Physicochemical properties and quality evaluation of white bread with “Baromi 2” rice flour as a wheat flour substitute

Rice flour was made from B2 supplied by the Rural Development Administration.

The patent application is named “Garumi2.” The hardness value of “Garumi2” is 2.9±0.09 kg. The physicochemical properties of the rice flour produced with the test mill “Buhler MLU-202” include average particle size (μm) 81.8±1.31, damaged starch (%) 5.6±0.11, whiteness (CIE value) 92.3±0.22, ash content (%) 0.59±0.02, protein content (%) 6.6±0.16, and amylose content (%) 18.0±0.59 (Jung et al., 2020).

White pan bread samples were prepared using the AACC standard straight dough method with slight modifications (AACC, 2000). The ingredients were as follows: 100 g of hard flour (Daehan Flour Mills Co., Ltd, Incheon, Korea), 4 g of refined sugar (CJ. Inc., Seoul, Korea), 4 g of whole dry milk (Seoul Dairy Cooperative, Seoul, Korea), 6 g of shortening (Samyangsa Co. Inc., Incheon, Korea), 2 g of sea salt, and 0.7 g of yeast (Saf-instant gold, Lesaffre Pte Ltd, France) for the control (RC) sample. For the tests, 20% (R1), 40% (R2), 60% (R3), 80% (R4) and 100% (R5) concentrations of B2 rice flour were added instead of wheat flour (Table 1). We mixed the ingredients for 3 min at low speed and then for 7 min at high speed (SK-20 Mixer, SK Mixer Co., Ltd. Saitama, Japan). The mixed dough was allowed to ferment for 1 h at 27°C with 75%–80% humidity. After sheeting, molding, and proofing for 60 min at 38°C with 85%–95% humidity, the dough was baked at 170°C (upper heat) and 210°C (lower heat) for 35 min (Fig. 1). The finished products were cooled until their internal temperature reached 30°C before conducting the experiments.

Fig. 1. Schematic diagram of the white pan bread-making process.

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The dough’s pH was measured using a pH meter after mixing 5 g of dough and 45 mL of distilled water (S20 SevenEasy, Mettler Toledo, Leicester, UK). The dough’s fermentation expansion was assessed by placing 10 g of bread dough into a 50 mL conical tube, smoothing the surface evenly, and subjecting it to fermentation for 120 min at 20 min intervals (temperature 27°C, humidity 85%) when measurements were taken. All samples started fermentation at the same volume (9 mL); the highest point of dough expansion was measured every 20 min.

The specific volume of the bread was measured 24 h after product molding using the VOLSCAN VSP300 (Stable Micro Systems Ltd, GU, UK) under the condition of 4 mm/sec. The baking loss rate was calculated using the following formula: the bread dough’s weight and the bread’s weight. All samples were measured five times.

The finished bread’s moisture content was assessed using the Infrared Moisture Analyzer FD-720 (Kett, Tokyo, Japan). A 3.000±0.005 g test sample was placed in an 8 cm diameter aluminum pan. The drying process temperature was maintained at 105°C. Measurements lasted approximately 8 minutes, and results were expressed as a percentage. Each variant was tested in triplicate.

The color measurement of the crust and crumb of white pan bread was conducted using a Chromameter CR-400 (Konica Minolta, Tokyo, Japan) with the CIE L*, a*, b* system. Calibration was performed with a standard white plate, and the measurement diameter was set at 0.8 cm. Each sample had five readings taken. The L* coordinate indicates surface brightness on a scale from 0 to 100, while a* measures color saturation, with positive values representing red and negative values representing green. Positive values denote yellowness for the b* parameter, and negative values indicate blueness. The crust’s Browning Index (BI) was calculated using the L*, a*, and b* parameters (Kupiec et al., 2021).

White pan bread crust (see image at the top - A) and its cross-section (see image at the bottom - B): RC-contains 0% B2, R1 contains 20% B2, R2 contains 40% B2, R3 contains 60% B2, R4 contains 80% B2, and R5 contains 100% B2.

The TPA for white pan bread and sweet bread was conducted using the CTX Texture Analyzer (AMETEK Brookfield, MA, USA) with these settings: table speed at 120 mm/min, a maximum load of 2 kg, a compression rate of 50%, and performed under 2-bite conditions. The samples analyzed were 20 mm thick slices taken from the center of the bread. Four parameters were assessed: hardness (N), springiness (mm), cohesiveness, and chewiness (J). Measurements were carried out at 24-, 48-, and 96-hr postmanufacturing.

RC-contains 0% B2; R1-contains 20% B2; R2-contains 40% B2; R3-contains 60% B2; R4-contains 80% B2; R5-contains 100% B2. Each experiment was conducted in triplicate, and the results are presented as the mean±SD.

The results were analyzed using the Statistical Package for the Social Sciences. ANOVA was applied, followed by the Tukey test for post hoc comparisons. The statistical significance level was p<0.05.

The external structure and attributes of bread containing B2 are displayed in Table 2, Fig. 2, and Fig. 3. Fermentation expansion measurements for the B2-enriched bread products are depicted in Fig. 3. Over a 120-minute fermentation period, the dough volume increased by 23.83 mL for the control group (RC), while R1 showed an increase of 25.33 mL, R2 saw an 18.00 mL rise, R3 increased by 10.5 mL, R4 by 7.25 mL, and R5 by 3.5 mL. The decline in fermentation expansion at higher B2 substitution levels is linked to the reduced gluten content, which is crucial for dough rising. Nonetheless, the data indicates that fermentation expansion can be enhanced at an optimal substitution level. Notably, substituting 20% B2 in the dough resulted in fermentation expansion exceeding that of the control group without negatively impacting the dough’s physical properties (Jun et al., 2019).

Fig. 2. Cross-sectional analysis of white pan bread with “Baromi2”.

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Fig. 3. Fermentation capacity of white pan bread with “Baromi2”.

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Table 2 indicates that increasing the B2 content decreased the bread’s volume and height, accompanied by a rise in weight. The moisture content analysis showed no substantial difference between RC and R1, although a trend was observed where moisture content tended to rise with a higher proportion of rice flour.

This is probably caused by the dough’s diminished ability to retain gas, attributed to lower gluten development, resulting in lesser oven spring and heightened moisture retention (Kraithong et al., 2018). Consequently, the baking loss rate lessened with increased B2 content. The specific volume is a critical measure of bread quality (Monteiro et al., 2021). This study found that the volume of bread with B2 diminished as the substitution level of B2 rose. Additionally, the baking loss rates analysis indicated no significant difference between RC (15.42%) and R1 (14.47%), while R2 showed a lower value (14.01%).

The analysis focused on the bread’s color and varying B2 levels to determine how increased B2 affects it (see Fig. 2, Table 3). Measurements of the crumb color indicated that as the B2 content rose, the L* value of the crumb also increased, with the * value remaining stable across all samples. The Tb* value showed a positive trend alongside the rising B2 content. In summary, while the L* value of the crumb decreased, the b* value increased, suggesting that higher B2 content contributes to a darker crumb color. Conversely, as the B2 content rose, the L* value of the crust improved, while both the a* and b* values declined. The BI, derived from the crust’s L*, a*, and b* values, demonstrated that the crust color lightened with increased B2 substitution levels. These findings indicate that the rate of B2 substitution for wheat flour notably influences the color of the final product.

Rice flour contains lower protein levels than wheat flour, which is naturally rich in carotenoids, contributing to its darker color (Choi, 2010; Baek & Koh, 2014). Changes in crust color can be linked to the higher B2 content. These results are consistent with earlier studies on rice flour-based bread, which indicated that products with rice flour had greater L* values than the control group. Furthermore, from sample R3 onward, the bread crumb displayed larger pores and inadequate fermentation, rendering these samples inappropriate for further analysis.

Texture parameters like hardness, cohesiveness, springiness, and chewiness are essential quality indicators for baked goods consumers (Roman et al., 2020). Among these factors, a rise in hardness is the most noticeable and frequently observed parameter linked to staling (Kim et al., 2019). The texture of the bread with B2 was evaluated 24 hr after baking. Hardness was measured after 4 days of storage, precisely at 24-, 48-, and 96-hr after baking (Table 4). At the 24-hr mark, the texture assessment revealed no significant differences in cohesiveness, springiness, or chewiness among the samples. Regarding hardness at 24 hr after baking, RC measured 37.00 N and R1 measured 38.25 N, while a sharp increase was noted for R2 at 65.87 N. After 48 hr, RC recorded a rise to 45.47 N (up 22.89% from the previous day), R1 reached 52.67 N (up 37.73%), and R2 saw a minimal increase to 75.15 N (1.08%). By 96 hr, RC jumped to 93.45 N (up 105.52% from the prior day), R1 rose to 72.87 N (up 38.37%), and R2 demonstrated a significant increase to 126.35 N (98.01%).

In general, aging occurred at a slower pace with increased B2 content. Even though R2 showed a reduced aging rate, its level was notably high. When R1 is compared to RC, the hardness increase rate saw a reduction of 22.01%. These findings imply that substituting 20% of B2 enhanced the aging rate and the overall aging extent. According to Nivell et al. (2020), the gluten network in bread influences the product’s hardness, while the amylose network helps reduce it. This study concludes that substituting 20% of the wheat flour with B2 in baked goods reduced gluten content, which improved both the degree and rate of retrogradation, ultimately enhancing product quality.