Note
Effect of Pre-cooking Grinding Conditions on Rice Texture After Cooking
2025 Volume 26 Issue 3 Pages 105-112
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2025 Volume 26 Issue 3 Pages 105-112
We compared the textural and sensory properties of rice materials prepared by cooking rice paste obtained by grinding rice in water or a slurry obtained by mixing dry- or wet-milled rice flour with water. Rice paste showed less starch damage and had a smaller average particle size than rice flour. When cooked, the rice paste and rice flour slurry formed a gel-like material. The rice paste material was significantly softer and less adherent than the rice flour material. Dynamic viscoelasticity measurements showed that the rice paste material tended to be less viscous than the rice flour material. Sensory evaluation revealed that the rice paste and wet-milled rice flour materials had a good appearance and were evaluated as smooth, whereas the dry-milled rice flour material was perceived as less sticky, but with a rougher texture. Owing to their characteristics, these novel materials have application potential in nursing foods and gluten-free foods.
食生活の多様化や欧米化を背景に，日本の米の消費量は減少傾向が続いている．この状況を改善するため，米の新たな素材開発が期待されている．我々は以前，浸漬した米を炊飯前に磨砕し，ペースト状としたものを炊飯することで，新たなテクスチャー特性をもつ米素材が得られることを見出した．本研究では，米粉に水を加えてスラリー状とし，これを炊飯しても同様な素材が得られるのではないかと仮定し，炊飯前の磨砕方法の違いが炊飯後の米のテクスチャーに及ぼす影響を明らかにすることを目的とした．米を水中で磨砕して調製した米ペースト，および乾式または湿式で粉砕した米粉と水を混ぜて調製した米粉スラリーを炊飯し，得られた米素材のテクスチャー特性および官能特性を比較した．その結果，炊飯前に磨砕した米ペーストは米粉に比べて澱粉損傷度が低く，平均粒子径が小さかった．炊飯処理を行ったところ，米ペースト，米粉スラリーのいずれもゲル状の米素材が得られた．米素材のテクスチャー特性は，米ペースト素材が米粉素材に比べて有意にやわらかく，付着性が低かった．動的粘弾性においても，米ペースト素材は米粉素材に比べて粘性が低い傾向にあった．官能評価の結果，米ペースト素材と湿式粉砕による米粉素材は外観が良く，なめらかであると評価された．一方，乾式粉砕による米粉素材はざらつきがあるものの，べたつきが少ないと評価された．これらの結果，米粉に水を加えスラリー状としたものを炊飯することでも，米ペーストを炊飯した場合と同様のゲル状素材を得られることがわかり，米粉の用途を拡げる知見を得ることができた．今後，これらの素材の特性を利用した介護食品やグルテンフリー食品への応用が期待される．
Rice consumption in Japan peaked at 118.3 kg (per person per year) in 1962 but has since continued to decline to a low of 50.9 kg in 2022 [i]. This trend can be explained by the diversification and westernization of diets. According to a household survey conducted by the Statistics Bureau of the Ministry of Internal Affairs and Communications, households have spent more on bread than on rice since 2014 [ii]. Because of the decline in the consumption of cooked rice, the use of processed rice (e.g., flour, puree, gel) is being considered [iii].
Rice is naturally gluten-free and rice flour is being used as a substitute for wheat flour in gluten-free foods such as bread [1], pasta [2], cupcakes [3], etc. worldwide. Compared with wheat flour, rice flour has a bland taste and white color, and it is highly digestible, with hypoallergenic properties [4]. Therefore, rice flour is expected to be used in various processed products in the future.
Rice flour is made by grinding raw rice grains into a powder, whereas rice puree is obtained by steaming rice in water and then blending it at high speed [iv][5], and rice gel is prepared from high-amylose rice by high-speed shearing after cooking [6]. Like rice flour, rice purees and gels have been used as substitutes for wheat flour in various foods, such as bread [7,8], noodles [9], and confectionery [6]. Bread dough made with rice puree has been found to have better consistency, and the resultant bread has better expansion properties than bread made with rice flour [7]. Therefore, the development of new rice materials may provide new directions not only for rice processing and enhancing rice consumption, but also for meeting the demands of patients with celiac disease or gluten allergy.
We previously developed and reported a new rice material with characteristics distinct from those of conventional rice materials [10]. We soaked crushed rice and then cooked the smooth paste to obtain a rice material with new textural characteristics [11]. This technology can be used to prepare rice materials with a wide range of physical properties, from liquid to gelatinous, by adjusting the amount of water used during cooking and is expected to be applied to a variety of processed products. The material has a smooth texture and is more coherent and less sticky than porridge paste, indicating that it can be used as a nursing food [12]. In addition, compared with rice gel obtained by high-speed shearing of rice after cooking, rice material made from rice paste generated by polishing grains before cooking is soft, smooth, and easy to eat [10].
Thus, for pre-cooking grinding-based rice materials with unique physical properties, a paste is prepared by wet grinding of rice grains and then cooked. In this study, we hypothesized that a similar material could be obtained by adding water to rice flour to form a slurry and then cooking the slurry. As rice flour is the most widespread rice processing material, if this technology could be adapted to rice flour, the potential applications of rice flour would expand. Therefore, the present study aimed to clarify the effects of different methods for grinding before cooking on various characteristics of rice after cooking.
Commercially available white rice “Koshihikari” (harvested in Toyama Prefecture in 2024, 12.7% moisture) was used.
2.2 Preparation of rice pasteOne hundred grams of rice was soaked in 200 mL of water at 20°C for 60 min. The soaked rice and soaking water were transferred into a mixer (MX-X701; Panasonic Corp., Tokyo, Japan) and ground at low speed (7900 rpm) for 4 min to prepare a rice paste.
2.3 Preparation of rice flour slurryRice flour was prepared using two milling methods: dry milling and wet milling. For dry milling, rice was milled at 4500 rpm using a pin-type milling machine (PHV-20; Meino Co., Ltd., Nagoya, Japan) and passed through a 120-mesh sieve. For wet milling, soaked rice was drained and then milled at 4000 rpm using an airflow milling machine (SPM-R430; Nishimura Machine Works Co., Ltd., Osaka, Japan). To prepare rice flour slurry, 200 mL of water was added to 100 g of rice flour and the mixture was stirred to homogeneity and left to soak at 20°C for 60 min.
2.4 Measurement of starch damageThe damaged starch content of rice paste and rice flour was measured according to the method of the American Association of Cereal Chemists (AACC method 76-31, 1992) using a starch damage assay kit (Megazyme International Ireland, Wicklow, Ireland). The kit measures the glucose produced through enzymatic digestion of sample with mold α-amylase, which specifically degrades damaged starch. Measurements were taken three times for each sample (100 mg/replicate), and the average value was used.
2.5 Particle size distribution determinationThe particle size distribution of rice paste and rice flour was analyzed using a laser diffraction particle size analyzer (PSA 1190; Anton Paar GmbH, Graz, Austria). Rice paste (0.2 g) was analyzed wet with water as a dispersant, and rice flour (10 g) was analyzed dry. Measurements were taken three times for each sample, and the average value was used.
2.6 Rice cooking and processingRice paste and rice flour slurry were placed in a rice cooker (NP-GL05; Zojirushi Mahobin Co., Ltd., Osaka, Japan) and cooked in “white rice quick” mode for approximately 20 min. After cooking, the rice material was stirred uniformly with a rice paddle. The rice material prepared from rice paste is referred to as “rice paste material,” and the rice materials prepared from dry or wet milled rice flour slurry are referred to as “dry-milled rice flour material” and “wet-milled rice flour material,” respectively.
2.7 Moisture content measurementThe moisture content of the rice materials after cooking was determined gravimetrically [13] using thermal drying at 135°C under atmospheric pressure for 3 h. Measurements were taken three times for each sample (2 g/replicate), and the average value was used.
2.8 Texture measurementTexture measurements were performed on 20 g of rice material in a stainless-steel petri dish (40 mm in diameter and 15 mm high) using a uniaxial compression testing device (RE-33005; Yamaden Co., Ltd., Tokyo, Japan). The measurement conditions were in accordance with the test method for food for special dietary uses for dysphagia [v]: uniaxial compression was applied using a cylindrical plunger (20 mm diameter, 25 mm height) with a clearance of 5 mm (compression ratio 66.7%) and a compression speed of 10 mm/s. The hardness, cohesiveness, and adhesiveness were determined. Measurements were taken five times for each sample, and the average value was used. Before measurements, the samples were incubated in a thermostatic chamber at 20°C for 1 h to ensure that the central temperature reached 20 ± 2°C.
2.9 Dynamic viscoelasticity measurementThe dynamic viscoelasticity of the rice materials was measured using a rheometer (MCR 702; Anton Paar). A 25-mm-diameter parallel plate was used as a fixture (parallel plate setup). The measurement conditions were as follows: gap (sample thickness), 2.0 mm; frequency, 10 rad/s; and strain range, 0.01%-600%. The storage modulus (G’), loss modulus (G’’), and loss tangent (tanδ) were determined. Measurements were conducted at 20°C.
2.10 Sensory evaluationSensory features were evaluated by 13 panelists who were faculty members and students of Jissen Women’s University in three samples for each rice material. Sensory evaluation was based on a ranking system with the following six items: (1) appearance (the best-looking item was ranked first), (2) softness (the softest texture was ranked first), (3) smoothness (the smoothest texture was ranked first), (4) roughness (the least rough texture was ranked first), (5) stickiness (the least sticky texture was ranked first), and (6) ease of eating (the easiest to eat was ranked first). For each item, the rice materials were ranked using a three-point scale, corresponding to first, second, and third place. In addition, the evaluation form included a free-response questionnaire interrogating the panelist about their impressions. The sensory evaluation was approved by the Research Ethics Review Committee of Jissen Women’s University (approval No.: H2024-2).
2.11 Statistical analysisData were statistically analyzed using one-way analysis of variance (ANOVA) followed by Tukey’s multiple comparison tests. Statistical significance was set to p < 0.05. For the sensory evaluation, Friedman’s test was used for repeated measures analysis of variance by ranks, and the Scheffé test was used for multiple comparisons.
The degree of starch damage and average particle size of rice paste and rice flour are shown in Table 1. Starch damage was lower in rice paste than in rice flour. Among the rice flours, starch damage was lower after wet milling than after dry milling. The degree of starch damage in rice flour varies depending on the milling method and grinding intensity, but is reportedly 8%-17% for dry milling and 1%-10% for wet milling [14-17]. The degree of starch damage in the rice flours prepared in this study was generally within these reported ranges, which are standard for rice flours. The degree of starch damage determines the quality of processed products based on wheat or rice flour. The higher the degree of starch damage in rice flour, the poorer the puffiness of rice-flour bread [14,18]. The degree of starch damage increases due to mechanical impact, frictional heat, and high-temperature drying during milling. Compared with wheat, rice has a very hardy endosperm; therefore, dry milling requires a large amount of energy, resulting in a high degree of starch damage [15]. In the wet milling approach, rice is first soaked in water to soften its structure through water absorption and then milled by airflow. Therefore, the heat energy produced during milling is consumed for drying, and the increase in product temperature can be suppressed, thus reducing thermal damage [19]. The rice paste prepared by grinding pre-soaked rice grains showed a very low degree of starch damage, likely because of the suppression of mechanical impact and frictional heat in grinding in water.
| Milling method | Starch damage degree (%) | Particle size (μm) | |
|---|---|---|---|
| Rice paste | 1.1 ± 0.0 a | 20.0 ± 0.7 a | |
| Rice flour | Dry milling | 8.6 ± 0.3 c | 53.7 ± 0.4 c |
| Wet milling | 3.8 ± 0.0 b | 47.5 ± 0.3 b |
Values are mean ± standard deviation (n ＝ 3).
Different letters in the same column indicate significant differences (p < 0.05, Tukey’s test).
The average particle size was smaller for rice paste than for rice flour. Figure 1 shows the particle size distributions. Rice paste showed a peak in the 5-10 μm range, which is roughly consistent with the size of rice starch grains, whereas rice flour showed a broad peak in the 10-100 μm range for both dry- and wet-milled flours, suggesting that the rice grains may not have been sufficiently ground. When analyzing rice and wheat flours, Kainuma and Tanaka [20] microscopically observed numerous aggregates of multiple starch granules. When comparing rice flours, they found that the average particle size was smaller, and the particle size distribution sharper, after wet milling than after dry milling. The sturdy endosperm of rice hampers dry milling. Increasing the milling strength for the purpose of rice flour refinement increases the degree of starch damage [21]. Wet milling produces finer particles than dry milling because of the softening of the grain structure due to water absorption [22]. As for the performance of the milling machines used in this study, the average reported particle size range is 50-250 μm for dry milling and 25-45 μm for wet milling [vi]. These ranges are in line with those in previous studies comparing the average particle size of rice flours prepared by dry and wet milling [1,14,16,22] and with those of the rice flours prepared in this study, and are standard for rice flours. In contrast, when using rice paste, the average particle size can be reduced to 20.0 μm using a simple household mixer, or even to <10 μm by using an ultrafine grinder or stone mill [20,23]. Rice grinding in water (pasting), which minimizes starch damage and enables low-cost particle size reduction, has significant industrial advantages in food processing.
Particle size distributions of rice paste and rice flours.
Figure 2 shows the appearance of the rice materials prepared by cooking rice paste or rice flour slurry. Both types of materials were cohesive, with similar moisture contents: 68.7% for the rice paste material, 68.1% for the dry-milled rice flour material, and 68.3% for the wet-milled rice flour material. No water separation was observed in any of the materials, and they all had a homogeneous gel-like texture. The surface gloss of the rice paste material was better than that of the rice flour material. The rice paste material was more fluid and smoother when lifted with a spoon than the rice flour material.
Appearance of rice materials prepared by cooking rice paste and rice flour slurry.
Left panel: rice paste material, middle panel: dry-milled rice flour material, right panel: wet-milled rice flour material.
The texture measurement results are provided in Table 2. The hardness of the rice paste material was significantly lower than that of the rice flour material, and the wet-milled rice flour material tended to be softer, albeit not significantly, than the dry-milled rice flour material. Cohesiveness did not differ among the materials, but adhesiveness was significantly lower in the rice paste material than in the dry-milled rice flour material. The differences in hardness and adhesiveness despite the similar moisture content may be due to a significant effect of the average rice particle size before cooking (Table 1). Saito [24] prepared dumplings using rice flour of different grain sizes and reported that, the finer the grain size, the softer the dumpling. The author compared the hardness of dumplings made of rice flour of different grain sizes to the strength of concrete, stating that small grains play the role of cement-like adhesive, medium grains that of sand, and large grains that of pebbles. The degree of starch damage reportedly is positively correlated with the viscosity of flour; the higher the degree of starch damage, the higher the viscosity [25]. Yu et al. [17] noted that setback viscosity increased with increasing starch damage, suggesting enhanced sensitivity to shear and faster recombination of amylose molecules released from swollen particles. They also suggested that rice flours with a high damaged starch content have poor gelatinization stability and are prone to retrogradation. This close relationship between viscosity and adhesiveness in gelatinous foods explains the increased adhesiveness in dry-milled rice flour materials with higher levels of starch damage in this study (Table 1).
| Milling method | Hardness (×103 Nm-2) |
Cohesiveness | Adhesiveness (×103 Jm-3) |
|
|---|---|---|---|---|
| Rice paste material | 46.9 ± 0.7 a | 0.8 ± 0.0 a | 14.6 ± 1.2 a | |
| Rice flour material | Dry milling | 57.4 ± 3.6 b | 0.8 ± 0.0 a | 17.8 ± 1.4 b |
| Wet milling | 53.2 ± 3.2 b | 0.8 ± 0.0 a | 16.6 ± 1.4 ab |
Values are mean ± standard deviation (n ＝ 5).
Different letters in the same column indicate significant differences (p < 0.05, Tukey’s test).
The dynamic viscoelasticity of the rice materials is shown in Fig. 3. Both rice paste and rice flour materials showed a linear range from 0.1% to 10% strain, with G’ > G’’. Above 10% strain, G’ and G’’ gradually decreased with increasing strain. The tan δ (＝ G’’/G’) of the rice materials was around 0.1% up to 10% strain, indicating a rather solid gel. Wang et al. [16] prepared pastes and measured the dynamic viscoelasticity of dry- and wet-milled rice flours with various levels of starch damage and average particle sizes. They found no significant difference in G’, which indicates elasticity; however, G’’, which indicates viscosity, was higher for dry-milled rice flour, which had a higher degree of starch damage and larger average particle size. Consistent herewith, in the present study, G’’ was the highest for dry-milled rice flour. The component (reducing sugar content) leached into the rice soaking solution reportedly affects the physical properties and taste of rice after cooking [26]. As the rice paste material was prepared by soaking rice grains, whereas the rice flour material was prepared by soaking rice flour, differences in the components in the soaking solution may have affected the physical properties of the rice.
Dynamic viscoelasticity of rice paste and rice flour materials.
The results of the sensory evaluation of the rice materials are shown in Fig. 4. Significant differences in appearance, softness, smoothness, and roughness were observed among the rice materials. The rice paste and wet-milled rice flour materials had a better appearance and were softer, smoother, and less rough than the dry-milled rice flour material. The softness increased in the order of dry-milled rice flour material, wet-milled rice flour material, and rice paste material, in agreement with the texture measurement results (Table 2, Fig. 3). Smoothness and roughness respectively increased and decreased in the order of dry-milled rice flour material, wet-milled rice flour material, and rice paste material, and are considered to be influenced by the average particle size (Table 1). The threshold for tactile roughness in food products is generally 10-25 μm [27], and particle size should be ≤20 μm to suppress roughness [28]. The average particle size of rice flour used to make dumplings and other products is approximately 100 μm [29,30]; hence, rice flour products often have a powdery or rough texture. The average particle size of the rice flour prepared in this study was 53.7 μm for dry milling and 47.5 μm for wet milling, which is smaller than that of ordinary rice flour. However, 12 of the 13 panelists rated the dry-milled rice flour material as the roughest. The average particle size of dry-milled rice flour was significantly larger than that of wet-milled rice flour (Table 1), and the particle size distribution showed a maximum peak at 100-110 μm (Fig. 1). In contrast, the particle size distribution of wet-milled rice flour showed a maximum peak at 50-60 μm, and the waveform was sharper. Therefore, the dry-milled rice flour was rougher than the other rice materials, whereas the wet-milled rice flour had a particle size distribution closer to the threshold for roughness, which is thought to be the reason why most panelists perceived it as smooth. Although there was no significant difference between the rice paste and wet-milled rice flour materials, many panelists rated the rice paste material as smoother and less rough. Reducing the average particle size of rice flour to <20 μm is difficult and costly in terms of energy and capital. However, these limitations can be addressed by using rice paste to prepare rice material that are smooth, with low roughness.
Sensory evaluation of the rice materials.
Rice paste material, 
dry-milled rice flour material, 
wet-milled rice flour material. *p < 0.05, **p < 0.01 (Scheffé’s multiple range test, n ＝ 13)．
Stickiness and ease of eating did not significantly differ among the rice materials; however, in the free response questionnaire, many respondents indicated that the dry-milled rice flour material was less sticky and easier to eat (i.e., easier to swallow) than the other materials. The relationship between oral perception of particles and palatability varies among foods [27]. Katsuta [31] prepared dumplings using rice flours of different grain sizes and conducted a sensory evaluation. They reported that dumplings made of rice flour of 74-104 μm particle size had the best consistency and texture. The grinding method can be selected to suit the application and preference. For example, rice paste can be used to produce smooth and less coarse rice products, and dry-milled rice flour for less sticky and easier-to-eat rice products.
In this study, we only tested rice paste and rice flour of a certain particle size, and the same amount of water was added during the preparation of all rice materials; however, changes in these factors are expected to result in textural changes, which requires further research. Further, the effects of rice variety and polishing ratio remain to be investigated.
Japanese households cook rice daily using a rice cooker. The results of this study revealed that rice materials with almost the same mechanical properties can be obtained by cooking rice paste prepared by grinding rice in water or a slurry obtained by mixing rice flour with water in a household rice cooker. Rice paste and wet-milled rice flour yield rice materials that are smooth and less coarse, whereas dry-milled rice flour yields rice materials that are coarser but less sticky. In the future, it is expected that the characteristics of these materials will be exploited in nursing foods and gluten-free foods.
i) https://www.maff.go.jp/j/zyukyu/fbs/ (Jun. 16, 2025)
ii) https://www.e-stat.go.jp/stat-search/files?stat_infid=000040270652 (Jun. 16, 2025)
iii) https://www.maff.go.jp/j/seisan/keikaku/komeko/k_houritu/ (Jun. 16, 2025)
iv) https://cs-n.jp/rice-puree/ (Jun. 16, 2025)
v) https://www.caa.go.jp/policies/policy/food_labeling/foods_for_special_dietary_uses/notice/assets/food_labeling_cms206_241210_11.pdf (Jun. 16, 2025)
vi) https://www.maff.go.jp/j/seisan/keikaku/komeko/ricemill.html (Jun. 16, 2025)
