Gel properties of rice varieties in relation to bread baking potential

ABSTRACT

The properties of rice flour gel relative to yeast-leavened bread baking potential have been investigated. Twelve rice varieties were dry-milled to flour. Firm gels with a jelly-like consistency, only formed in high and intermediate amylose content (AC) varieties, including Goamibyeo, Milyang260, Suweon517, and Milyang261. These varieties maintained their shape after baking, yielding bread volumes comparable to that of wheat bread. Eight low AC varieties that did not form hard gels were unsuitable for bread-making due to shrinkage during cooling. Specific bread volumes correlated positively (p < .01) with gel hardness, elasticity, and cohesiveness. Crumb firmness correlated negatively (p < .01) with gel hardness and elasticity. Gel fracturability also correlated (p = .015) with crumb firmness. Hard gel formation was the primary determinant of rice suitability for yeast-leavened bread. Secondarily, the gel should be elastic and non-brittle. The physical properties of rice flour gels can be useful for predicting rice flour bread-making potential.

KEYWORDS:

• rice
• gel
• bread making potential
• Crumb firmness

Introduction

Yeast-leavened rice bread is heavy and sticky and has little volume expansion when baked. As rice does not contain visco-elastic proteins similar to wheat gluten and the ratio of amylose to amylopectin differs widely among rice varieties.^[1] Carbon dioxide gas retention during fermentation and baking affects the quality and texture of rice bread.^[2] To overcome the problems associated with yeast-leavened rice bread, the addition of bakery additives, such as gum,^[3,4] surfactant,^[5] bakery supplements,^[6,7] active gluten,^[8] enzymes,^[9,10] and glutathione,^[11] has been investigated.

Amylose content (AC) permits the selection of appropriate rice varieties for baking purposes because the ratio of amylose to amylopectin is a major factor affecting volume expansion during the popping of heated raw rice and the texture of yeast-leavened rice bread.^[2,5,12] However, if the AC is high, even if the volume of the bread is large, the texture becomes too hard, making it unsuitable for bread. In addition to the AC, other factors, such as the soluble starch content and gelatinized viscosity,^[1] flour grinding method,^[12–14] damaged starch content, and water absorption rate of flour,^[1,15] also affect the baking properties. Therefore, breeding studies have been carried out to develop rice varieties appropriate for baking when considering the above-mentioned factors.^[16,17]

AC is an important factor in the formation of gel networks. Amylose forms double helices that connect during gel formation.^[18,19] Higher AC content^[20] and longer amylopectin branches^[21] form harder gels. Gel made from Japonica rice grown in Korea has low AC and is sticky and too soft.^[22,23] The physical properties of a gel are related to the quality of the end products.^[24,25] Correlations between the properties of wheat flour gels and the quality of wheat noodles have been reported.^[24] The qualities of vermicelli-rice noodles relative to rice flour gel properties have also been investigated.^[25]

Bean and Nishita^[1] have reported amylose content, gelatinization temperature, amylograph viscosity of paste, and eating equality of milled rice as useful characteristics for predicting the bread-making properties of rice. We investigated the relationship between the baking properties of rice flour and the physical properties of rice gel to determine the baking potential of rice varieties with a small amount of sample, even during the breeding stage. This study can be used as an indicator to determine the bread-making functionality of rice.

Materials and methods

Materials

Rice grown at the Department of Functional Crop, NICS, Milyang, Korea, including four indica lines (Hanareumbyeo, Milyang240, Dasanbyeo, and Milyang260) and eight japonica lines (Manmibyeo, Jinsumi, Milyang241, Seolgaeng, Chuchung, Goamibyeo, Suweon517, and Milyang261) were used after 95% polishing. The polished rice was dry-milled using a centrifugal disc mill (KCFM-48, Korea-medi Ltd., Daegu, Korea) adjusted to a grinding plate spacing of 12 mm. Pulverized rice flour was passed through a 115-mesh standard sieve (<125 μm), immediately sealed in airtight plastic bags, and stored at 4°C until further analysis. Commercial bread flour (hard wheat flour, Daehan flour, Seoul, Korea) was used as the control. Other baking additives, such as white sugar(Cheiljedang Co., Seoul, Korea), salt (Sajohp Co., Seoul, Korea), yeast (Saf Levure-instant, Lyons, France), hydroxy propyl methyl cellulose (CN40H, Samsung fine chemicals Co. Ltd., Ulsan, Korea), shortening (Alps shortening-200, Samyang wellfood Co., Incheon, Korea), and vegetable oil (Soybean oil, Ottogi Co., Anyang, Korea) were purchased commercially.

Protein and amylose content analysis

The protein content of the rice flour was analyzed using a protein assay kit (DC protein assay reagents package, Bio-rad laboratories Inc., CA, USA) according to Lowry method.^[26] The AC assay procedure was a modification of the concanavalin-A method described by Yun and Matheson^[27] using a Megazyme kit (K-AMYL 04/06, Megazyme International Ireland, Wicklow, Ireland).

Preparation of gel

Rice flour was gelatinized under the same conditions as those used for the AACC method 61–02^[28] using rapid visco analyzer (RVA, RVA Tecmaster, Newport Scientific Pty Ltd., Warriewood, Australia). Rice flour (3.5 g, 14% moisture basis) was weighed directly in an aluminum sample canister, and 25 mL distilled water was added. A programmed heating and cooling cycle were used, in which the sample was held at 50°C for 1 min, heated to 95°C for 3.8 min, and held until 7 min and 18s from the start of the RVA operation. After terminating the reaction at 95°C without dropping the temperature again, the hot gelatinized suspension was poured into a cylindrical glass tube (20 × 25 mm), allowed to cool for 30 min, immediately sealed in an airtight plastic container, and stored at 4°C for 1 day.

Baking test

Yeast-leavened bread made with 100% rice flour without adding wheat flour was prepared according to the methods described by Han et al.^[15] Loaf volume and weight were determined at ambient temperature 2 h after baking. The loaf volume was determined by rapeseed displacement, and the specific volume (mL g⁻¹) of the bread was calculated as the ratio of the volume (mL) to the weight (g) of the bread.

Analysis of gel and bread texture

The mechanical properties of the gel were measured using a texture analyzer (TA-XT express, Stable Micro Systems Ltd., Surrey, UK) equipped with a 10 kg load cell. Texture profile analysis (TPA) of gel that maintained its shape without a glass tube consisted of 70% deformation through the gel after its surface was compressed with a 45 mm aluminum cylindrical probe. The measurement conditions were 0.5 mm/s test speed and 1.0 g trigger force. The hardness (maximum force of the 1st compression), fracturability (force at the first peak), adhesiveness (negative area of the 1st compression), springiness (distance of the detected height during the second compression divided by the original compression distance), and cohesiveness (area of work during the second compression divided by the area of work during the first compression) of the gel were determined from the force-time curve of TPA. At least 10 gels were analyzed for each rice variety.

Bread firmness was measured according to the AIB standard procedure for white pan bread^[29] using a texture analyzer (TA-XT express, UK) equipped with a 10 kg load cell. Fresh bread crumb firmness was measured after cooling at room temperature for 2 h. To determine the degree of aging of the bread during the storage period, the remainder of each loaf was stored in a vinyl zip-lock bag at refrigeration temperature for 24 h. Crumb texture firmness was determined by compression using an aluminum cylinder (20 mm) probe at the center of a slice cut to a thickness of 20 mm from the center of the loaf. The firmness ratio was calculated as the ratio of the hardness of bread crumbs stored for 24 h to that of fresh bread crumbs cooled for 2 h after baking.

Statistical analysis

Statistical analyses of the experimental results were performed using SAS version 8.02. (SAS Institute, Cary, NC, USA). The experiments were repeated five or more times, and values are presented as the mean ± standard deviation obtained from replicates. Data were analyzed using analysis of variance (ANOVA) and Duncan’s multiple-range test. Differences among rice varieties were considered significant at P < .05. Gel maintaining or not its shape like jelly was analyzed according to protein and amylose contents by independent sample t-test. The significance of the relation between baking properties, such as bread volume and firmness, and TPA properties of the gel was analyzed using Pearson’s correlation coefficient. Independent sample t-test was also performed to compare the properties of gel between the following two bread groups: hard fresh crumb (hardness was 0.1 kg or more; varieties, Suweon517 and Milyang261) and soft fresh crumb (hardness was 0.1 kg or less; varieties, Milyang260 and Goamibyeo). For TPA of the gel and crumb firmness of bread, only those varieties that maintained the gel and bread shape after removing the glass tube and after 2 h of cooling, respectively, were considered.

Results and discussion

Flour and gel properties

The protein content of the rice flour was in the range of 5.46–8.28%, with Goamibyeo having the highest and Milyang240 having the lowest protein content, as shown in Table 1. The AC ranged from 10.93% to 31.79%. According to the classification based on AC,^[30] Milyang261 was classified as high (25%<) and Milyang260, Goamibyeo, and Suweon517 were classified as intermediate (20–25%), and the remaining eight varieties, including Dasanbyeo, were classified as low (<20%) AC rice. Gel made from high and intermediate-AC lines including Milyang260, Goamibyeo, Suweon517, and Milyang261 maintained their shape and firm gels with a jelly-like consistency as shown in Figure 1. The other eight cultivars, which had a relatively low AC (less than 20%), did not form a firm gel when the cylindrical tube was removed, and their gel could not maintain the shape and flowed down.

Figure 1. Rice gels. Gel A: soft gel that could not maintain its shape without a glass tube. Gel B: firm gel with a jelly-like consistency maintained its shape without a glass tube.

Table 1. Protein and amylose content of rice.

Rice	Protein(%)	Amylose(%)
Manmibyeo	6.24 ± 0.10^e	10.93 ± 1.46 ^f
Hanareumbyeo	6.79 ± 0.10 ^cd	15.03 ± 1.72^e
Jinsumi	6.43 ± 0.00^de	15.12 ± 1.45^e
Milyang241	6.07 ± 0.60^e	15.34 ± 3.85^e
Seolgaeng	6.21 ± 0.04^e	15.38 ± 3.05^e
Chuchung	6.84 ± 0.15^c	16.81 ± 1.66^de
Milyang240	5.46 ± 0.15 ^f	18.07 ± 0.50^cde
Dasanbyeo	7.95 ± 0.16^a	18.70 ± 0.09 ^cd
Milyang260	7.31 ± 0.14^b	20.82 ± 1.03^bc
Goamibyeo	8.28 ± 0.19^a	22.94 ± 1.56^b
Suweon517	6.64 ± 0.13 ^cd	22.99 ± 3.11^b
Milyang261	7.31 ± 0.15^b	31.79 ± 1.48^a

Data are expressed as mean ± standard deviation. ^a,bDifferent letters within a column indicate significantly different values (p < 0.05; Duncan’s multiple range test).

Statistical analysis between gel-forming ability with a jelly-like consistency and protein and amylose content is shown in Table 2. Both protein (p < .0008) and amylose (p < .0000) content showed significant differences with a gelling property of rice flour. Although rice proteins are considered valuable in essential amino acids, hypoallergenic and hypocholesterolemic effects, little is known about these functional properties such as foaming, gelling and emulsifying abilities for food processing applications.^[31] According to Baxter et al.,^[32] rice glutelin had poor solubility but good gelling properties. Detchewa et al.^[33] reported that rice protein itself had a poor gel-forming ability, but a low degree of hydrolyzed protein increased gel-forming properties. Our result (Table 2) showed the total protein content was significantly (p < .0008) related to the gel-forming ability with a jelly-like consistency. These conflicting results require more research on rice protein and gel-forming ability in the future. The amylose-amylopectin ratio is known to be a major factor influencing the physicochemical properties of starch. Amylose can form a firm gel, whereas amylopectin displays low syneresis and high resistance to retrogradation.^[34] Our results also showed that rice with low AC did not produce a firm gel and amylose content showed a very significant (p < .0000) difference with the gel-forming ability with a jelly-like consistency of rice flour.

Table 2. Comparison of amylose and protein content to gel-forming properties.

Variable	Gel A	Gel B	p-value
Protein content	6.50 ± 0.73	7.39 ± 0.62	0.0008
Amylose content	14.17 ± 2.59	25.06 ± 4.89	0.0000

Gel A: soft gel that could not maintain its shape without a glass tube. Gel B: firm gel with a jelly-like consistency maintained its shape without a glass tube.

As for rheological properties, gels from a relatively low AC (less than 20%), did not form a hard gel and flowed down was not proper for TPA analysis, and gels formed in high and intermediate amylose content (AC) varieties, including Goamibyeo, Milyang260, Suweon517, and Milyang261 were analyzed. As shown in Table 3, the gel hardness was in the following order: Suweon517 < Goamibyeo < Milyang261 < Milyang260. Milyang261 and Suweon517 showed high fracturability. The springiness of the gel was significantly (p < .05) higher in Milyang260 and Goamibyeo, and was lower in Suweon517 and Milyang261. Adhesiveness and cohesiveness were not significantly different between the varieties. Gels made from Suweon517 and Milyang261 were hard and brittle but had no elasticity. However, gels made from Milyang260 and Goamibyeo were hard, elastic, and non-brittle.

Table 3. Texture profile of rice gels.

Rice	Hardness (kg)	Fracturability (kg)	Adhesiveness (g.s)	Springiness	Cohesiveness
Manmibyeo	-	-	-	-	-
Hanareumbyeo	-	-	-	-	-
Jinsumi	-	-	-	-	-
Milyang241	-	-	-	-	-
Seolgaeng	-	-	-	-	-
Chuchung	-	-	-	-	-
Milyang240	-	-	-	-	-
Dasanbyeo	-	-	-	-	-
Milyang260	0.35 ± 0.32^a	0.0 ± 0.0^b	−21.1 ± 12.9	0.63 ± 0.16^a	0.18 ± 0.09^a
Goamibyeo	0.25 ± 0.40^b	0.0 ± 0.0^b	−21.7 ± 11.6	0.61 ± 0.07^a	0.24 ± 0.03^a
Suweon517	0.12 ± 0.18^c	0.84 ± 0.01^b	−96.4 ± 97.2	0.27 ± 0.06^b	0.17 ± 0.02^a
Milyang261	0.28 ± 0.87^ab	0.24 ± 0.11^a	−23.7 ± 12.4	0.30 ± 0.08^b	0.16 ± 0.02^a

Data are expressed as mean ± standard deviation. ^a,bDifferent letters within a column indicate significantly different values (p < 0.05; Duncan’s multiple range test). -Not determined because gel could not maintain its shape without a glass tube.

Baking Properties

The yeast-leavened bread made from low-AC lines, such as Hanareumbyeo, Jinsumi, and Seolgaeng, showed adequate volume expansion after baking; however, the inside crumbs shrank from the outside crumbs to produce large holes, thus making it impossible to measure bread firmness, as shown in Table 4. The bread made from Manmibyeo showed adequate volume expansion, but the inside crumbs shrank, making it impossible to measure crumb hardness after 24 h of storage. The yeast-leavened bread made from other low-AC varieties, such as Milyang241, Chuchung, Milyang240, and Dasanbyeo, had a very small loaf volume. As shown in Figure 2, bread made from high-AC lines including Milyang260, Goamibyeo, Suweon517, and Milyang261 maintained their shape and showed relatively uniform porosity and adequate loaf volume. Compared to wheat bread (4.85), the specific loaf volume of Goamibyeo, Suweon517, and Milyang261 bread was smaller (range: 2.18–3.19 mL/g), and that of Milyang260 bread (4.39) showed no significant (p < .05) difference. Rice protein cannot form a gluten network in the same way as wheat proteins.^[35] The specific loaf volume of gluten- and additive free rice bread was significantly correlated with amylose content, but not protein content.^[36] Thus, to improve the quality of rice bread, the exogenous protein enrichment has been reported.^[37] In our study, there was no significant (p < .05) correlation between specific loaf volume of rice bread and protein content (data are not shown).

Figure 2. Rice breads.

Table 4. Specific loaf volume and crumb firmness of yeast-leavened rice breads.

Rice	Bread Shape	Specific loaf volume (mL/g)	Crumb firmness (kg)
			Fresh	Stored for 24 h	Ratio
Wheat flour	Maintain	4.85 ± 0.13^a	0.09 ± 0.01 ^cd	0.25 ± 0.06^b	2.78
Manmibyeo	Collapse	3.66 ± 0.46^dc	0.08 ± 0.00 ^cd	-
Hanareumbyeo	Collapse	3.86 ± 0.52^bc	-	-
Jinsumi	Collapse	2.95 ± 0.41^e	-	-
Milyang241	Maintain	1.17 ± 0.01 ^g	0.08 ± 0.01 ^cd	1.29 ± 2.14^b	16.13
Seolgaeng	Collapse	4.66 ± 0.16^a	-	-
Chuchung	Maintain	1.03 ± 0.06 ^g	0.38 ± 0.01^ab	4.51 ± 0.83^a	11.87
Milyang240	Maintain	1.01 ± 0.07 ^g	0.38 ± 0.03^ab	4.93 ± 1.49^a	12.97
Dasanbyeo	Maintain	1.15 ± 0.11 ^g	0.42 ± 0.01^a	5.55 ± 0.24^a	13.21
Milyang260	Maintain	4.39 ± 0.25^ab	0.04 ± 0.03^d	0.25 ± 0.02^b	6.25
Goamibyeo	Maintain	2.96 ± 0.20^e	0.06 ± 0.06 ^cd	0.37 ± 0.06^b	6.17
Suweon517	Maintain	2.18 ± 0.29 ^f	0.33 ± 0.02^b	1.82 ± 0.15^b	5.52
Milyang261	Maintain	3.19 ± 0.18^de	0.13 ± 0.01^c	0.50 ± 0.03^b	3.85

Data are expressed as mean ± standard deviation. ^a,bDifferent letters within a column indicate significantly different values (p < 0.05; Duncan’s multiple range test). Ratio of the hardness of bread crumbs stored for 24 h to the hardness of fresh bread crumbs. -Not determined because bread could not maintain its shape after cooling.

The fresh crumb firmness of Manmibyeo, Milyang260, Milyang241, and Goamibyeo bread was in a relatively lower range (0.04–0.08), and the other varieties showed higher value (0.13–0.42) than that of wheat bread (0.09), as shown in Table 3. After 24 h of storage, the firmness of bread made from four rice varieties, viz. Milyang260, Goamibyeo, Suweon517, and Milyang261, were not significantly (p < .05) different from that of wheat bread, but the firmness of Milyang241, Chuchung, Milyang240, and Dasanbyeo bread was increased significantly. Their bread crumbs were very sticky and chewy when fresh. However, their texture became very hard a few hours after the baking (data not shown). The crumb firmness ratio, calculated as the ratio of crumb hardness of bread stored for 24 h to that of fresh bread, was generally higher for rice bread (5.52–16.13) than for wheat bread (2.78). Among rice breads, high-AC varieties, such as Milyang260, Goamibyeo, Suweon517, and Milyang261, showed a relatively low firmness ratio (3.85–6.25) compared to low-AC varieties, such as Milyang241, Chuchung, Milyang240, and Dasanbyeo (11.87–16.13). Milyang261 had the lowest ratio (3.85); however, the hardness of fresh bread was high, with sandy and harsh crumb characteristics. Suweon517 showed the hardest texture, which was found to be the highest degree of bread firmness during storage among the high-AC varieties. Therefore, it can be seen that intermediate- or high-AC rice has a lower crumb hardness ratio compared to that of low-AC rice, resulting in slower bread staling. Perdon and Juliano^[2] also reported that rice with an intermediate AC (20–25%) was preferred for preparing fermented rice cakes because of the ability of its better to retain more CO₂ during steaming, resulting in a larger volume expansion of the cake as well as its soft texture.

Analysis of the relation between the gel and bread-baking properties of rice

Four varieties of rice, including Goamibyeo, Milyang260, Suweon517, and Milyang261, which formed a hard gel and retained their shape after baking and storage for 24 h, were statistically analyzed to determine the relationship between the mechanical gel properties and bread baking properties of rice flours. Pearson’s correlation analysis (Table 5) showed a positive correlation (p < .01) between the hardness, springiness, and cohesiveness of the gel and bread volume. There was a negative correlation between the bread firmness of fresh or stored bread and the hardness (p < .01), adhesiveness (p < .05), and springiness (p < .01) of the gel. Because the hardness and elasticity of the gel showed a significant (p < .01) correlation with both bread volume and crumb firmness, they are expected to be a very useful criterion for determining the baking potential of rice varieties for yeast-leavened bread.

Table 5. Correlation coefficients between mechanical gel properties (TPA) and specific loaf volume/crumb firmness.

Gel texture (TPA)	Specific loaf volume	Crumb firmness
		Fresh	Stored for 24 h
Hardness	0.80**	−0.80**	−0.85**
Fracturability	−0.39	0.37	0.35
Adhesiveness	0.46	−0.62*	−0.61*
Springiness	0.77**	−0.64**	−0.69**
Cohesiveness	0.65**	−0.26	−0.27

*p < 0.05. **p < 0.01

Among the four varieties that maintained the shape and volume of the bread, the rice flours were classified into hard fresh crumb (Milyang261 and Suweon517) and soft fresh crumb (Goamibyeo and Milyang260) groups, and their gel properties were compared. As shown in Table 6, the hardness (p = .060), adhesiveness (p = .232), and cohesiveness (p = .178) of the gels were not significantly different among these two groups with different crumb hardness. However, fracturability (p = .015) and springiness (p = .000) were significantly different among these groups. In other words, rice varieties that can make a non-brittle and highly elastic gel are considered suitable for making soft, less firm yeast-leavened rice bread. The brittleness of the gel showed a significant (p = .015) correlation with the firmness of fresh bread when classified according to crumb hardness. Based on the above results, the primary condition for determining a rice variant’s suitability for yeast-leavened bread-making is the capacity to make a firm gel that is elastic and not brittle.

Table 6. Comparison of mechanical gel properties (TPA) with crumb hardness of fresh bread.

Gel texture	Soft crumb	Hard crumb	p-value
Hardness (g)	302.37 ± 65.40	198.42 ± 100.57	0.060
Fracturability (g)	0.00 ± 0.00	163.20 ± 109.84	0.015
Adhesiveness (g.s)	−21.38 ± 10.97	−60.05 ± 73.69	0.232
Springiness	0.62 ± 0.11	0.28 ± 0.07	0.000
Cohesiveness	0.21 ± 0.07	0.17 ± 0.02	0.178

Hard crumb: hardness was 0.1 kg or more; soft crumb: hardness was 0.1 kg or less. p-value of independent sample t-test.

Conclusion

This study investigated the relationship between the gel properties and baking performance of rice to determine the yeast-leavened bread-making potential of rice varieties that can be used for breeding studies. Among the 12 varieties of rice, the low AC lines, namely Milyang240, Dasanbyeo, Milyang241, Chuchung, Manmibyeo, Hanareumbyeo, Jinsumi, and Seolgaeng, did not form a hard gel and the loaf volume was very small; even when the loaf volume was adequate, the bread shrank from the outside crumbs to produce large holes during cooling and storage, making it impossible to obtain normal bread. In contrast, the intermediate-AC lines, namely Goamibyeo, Milyang260, Suweon517, and Milyang261, formed a hard gel, and the loaf volume was adequate for normal yeast-leavened bread. The loaf volume of bread was significantly (p < .01) correlated with gel hardness, elasticity, and cohesiveness. The firmness of both fresh and stored bread was significantly correlated with the hardness (p < .01), springiness (p < .01), and adhesiveness (p < .05) of the gel. When the fresh bread was classified based on the crumb hardness, brittleness (p = .015) of the gel also showed a significant relation with bread hardness. Therefore, AC and formation of a hard gel are the primary factors that predict the baking potential of yeast-leavened rice bread. End-use quality of yeast leavened rice bread can be predicted based on gel properties, such as hardness, springiness, and brittleness.

Disclosure statement

No potential conflict of interest was reported by the author(s).