Characterization of Rice Flour Milled with Water and Effects of Soaking Conditions (2024)

Introduction

As of 2012, the per capita per annum rice consumption was 56.3 kg in Japan, less than 50% of its peak consumption of 118.3 kg in 1962 (i). In 2010, household expenses on bread surpassed those on rice for the first time (ii). As trends in food consumption are rapidly changing, there is a limit to the degree that the consumption of rice can be stimulated given only the conventional uses of rice. Therefore, it is necessary to develop broader uses for rice.

Japan's calorie-based food self-sufficiency rate has remained below 40% since 1998. This prompted the Japanese Ministry of Agriculture, Forestry and Fisheries to implement its “FOOD ACTION NIPPON” initiative in 2008, which aimed to increase food self-sufficiency by boosting the consumption of domestic agricultural produces. The initiative has set a goal of increasing rice production for use as rice flour to 500,000 metric tons by the year 2020. This increase is due in part to the increased ratio of rice flour produced for novel purposes, which amounted to 23,797 of the 90,362 metric tons produced in 2013, or 26% of the total rice flour production (iii).

The outer and inner layers of rice grains differ in hardness; the hard outer layer has a dense structure, and conventional milling produces coarse particles (Yoshii, 2010). For this reason, dry jet milling is employed to mill rice to the fineness of wheat flour (Shishido and Egawa, 1992). However, dry jet-milled rice flour tends to show increased starch damage, gelatinization, and saturation moisture content with decreasing particle size (Shoji et al., 2012), and such rice flours have various deficiencies, such as poor rising ability in bread making (Araki et al., 2006).

Soaking softens the rice via water absorption (Seki and Kainuma, 1982) and makes it easy to crush. In Southeast Asia, rice is processed into noodles by grinding pre-soaked rice with a stone mill or other device, after which the water is strained off and the flour processed into rice flour products (Uchimura et al., 1991). Kainuma et al. (2009) detailed their attempts at adding soaked and ground rice directly to bread dough in the form of a rice paste (particle size close to 6 µm), similar to starch microparticles. The paste was found to possess favorable properties such as lower water absorption and lower starch damage rate (Arai et al., 2013) than rice flour obtained by dry jet milling.

The authors proposed the possibility of conveniently and cheaply producing a high-quality, high moisture content rice flour by pulverizing rice in the presence of ample water and then reducing the moisture using centrifugal dehydration or a similar method. Rice flour made from glutinous rice (Shiratamako) is one such type of rice flour milled with water (Arisaka, 2010). However, there are few reports on its production conditions, and existing reports go no further than the relationship between glutinous rice flour particle characteristics and the clearance and rotational speed used in milling with water (Eguchi, 2013).

In this study, we have therefore prepared high-moisture rice flour milled with water by soaking non-glutinous polished rice to allow for water absorption, then pulverizing the soaked rice using an electric ceramic mill and finally dehydrating the product by centrifugation. We investigated the yield of rice flour and the effect of soaking time and temperature on the rate of starch damage, and finally considered possible uses for rice flour produced from non-glutinous rice under high-moisture conditions.

Materials and Methods

MaterialsThe ‘Haenuki’ rice cultivar produced by Yamagata Integrated Agricultural Research Center in 2012 was used.

Production method for rice flour milled with waterRice polished at a standard polishing rate was washed and soaked for a certain period of time to allow for water absorption. After water absorption, the rice was transferred to a sieve and the weight of polished rice after water absorption was determined once the rice was drained well. Subsequently, water was added so that the combined weight of the soaked rice and water was twice the weight of the polished rice pre-absorption. The rice was then pulverized using an electric ceramic mill (Sawa-Boy NSG-08F; Nagasawa Kikai Seisakusho Co., Ltd., Kawaguchi, Japan) at 1,400 rpm, while it was poured along with the water into the hopper.

The resulting slurry was passed through an approximately 200-µm sieve, collected in a straining bag for ginjo-quality sake (2B cotton, folding radius 270 × H 850 cm) and tied closed with an elastic band. The slurry was hung to gravity-drain for about 2 h, after which the remaining mass was desiccated by centrifugal hydro-extraction. The resulting rice flour was designated as the rice flour milled with water. This preparation was repeated twice and the data were averaged. All data showed good reproducibility.

Moisture content of the rice flour milled with waterThe moisture content of the rice flour milled with water was determined by drying under heat and atmospheric pressure (105°C, 5 h).

Observation of particle surfaceThe surface of the rice flour milled with water was assessed using scanning electron microscopes (JSM6301F, JEOL Ltd., Japan; S-3400N, Hitachi High-Technologies Co., Japan). Dry jet-milled rice flour and wet jet-milled rice flour, which were purchased from a market, were also assessed for comparison. The raw material of the dry jet-milled rice flour showed approximately 15% moisture content and was pulverized by jet milling. For the wet jet-milled rice flour, the rice was allowed to absorb water by soaking and was then pulverized by jet mill. After milling, the product had a 12% moisture content.

Evaluation of starch damageUsing a starch damage test kit (Megazyme International Ireland, Bray, Ireland), the starch damage rate was determined by measuring the amount of sugar degradable by α-amylase. Dry jet-milled rice flour and wet jet-milled rice flour were also assessed for comparison.

Measurement of particle distribution of the rice flour milled with waterThe particle size of the rice flour was measured using a laser diffraction/static light scattering particle size analyzer (LS13320; Beckman Coulter, Inc., Miami, USA).

Results and Discussion

To determine the optimal soaking condition for the preparation of high-moisture rice flour milled with water, rice was soaked at room temperature (20 – 25°C) for between 2 – 8 h, and the effect of soaking time on yield and moisture content of the resultant rice flour was determined. Observation during water milling revealed very few unground substances remained on the 200-µm sieve, and there were no notable difference among soaking times. The yield of rice flour milled with water after various soaking periods, pulverizing, and straining was 94.1 – 97.8% (Fig. 1), indicating a slight increasing trend in the yield with increased soaking time. The moisture content of the resulting rice flour was nearly constant at 42.9 – 45.9%, indicating no effect of soaking time (Fig. 1).

Because the yield was constant under all durations, subsequent water-milling was performed on rice flour samples soaked for less than 2 h. The yields and moisture contents of rice flours are shown in Fig. 2. Soaking at room temperature for a short period of time greatly increased the yield of wet-milled rice flour; the yield increased from 47% with no soaking time (i.e., 0 min) to 87% after 15 min of soaking. From 30 min onwards, the yield basically stabilized at values of 90% and greater. These results suggest that the maximum yield is reached at approximately 30 min of soaking time at room temperature.

The appearance of the wet-milled samples indicated that the amount of coarse residues remaining on the sieve increased with very brief soaking times. In addition, the loss of components to the water seemed to increase, as clouding of the liquid running from the straining bag was visible. As the soaking time increased from 15 to 45 min, the coarse residue remaining on the 200-µm sieve decreased, and the clouding of the liquid runoff also gradually decreased.

The moisture content of rice flour with a 0-min soaking time was 56.4%, and the moisture content decreased with prolonged soaking time before leveling off at about 45% for 60 and 120 min.

Particle size distribution of rice flour milled with water and microscopic observationWe next investigated the particle size of rice flour milled with water (Fig. 3) soaked for 30 min or more. The resulting rice flour milled with water had a broad distribution with a large peak around 6 µm, and additional peaks at 30 and 80 µm. The analysis revealed that the majority of particles were 10 µm or smaller in size, with a minor amount of coarse particles. Soaking time had little influence on the particle size distribution, as distributions for 0.5 – 1 h soaking time had a similar pattern to that for 2 h. Particle size distribution analysis indicated that there was only a minor difference in particle distribution, even for water-milling with short soaking time.

The finer starch granules in rice are 5 – 10 µm in size (Kawagoe, 2013), and the peak particle size distribution of rice flour milled with water has been reported to correspond to this particle size.

The rice flours were examined under an electron microscope (Fig. 4). Rice flours prepared by dry jet milling and wet jet milling were also examined for comparison. At 3000× magnification, the appearance of rice flour milled with water was very fine with uniform particle size compared to both dry and wet jet-milled rice flours. Dry jet-milled rice flour had a variable particle population, in which large particles were intermingled with finely divided parts. Observation at a higher magnification revealed that the particle surface was melted by the heat, making it impossible to observe the multi-grain structure of the starch. Wet jet-milled rice flour particles were somewhat finer and more uniform than those produced under the dry method. No signs of melting were observed on the rice flour surface at a higher magnification, and the granular structure of starch was visible. Polyhedral particles of 5 – 10 µm have been observed in rice flour milled with water at 10,000× magnification (Kawagoe, 2013). Rice flour milled with water particles in this study were extremely fine and uniform in size, and their form closely resembled that of starch granules. From the microscopic observation, it was also speculated that many of the rice flour particles had been reduced to the level of starch granules.

Kainuma and Tanaka (2009) reported that rice paste prepared by a grinding mill after 10 h of soaking under refrigeration consisted of small, uniform particles 6 µm in size, which are presumed to be single-starch grains. In our study, we showed that similarly fine rice flour can be prepared with much lower soaking time at room temperature. These rice flours milled with water are much finer, smaller-grained rice flours than those prepared by dry or wet jet-milling (Shoji et al., 2012), and are also likely to possess different properties.

As described above, we obtained data regarding yield, particle size, and particle form in relation to soaking conditions for rice flour prepared by water-milling. One characteristic of wet-milling is the low rate of starch damage (Ogawa and Nagai, 2011). We therefore compared the starch damage rate between rice flour milled with water prepared with various soaking times with dry and wet jet-milled rice flours (Table 1), which was around 10% and 3.5%, respectively. In contrast, the starch damage rate of rice flour milled with water was 4.2% at 20-min soaking time, which was lower than that of dry jet-milling but somewhat higher than that of wet jet-milling. Extending the soaking time to 50 min or more resulted in a starch damage rate of approximately 1.8%, which was lower than that for wet jet-milling. Soaking times of 35 min and 50 min showed a slight difference in the starch damage rate. Considering that the water content is saturated after 35 min of soaking, the data do not appear to coincide with the result in Fig. 7 This result indicates that other factors are related to starch damage rate. One possibility is the delayed interaction between water molecules and rice components. Soaking rice for 50 min or more therefore produces rice flour that is much finer than the dry- and wet jet-milled rice flours (Shoji et al., 2012), and moreover has an extremely low starch damage rate.

The relatively high starch damage rate at the very short soaking time may be explained by incomplete water absorption. Milling of polished rice where the interior structure has not yet been thoroughly permeated by water leads to a condition similar to dry-milling. Local rises in temperature caused by such a condition may also have caused the starch to gelatinize (Shoji et al., 2012). The occurrence of starch gelatinization during the milling process increases the water retention of the gelatinized part as well as its solubility in water. This may have led to the leaching of partially gelatinized starch into the water, reducing the yield and increasing the amount of water contained in the yielded rice flour.

According to Matsumoto et al. (1967), the absorbed water reaches 22% of rice weight at 40 min and 23% at 60 min at 22 – 26°C water temperature; thus, at 40 min saturation has already been reached. In addition, Seki et al. (1982) showed that over 90% of the saturated moisture content is absorbed by 30 min of soaking, and microscopic observation revealed that 92 – 96% of rice grains show complete absorption after 60 min. However, the hardness of water-absorbed rice is greatly reduced with soaking > 30 min and even further softened with > 2 h of soaking. The results of our study showed that the soaking time at which the rice flour yield is high and starch damage is low is around 1 h at room temperature (20–25°C). This is in line with the time required to reach moisture saturation reported in other studies. These results indicate that sufficient water absorption and even distribution of water in the rice grains are necessary factors for increased yield and decreased starch damage.

Properties of rice flour milled with water obtained with short soaking time and increased temperatureTo test the idea that water saturation could be reached in shorter time by raising the soaking water temperature, thereby reducing soaking time, the rice was soaked in water at various temperatures and yield, moisture content, particle size distribution, and starch damage rates of the resulting rice flours milled with water were compared.

The yield and moisture content of rice flour milled with water soaked at 30°C and 40°C are shown in Fig. 5. At 30°C, the yield of rice flour milled with water was 75.1% after 15 min of soaking and 98.4% after 30 min; a similar trend was seen at 40°C. As for the moisture content, 15 min of soaking resulted in 49.4% at 30°C and 51.3% at 40°C, and was lower after 30 min of soaking, resulting in 44.7% at 30°C and 45.3% at 40°C. Extending the soaking time even further resulted in similar moisture content values. Comparison of temperature conditions revealed that 30°C resulted in a lower moisture content and slightly higher yields; however, these differences were marginal.

Figure 6 shows the particle size distribution of rice flour milled with water soaked at 30°C and 40°C. The largest distribution was observed at close to 6 µm, and the overall pattern of particle size distribution was similar to that of water-milling with a soaking temperature of 20 – 25°C. This indicates that the rice flour is extremely fine, mainly comprised of particles at the starch granule level, which is also the case for soaking at room temperature. This suggests that at least within the ranges examined in this study, soaking temperature and time do not have a notable influence on particle size distribution.

Starch damage rates of rice flour milled with water made with rice soaked at 30 and 40°C are shown in Figure 7. In the case of 30°C, 15 min soaking resulted in a high damage rate of 6.3%, which decreased with increasing soaking time to 3.3% at 30 min and 2.3% at 45 min. At 40°C, damage rates were high at 8.2% at 15 min, 4.7% at 30 min, and 3.1% at 45 min. Damage was clearly higher at 40°C compared to 30°C; however, both temperatures tended to produce higher damage overall compared to soaking at room temperature. Presumably, in the case of rice flour milled with water prepared at moderate temperature, the rice grains are ground together with water while the grains themselves are fully perfused. The heat produced from grinding is absorbed by the amply present water, preventing a rise in temperature and maintaining a low level of starch damage, resulting in the pulverization of rice grains to a state close to that of non-gelatinized starch granules. It is therefore desirable to keep the temperature of soaking and grinding low, as this would help to remove the heat produced from grinding.

Changes in the starch damage rate and moisture content of rice flour milled with water appeared to be correlated; namely, rice flour with a low starch damage rate showed high moisture absorption and therefore a high moisture content (Fig. 8). These results indicate that when the rice structure becomes sufficiently permeated with water, by soaking the polished rice in water, rice can be milled easily without causing starch damage. Rice thoroughly perfused with water has a considerably softened structure, and milling such rice is presumed to result in a pulverization state in which the starch granules, proteins, and cell walls are almost completely detached from each other, creating a hom*ogenous particle size distribution. Judging from the weight of the rice, polished rice is saturated with water after 50 min of soaking at 20°C. The yield of rice flour milled with water is thought to be reduced due to the pre-gelatinization of the surface starch of the rice flour, which promotes hydration, increasing solubility and resulting in starch leaching into water. Therefore, soaking time sufficient to prevent such pre-gelatinization of rice flour at room temperature was determined to be more than 50 min.

According to the nuclear magnetic resonance study by Horigane et al. (2006) of the moisture distribution of rice grains during water absorption, 45 – 60 min was required for complete water absorption in polished ‘Koshihikari’, with a polishing efficiency of 90% . This is in keeping with the results of our experiments.

The effect of increasing water temperature in order to shorten the soaking time was investigated. However, it appears that raising the temperature caused an increase in starch damage. For this reason, it is preferable to maintain a low soaking and milling water temperature, so as to absorb the heat produced by milling. In our case, it is possible that the damage was due to the unaltered milling temperature after soaking at a constant temperature. However, with regards to an effective production process, having to decrease the soaked rice temperature before milling would consume more time than would be saved by raising the soaking temperature. It is therefore considered undesirable to raise the soaking temperature when preparing rice flour milled with water.

In conclusion, we determined that the ideal condition for soaking polished rice for the preparation of wet-milled rice flour is 50 – 60 min at room temperature, as it is important that the absorbed water permeates the rice structure evenly. These soaking conditions allowed for the high-yield production of fine, high-moisture rice flour with low starch damage.