Pre-Germinated Brown Rice Substituted Bread: Dough Characteristics and Bread Structure

Abstract

Pre-germinated brown rice (PGBR) having the germ length of 0.5–1.0 mm is produced as a healthy food by immersing the brown rice in water. In this article, various additives were used for making PGBR breads, and suitable combinations of PGBR and additives for breadmaking were evaluated to provide PGBR bread with high functional properties. The 30% of the wheat flour was substituted with PGBR (PGBR 30), and combined additions of phytase (PHY), hemicellulase (HEM) and sucrose fatty acid ester (SE) to PGBR 30 improved the bread qualities with more suitable dough properties, as compared with the sample without their addition. During fermentation, the amounts of gas leaked from the PGBR 30 dough were suppressed by the additions. PHY and HEM hydrolyzed the phytate and hemicellulose in PGBR, and the maturity and extensibility of the PGBR 30 dough were caused by the activated yeast with formed phosphate and decomposed bran, making the large loaf volume and softness of breadcrumbs during storage. In addition, SE accelerated the dough tolerance to mixing or fermentation with the emulsifying ability. Therefore, the combined additions with PHY, HEM, and SE to PGBR 30 improved the dough and bread qualities.

Keywords:

• Rice
• Bread
• Dough
• Germination

INTRODUCTION

Rice flour is an ingredient to be commonly used for patients allergic to wheat flour because it lacks gluten-like proteins.[1] In addition, the rice bread has been a constant necessity for the patients suffering from celiac disease. But, rice flour does not contain gluten that makes viscoelastic dough, so it is unsuitable for yeast-leavened-bread-like products. As the amount of gas generation of rice flour dough during fermentation is not sufficient, various hydrocolloids, such as hydroxypropylmethyl cellulose (HPMC), locus bean gum, guar gum, carrageenan, and xanthan gum are practically used.[2] In addition, rice flour bread still has the problem of a short shelf life due to its quick retrogradation.[3]

Recently, pre-germinated brown rice (PGBR) having the germ length of 0.5∼1.0 mm was produced as one of healthy foods by immersing the brown rice (BR) in water. The PGBR has been reported to contain more vitamins, minerals, fibers and physiologically activated materials than un-germinated BR does.[4] In addition, the PGBR can be easily cooked with a conventional rice cooker as compared with BR, and more amounts of gamma-aminobutyric acid (GABA) were detected in PGBR than in BR.[5] Celiac disease or wheat allergy is becoming quite serious as mentioned above, therefore daily diet supplemented with PGBR, which has various functional materials, would promote our health.

The authors have already reported the dough properties and baking qualities of PGBR,[5–7] but for the practical application, some more improvements are still needed. In addition, applications of rice flour for cake or bread making and functional or processing properties of rice bran have been studied.[3,8–13] Other materials, such as amaranth, acha grain flours and tomato seed meals are also applied for breadmaking,[14–16] nevertheless, the number of the studies is not close to that of wheat flours at all. Various additives (enzymes, emulsifiers, and hydrocolloids) have been used for retarding the firmness of wheat bread,[6,17–21] but improving effects of additives on the qualities of rice breads have not been sufficiently studied. As to the reasons for poor baking properties of PGBR, much amount of hemicellulose existed in the bran of PGBR and lack of gluten protein were considered. In addition, PGBR included abundant amounts of phytic acid,[22,23] and they have excellent benefits on the body, such as anti-oxidative effect, protecting cardiovascular disease and preventing platelet aggregation, however would suppress the absorption of metallic ions into the body. Namely, the phytic acid has a strong ability to chelate multivalent metal ions, such as zinc, calcium and iron.[24] Therefore, in this study, some additives including enzymes, such as phytase (PHY) and hemicellulase (HEM) were used for making PGBR-substituted bread to improve the baking qualities. In addition, a sucrose fatty acid ester (SE, S-1670) that has been known as general additive for improving the rheological properties of the dough was also used to compensate insufficient effects of enzymes on the baking qualities. The aim of the present study is to make the PGBR-substituted bread with the more functional properties and favorable qualities, as compared with the conventional rice bread.

MATERIALS AND METHODS

Flours and Chemicals

The flour used was a hard-type commercial wheat flour; ‘Hermes’ donated from Okumoto Flour Milling Co., Ltd. (Osaka, Japan). Its protein, ash and moisture contents were 11.8, 0.38, and 13.8%, respectively. Rice grains Koshihikari harvested in Nagano prefecture, Japan in 2004 was used for preparation of PGBR flour (Asahi Yeast Co., Ltd., Nagano, Japan). The same yeast culture in the PGBR flour was provided from Asahi Yeast Co., Ltd. (Nagano, Japan) as used previously.[6,7] As additives for breadmaking of PGBR flour, some enzymes and emulsifiers were used as follows: PHY and HEM[18] from Amano Enzyme Inc. (Nagoya, Japan) and SE donated from Mitsubishi-Kagaku Foods Co. (Tokyo, Japan). The origin of PHY (3000U/g) and HEM (90,000U/g) was Aspergillus niger. Both the enzymes contained only negligible amount of amylase.

Breadmaking

The bread making formula and procedures were carried out with a slight modification of AACC Method 10–10B.[25] In this study, 15 g of yeast, 18 g of sugar and 4.5 g of NaCl were used and 10–30% of the wheat flour was replaced by PGBR flour. The optimal amount of water for the flour was determined from the value of water absorption by a farinograph mixing. These ingredients were mixed for 20 min by a KN-200 mixer (Taisho Denki Co., Ltd., Osaka, Japan) and then the dough was subjected to the first fermentation at 30 °C and 85% relative humidity (RH) for 60 min, followed by the punching. The punched dough was subjected to the second fermentation for 30 min at 30 °C and 85% RH. Then the dough was divided into 3 pieces (130 g/piece), rounded and covered with canvas cloth to go through bench time at room temperature for 20 min. The prepared dough samples were molded using a SM-230 molder (Baker's Production Co., Ltd., Osaka, Japan), and placed in a baking pan. The dough was proofed in the pan for 48 min at 38°C and 90% RH, followed by baking at 200°C for 20 min. Loaf volume was measured by the rapeseed displacement method. The loaves were sliced in half, and the appearance of the inside was examined visually, and the surface of breadcrumbs was photocopied to determine the gas cell distribution.

Characteristics of Dough and Bread

Farinograph data of various doughs were determined according to AACC Method 54–21.[25] The mixing was conducted at the standard speed of 63 rpm at 30 °C using a 50 g stainless steel bowl equipped in a Brabender apparatus. For extensigraph data, dough samples were prepared by mixing 300 g of flour with water containing 2 % (w/w) of NaCl at 30 °C using a farinograph according to the procedure of AACC method 54–10.[25] The dough whose consistency was arrived at 500 B. U. in farinograph mixing was obtained and allowed to store for 45, 90, and 135 min in a cabinet whose temperature was adjusted to 30°C. For each time, the resistance (R) and extensibility (E) of doughs were measured using an extensigraph. The amounts of CO₂ gas generated in dough samples during fermentation under the constant temperature of 30°C were measured according to the official manual by a fermograph (Atto Co., Ltd., Tokyo, Japan). The amounts of total and inner gas in the dough samples were calculated from the increase in the volume of water during incubation. For the examination of viscoelastic properties of dough, the dough added with the optimal amount of water was mixed for 10 min in the farinograph and analyzed by the same Fudoh Rheometer (Rheotec Co., Ltd., Tokyo, Japan) as reported previously.[6,18,26] Regarding bread quality, the staleness of breadcrumbs during storage was measured from the value of compression stress using the same rheometer as described above. The data obtained were processed using a Reosoft TR-6 computer program (Rheotec Co., Ltd., Tokyo, Japan).[27–29]

Scanning Electron Microscopy (SEM)

SEM of a Hitachi apparatus (Hitachi Model S-800, Osaka, Japan) was essentially the same as described previously.[19,30] The dough for SEM observation was prepared as the same method reported previously,[5,19,30] and the SEM apparatus was operated at 10 kV.

Characteristics of Phytate in Dough and Bread Containing PGBR Flours

The amount of phytic acid in 30% substituted-PGBR dough and bread samples was measured using spectrophotometric method.[31] One or two grams of samples were taken in a test tube with 20 mL of 2.4% HCl aqueous solution, and stirred vigorously for 1hr. Then the suspension was centrifuged at 3000 g for 15 min at room temperature. Ten mL of the supernatant diluted 5 times was passed through an anion exchange column (Dowex AG-1-X8 chloride form, 200–400 mesh), and the column was washed with 15 mL of 0.1 M NaCl and the eluted solution was discarded. Then inorganic phosphorus in the sample was eluted with 15 mL of 0.7 M NaCl and the eluate was pooled. Three mL of the solution was added to 1 mL Wade reagent (0.03% FeCl₃ 6H₂O and 0.3% sulfosalicylic acid in distilled water), and then the liberated phosphorus was immediately measured at 500 nm using a Shimazu spectrophotometer model UV-160 A (Kyoto, Japan).

Statistical Analysis

Values were obtained as the means ± standard deviation of at least three experiments, followed by analysis of variance (ANOVA) and significant differences by Duncan's multiple-range test (P < 0.05) using SPSS (Versin 11.0; SPSS Inc,. Chicago, IL).

RESULTS AND DISCUSSION

Characteristics of PGBR-Bread with Enzymes

Suitable amounts of PHY and HEM were evaluated on the preliminary baking test using 50–400 ppm and 50–100 ppm, respectively. Consequently, the amounts of PHY and HEM were 150 ppm and 50 ppm for favorable bread qualities, respectively. When the wheat flour was substituted with PGBR at 20 or 30% amounts, the specific volume distinctly decreased. However, addition of PHY or HEM to 20 or 30% PGBR-substituted wheat flours (PGBR 20 or PGBR 30) without enzymes increased the specific volume of bread, as compared with the control sample (data not shown). The combined addition of PHY and HEM to the PGBR-substituted flours did not have more increasing effects on the loaf volume, as compared with the samples with PHY or HEM alone. As to the softness of breadcrumbs during storage, the addition of HEM to 20 or 30% PGBR-substituted flours dramatically softened the breadcrumbs regardless of the combination with PHY. This result was considered to relate to the higher degradation of PGBR by HEM as reported previously[32] and HEM improved bread qualities substituted with polished wheat flours containing bran and germ as the present PGBR did.[32] This improving effect on the storage properties of breadcrumbs was obvious for 20% PGBR substitution than 30% PGBR substitution (data not shown).

Characteristics of PGBR-Dough with Emulsifiers

Farinograph and viscoelasticity results

The 30% substitution of PGBR to wheat flour (PGBR 30) decreased development or stability times with higher water absorption and weakness values (Table 1). Combined addition with PHY and HEM to the PGBR 30 made longer stability time and lower weakness value. The addition of SE to the PGBR 30 also lengthened the stability time and showed lower weakness value than that of the PGBR 30 without additives. Moreover, the three combinations with PHY, HEM and SE decreased the weakness value, as compared with the PGBR 30 dough. The present improving effect of SE coincided with previous results of polished wheat flour doughs, as reported before.[19] As to the viscoelastic properties of wheat doughs containing PGBR, the dough became harder, because the dough properties resulted in higher values of stress, modulus of elasticity and adhesiveness than that of the control sample, except for PGBR 30 with enzymes. In the case of viscoelasticity of mixed doughs with PHY, HEM, and SE, the value of stress significantly decreased having similar values of modulus of elasticity and adhesiveness to those of the control sample (Table 2). Therefore, the PGBR 30 with PHY, HEM and SE was considered to have suitable rheological properties with favorable hardness, extensibility and viscosity for handling dough and making bread.

Table 1 Farinograph data of wheat flour containing pre-germinated brown rice-powder added with or without enzyme and emulsifier

Additives	Arrival time(min)	Developmenttime (min)	Stability time(min)	Weakness(BU)	Water absorption (%)
Control	1.8	24.3	29.8	10.0	65.0
PGBR 30	5.0	9.2	6.0	63.3	70.0
PGBR 30 + ENZ	1.7	9.8	8.1	22.5	67.8
PGBR 30 + SE	4.0	6.3	7.0	50.0	70.0
PGBR 30 + MIX	3.5	6.0	6.0	30.0	69.4
Control: Hermes 100%; PGBR 30: Hermes 70% + PGBR 30%; ENZ: Phytase 150 ppm + Hemicellulase 50 ppm; SE: S-1670 0.5%; MIX: Phytase 150 ppm + Hemicellulase 50 ppm + S-1670 0.5%. n = 2.

Table 2 Viscoelastic properties of wheat dough containing pre-germinated brown rice-powder added with or without enzyme and emulsifier

Additives	Stress g (10^2 N/m^2)	Modulus of elasticity(10^4 N/m^2)	Viscosity coefficients(10^4 N/sm^2)
Control	157.5^b	5.10^b	1.33^a
PGBR 30	203.1^e	5.85^d	1.39^b
PGBR 30 + ENZ	167.1^c	4.77^a	1.43^c
PGBR 30 + SE	185.9^d	5.76^c	1.54^d
PGBR 30 + MIX	141.2^a	5.12^b	1.34^a
Values followed by the same letter in the same column are not significantly different at P < 0.05. The dough was mixed for 10 min in a farinograph. Abbreviations are the same as in Table 1. n = 3.

Extensigraph results

The 30% substitution of PGBR for wheat flour distinctly increased the value of R, whereas that of E decreased, resulting in the higher value of R/E. This tendency disappeared by additions of PHY and HEM (Table 3). But, the maturity of PGBR 30 with enzymes was sufficiently greater than that of the control. Addition of SE to PGBR 30 showed the same tendency as that for PHY and HEM addition, however the combinations with PHY, HEM, and SE significantly increased the value of R/E, as compared with the control, and caused dough properties with good maturity and extensibility. Therefore, HEM hydrolyzed the bran materials in PGBR as reported previously,[32] and SE played the interaction between the water, gluten, starch and bran in the dough, and thus formed network structure of the dough would improve the maturity of doughs. In addition, SE improved the polished wheat flour doughs with barn and germ in the previous report.[19]

Table 3 Effects of pre-germinated brown rice-powder and various additives on the maturity of dough samples

	R (BU)			E (cm)			R/E (BU/cm)
Sample	45	90	135	45	90	135	45	90	135
Control	387.5ab	526.3a	504.3a	19.3c	17.9c	16.4d	20.1a	29.4a	30.9a
PGBR 30	631.2c	703.3b	709.7b	9.7a	9.6a	8.7a	65.5c	74.9c	86.1c
PGBR 30 + ENZ	374.0a	564.3a	508.3a	13.4b	11.4b	11.8c	28.2b	49.8b	43.0ab
PGBR 30 + SE	452.0b	542.0a	570.0a	14.2b	11.1b	11.4bc	31.9b	50.1b	50.8b
PGBR 30 + MIX	410.0ab	563.2a	597.3a	12.8b	11.6b	10.3b	32.3b	48.8b	58.1b
R, resistance to extensibility; E, extensibility. Abbreviations are the same as in Table 1. n = 2.

Fermograph results

When the wheat flour contained 30% of PGBR, the total amount of CO₂ produced in the dough samples during fermentation was the same as that of wheat flour dough sample (Figure 1A). Addition of additives suppressed the production of CO₂, as compared with the control sample. This decreasing tendency was the most prominent for the PGBR 30 with SE. The additions of HEM and PHY to PGBR 30 suppressed the production of total CO₂ in the dough during fermentation than the PGBR 30 without additives, and the values did not change by combination of SE. However, the amounts of gas leaked from the fermented dough samples were significantly suppressed by additions of HEM, PHY and SE (Figure 1B). There were not clear differences in the results between PGBR 30 + ENZ and PGBR 30 + SE. Therefore, combined additions of HEM, PHY, and SE to PGBR 30 made the membrane of inner CO₂ gas in the dough more stable, suppressing the collapse or consumption of gas cells in the dough during fermentation, as compared with other conditions. The present PGBR dose not contain proteins similar to gluten, and the dough strength of PGBR 20 or PGBR 30 is weaker than the control with all wheat flours. Therefore, the total amounts of gas decreased when PGBR was substituted for wheat flour. However, the amount of gas leaked was suppressed by additives, as reported previously using polished wheat flours with SE.[19]

Figure 1 Fermentation of doughs with pre-germinated brown rice-powder added with or without enzyme and emulsifier. Abbreviations are the same as in Table 1. n = 3. Values followed by the same letter after fermentation for 180 min are not significantly different (P < 0.05).

SEM Results

SEM images of various dough samples containing HEM, PHY and SE are shown in Figure 2. Substitution of PGBR for wheat flour made the relatively more dried dough appearance than that of the control without substitution (Figure 2A,B). But, this network structure disappeared by additions of HEM and PHY. Dough appearance of PGBR 30 with HEM and PHY seemed to contain more viscous gluten substance, and the gluten adhered starch granules substantially, as compared with the control sample (Figure 2C). Nevertheless, SE addition might not change the dried appearance of PGBR 30 dough as compared with the enzyme addition (Figure 2D), and this result was associated with rheological properties as shown in Tables 1 and 2. As PGBR 30 included HEM, PHY and SE, the dough structure was similar to the control sample made from wheat flour (Figure 2E), and this dough structure should be correlated to the better baking results as described above. The same effects of enzyme and emulsifier were obtained in the previous study.[19]

Figure 2 SEM images of doughs with pre-germinated brown rice-powder added with or without enzyme and emulsifier. The dough was mixed for 10 min in a farinograph. Abbreviations are the same as in Table 1.

Characteristics of PGBR-Bread with Emulsifiers

Bread quality

The SE distinctly improved the specific volume as compared to PHY and HEM do. And the increasing effects of enzymes on the specific volume were much stronger by the combined addition of SE (Figure 3B). In addition, the cross-sectional views of breadcrumbs including PGBR and various additives are shown in Figure 4. The combined additions of PHY, HEM, and SE distinctly improved the appearance of network structure, as compared with PGBR 30 bread without additives. This tendency was obtained for storage properties of breadcrumbs, and PBGR 30 with PHY, HEM, and SE could keep the similar softness to that of the control bread during storage for three days (Figure 3A). The improving effects of HEM and SE on the baking qualities were also observed using whole wheat flour breads containing bran and germ.[18,19]

Figure 3 Bread qualities of bread samples containing 30% pre-germinated brown rice-powder added with or without enzyme and emulsifier. Abbreviations are the same as in Table 1. n = 3. Values followed by the same letter are not significantly different (P < 0.05)

Figure 4 Cross-sectional views of bread made with pre-germinated brown rice substitution for wheat flour with or without enzyme and emulsifier. Abbreviations are the same as in Table 1.

Change of phytic acid in the dough or bread samples during baking processes

The wheat flour contained 0.016 and 0.014% phytate in the dough and bread samples, respectively on the dry basis (data not shown). The PGBR 30 sample increased the amount, and the values for dough and bread were 0.420 and 0.460%, respectively. When PHY was added to PGBR 30, the amount of phytate distinctly decreased, and this tendency appeared much stronger for bread sample (0.15%) than dough sample (0.21%). Generally, the phytate in the dough has been reported to decrease during fermentation,[33,34] and the phosphate produced by the hydrolysis of phytate is considered to become a source of nutrition for yeast to increase its life activity (vital activity). As a result, the activated yeast might produce more amounts of gas in the dough during fermentation. Since the PGBR is not wheat but rice, it does not have the suitable amounts of gluten as does the control of dough and bread qualities. In addition, as PGBR contained the more amounts of bran, it might damage the mixing properties, resulting in the formation of dried and not extensible gluten structure. Therefore, addition of PGBR actually made poor baking properties and then was considered to be impossible to apply to breadmaking. Because PHY and HEM hydrolyzed the phytate and hemicellulose in PGBR, the maturity and extensibility of dough were caused by activated yeast with formed phosphate and decomposed bran. In addition, SE affected the property of pentosan that is part of dietary fibers.[35] Actually, SE did not remarkably change the properties of pentosans, but the addition of SE to wheat flours substituted with polished flours might entangle the water-insoluble pentosan in the doughs with high hydrophilicity, resulting in the decrease in the rigid structure of water-insoluble pentosan.[35] From these results, these additives made the large loaf volume and softness of breadcrumbs during storage.

CONCLUSIONS

PHY and HEM improved the dough and bread qualities of PGBR 30, but the results could not reach to the level of the properties achieved by the control sample made from wheat flours. Improving effects of HEM were relatively larger than that of PHY, and these combined addition was not effective on the final qualities of bread such as loaf volume and softness. In contrast, SE addition to PGBR 30 distinctly increased the baking qualities and showed the synergistic improvement by the combined addition of HEM and PHY. The SE would compensate insufficient points on the baking qualities that could not completely improved by HEM and PHY, giving more extensible or tolerant properties to the weak PGBR 30 dough by its emulsifying ability, resulting in the formation of favorable PGBR 30 bread.

ACKNOWLEDGMENTS

The authors wish to thank the Okumoto Flour Milling Co., Ltd. (Osaka, Japan) for supplying wheat flour; Asahi Yeast Co., Ltd. (Nagano, Japan) for providing rice grain and yeast; the Amano Enzyme Inc. (Nagoya, Japan) for providing enzymes; and Mitsubishi-Kagaku Foods Co. (Tokyo, Japan) for donating sucrose fatty acid ester.