Physical properties and cooking quality of extruded restructured rice: impact of water temperature and water level

Rice is the main food for approximately 3.5 billion people worldwide, especially in Asians, who have consumed more than 90% of the total rice produced. Restructured rice is another designation of artificial rice is an effort to diversify staple foods processed from carbohydrate-based raw materials with the addition of certain substances to improve the quality of staple foods. Restructured rice can be done with several techniques, such as using extrusion. This study aimed to investigate the effect water content (35%, 37.5%, 40%, 42.5%, and 45%) and temperature (100C and 26C) on the surface and characteristics of restructured rice using a pasta extruder. Results of this study showed the treatment with water content 40% and temperature 100C to be the best, more precisely seen from the results of laser microscope, color, WAI, WSI, WAR, Cooking losses, and cooking time. Pores and surfaces of restructured rice are almost the same as milled rice. Water absorption index (WAI) value = 2.273±0.10 g/g, WSI = 2.114±0.11%, WAR = 150.99±0.77%, CL = 1.92±0.10% and cooking time 4 mins. Suggestions for this research are further studies such as fortification with other ingredients using a pasta extruder technology and are expected to be implemented and commercialized.


Introduction
Rice (Oryza sativa L.) is one of the most widely produced and consumed grains in the world because it is the main food source for more than half the world's population. Rice is also an important source of energy for human consumption because it has a high content of starch and a source of minerals, protein, and vitamins (Walter et al., 2005). Rice is the main food for approximately 3.5 billion people in the world, especially to Asians, who have consumed more than 90% of the total rice produced (Mohanty, 2013). Rice has specific characteristics, such as hypoallergenic, ability to bring flavor and taste, making it the desired seed for use in value-added products. Some examples of these products include tortillas, gluten-free rice bread, drinks, low-fat sauces, processed meats, puddings, salad dressings, and artificial rice. Changing rice flour can change its functional properties in such a way that it can be used as a substitute for products that have other added values (Wang et al., 2013).
Restructured rice is another designation of artificial rice is an effort to diversify staple foods processed from carbohydrate-based raw materials with the addition of certain substances to improve the quality of staple foods, so that can be used as food functional. Restructured rice can be done with several techniques, such as using extrusion. According to Mishra et al. (2012) extrusion technology can be classified into two categories: hot extrusion and cold extrusion. Both processes have a dough shaped material made from the main components (mostly rice flour), additive mixtures, and water through single or twin-screw extruders, while Alavi et al. (2008) reported does not utilize any additional thermal energy input other than the heat generated during the process itself, and is primarily a low temperature (below 70 o C), forming process resulting in grains that are uncooked, opaque, and easier to differentiate from regular rice kernels. Much research on rice production was restructured using cooking/hot extruders. This research to study about the production of restructured rice using a pasta extruder because it is easier to operate and simpler to produce than cooking extruder, but the problem is to know how much composition is right to produce good restructured rice because pasta extruder does not use heat energy in the process.
Many studies on cooking extrusion have extrusion effects on the quality parameters of extruded products (Dalbhagat and Mishra, 2019), texture evaluation of extruded rice (Wang et al., 2013), micronutrient stability (Dalbhagat and Mishra, 2019), and cooking characteristics in food extrusion (Singh et al., 2005) have been reported. However, this review brings together recent research on extruded rice and rice-based products and the effects of water content and temperature parameters using a pasta extruder.
Research about the effect of water content and temperature on production of restructured rice using pasta extruder has never been done. So, it is necessary to research to obtain an appropriate formulation in the use of the water content and temperature to produce restructured rice with good physical, chemical, and cooking characteristics. This study is used to prepare the restructured rice that will be fortified with other ingredients using a pasta extrusion technology. Various ingredients are added to rice flour to improve the nutritional, texture or sensory attributes of rice-based extrudates. To improve texture, rheology and functional properties and to reduce the loss of cooking the final product, proteins such as egg white powder, lupine, casein, etc. Added as a structuring agent (Dalbhagat and Mishra, 2019).

Preparation of rice flours
Japonica rice (Oryza sativa) variety Tainan 11: TN-11 was grounded using small grinder (RT-04, Mill Powder Tech Co., Ltd., Taiwan), and sieved through an 80-mesh using sieve shakers (AS 200, RETSCH Ltd., Germany), test sieve no 80 Mesh (Standard ASTM E11-09 RETSCH Ltd., Germany) used to measure flour. Flour is packed in a polyethylene bag and stored for 24 hrs at 4°C to avoid microbial growth.
Restructured rice was made with independent variables, namely the first of the Water Content (X 1 ) with five levels, 35% (M1), 37.5% (M2), 40% (M3), 42.5% (M4), and 45% (M5). Then in the second treatment, Temperature (X 2 ) with two levels, 100 o C (T1) and 26 o C (T2).In this study, completely randomized design (CRD) with two factors and three replications has been used. The treatment combination is determined by five levels of water content (X 1 ), two levels of temperature (X 2 ), and control (Japonica Rice).

Research implementation
Water is heated using cooking deck (UF-2050, Sony, Japan) at a temperature of 100 o C and 26 o C in a beaker glass and closed using aluminum foil and equipped with a thermometer to measure temperature. The water was weighed according to the treatment in the research, namely water contents 35%, 37.5%, 40%, 42.5%, and 45%. Then added with xanthan gum 1% (w/b), emulsifier 1% (w/b), and shortening 2.5% (w/b) in water that has been heated and weighed then stir until evenly. Afterwards, take some rice flour (50%) and mix with water that has been weighed, heated, added xanthan gum, emulsifier, and shortening. After that blend until homogeneous, then add remaining rice flour (50%), and solution Ca(OH) 2 2% by 8% (w/b) then blend until evenly. Then the dough is placed in Pasta Extruder La Monferrina Model P6 which is shaped like a grain of rice and the dough is cut into fragments. Then, immediately place the restructured rice into the air dryer with a temperature of 40 o C for 2 hrs. After getting a dry rice FULL PAPER Figure 1. The experimental flow chart in the study kernel (DRK), the surface is analyzed by a laser microscope, then analyzed color, water index absorption, and water solubility index. Then, dry rice kernel is cooked to analyze cooking characteristic (water absorption ratio, cooking loss, and cooking time). For more details, the flow chart can be seen in Figure 1.

Analysis method 2.4.1 Restructured rice surface
The surface of the restructured rice was observed using a Laser Microscope (VK-X series, Keyence, USA) with 1000x magnification.

Color analysis
The restructured rice is ground and then sieved using a 80 mesh. At first, Color Meter (ColorQuest XE, Hunter Lab, Inc., USA) with calibration is prepared, then 50 grams of sample was placed in machine space and then run. The color test is done with color system L* (Lighting color), a* (red color), and b* (yellow color). A colorimeter is calibrated first with the white standard contained in the tool. The results of the analysis of the resulting white degrees in the form of values L*, a*, b*. Measurement of the total degree of color is used as a white base color.

Water absorption index (WAI) and water solubility index (WSI)
WAI and WSI are determined using the method by (Dalbhagat and Mishra, 2019). First, the sample flour (2 g) was diluted in 20 mL of distilled water in 50 mL centrifuge tubes then mixed for 10 mins and centrifuged at 3000 rpm for 20 mins. After that, the supernatant is placed in an aluminum dish then dried the supernatant in an oven 105 o C for 24 hrs, then the weight of the gel and dried supernatant is recorded. WAI and WSI were determined by the following formula:

Water absorption ratio (WAR), cooking losses, and cooking time
The ratio of water absorption and loss of solids during cooking is determined using the method by (Singh et al., 2005). Rice samples (5 g) were cooked in 20 mL boiling distilled water for 10 mins. After cooking is complete, the dissolved solids are moved into an aluminum dish and dried in an oven at 105°C for 24 hrs. Then the cooked weight is taken and WAR is calculated according to the formula. Dry dissolved solids are weighed, and cooking losses are calculated according to the formula. The cooking time of the sample is determined using the method by (Singh et al., 2005). Rice samples (5 g) were placed in 20 mL of boiling water. At 1 minute intervals, ten rice grain samples were taken and pressed between the sliding glass to find out the presence of the white core. The loss of the white core indicates that the rice is ripe. WAR and Cooking Losses were determined by the following formula:

Statistical analysis
Data obtained from the study were analyzed by ANOVA using the SPSS Statistic version 22 software. Significantly different data was received by the Duncan's Multiple Range Test (MRT) with a confidence interval of 5%.

Restructured rice surface
The surface 3D resulted from the effect of water content and temperature on the restructured rice, which was observed using Laser Microscope, as shown in Figure 2.
Observations using a laser microscope on the surface of the restructured rice that is treated at a moisturized feed temperature of 26 o C compared with a temperature of 100 o C. The picture shows that the treatment of temperature 26 o C surface of restructured rice is more porous, so it is broken more easily. This is related to the temperature of starch gelatinization and the effects of the FULL PAPER Figure 2. Laser Microscope on the surface of japonica rice (commercial) and restructured rice 1000x magnification. gelatinization process in the dough. At 100 o C, the gelatinization process can strengthen the restructured rice granules, and the surface is stronger and smoother. Budi et al. (2013) reported that at extrusion temperatures of 70℃, 80℃, and 90℃, the water content of the dough is 35%, 40%, and 45%, the extrusion process causes the starch in analog rice to be fully gelatinized (degree of gelatinization 100%). While at room temperature, starch gelatinization does not occur so that the surface is flat and causes little damage to the surface of the restructured rice. And the more the water content increases, the more the density level of the restructure rice also increases. Similar research has been conducted by Zhuang et al. (2010) that the bulk density of extrudates varies not significantly with temperature changes 30 o C and 50 o C. It is also evident that the extrudates structure becomes denser due to the feed water content increased from 28% to 36%. Lower screw extruder speed becomes relatively looser structures are formed in the extrudates. With a higher screw extruder speed, retention time is shortened, and feed gelatinization rate is low, and mixing of dough is not homogeneous exists, so there is a relatively denser structure in it extrudates.

Color
The effect of water content and temperature on the color of the restructured rice can be seen in Figure 3. The result is that the lightness (L) of the restructured rice tends to be lower than milled rice due to the addition of other components such as xanthan gum, emulsifier, and shortening. Temperature tends to affect the red color (a) because there are some component changes during the process of mixing the dough in hot water, such as denaturation of proteins and starch gelatinization.
Temperature and water content do not affect the yellowish color (b). This is because the material used is rice flour does not contain a reducing sugar component so that no Maillard reaction occurs.

Physical characteristics
The effect of water content and temperature on the water absorption index (WAI) and the Water-Soluble Index (WSI) of restructured rice can be seen in Table 1. Table 1 shows the result for the WAI which is not significantly different between control, M1T1, and M1T2. The value for WAI ranges between 1.836 and 2.614. The temperature of water mixed in the dough showed results affect the WAI, the temperature of 100 o C yield a lower WAI compared to the temperature of water 26 o C. Increased water content results in a higher WAI. Water absorption index (WAI) is the ability of a material to absorb a certain amount of water. WAI can be used as a functional indicator to know the degree of cooking of extruded products. We can notice the increase in the amount of starch gelatinized in the extrusion process. High temperature and pressure will cause more and more starch to undergo dextrinization. Dextrinized starch plays a role in water absorption. This affects WAI which includes several extrusion process input variables, such as extruder type, feed water content, feed rate, screw speed, last barrel temperature, screw configuration, screw compression ratio, and die size. Water is very influential as a plasticizer in the extrusion process, which can reduce the degradation of rice starch and can increase the water absorption capacity of an extrudate (Bryant et al., 2001;Liu et al., 2011). And also, Liu et al. (2011) reported that increased screw speed and moisture could increase WAI in rice. Meanwhile Hagenimana et al. (2006) reported at high temperature and low moisture, WAI decreased significantly due to dextrinization gelatinization in rice analogs. FULL PAPER Figure 3. Effect of water content and water temperature on the color of restructured rice.  Table 1 shows the result of the WSI which is significantly different between control and treatments. The value for WAI ranges between 0.987 and 2.279. The temperature of water mixed in the dough showed to affect the WSI, the temperature of 100 o C yield a high WAI compared to the temperature of water 26 o C, and increased water content results in a higher WSI. The number of water-soluble materials in a certain amount of water. After starch repair, gelatinization will occur the degradation of amylose and amylopectin to produce smaller molecules. This degradation occurs during the extrusion of materials at high temperature and pressure conditions. These relatively small molecules are more easily dissolved in water. Onwulata et al. (2001) reported higher water will cause a higher material flow rate in the barrel to reduce shear and starch degradation whereas Stojceska et al. (2009) reported if lower water was associated with high starch degradation due to high shear. The increase in WSI value is thought to be due to the disintegration of the amylopectin and amylose chains in the extrusion process, thereby increasing the total dissolved solids (Sandrin et al., 2018).

Cooking properties
The effect of water content and temperature on water absorption ratio (WAR), cooking loss, and cooking time of restructured rice can be seen in Table 2. Increased water content increases in WAI value. The results of this study prove that water penetration into the grains of restructured rice is easier during cooking, due to lower gelatinization temperatures, starch tissue that is weak and porous so that water is more easily absorbed in restructured rice. In a similar experiment by L. Wang et al. (2016) on brown rice pasta which reduces water absorption due to temperature rise, however, it remains the same as the screw speed increases. Da Silva et al. (2016) reported that gelatinization that occurs in starch can inhibit water absorption. And also, Singh et al. (2005) reported that solid and dense structures reduce water absorption from natural rice seeds during the cooking process. Meanwhile, in a study Kaushal et al. (2012) it was found that the capacity of water absorption increases during the cooking process due to the loss of amylose leaching and crystallinity. Table 2 shows the result for the Cooking loss which is not significantly different between control, M5T1, and M2T2. The value for WAI ranges between 1.59 and 2.55. Water content and temperature affect cooking loss value from restructured rice. The temperature of 100 o C used in the dough produces a lower cooking loss compared to the temperature of 26 o C. The increase in water content above 40% resulted in the decrease in the cooking loss value. The amount of cooking loss is influenced by factors such as incomplete starch gelatinization under mild extrusion conditions (eg, lower temperatures with combined effects of water) or excessive starch degradation under severe extrusion conditions (eg, low temperatures in combination with screw speed, water, and feed rate). Besides that, at 40% water and 100 o C temperature produce a restructured rice that has a finer surface structure and its porosity is very tight (M3T1 Figure 2). Wang et al. (2016)  properties of brown rice pasta. The pasta produced using an extrusion temperature of 120°C and a screw speed of 120 rpm has the best quality with a cooking loss of 6.7. According to Da Silva et al. (2016) It can be attributed that the role of gelatinized starch can be used as a binding agent in extruded products. Table 2 shows the result for the Cooking time which is significantly different between control and treatments. The effect of water content and temperature on cooking Time of restructured rice can be seen in Table 2. The temperature of 100 o C and 60 o C used in the dough has an average cooking time of 3-6 mins on the treatments, while the control has value 10.5 mins. An increase in feed water content increases the cooking time of restructured rice. The higher the water content, the longer it takes to cook. This is because the water absorption into the grains of the restructured rice takes longer due to the dense granules. The higher the temperature in the dough, the cooking process is shorter because the starch gel has occurred before cooking. Saini (2015) reported that the nature of analog rice-forming produces the amount of water absorbed during cooking. Besides, processing parameters such as feed humidity and barrel temperature can indirectly contribute to cooking time.

Conclusion
Water content and temperature in the restructured rice using pasta extruder affect the surface shape. In the study, the best results were treated with the water content of 40%, and a temperature of 100 o C, pores, and surfaces of restructured rice are almost the same as milled rice. Water absorption index (WAI) value = 2,273±0.10, WSI = 2,114±0.11, WAR = 150.99±0.77, CL = 1.92±0.10 and cooking time 4 mins. Suggestions for this research are for further studies such as fortification with other ingredients using a pasta extruder technology and are expected to be implemented and commercialized.