time of 4 min. Color values were used as a quality index of parboiled rice. DSG was strongly correlated with color value and the quality of parboiled rice. Finally, we conclude that steaming for about 4 min is appropriate to optimize all the required qualities of rice. These results can be used to predict DSG values or quality properties of parboiled rice following steaming under different time conditions. This leads us to the conclusion that quality properties can be applied as a simple predictive tool of the amount of starch gelatinization during steaming. Also, these results can be used for a real industrial scale, which is worthy of being investigated in future.
ACKNOWLEDGEMENT
The authors thank the department of Food Science & Technology at Tokyo University of Marine Science & Technology, Japan, for use of the micro DSC instrument.
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