Effects of different types of hydrocolloids on the quality improvement of gluten-free rice flour bread made with soymilk

Abstract

The leavening of gluten-free rice flour bread is enhanced when soymilk is used instead of water. However, as the amount of batter is increased to bake bread in the shape of a square, the crumb becomes less dense. This study aimed to investigate the effects of hydrocolloids (xanthan, guar, curdlan, methylcellulose, and hydroxypropyl methylcellulose) on crumb hollowing. When 0.5 % methylcellulose or hydroxypropyl methylcellulose was added to gluten-free bread made with soymilk, the crumb could be baked without hollowing. The addition of 1 % or more of cellulose derivatives is needed to ensure satisfactory leavening of gluten-free bread. By combining cellulose derivatives with soymilk, a homogeneous crumb structure could be achieved with the addition of 0.5 % of cellulose derivatives. These results suggest that cellulose derivatives are effective for preventing bread from losing density, as they contribute to bubble stabilization via their interfacial activity and promote superior viscosity.

Introduction

Wheat is one of the three major food allergens responsible for food allergies, and cases of wheat allergy have been reported in many countries including Japan (Morita et al., 2012). Wheat allergy has multiple causative agents. For example, people who are sensitive to omega-5 gliadin (Matsuo et al., 2005), a type of protein that makes up gluten, cannot consume bread containing gluten. Therefore, numerous formulations for preparing gluten-free bread using rice flour, which can be ingested by allergy sufferers with gluten sensitivity, have been reported (Kawamura-Konishi et al., 2013; de la Hera et al., 2013; Cornejo and Rosell, 2015).

The authors (Nozawa et al., 2014) previously reported the effectiveness of soymilk for the leavening of rice flour, which is naturally gluten-free. When batter is prepared by adding soymilk instead of water to rice flour, the major soy proteins, such as glycinin and β-conglycinin, form a foam film around the CO₂ produced by fermentation, suppressing gas bubble coalescence and breakup, thereby resulting in better leavening compared with water. Furthermore, the use of soymilk increases batter viscosity, which reduces the amount of CO₂ escaping from the batter surface during fermentation.

However, although suitable leavening was achieved when batter using soymilk was baked in muffin cups, when the batter amount was increased to bake loaves in loaf pans, a large internal hollowing was generated in the upper part of the bread crumb. It is thought that this “hollowing” occurred because of gas bubble coalescence and breakup brought on by the delayed heat coagulation of the soy proteins forming the foam film, attributed to the slower heat transfer at the center of the loaf pans compared with that at the center of the muffin cups. Therefore, to increase the thermal stability of the foam film and prevent gas bubble coalescence, dried albumen containing ovalbumin, which has higher thermocoagulation ability compared to soy protein, was added to rice flour (2.5 %). This caused thermocoagulation of the foam film before the water around it evaporated, effectively preventing the hollowing of the bread crumb (Nozawa et al., 2016).

However, although gluten-free bread supplemented with dried albumen is suitable for people with wheat allergy, it is not suitable for people with egg allergy. Therefore, for improved versatility, dried albumen must be replaced with an allergen-free component. To this end, in the present study, we focused on hydrocolloids, which are already used to improve the leavening of gluten-free bread. The purpose of this study was to investigate the effects of hydrocolloids on preventing bread crumb hollowing, with the aim of producing gluten-free bread suitable for people with wheat and egg allergy.

Materials and Methods

Bread materials    Gluten-free bread was made using rice flour (powder rice type D; Niigata Seifun, Niigata, Japan), organic soymilk (Marusan-AI, Aichi, Japan; protein, fat, ash, and water contents were 4.6 %, 2.8 %, 0.5 %, and 90.7 %, respectively), granulated sugar (Fuji Nihon Seito, Tokyo, Japan), refined salt (Salt Industry Center of Japan, Tokyo, Japan), and freeze-dried instant yeast (Nisshin Foods, Tokyo, Japan). Other components included dried albumen (M Type; Kewpie Egg, Tokyo, Japan), xanthan gum (XG; Echo Gum, DSP Gokyo Food & Chemical, Osaka, Japan), guar gum (GG; Supergel CSA 200/50, Sansho, Osaka, Japan), curdlan (CL; Curdlan NS, MC Food Specialties, Tokyo, Japan), methylcellulose (MC; MCE-4000, Shin-Etsu Chemical, Tokyo, Japan), and hydroxypropyl methylcellulose (HPMC; SFE-4000, Shin-Etsu Chemical). The hydrocolloids selected were XG, which confers small temperature-related changes in viscosity, GG, which confers viscosity decreases with increases in temperature, MC and HPMC, which form a thermo-reversible gel and promote interfacial activity, and CL, which forms a thermo-irreversible gel.

Bread-making formulation    Bread was prepared using a half-loaf mold according to a previous paper (Nozawa et al., 2016). The basic batter ingredients were as follows: 200 g rice flour (baker's percentage of 100 %), 220 g soymilk (110 %), 4 g sugar (2 %), 2.5 g salt (1.25 %), and 2.5 g yeast (1.25 %). For the positive control bread, 5 g dried albumen (2.5 %) was added to the basic batter ingredients. On the other hand, when adding hydrocolloids instead of dried albumen, the amount was 1 g (0.5 %). When hydrocolloids are used to improve the leavening of gluten-free bread, the effective amount is often reported as 1 to 5 % (Crockett, et al., 2011; Moreira et al., 2011; Hager and Arendt, 2013; Morreale et al., 2018). However, as gluten-free bread produced by our method (Nozawa et al., 2016) already benefits from improved leavening through the use of soymilk, the addition of 0.5 % hydrocolloids, which is not considered to affect leavening significantly (Sabanis and and Tzia, 2011), was adopted.

After adjusting the ingredients to the batter temperature of 10 °C, they were placed in a bowl with ice water and mixed for 5 min with an electric mixer (THM 26M; Tescom, Tokyo, Japan) at a mixing rate of 800 rpm. The resulting batter was poured in 300-g portions into a half-loaf aluminum mold (95 mm × 95 mm × 95 mm) and yeast-fermented at 38 °C and 80 % relative humidity (RH) for 120 min in an incubator. After yeast fermentation, the batter was baked at 170 °C in a gas convection oven (RCK-10; Rinnai, Nagoya, Japan) for 40 min. After baking, the bread was removed and cooled in an incubator at 20 °C and 60 % RH for 2 h.

Batter viscosity    The batter used for viscosity measurement was prepared with all of the above-mentioned ingredients except the yeast. The shear measurement of the batter was performed with a cone-plate-type rotational viscometer (VT550; Thermo Electron GmbH, Karlsruhe, Germany) under the following conditions: plunger, PK1 (1°); shear rate, 0–200 s⁻¹; measurement time, 120 s; and stage temperature, 25–75 °C. Viscosity at 100 s^-1 was read from the rheogram using the supplied software.

Rate of increase in batter volume    A 50-g portion of batter was added to a graduated cylinder (200 mL) and yeast-fermented at 38 °C and 80 % RH for 120 min. The rate of increase in batter volume was calculated using the following formula: Rate of increase in batter volume (%) = (volume after fermentation [mL] / volume before fermentation [mL]) × 100.

Rapid Visco-Analyzer (RVA) measurement    The pasting properties of the rice flour added with hydrocolloids were determined using a Rapid Visco-Analyzer (RVA-4; Perten Instruments, Hägersten, Sweden). Rice flour (3 g, 14 % moisture basis) and hydrocolloid (0.015 g, 0.5 % to rice flour) were transferred into an aluminum canister, and 25 g of water was added. The obtained slurry was kept at 50 °C for 1.0 min, heated to 95 °C in 3.8 min, and then held at 95 °C for 2.5 min. The slurry was then cooled to 50 °C in 3.8 min and held at 50 °C for 1.4 min (Nozawa et al., 2016). The paddle rotation speed was 960 rpm for the first 10 s, and was then reduced to 160 rpm throughout the remainder of the measurement. The pasting properties were calculated from the RVA-curve using the supplied software.

CO₂ concentration escaped from the culture solution    In accordance with a previous paper (Nozawa et al., 2014), CO₂ escape from the culture solution surface was measured. Sugar (1 g), salt (0.625 g), and yeast (0.625 g) were added to water (50 g) and stirred in a beaker to prepare a culture solution. This culture solution was placed in a desiccator (diameter, 185 mm; depth, 200 mm) with a CO₂ monitor (pSENSE-RH EQC; Sakaki, Osaka, Japan) and yeast-fermented at 38 °C for 80 min. Further, the culture solutions to which MC, HPMC, and XG were added were also measured. The CO₂ escape in the desiccator was calculated using the following formula: Escaped CO₂ concentration (ppm) = CO₂ concentration after fermentation (ppm) − CO₂ concentration before fermentation (ppm).

Specific loaf volume (SLV)    Each loaf of bread was weighed and then measured for volume using the rapeseed displacement method. Specific loaf volume (SLV; mL/g) was calculated as the ratio of the volume and mass of the bread.

Moisture content    The moisture content of the bread crumb was measured at 120 °C in automatic ending mode using a moisture analyzer (MOS-120H; Shimadzu, Kyoto, Japan).

Texture measurements    In accordance with a previous paper (Nozawa et al., 2016), the hardness and cohesiveness of the bread crumb were analyzed. The bread loaves were sliced in half at 24 h after baking, and four sample cubes, 20 mm on each side, were obtained from the bottom of each loaf using an ultrasonic cutter (USC-3305; Yamaden, Tokyo, Japan). Hardness and cohesiveness were measured using a universal testing machine (RE2-33005s, Yamaden) with a cylindrical plunger (diameter, 40 mm) at a rate of 1 mm/s and 50 % compression and calculated from the texture profile curve using the supplied software.

Statistical analysis    All measurements were performed in triplicate or more, and each value was expressed as the mean ± standard deviation. The results were evaluated using analysis of variance (ANOVA) and the means were compared using Tukey's test, with significance set at p < 0.05.

Results and Discussion

Internal phase of bread with added hydrocolloids    First, we observed the internal phase of bread made using dried albumen and five types of hydrocolloids as additives (Fig. 1). As described previously (Nozawa et al., 2016), in the bread made without the addition of dried albumen, a cavity was seen; however, no cavity was observed in the bread made with the addition of 2.5 % dried albumen. Regarding the hydrocolloids, cavities were observed in the breads made with GG, XG and CL, but not in those made with MC and HPMC.

Fig. 1.

Cross-section image of gluten-free rice flour bread made with soymilk: Non-additive, no added dried albumen, or hydrocolloid. Dried albumen: 2.5% dried albumen added to rice flour; Hydrocolloid: 0.5% hydrocolloid added to rice flour. XG, xanthan gum; GG, guar gum; CL, curdlan; MC, methylcellulose; HPMC, hydroxypropyl methylcellulose. The areas outlined with a dotted line indicates the cavity of the bread crumb.

These results suggest that hollowing of the bread crumb can be prevented by adding cellulose derivatives such as MC and HPMC instead of dried albumen. Increasing the amounts of XG, GG, and CL from 0.5 % to 2 % failed to prevent cavity formation (photo image omitted), which suggested that a function specific to cellulose derivatives helps prevent bread crumb hollowing.

Influence of hydrocolloids on batter viscosity and volume    To clarify the mechanism underlying the prevention of bread crumb hollowing owing to the addition of cellulose derivatives, we first investigated the influence of hydrocolloids on batter viscosity and rate of increase in batter volume during fermentation (Table 1).

Table 1. Apparent viscosity of batter and increase rate of batter volume during yeast fermentation.

Property		Non-additive	Additive
			Hydrocolloid
			XG	GG	CL	MC	HPMC
Apparent viscosity of batter	(Pa·s)	600 ± 184d	3130 ± 101b	4345 ± 184a	738 ± 77d	2902 ± 306b	2331 ± 123c
Rate of increase in batter volume (%)	40 min	127 ± 5b	123 ± 3b	142 ± 6a	130 ± 5b	124 ± 4b	128 ± 3b
	80 min	286 ± 7a	255 ± 6b	250 ± 8b	282 ± 11a	243 ± 9b	257 ± 5b
	120 min	431 ± 9a	348 ± 2c	258 ± 8e	400 ± 3b	341 ± 8cd	333 ± 3d

Abbreviations of hydrocolloid are the same as in Fig. 1. Apparent viscosity of batter was measured at 40 °C and at a shear rate of 100 s⁻¹. Data are expressed as the mean ± standard deviation (apparent viscosity of batter, n = 6; increase rate of batter volume, n = 4). Means in a row with different letters are significantly different (p < 0.05).

The apparent viscosity of the batter at 40 °C, which is close to the fermentation temperature, was the lowest when no hydrocolloids were added; with the addition of hydrocolloids, the viscosity increased in the order of CL, HPMC, MC, XG, and GG. The rate of increase in the batter volume during fermentation increased over time for all hydrocolloids except for GG, which was associated with the highest apparent viscosity. However, the batter volume at the start of baking (120 min) was greatest for no additives (201.5 ± 4.6 mL), followed in descending order by CL (198.7 ± 0.9), XG (175.5 ± 2.7), MC (170.3 ± 3.3), HPMC (158.8 ± 6.1), and GG (175.5 ± 3.6), indicating that batter volume is approximately negatively correlated with batter viscosity (r = −0.88).

One of the purposes of adding hydrocolloids to gluten-free bread is to increase the batter viscosity (Mir et al., 2016). However, it is surmised that the further increase in viscosity through the addition of hydrocolloids caused a decrease in batter volume, as the batter prepared in our formulation already showed increased batter viscosity through the use of soymilk. On the other hand, cavities in the bread crumb developed in both the non-additive batter with low viscosity and high volume, and in the batter with added GG with high viscosity and low volume (Fig. 1). Gallagher (2009) stated that the addition of hydrocolloids increases batter viscosity, which delays the rising of gas bubbles generated in the bread crumb and the setting of the starch, resulting in the homogenization of the gas bubbles before gelatinization. However, from the results in Fig. 1 and Table 1, it is inferred that factors other than viscosity are also involved in the prevention of hollowing by cellulose derivatives, given the fact that no relationship was observed between batter viscosity and the hollowing of the bread crumb.

Influence of hydrocolloids on the pasting properties of rice flour    Next, to investigate the influence of hydrocolloids on batter viscosity during baking, 0.5 % hydrocolloid was added to rice flour, and the pasting properties of the resulting batter were measured using an RVA (Table 2).

Table 2. Effects of hydrocolloid addition on the pasting properties of rice flour.

Property		Non-additive	Additive
			hydrocolloid
			XG	GG	CL	MC	HPMC
Viscosity (RVU)	Peak	300.3 ± 3.8b	303.4 ± 2.8b	324.9 ± 4.6a	304.8 ± 3.0b	306.4 ± 6.7b	301.4 ± 1.8b
	Trough	201.6 ± 14.1	220.1 ± 14.4	215.8 ± 13.2	199.8 ± 0.4	201.0 ± 8.3	191.2 ± 5.6
	Breakdown	106.8 ± 4.3	83.3 ± 14.7	109.2 ± 16.2	104.9 ± 3.3	105.4 ± 5.9	110.3 ± 7.4
	Final	296.8 ± 9.3	314.7 ± 12.1	316.5 ± 11.2	296.8 ± 3.8	297.4 ± 6.7	293.0 ± 11.3
	Set back	95.3 ± 7.3	94.6 ± 3.3	100.8 ± 4.2	97.0 ± 4.8	96.4 ± 4.4	101.8 ± 7.3
Pasting temperature (°C)		75.2 ± 1.4b	73.1 ± 2.3b	74.4 ± 0.8b	74.9 ± 0.4b	81.7 ± 3.0a	80.9 ± 2.5a
Peak time (min)		6.6 ± 0.1	6.6 ± 0.1	6.6 ± 0.1	6.6 ± 0.1	6.5 ± 0.1	6.5 ± 0.1

Abbreviations of additive are the same as in Fig. 1. RVU indicates a rapid viscosity unit. Data are expressed as the mean ± standard deviation (n = 3). Means in a row with different letters are significantly different (p <0.05).

Compared with the non-additive, peak viscosity was significantly higher in GG only; thus, it was surmised that the high viscosity of GG itself was a factor (Rosell et al., 2011). No significant differences in the viscosities of trough, breakdown, final, and set back were observed. Therefore, the addition of 0.5 % hydrocolloid was not found to have a significant effect on the pasting properties of rice flour.

The pasting temperature of MC and HPMC shifted about 5 °C higher compared with the non-additive and the other hydrocolloids. HPMC and MC showed almost the same peak time as did the non-additive and the other hydrocolloids. Sucrose in the proportion used in a cake recipe can increase the gelatinization temperature of wheat starch by about 10–15 °C (Baeva et al., 2000; Hicsasmaz et al., 2003). The hydrocolloids used in this study may have also played a role in suppressing starch swelling and delaying gelatinization. Although this effect was not observed in XG, GG, and CL due to the small amount of hydrocolloids added (0.5 %), MC and HPMC inhibited the swelling of starch even at low concentrations, and thus the pasting temperature was significantly higher than that of the non-additive and the other hydrocolloids. Despite the suppression of starch swelling, the peak time of MC and HPMC was the same as that of the other hydrocolloids. These cellulose derivatives gel at around 55 °C or higher (Bousquières et al., 2017), which is presumably because of the rapid increase in viscosity. These results suggest that the gradual increase in batter viscosity in GG, XG, and CL caused gas bubbles to coalesce and break, resulting in the formation of cavities in the crumb. In MC and HPMC, since the batter viscosity increased rapidly at high temperatures, the coalesced gas bubbles were fixed without breaking, and thus, cavities were not likely to occur.

CO₂ retention of batter with hydrocolloids    Not only the increase in batter viscosity, but also the improvement in the dispersibility of gas bubbles prevents the emission of CO₂ from the batter surface. To predict the amount of CO₂ escaped from the batter surface, a culture solution made by adding yeast, sugar, salt, and MC or HPMC to water was fermented in a desiccator, and the concentration of CO₂ that escaped from the culture solution surface into the desiccator was measured (Fig. 2). Further, to investigate the influence of viscosity in regard to CO₂ emissions, measurements were also performed with a culture solution of XG, which produces a batter viscosity approximating that produced by MC (Table 1).

Fig. 2.

Concentrations of CO₂ escaped from the surface of culture solution. Abbreviations of hydrocolloid are the same as in Fig. 1. Data are expressed as the mean ± standard diviation (n = 3). Means carrying different letters are statistically significant (p < 0.05).

In the culture solution with the added hydrocolloids, the concentration of CO₂ in the desiccator was significantly lower than that in the solution with the added hydrocolloids. This was surmised to be because the viscosity of the culture solution with hydrocolloids was higher than that of the culture solution without hydrocolloids.

Although MC and HPMC produced batter viscosity that closely approximated that produced by XG, their use resulted in a smaller amount of escaped CO₂ compared with XG. Thus, it is surmised that in the case of the culture solution with cellulose derivatives added, the surface activity of the cellulose derivatives disperses the gas bubbles (Sadahira et al., 2015), making it difficult for gas bubble breakup due to coalescence and surface rise to occur, so that the concentration of escaped CO₂ is lower than that with XG.

In our formulation, the glycinin and β-conglycinin in soymilk also form a foam film that stabilizes gas bubbles (Martin et al., 2002; Nozawa et al., 2014). It is considered that the hollowing of the bread crumb did not occur because the cellulose derivatives bonded with these proteins, forming a stronger foam film around the gas bubbles that maintained them in a dispersed state (Crockett et al., 2011).

Characteristics of bread with added cellulose derivatives    These results indicated that cellulose derivatives, similarly to dried albumen, can help prevent the hollowing of the bread crumb. Next, the SLV, moisture content, and texture of bread added with cellulose derivatives were compared with those of bread with dried albumen (Table 3).

Table 3. Characteristics of bread with the addition of dried albumen and cellulose derivative.

	Property		Dried albumen	Cellulose derivative
				MC	HPMC
SLV (mL/g)			2.52 ± 0.06b	2.65 ± 0.01a	2.45 ± 0.04b
Moisture content (%)	Day 1		51.3 ± 0.2b	51.3 ± 0.1b	51.8 ± 0.2a
	Day 3		50.8 ± 0.4	50.9 ± 0.4	51.2 ± 0.6
Texture parameter	Day 1	Hardness (×103 Pa)	3.15 ± 0.35a	2.71 ± 0.34b	2.18 ± 0.43c
		Cohessivness	0.76 ± 0.01	0.75 ± 0.03	0.75 ± 0.01
	Day 3	Hardness (×103 Pa)	8.29 ± 1.32a	6.11 ± 1.03b	4.83 ± 0.52c
		Cohessivness	0.55 ± 0.06a	0.51 ± 0.07b	0.46 ± 0.03c

Abbreviations of cellulose derivatives are the same as in Fig. 1. The amounts of dried albumen and cellulose derivatives added are 2.5 % and 0.5 % to rice flour, respectively. Data are expressed as the mean ± standard deviation (SLV, n = 3; moisture content, n = 3; texture parameter, n = 16). Means in a row with different letters are significantly different (p <0.05).

SLV was found to be significantly higher in the case of bread made with MC than in bread made with dried albumen, and improved leavening was observed. It is thought that MC makes gas bubbles less likely to collapse during baking and improves SLV because it forms a temporary network, owing to the hydrophobic interaction between side chains. On the other hand, SLV in the case of bread made with HPMC was not significantly different from that in bread made with dried albumen and was significantly lower than that of MC. This was thought to be due to the fact that HPMC, which contains the hydroxypropyl group, is more prone to swelling and has a more fragile molecular structure for retaining gas bubbles compared with MC. Based on the above, bread made with cellulose derivatives was found to have identical or better leavening than bread made with dried albumen.

The moisture content of the bread crumb made with HPMC was slightly higher than that of bread made with dried albumen at 1 day, but no difference in moisture content was observed at 3 days; thus, the addition of cellulose derivatives had no effect on the moisture content of bread.

The hardness of the bread crumb made with MC and HPMC was significantly lower than that of the bread made with dried albumen at 1 and 3 days. In particular, the decreased hardness of the bread made with HPMC was surmised to be the result of the suppressed gelation of the amylose eluted during baking by the hydroxypropyl group (Goesaert et al., 2008).

The cohesiveness did not differ significantly between dried albumen, MC, and HPMC at 1 day after baking. However, at 3 days after baking, the cohesiveness of MC and HPMC were significantly lower than that of dried albumen, especially in the case of HPMC. MC and HPMC absorb not only free water after baking, but also the water hydrating the gelatinized starch during storage. Therefore, it was surmised that the structure of cell walls made by gelatinized starch became fragile and less cohesive. HPMC in particular was deemed to promote decreased cohesiveness to a greater extent than MC because of the large amount of swelling caused by the hydroxypropyl group (Pérez et al., 2007). Since the protein in dried albumen undergoes heat denaturation during baking, which causes it to lose its water-holding property, it is presumed that it does not absorb water from the gelatinized starch after baking; thus, a decrease in cohesiveness is less likely to occur compared with cellulose derivatives.

Conclusion

By adding 0.5 % cellulose derivatives such as HPMC or MC to gluten-free rice flour bread made with soymilk, bread with excellent leavening was achieved. Usually, the addition of 1 % or more cellulose derivatives is required to ensure suitable leavening of gluten-free bread. However, by combining cellulose derivatives with soymilk, suitable bread can be made even with the addition of just 0.5 % cellulose derivatives. Therefore, the combined use of cellulose derivatives with soymilk is considered to be effective in terms of reducing the use of food additives. Further, the reason that cellulose derivatives are more effective than other hydrocolloids in preventing the hollowing of the bread crumb is that interfacial activity contributes to the stabilization of gas bubbles.

Acknowledgements    We are grateful to DSP Gokyo Food & Chemical Co., Ltd., Sansho Co., Ltd., MC Food Specialties Inc., and Shin-Etsu Chemical Co., Ltd. for providing the hydrocolloids.

Conflict of Interest    The authors declare no conflict of interest.

Abbreviations

MC

methylcellulose

HPMC

hydroxypropyl methylcellulose

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