Influence of Moisture Content, Variety and Parboiling on Milling Quality of Rice Grains

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1 Rice Science, 2014, 21(2): Copyright 2014, China National Rice Research Institute Published by Elsevier BV. All rights reserved DOI: /S (13) Influence of Moisture Content, Variety and Parboiling on Milling Quality of Rice Grains Abozar NASIRAHMADI 1, Bagher EMADI 1, Mohammad Hossein ABBASPOUR-FARD 1, Hamid AGHAGOLZADE 2 ( 1 Department of Agricultural Machinery, Ferdowsi University of Mashhad, P. O. Box Mashhad, Iran; 2 Haraz Extention and Technology Developement Centre, P. O. Box, 413 Mahmoodabad, Iran) Abstract: The effects of moisture content (8%, 10% and 12%), variety (Tarom and Fajr) and parboiling on milling quality of rice as a function of milling recovery (MR), head rice yield (HRY), degree of milling (DOM) and whiteness were investigated. The parboiled grains was prepared with three soaking temperatures of 25 ºC, 50 ºC and 75 ºC and three steaming times of 10, 15 and 20 min. As a result of parboiling, the increasing rates of MR and HRY values were 7.8% and 14.3% for Tarom and 9.8% and 10.0% for Fajr, respectively, and the decreasing rates for DOM and whiteness were 6.6% and 10.8% for Tarom and 6.8% and 10.5% for Fajr, respectively. Moreover, decreasing moisture content to 8% maximized MR (75.8% for Tarom and 74.3% for Fajr) and HRY (65.8% for Tarom and 57.0% for Fajr) while increasing that to 12% revealed maximum values of DOM (6.1% for Tarom and 6.2% for Fajr) and whiteness (24.8% for Tarom and 28.2% for Fajr). Key words: degree of milling; head rice yield; milling recovery; whiteness; moisture content; parboiling; milling quality Rice (Oryza sativa L.) as one of the main agricultural produces of the world supplies the food for more than half of the world s population (Courtois et al, 2010). The global production of rice had been increased from 520 million to about 680 million tons from 1990 to 2009 (FAOSTAT, 2009). Processing rice like the other agricultural produces is accompanied with high losses. For example, milling loss because of poor performance of milling equipment and operator ineptitude was reported as 45% (FAO, 2011), which regarding to the high volume of rice production reveals the importance of the subject. Therefore, reducing the processing losses can be one of the feasible ways of compensating the increasing demand. The initial step in rice processing is dehusking of kernels which results in brown rice. Polishing or removing of bran to yield white rice is the second step. Although white rice is less nutritious than brown rice, some consumers prefer white rice because of its color and easy digestion (Gujral et al, 2002). Experts apply various terms to evaluate different steps of rice processing. For example, milling recovery (MR) and head rice yield (HRY) are defined Received: 30 January 2013; Accepted: 20 June 2013 Corresponding author: Bagher EMADI (emadi-b@ferdowsi.um.ac.ir) as the percentages of total milled rice (broken + head) and head rice based on the paddy (rough rice) weight, respectively (Pan et al, 2007). Head rice is expressed as milled grains that are 3/4 or more the original grain length. The high values of MR and HRY are essential for the effective rice breeding programs (Gravois, 1998). The cooking quality and hence the market price of broken rice is less than those of head rice (Iguaz et al, 2006). Degree of milling (DOM) is expressed as the extent of bran removal from brown rice. This parameter is essential to estimate the nutritional value of rice including the amount of proteins, vitamins, and minerals. Additionally, DOM is a valuable factor in terms of economic return of the milled rice (Liu et al, 1998; Gujral et al, 2002). The economic value of rice is also affected by its milling quality (MQ) (Sun and Siebenmorgen, 1993; Pan et al, 2005). There is no exact definition for MQ. Therefore, many studies have been conducted on MQ. Sun and Siebenmorgen (1993) noted that HRY and DOM are two indices which can be used to determine MQ. They found that milling time and kernel thickness affect HRY and DOM. Yadav and Jindal (2008) used HRY and whiteness to define MQ of 10 rice varieties. They showed that whiteness increases with an increase in milling duration. MQ affects three categories including trade quality (HRY and whiteness), cooking quality (amylose content,

2 Abozar NASIRAHMADI, et al. Milling Quality Affected by Moisture Content, Variety and Parboiling 117 gelatinization temperature, gel consistency, aroma and flavor) and nutritional quality (thiamine and lysine contents) (Karbassi and Mehdizadeh, 2008; Mehdizadeh and Zomorodian, 2009). Ondier et al (2010) also mentioned that MQ is related to HRY and whiteness. One noticeable portion of rice processing losses is qualitative losses including cracked kernels, damaged kernels and broken kernels (Zareiforoush et al, 2010). The parboiling of rice as an optional and promising operation can reduce these losses. Parboiling was defined by Ali and Pandya (1974) as a hydrothermal process which causes gelatinization of rice-starch within grains. This process includes soaking rice in hot/cold water, steaming with hot vapor and finally drying rice under different conditions. Reviewing the literature indicates that parboiling process has many advantages such as increasing nutrition value, HRY and MR and extending storage life. However, less whiteness and DOM values are limitations of the parboiling process as reported by some researchers (Bhattacharya, 1996; Pillaiyar et al, 1996; Gujral et al, 2002; Sridhar and Manohar, 2003; Soponronnarit et al, 2006; Lamberts et al, 2008; Dors et al, 2009). Bhattacharya (1996) reported that parboiling can decrease the whiteness of rice while the pressure of soaking and the time of steaming affect rice color. Gujral et al (2002) investigated the effects of weight of milled sample and milling chamber on DOM of three Indian parboiled and unparboiled rice varieties. They reported that DOM reduces and HRY increases as the result of parboiling. In another research (Lv et al, 2009), it has been revealed that parboiling increases the yellowish color of polished rice grains. Liu et al (2009) explored the relationship between DOM and physical properties of 10 Chinese brown rice. They showed that DOM is affected by different physical properties of brown rice. The present study aimed to research the effects of moisture content (8%, 10% and 12%) on MQ defined as a function of MR, HRY, DOM and whiteness of two rice varieties for parboiled and unparboiled samples. MATERIALS AND METHODS Two most cultivated cultivars of long-grain Iranian rice (Tarom and Fajr) as case studies were provided by the Haraz Extension and Technology Development Center (Mazandaran, Iran). The rice panicles were harvested by hand and threshed with a small-plot combine. Moisture content of rice grains was measured based on the oven-drying method at 105 C for 24 h (Mohapatra and Bal, 2007). The average initial moisture content of rough rice was obtained as 23% after threshing. The grains were dried at room temperature for 48 h to achieve moisture content of about 15%. About 45 kg of grain samples of each variety were divided and packed into polyethylene bags (each 1.5 kg) and stored in a refrigerator at 10 ºC until the time of experiments. Unparboiled samples were dried in a standard hot air oven at C for up to 24 h and different time intervals, until achieving 12%, 10% and 8% of moisture content. Parboiling of grains Two-thirds of the collected samples were parboiled using a local method with the following procedure: Soaking: The grains were soaked in water at 25 ºC, 50 ºC and 75 ºC for 48, 6 and 3 h, respectively. The samples were then completely drained until there was no free water. Steaming: The samples were steamed for 10, 15 and 20 min at 100 C and atmospheric pressure for each soaking temperature. At the end of this process, the moisture content of samples was about 40% 45%. Drying: Parboiled samples were left on the mats (made from agricultural by-products) in a laboratory for 48 h until the moisture content reduced to 15%. The parboiled samples were then dried in a standard hot air oven at C for h with different time intervals, until achieving 12%, 10% and 8% of moisture content levels. Samples were labeled according to soaking temperature and their steaming time, respectively. For example, the sample with initial soaking temperature of 25 C and steaming time of 10 min was given the label of Milling of kernels To obtain brown rice, three 500 g sub-samples of unparboiled and parboiled rice of both varieties were taken and dehusked using a laboratory rubber roll type rice husker (ST50, Yanmar, Japan). The whole brown rice kernels were weighed using an electronic balance (GF-6000 AND, Japan) of ±0.001 g accuracy and subsequently were milled for 30 s at room temperature using a laboratory friction and abrasion vertical type whitener (VP-31, Yamamoto, Japan). The milling pressure was adjusted to 300 g/cm 2. During whitening (milling) of the kernels, the milling pressure was kept uniformly for all the samples. The head rice and broken kernels were separated using a laboratory rice grader (TRG058, Satake, Japan). A mesh of 4.75 mm was used for separating broken rice and another mesh of 5.25 mm was applied for separating head rice.

3 118 The degree of whiteness of milled rice samples was measured with a laboratory whiteness meter (C-300-3, Kett Electronic, Japan). The samples of white rice were placed in a small container of the whiteness meter and the whiteness was individually measured and recorded. The equipment was calibrated with a calibration plate having white index of MR based on paddy weight was determined as following (Pan et al, 2007): MR (%) = (W t / W p ) 100, where W t (g) is the weight of total rice grains (head + broken) after milling, and W p (g) is the weight of grains before milling. HRY based on grain weight was calculated using the following equation (Pan et al, 2007): HRY (%) = (W d / W p ) 100, where W d (g) is the weight of head rice after milling. DOM based on paddy weight was determined as (Gujral et al, 2002): DOM (%) = (W b W t ) / W p 100, where W b (g) is the weight of brown rice. Statistical analysis The data extracted from experiments were analyzed using SPSS (version 16) and Excel software for plotting the necessary charts. Analysis of variance of means was conducted using Duncan s multiple tests. The reported data for MR, HRY, DOM and whiteness are the average of three replications. RESULTS MR Analysis of variance of the effects of independent variables on milling quality elements (MR, HRY, DOM and whiteness) of unparboiled and parboiled Rice Science, Vol. 21, No. 2, 2014 grains for both varieties is shown in Table 1. MR values of unparboiled and parboiled samples were significantly affected (P < 0.01) by the independent variables. Fig. 1 shows the effect of different moisture contents and treatments of parboiling on MR for both varieties. Comparison of the MR values for unparboiled grains of the two varieties revealed that MR was higher for Tarom than that for Fajr in all levels of moisture content. The values of MR decreased with increasing moisture content from 8% to 12%, for both parboiled and unparboiled samples. HRY HRY values of unparboiled grains were significantly (P < 0.01) affected by variety, moisture content and their interaction (Table 1). Also, HRY values of parboiled grains were significantly (P < 0.01) affected by variety, moisture content, soaking temperature and steaming time. HRY increased with the decrease in moisture content from 12% to 8% for both parboiled and unparboiled grains (Fig. 2). The unparboiled Fajr grains had relatively higher slopes than unparboiled Tarom ones in all intervals of moisture content. The highest and the lowest values of HRY for parboiled Tarom and Fajr were found both in samples and 50-10, respectively. It can be observed from Fig. 2 that parboiling caused 14.3% and 10.0% increase of HRY compared to unparboiled Tarom (51.5%) and Fajr (47.0%) varieties in the same level of moisture content, respectively. DOM DOM was significantly affected (P < 0.01) by variety, moisture content, soaking temperature, steaming time Fig. 1. Effects of moisture content, variety and parboiling on milling recovery.

4 Abozar NASIRAHMADI, et al. Milling Quality Affected by Moisture Content, Variety and Parboiling 119 Table 1. Analysis of variance indicating the effects of independent variables on milling quality of unparboiled and parboiled rice grains. Variable MR (%) HRY (%) DOM (%) Whiteness Unparboiled grains Variety (A) 1.3E3** 2.7E3** 78.6** 7.8** Moisture content (B) 2.7E3** 1.4E3** 367.8** 12.3** A B 26.24** 32.65** 15.0** 0.6 Parboiled grains Variety (A) 112.3** 20.7** 64.8** 2.0E3** Moisture content (B) 439.5** 6.1** 184.5** 37.1** A B 26.2** 3.4* 3.5* 4.0* Soaking temperature (C) 124.5** 35.0** 106.1** 219.9** Steaming time (D) 525.2** 15.4** 83.6** 53.4** C D 72.3** 3.0* 26.0** 1.4 MR, Milling recovery; HRY, Head rice yield; DOM, Degree of milling. ** and *, Significant at P < 0.01 and 0.05, respectively. and interaction between soaking temperature and steaming time (Table 1). After parboiling, the DOM values of the two varieties decreased (Fig. 3). While the maximum obtained values of DOM for parboiled Tarom and Fajr varieties were 6.1% and 6.2%, it was 12.7% and 13.0% for their unparboiled samples, respectively (both at 12% moisture content). For parboiled Tarom and Fajr, the lowest DOM was obtained as 4.8% and 5.0%, respectively, in the cases of 25-20, samples, both at moisture content of 8%. Whereas for unparboiled Tarom and Fajr, the lowest DOM was achieved as 9.7% and 10.2%, respectively. DOM increased with increasing moisture content for both varieties (Fig. 3). When moisture content increased from 8% to 12%, DOM increased from 9.7% to 12.7% and 10.2% to 13.0%, for unparboiled Tarom and Fajr varieties, respectively. The steepness slope of the lines shows the rate of change in DOM compared with moisture content (Fig. 3). Generally DOM increased with the increase of moisture content of two unparboiled rice varieties, but the rate of change of DOM with moisture content of 10% 12% was different from that of 8% 10% for both varieties. It confirms the significant effect (P < 0.01) of the interaction between steaming time and soaking temperature on the resulted values of DOM for both studied varieties. The DOM values of parboiled Tarom samples 25-10, and showed significant increase (P < 0.01). The parboiled samples 25-20, and also exhibited significant increase (P < 0.05). However, the DOM values of parboiled Tarom samples 25-15, and showed no significant differences (P > 0.05). The DOM values of parboiled Fajr samples 25-20, 50-10, 50-15, 50-20, 75-10, 75-15, and also showed significant increase (P < 0.01). There were no significant differences (P > 0.05) for the two other samples of parboiled Fajr and Whiteness Whiteness is another important factor influencing rice quality. The moisture content, variety, soaking temperature and steaming time had significant effect (P < 0.01) on whiteness of parboiled samples, whereas the interaction of variety and moisture content on whiteness was significant at P < 0.05 (Table 1). The whiteness of both varieties decreased after parboiling process (Fig. 4). The whiteness of parboiled Tarom varied from 20.7% (75-20) to 24.8% (50-15) (Fig. 4). The highest whiteness of parboiled Tarom obtained at 12% moisture content was 10.8 percent point lower than that of unparboiled Tarom (35.6%) at the same Fig. 2. Effects of moisture content, variety and parboiling on head rice yield.

5 120 Rice Science, Vol. 21, No. 2, 2014 Fig. 3. Effects of moisture content, variety and parboiling on degree of milling. Fig. 4. Effects of moisture content, variety and parboiling on whiteness. moisture content. In parboiled Fajr, the whiteness varied from 22.6% (75-20) to 28.2% (50-15). The highest value of whiteness for parboiled Fajr was 10.5 percent point less than that of unparboiled Fajr (38.7%) at the same level of moisture content (12%). An increasing trend was observed for the whiteness of parboiled Fajr when moisture content varied from 8% to 12%. The effect of moisture content and variety on whiteness of unparboiled samples was also significant (P < 0.01), whereas the interaction effect of these two parameters had no significant effect on whiteness (Table 1). In unparboiled samples, the maximum whiteness occurred at 12% moisture content for both varieties. For unparboiled Fajr, whiteness increased from 37.8% to 38.7% with the increase of moisture content from 8% to 12%. For unparboiled Tarom, whiteness increased from 33.8% to 35.6% with the increase of moisture content from 8% to 12%. MR and HRY DISCUSSION As mentioned before, MR values of unparboiled samples were higher for Tarom than those for Fajr in all levels of moisture content. This may be related to the different behaviors of rice varieties during separation of husk from kernel, like: kernel shape, porosity of starch granules and presence of chalkiness. For example, Razavi and Farahmandfar (2008) reported existing significant differences (P < 0.05) of 1000-grain weight, unit mass, bulk density and porosity between Tarom and Fajr in the case of rough rice. The mechanical properties of their kernel and bran may also be different, which needs more research. Observing decreased values of MR with increasing moisture content from 8% to 12%, for both parboiled and unparboiled samples,

6 Abozar NASIRAHMADI, et al. Milling Quality Affected by Moisture Content, Variety and Parboiling 121 revealed that their properties could be influenced by varying the values of moisture content. These results agree with the results of pervious research (Thompson et al, 1990; Andrews et al, 1992). It was observed that parboiling can improve the MR (up to 75.8% for Tarom and 74.3% for Fajr) of both varieties. Regarding to the maximum achieveable MR of current commercial millers which is 65% (Rice Knowledge Bank, 2012), parboiling is strongly recommented to reduce milling losses. Similar results have also been reported by others (Imoudu and Olufayo, 2000; Miah et al, 2002). As it is shown from the data of this research, the best results for MR were obtained at 8% moisture content for all treatments. However, Imoudu and Olufayo (2000) recommended 12% moisture content for achieving the highest MR. The highest (75.8%) and the lowest values (68%) of MR for parboiled Tarom were observed in the samples labeled and 25-10, respectively. These values for Fajr occurred in the samples (74.3%) and (64.5%), respectively. The obtained close values of parboiling characteristics for both varieties reveals the importance of investigating optimum values for each variety of grains which are going to be parboiled. Decreased HRY values with increasing moisture content which was found in this study for both parboiled and unparboiled grains, has been already reported by previous researchers (Banaszek and Siebenmoregen, 1990; Thompson et al, 1990; Andrews et al, 1992). As mentioned before, parboiling resulted in increased HRY values for both varieties. Similar trends have been reported by others (Gujral et al, 2002; Sareepuang et al, 2008). They may be attributed to gelatinization process, which contributes to improve stronger structure of rice starch (Soponronnarit et al, 2006). In general, HRY of Tarom was somehow higher than that of Fajr in all levels of moisture content. This may be related to the difference of physical, mechanical and morphological properties between the studied varieties. Razavi and Farahmandfar (2008) already revealed the existence of difference in physical and frictional properties of these two varieties. They reported that the unit mass of Tarom is significantly (P < 0.05) higher than that of Fajr in the cases of rough, brown and milled rice while the volume value of Tarom is significantly (P < 0.05) higher than that of Fajr only for brown and milled rice cases. It is clear that for a high enough HRY, the moisture content should be less than 10%. The best results for HRY were obtained at 8% moisture content for all treatments and also the control sample, and hence to have higher HRY and lower loss, it is better to keep down moisture content to 8%. DOM and whiteness Decreased DOM of rice after parboiling may be attributed to more bran removal from unparboiled rice than parboiled rice (Miah et al, 2002), because parboiling increases milling resistance of rice (Gujral et al, 2002). A similar effect of parboiling on DOM has been reported by others (Gujral et al, 2002; Miah et al, 2002). It is justified that by increasing moisture content, the bran can be easily removed from the brown rice (Andrews et al, 1992; Reid et al, 1998). Moreover, the DOM of unparboiled rice was higher for Fajr than Tarom in all levels of the studied moisture content. These differences between the varieties may be related to the morphological variations for removal of bran layers or may associate with their physical properties (Liu et al, 2009). For the studied varieties of Tarom and Fajr, Razavi and Farahmandfar (2008) revealed that there is a significant difference (P < 0.05) between physical properties (including mass, volume and grain weight) of brown and milled rice. Decreasing whiteness because of parboiling, which was revealed in this research for both studied grains, has been reported by others (Ali and Pandya, 1974; Bhattacharya, 1996; Pillaiyar et al, 1996). Whiteness was also affected by moisture content. Similar behavior is reported by Yan et al (2005). They found a direct correlation between increased moisture content and whiteness of embryo rice (rice with embryo). It was concluded that unparboiled Fajr had higher whiteness than unparboiled Tarom in all the moisture content levels (Fig. 4). In addition to different physical properties of these varieties (Razavi and Farahmandfar, 2008) which may cause this behavior, different mechanical properties of their bran may also contribute to the observed results. For example, frictional behavior of Tarom and Fajr on rubber and glass is significantly different (P < 0.05), which confirms the tissue of their bran could be different. These results showed that from whiteness point of view, for both unparboiled and parboiled varieties, moisture content of 12% is the best level and is favorable to produce high quality of rice. CONCLUSIONS The effects of milling moisture content, variety and different parboiling treatments on milling quality parameters of two rice varieties Tarom and Fajr were

7 122 Rice Science, Vol. 21, No. 2, 2014 investigated through MR, HRY, DOM and whiteness. The highest values of MR and HRY were obtained at 8% moisture content for both varieties and the values were higher for Tarm than for Fajr. The maximum values of MR and HRY were 75.8% and 65.8% for Tarom and 74.3% and 57.0% for Fajr, respectively. The maximum values of DOM and whiteness belonged to Fajr at 12% moisture content. They were 6.2% and 28.2% for Fajr and 6.1% and 24.8% for Taro, respectively. ACKNOWLEDGEMENTS The authors would like to thank Ferdowsi University of Mashhad, Iran for providing the laboratory facilities and financial support (Grant No ). REFERENCES Ali N, Pandya A C Basic concept of parboiling of paddy. J Agric Eng Res, 19(2): Andrews S B, Siebenmorgen T J, Mauromoustakos A Evaluation of the McGill No. 2 rice miller. J Cereal Chem, 69(1): Banaszek M M, Siebenmoregen T J Head rice yield reduction rates caused by moisture. 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