Food Science and Technology Research
Online ISSN : 1881-3984
Print ISSN : 1344-6606
ISSN-L : 1344-6606
Original papers
Sweet Potato Flour Decreases Firmness of Gluten-free Rice Bread
Noriaki Aoki
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2018 Volume 24 Issue 1 Pages 105-110

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Abstract

Rice flour of the rice cultivar “Mizuhochikara” is suitable for making gluten-free bread without additives such as hydrocolloids or enzymes, but the bread becomes firm more rapidly than wheat bread. The firming rate of rice bread can be reduced by partially replacing the rice flour with sweet potato flour. Herein, when the replacement ratio or β-amylase activity in sweet potato flour was higher, the firming rate was lower. Rice bread made by 5% replacement of rice flour with sweet potato flour with the highest β-amylase activity had a firming rate that was more than three times slower than that of rice bread without replacement. The replacement of more than 5% of rice flour by sweet potato flour produced inferior shaped bread. Therefore, replacing up to 5% of rice flour with sweet potato flour with high β-amylase activity is a better way to reduce the firming rate of gluten-free bread.

Introduction

Many people should avoid wheat products like bread owing to the presence of allergies or celiac disease, as two examples. Wheat accounts for 8% of cases of food allergy, and is the third most prevalent allergen in Japan (Ebisawa et al., 2013). The prevalence of wheat allergy in Japanese adults is 0.21% (Morita et al., 2012). Celiac disease is an autoimmune disorder triggered by gluten. It is a more critical issue in Europe and the United States. The prevalence of celiac disease in the United States is reportedly 0.71% (Rubio-Tapia et al., 2012). Celiac disease is thought to be rare in Asia, but has been reported in Japan (Nakazawa et al., 2014).

For people with a gluten allergy or celiac disease, a gluten-free diet is recommended. Rice (Oryza sativa) is gluten-free and can be a material for gluten-free bread, which is typically made by adding hydrocolloids (Masure et al., 2016). However, gluten-free rice bread firms more rapidly than wheat bread during storage (Kadan et al., 2001). The resulting shorter shelf life is also an issue for other types of gluten-free bread (Hager et al., 2012).

Major causes of the bread firming are retrogradation of starch, recrystallization of amylose, and formation of amylopectin as the gelatinized starch cools (Morgan et al., 1997). One approach to retard bread firming is using a starch-hydrolyzing enzyme such as α-amylase or cyclodextrin glycosyl transferase (Gujral et al., 2003). Another approach is using food material containing a starch-hydrolyzing enzyme. Ito et al.. (2013) reported that replacing rice flour with sweet potato flour, which had the highest β-amylase activity among the tuber samples used, resulted in lower firmness of the bread made from rice flour and wheat gluten.

Here, I report that the firming of gluten-free bread can be reduced by replacing rice flour with sweet potato flour. The slow-firming bread can easily be made at home because all ingredients including sweet potato flour are commercially available, and the bread can be made using a bread machine.

Materials and Methods

Materials    Rice flour made from rice cultivar “Mizuhochikara” (Cuoca Planning Inc., Tokushima, Japan) was used to make gluten-free bread. Damaged starch content measured using a Starch Damage Assay kit (Megazyme, Bray, Ireland) was about 3.0%. Five commercial sweet potato flour formulations were purchased from four different companies. These samples were designated sweet potato flour No. 1 to No. 5 in increasing order of β-amylase activity. χ-amylase from sweet potato was purchased from Sigma-Aldrich (St. Louis, the United States).

β-amylase activity    β-amylase activity was measured using a β-amylase assay kit (Megazyme) following the manufacturer's instructions. p-Nitrophenyl-β-D-maltotrioside (PNPβ-G3) was used as the substrate (McCleary and Codd, 1989). Each sample was measured in triplicate.

Bread making    The model SD-RBM1001 bread maker (Panasonic, Osaka, Japan) was used to make gluten-free rice bread without additives including hydrocolloids or enzymes (pure rice bread). The pure rice bread included 230 g of rice flour, 20.7 g of sugar, 4.6 g of salt, 5.75 g of olive oil, 4.6 g of dry yeast, and 207 g of water. Formulations in which 1, 2, and 5% of the rice flour was replaced with sweet potato flour, and formulations in which 10 U, 20 U, and 30 U of β-amylase were also used for bread making. All gluten-free bread was made using the fast bake mode of the bread maker, with a bread making time of about two hours.

The finished bread was cooled for two hours to room temperature (approximately 28°C), wrapped in plastic wrap to prevent drying, put in a plastic bag, and stored in a 15°C incubator.

Evaluation of bread properties    Loaf volume was measured using the rapeseed displacement method. Specific loaf volume was calculated as volume per weight (mL/g). Bread hardness was evaluated with a compression test using a model RE2-3305C creep meter (Yamaden, Tokyo, Japan). A slice of bread 2.0 cm in thickness was compressed to 40% of its original thickness at a speed of 5.0 mm/s using a 16-mm diameter plunger with a flat surface. The hardness at 25% of the thickness was reported as bread firmness. Bread firmness was measured one, two, and three days after baking. Bread firming rate was defined as the increment of bread firmness per day (N/day).

Statistical analysis    Regression analysis and Tukey's test were performed using R statistical software. Statistical significance for Tukey's test was defined as P < 0.05.

Results

β-amylase activity    Table 1 presents β-amylase activity data of rice flour of the “Mizuhochikara” rice cultivar and five sweet potato flour samples. β-amylase activity was almost zero in rice flour. In contrast, the sweet potato flour samples displayed markedly different β-amylase activities. Amylase activity of sweet potato No.5 was more than three times higher than that of sweet potato No.1.

Table 1. β-amylase activities of flour samples
Sample β-amylase activity (Betamyl-3 U/g)
Rice flour −0.01 ± 0.02
Sweet potato flour 1 1.32 ± 0.04
Sweet potato flour 2* 1.35 ± 0.06
Sweet potato flour 3 2.05 ± 0.08
Sweet potato flour 4 2.76 ± 0.16
Sweet potato flour 5 4.10 ± 0.26

Sweet potato samples were designated No. 1 to No. 5 in order of increasing β-amylase activity.

Means with the same letter are not significantly different at P < 0.05 (Tukey's test).

Values represent the mean ± S.D. for three determinations.

*  Sweet potato flour No. 2 is derived from purple sweet potato.

Loaf volume of rice bread    Pure rice bread made from “Mizuhochikara” rice flour without replacement by sweet potato flour had the highest specific loaf volume (4.20 ± 0.15 mL/g) among the bread samples (Fig. 1 and Photo 1). The specific loaf volume of the bread made from a flour formulation where 1% of the rice flour was replaced with sweet potato flour did not show a significant difference compared to the specific volume of pure rice bread. However, these bread samples were smaller than pure rice bread (Fig. 1). In contrast, the specific loaf volume of the bread using the flour formulation in which 5% of the rice flour was replaced with sweet potato flour was significantly lower than that of pure rice bread (Fig. 1). Multiple regression analyses revealed negatively significant correlations between the replacement ratio and specific loaf volume, and between amylase activity and specific loaf volume (both P < 0.01; data not shown). Rice bread made from the flour replaced with 10% and 20% of sweet potato flour had a markedly inferior shape (Photo 1). Therefore, the maximum replacement percentage was 5%.

Fig. 1.

Specific loaf volume of pure rice bread and rice bread made with sweet potato flour. Means with the same letter are not significantly different at P < 0.05 (Tukey's test). “0% sweet potato flour” indicates pure rice bread made from “Mizuhochikara” rice flour without replacing it with sweet potato flour.

Photo 1.

Shape of bread

In order from left: Rice bread made from 0, 1, 2, 5, 10, and 20% replacement of the rice flour with sweet potato flour No.5.

Bread firmness, firming rate    Data concerning the firmness of rice bread one, two, and three days after baking are presented in Table 2. Firmness of rice bread made using the sweet potato flour No. 1, 2, 3, or 4 formulations was not significantly different from that of pure rice bread free from sweet potato flour one day after baking. Bread made with 2% or 5% replacement with sweet potato flour No. 5 had significantly lower firmness compared with pure rice bread even one day after baking.

Table 2. Change in bread firmness of the pure rice bread and rice breads made with sweet potato flour.
Samples Bread firmness (N)
Day 1 Day 2 Day 3
0% Sweet potato flour 0.46 ± 0.07 a 1.15 ± 0.12 a 1.56 ± 0.14 a
1% Sweet potato flour 1 0.34 ± 0.04 ac 1.05 ± 0.12 ab 1.23 ± 0.09 bc
1% Sweet potato flour 2 0.38 ± 0.03 ac 0.95 ± 0.02 acd 1.39 ± 0.10 ab
1% Sweet potato flour 3 0.39 ± 0.04 ac 0.84 ± 0.05 acd 1.29 ± 0.11 bc
1% Sweet potato flour 4 0.42 ± 0.06 ac 0.93 ± 0.14 bcd 1.21 ± 0.18 bd
1% Sweet potato flour 5 0.35 ± 0.03 ac 0.72 ± 0.12 ce 1.05 ± 0.06 cdf
2% Sweet potato flour 1 0.47 ± 0.07 ab 0.92 ± 0.17 acd 1.27 ± 0.15 bc
2% Sweet potato flour 2 0.49 ± 0.06 ab 1.03 ± 0.13 ac 1.39 ± 0.11 ab
2% Sweet potato flour 3 0.37 ± 0.02 ac 0.76 ± 0.06 bcd 1.07 ± 0.06 cdf
2% Sweet potato flour 4 0.40 ± 0.03 ac 0.81 ± 0.10 bcd 1.02 ± 0.11 cdf
2% Sweet potato flour 5 0.34 ± 0.06 bc 0.66 ± 0.06 de 0.83 ± 0.11 fg
5% Sweet potato flour 1 0.44 ± 0.06 ab 0.91 ± 0.07 acd 1.10 ± 0.06 bdf
5% Sweet potato flour 2 0.49 ± 0.02 ac 0.98 ± 0.10 ac 1.17 ± 0.03 bde
5% Sweet potato flour 3 0.38 ± 0.04 ac 0.75 ± 0.06 bce 0.91 ± 0.04 dg
5% Sweet potato flour 4 0.41 ± 0.03 ac 0.74 ± 0.05 ce 0.86 ± 0.04 efg
5% Sweet potato flour 5 0.29 ± 0.03 c 0.45 ± 0.06 e 0.60 ± 0.06 g

Means with the same letter (on the same day) are not significantly different at P < 0.05 (Tukey's test).

Values represent the mean ± S.D. for nine (0% sweet potato flour) or three (other samples) loaves.

In contrast, firmness of rice bread made from replaced flour with sweet potato flour No. 1, 3, 4, and 5 was significantly lower than that of pure rice bread three days after baking. Firmness of bread made with 1% or 2% replacement with sweet potato flour No. 2 did not show significant difference at three days after baking compared with that of pure rice bread.

The bread firming rate and increment of bread firmness per day are presented in Fig. 2. The information was calculated from the data shown in Table 2. Firming rates of rice bread made using formulations having 1% and 2% of the rice flour replaced with sweet potato flour No. 2 were not significantly different from pure rice bread. Rice bread made from 1% and 2% replaced flour with sweet potato flour No. 1, 3, 4, and 5 displayed significantly lower firming rates than pure rice bread. The firming rate of rice bread made with replacement of 5% of the rice flour by sweet potato flour was significantly lower than that of pure rice bread.

Fig. 2.

Firming rate of pure rice bread and rice bread made with sweet potato flour. Means with the same letter are not significantly different at P < 0.05 (Tukey's test). Firming rate of wheat bread made from same bread maker was 0.18 ± 0.03 N/day.

Among the samples with same percentage replacement of rice flour by sweet potato flour, there was a tendency of lower firming rate with increasing amylase activity. Multiple regression analyses revealed negatively significant correlations between the percentage replacement of rice flour and bread firming rate, and between β-amylase and bread firming rate (both P < 0.01, data not shown).

Effect of β-amylase on bread volume and firming    In order to clarify the relationship between β-amylase and bread properties, purified β-amylase was used for bread making. The specific loaf volume of the bread made by adding 20 U or 30 U of β-amylase was significantly lower than that of pure rice bread (Fig. 3A), but the values were more than 3.9. The firming rate of rice bread made by adding β-amylase was significantly lower than that of pure rice bread (Fig. 3B). There was a tendency of lower firming rate with increasing dosage of β-amylase.

Fig. 3.

Specific loaf volume and firming rate of pure rice bread and rice bread made with α-amylase. A: Specific loaf volume. B: Bread firming rate. Means with the same letter are not significantly different at P < 0.05 (Tukey's test).

Discussion

Rice cultivar “Mizuhochikara” is a high-yielding cultivar derived from Korean cultivar Miryang 23, Suwon 258, Taiwanese cultivar Tainung 67, and others (Sato et al., 2017). “Mizuhochikara” is suitable for making gluten-containing rice bread because of its high loaf volume (Aoki et al., 2012; Sato et al., 2017). In this study, pure rice bread made from “Mizuhochikara” rice flour had a high specific loaf volume (4.20 ± 0.15 mL/g; Fig. 1), indicating that this rice cultivar is suitable for gluten-containing and -free rice bread. Elsewhere, the specific loaf volume of gluten-free rice bread made without additives such as hydrocolloids or enzymes was reported as less than 2.0 (Hager et al., 2012; Hager and Arendt, 2013; Hamada et al., 2013; Renzetti and Arendt, 2009). The reasons for the high loaf volume of pure rice bread made from the “Mizuhochikara” cultivar are not unclear; one suggestion is the likely low content of damaged starch in rice flour (about 3.0%), which is necessary for making additive-free and gluten-free bread (Yano et al., 2017). Other components in rice that include starch or protein may also influence the high loaf volume because pure rice bread made from flour pulverized from other cultivars with similar low damaged starch content reportedly has lower loaf volume than that of “Mizuhochikara” rice flour (data not shown).

Rice bread made with replacement of 10% or 20% of the rice flour with sweet potato starch was about 10% smaller than pure rice bread (P < 0.01, data not shown). However, the bread was not inferior in shape, as was the bread made from replaced flour with sweet potato flour. The firming rate of the rice bread was almost same as that of pure rice bread (data not shown). Therefore, the effect of lowering firming rate by sweet potato flour is not due to starch in sweet potato, but due to other constituents. One likely constituent is amylase. Sweet potato possesses high β-amylase activity, and contains α-amylase (Ito et al., 2013). The results in Fig. 3 indicate that β-amylase is the most likely candidate for lowering firming rate. The main cause of slower firming is probably due to high amount of maltose produced by β-amylase during baking (Ito et al., 2013) and/or maltodextrins (Rojas et al.. (2001). Which of these amylase species is responsible for the effect is unclear. Use of sweet potato flour made from the null mutant of β-amylase (Kuamgai et al.., 1990) could help clarify the situation. Bread made from a formulation in which 1% or 2% of the rice flour was replaced sweet potato flour No. 2 did not show a significantly lower firming rate than pure rice bread three days after baking (Fig. 2). Sweet potato flour No. 2 is a flour made from purple sweet potato. The purple pigment such as anthocyanin that is present may have influenced bread firmness.

Firming rate of pure rice bread (0.55 ± 0.06 N/day) was about three times higher than that of wheat bread made using the same bread machine (0.18 ± 0.03 N/day). The finding that gluten-free bread firmed more rapidly than wheat bread is consistent with another report (Kadan et al., 2001). The firming rate of rice bread made from 5% substation of sweet potato flour No. 5 (0.15 ± 0.02 N/day) was similar value to that of wheat bread. This means that replacing sweet potato flour is useful for reducing bread firming of gluten-free rice bread.

However, rice bread made with replacement of 5% of the rice flour by sweet potato flour No. 5 had the smallest specific loaf volume (about 86.2% of pure rice bread) (Fig. 1). In order to improve the low loaf volume, sweet potato flour with higher amylase activity may be useful. The standardized partial regression coefficient between specific loaf volume and replacement ratio or β-amylase activity determined by multiple regression analysis was −0.133 and −0.0588, respectively. This indicates that the amount of rice flour that is replaced by sweet potato flour has a greater influence on loaf volume than β-amylase activity does. In contrast, the standardized partial regression coefficient between bread firming rate and flour replacement or β-amylase activity on bread firming rate was −0.0665 and −0.0596, respectively. This result indicates that both the degree of replacement and β-amylase activity equally affect bread firming rate. If a smaller amount of sweet potato flour with higher β-amylase activity is used, even if the total amylase activity is same, gluten-free rice bread with higher specific loaf volume and soft texture may be obtained. There are large varietal differences in β-amylase activities (Takahata et al.., 1994; Nakamura et al.. 2017). Cultivars with high activity such as “Beniharuka” may be suitable for the replacement.

In this study, I used only commercially available food materials and bread machine for bread making. Thus, the findings should be achievable at home. The lowering effect of sweet potato flour on firmness of gluten-free rice bread may be applicable to other types of gluten-free rice bread or some gluten-free products made from other cereal. Buckwheat, maize, and sorghum are materials of gluten-free bread, where the bread is also prone to firming (Hager et al., 2012). One of the causes of bread firming during storage is organization of starch molecules (Hug-Iten et al., 2003). Starch properties of these cereals are relatively similar to that of rice (Hager et al., 2012; Onyango et al., 2011). Therefore, sweet potato flour could be effective on these cereals.

Lowering the firmness of gluten-free bread will extend the shelf life of the bread. This will improve the bread quality and lower the cost, which is beneficial for both the bakery owners and consumers.

Acknowledgement    I thank Tomoyuki Oki for the useful discussions.

References
 
© 2018 by Japanese Society for Food Science and Technology
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