DIFFERENT STABILIZATION TREATMENTS OF RICE BRAN ADDED TO WHEAT FLOUR DETERMINE DIFFERENT PROPERTIES IN PARTIALLY BAKED WHEAT BREAD

Abstract

In the present study, wheat flour was replaced by 3, 6 and 9% of rice bran (RB) previously exposed to two types of heat treatment: extrusion (EXT) and steam cooking (STC). The aim was to investigate how RB affected the properties of wheat-flour dough and partially baked wheat bread, as a final product. The results of the study indicate that neither of the RB treatments had a negative influence on the rheological characteristics of the dough, but that its fermentation capacity and gas retention improved, especially in case of EXT-RB, which was exposed to more intensive heat treatment. In the final bread product, there was also a positive increase in the specific total volume for the EXT-RB bread, and a decrease for the STC-RB bread. Nevertheless, sensory analysis showed that the score for the RB-enriched bread decreased when RB content increased. We concluded that low concentrations of RB exposed to a more intensive heat stabilization process improves several rheological properties of the dough and the physical properties of the partially wheat bread. [PUBLICATION ABSTRACT]

Headnote

ABSTRACT

In the present study, wheat flour was replaced by 3, 6 and 9% of rice bran (RB) previously exposed to two types of heat treatment: extrusion (EXT) and steam cooking (STC). The aim was to investigate how RB affected the properties of wheat-flour dough and partially baked wheat bread, as a final product. The results of the study indicate that neither of the RB treatments had a negative influence on the rheological characteristics of the dough, but that its fermentation capacity and gas retention improved, especially in case of EXT-RB, which was exposed to more intensive heat treatment. In the final bread product, there was also a positive increase in the specific total volume for the EXT-RB bread, and a decrease for the STC-RB bread. Nevertheless, sensory analysis showed that the score for the RB-enriched bread decreased when RB content increased. We concluded that low concentrations of RB exposed to a more intensive heat stabilization process improves several rheological properties of the dough and the physical properties of the partially wheat bread.

- Keywords: rice bran, stabilization, partially baked, wheat bread -

INTRODUCTION

Rice is the second most cultivated cereal grain in the world, and the most important cereal crop of Asia, Australia and South America. In Europe rice is grown mainly in Italy and Spain. Rice bran (RB), which amounts to 8% of the total rice grain, is a by-product of white rice, the most common form for consumption. After its production, RB deteriorates very rapidly because of lipid hydrolysis caused by the action of endogenous lipases. This poses a serious problem if it is to be consumed by both animals and humans and means that widespread consumption of RB is unacceptable. In order to disable the enzyme action and ensure that RB is not damaged, different types and intensities of heat stabilization treatment have been assayed (DA SILVA et ed., 2006). The annual production of rice in Spain is around 750 million kg, which means approximately 60 million kg of RB. Since RB is not subjected to stabilization processes and cannot be consumed immediately, there is an enormous amount of potential waste and its disposal is an environmental problem.

Unlike other brans consisting mainly of fiber, RB does contain insoluble fiber but also significant amounts of protein and fat. RB has been extensively characterized in the past, especially in its lipid fraction (WARREN and FARRELL, 1990), and it has shown interesting properties from the nutritional point of view. RB is rich in tocopherols and especially in ?-oryzanol. It mainly consists of ferulic acid esters, sterols and triterpenic alcohols (GARCIA-LERMA et ed., 2009), all the important nutrients with potential health properties. RB intake has been shown to be effective at decreasing plasma LDL-cholesterol in hypercholesterolemic individuals (GERHARDT and GALLO, 1998). The lipid fraction of RB has also been shown to be responsible for decreasing cholesterol in humans (MOST et ed., 2005). No differences in the glycemic index have been observed after the consumption of corn bread and corn bread supplemented with RB (SCHNELL et ed., 2005). However, after the long-term consumption of whole RB or its fractions, the glucose metabolism in patients with type I and II diabetes mellitus has been shown to improve (QURESHI et ed., 2002). Finally, the antioxidant activity of RB is high in both lipophilic and hydrophilic fractions, much higher than has been observed for most other plant products (WU et ed., 2004).

For many years, there has been increasing interest in adding RB to bread (CARROLL, 1990; SHARP and KITCHENS, 1990). Most research has focused on increasing the nutritional value of bread, basically the protein, fiber and mineral content (AJMAL et ed., 2006; SADAWARTE et ed., 2007). Nevertheless, the reported levels of RB added were generally very high and many different types of bread were used, so it is difficult to extrapolate these data to other sorts of bread. In the present study, small doses of RB, exposed to two types of heat treatment were added to wheat flour so that we could assess the effects of RB addition on the properties of wheat flour dough and partially baked wheat bread, widely consumed in Europe.

MATERIALS AND METHODS

The RB used was of the Bomba variety and cultivated in the delta of the Ebro River (Spain). The RB obtained was ground and thermally treated by two different methods within 24 hours of procurement. One of the methods was an extrusion cooking (EXT) procedure (Bühler AG, Switzertland) and RB was extruded at 115°C and an end-plate pressure of 12.1 bar. The other method was a steam cooking (STC) procedure (FSH; Bühler AG, Switzerland) in which RB was stabilized by being subjected to steam at 100 °C and a subsequent residence time of at least 1 minute. After stabilization and cooling, RB was stored in closed paper bags until it was used.

Analysis of wheat flour and rice bran

The proximate composition of wheat flour (La Meta, Lleida, Spain) and the two types of RB (EXT and STC) was analyzed. Humidity was determined by drying the sample at 130°C for 2 h. Ash was analyzed by burning the sample at 525°C for 5 h. Protein was analyzed by the Kjeldahl AOAC method 920.87 (AOAC, 2005) in a semiautomatic system (Büchi, Switzerland). Total lipid content was determined by the Soxhlet AOAC method 920.39c (AOAC, 2005). Total, insoluble and soluble dietary fiber was analyzed using the AOAC Official Method 991.43 (AOAC, 2005). The protein solubility of RB was determined according to SHARMA et ed. (2004).

The particle size distribution of wheat flour and two types of RB (EXT and STC) was analyzed using an electromagnetic sieve shaker (CISA, Barcelona, Spain) with four meshed sieves with size openings of 500, 300, 150, and 125 μm. In each case 500 g of the sample was placed on the top screen (500 μm openings) and then shaken for 20 minutes. In order to determine the amount of sample retained on each screen, the screens were subsequently weighed.

All analyses were run in triplicate, and the average values were adopted.

Experimental conditions

In order to investigate the properties of dough and baked bread, mixtures of wheat flour replaced with 3, 6 and 9% of both types of RB (EXT and STC) were prepared. The dough and baked bread made with wheat flour without added RB were used as a control.

Dough characteristics

For each experimental condition, the viscoelastic behavior of the dough was determined by the Alveograph MA82 (Tripette and Renaud, France) according to the AACC method 54-30.02 (AACC, 2009). The parameters measured were: the strength or deformation energy (W), tenacity or resistance to extension (P) and dough extensibility (L). Proteolytic degradation was determined by comparing the W value right after analysis and after two hours of dough resting time (W2h) (ROSELL et al., 2002)

The mixing behavior of the dough was determined using the Farinograph 810-105 (Brabender, Germany), according to the AACC method 5421.01 (AACC, 2009). The following parameters were determined: water absorption (%), dough development time (DDT), stability and the decay at 10 minutes.

The gelatinization properties, measured as the gelatinization maximum or the peak viscosity in AU (Amylographic Units), were analyzed using the Amylograph ? (Brabender, Duisburg, Germany), according to the AACC method 2210.01 (AACC, 2009).

Fermentation behavior was measured by the Rheofermentometer F3 (Tripette and Renaud, France) according to the supplier's specifications. The following parameters of dough development were measured: maximum dough fermentation height (Hm) and the time at which dough reaches maximum height (T^. The gas parameters measured were: the time of maximum gas formation (GTJ, dough permeability when gas starts to escape from the dough (Tx), the volume of gas produced throughout fermentation (VC02), and the gas retained in the dough at the end of the assay (R), in both milliliters (ml) and percentage (%) of gas retained.

All assays were performed in triplicate, and the average values were adopted.

Breadmaking process

The wheat bread used for the test was produced industrially using a Sancassiano single spiral kneading system (15 min; 23°C final dough temperature) (Sancassiano, Alba, Italy) and a Mecatherm production line (Mecatherm, Barembach, France) in a straight process that takes into account the various experimental conditions described above. Water (57%), yeast (2%), salt (1.8%), and commercial bread improver (0.5%) with ascorbic acid, DATEM, a-amylase and xylanase (Leag-Eurogerm, Barcelona, Spain), were added to the wheat flour or mixtures of wheat flour with RB (flour basis; see section on experimental conditions). After kneading and make-up, the dough underwent a 90 min fermentation process and prebaking was performed for 12 min at 185°C. After the cooling phase (20 min), the partially baked bread was deep-frozen at -18°C, packed in plastic bags in a cardboard box, and kept in frozen storage at -18°±2°C. It was then stored in the freezer for 15 days until analyses had been completed. Before analysis, the bread was defrosted for 45 min and second baking was performed (16 min at 190°C) in a Salva convection oven (Salva, Lezo, Spain). Samples were analyzed after cooling at room temperature for two hours.

Bread loaf volume

Bread volume was determined using a laser sensor, with the volume analyzer BVM-L 370 (TexVol Instruments, Sweden). The value of volume index was measured following the AACC method 10-91 (AACC, 2009) according to GOMEZ et al. (2003). Specific volume was calculated as the ratio between the volume of bread and its weight. The final value obtained for each substitution level was an average of 20 repetitions (5 loaves χ 4 pieces).

Sensory analysis

The sensory analysis was carried out following the ISO-8587 Standards (ISO, 2006). A total of 30 people, habitual consumers of bread, were asked to test seven samples (control sample and six bread samples containing RB) and rank them from the one they liked the most to the one they liked the least. In this case, the high number of samples made the sensory analysis difficult for untrained panelists and the evaluation parameters had to be simplified to overall taste preferences only. Once the samples had been sorted according to the panelists' palatability, they were each scored on a scale of 7 (like the most) to 1 (like the least).

Statistical analysis

Analysis of variance (ANOVA) was applied to specific volume and sensory analysis data to evaluate the differences between the seven kinds of tested bread. The substitution level of wheat flour by two types of RB (EXT and STC) was considered. In order to discriminate among means, Fisher's least significant differences (LSD) test was used (95% confidence interval). The software used for statistical analysis was Statgraphics Centurion XVI V. 16.0.08 provided by Statsoftlnc., USA.

RESULTS

Wheat flour and rice bran characteristics

Table 1 summarizes the proximate composition of wheat flour and the two types of RB (EXT and STC). Our results were in agreement with previously reported results for this kind of product (CARROLL, 1990; PACHECO DE DELAHAYE et al., 2009). RB has a high content of soluble albumins and globulins and their denaturation during the heat treatment led to a decrease in its soluble form (PRAKASH and RAMANATHAN, 1994). According to the findings reported by SHARMA et al. (2004), the very low content of soluble protein in EXT-RB is due to the high temperature applied during the heat treatment.

Influence of rice bran on dough properties

The viscoelastic characteristics of the dough are shown in Table 2. The deformation energy (W) tends to decrease when the RB content of the mixture increases. This effect is greater than expected due to the dilution of gluten, especially in RB-STC. For a given W value, the P/L ratio is a good predictor of dough behavior. This kind of behavior was specific for each mixture and determined by interactions between different components of wheat flour and RB. In studies carried out on mixtures of wheat flour and fibers, the alveographic parameters were observed to depend heavily on the type of fiber (GOMEZ et al., 2003; WANG et al., 2002). Another important aspect is the degradation of dough due to proteolysis, expressed as the loss of dough strength after 2 hours of dough formation (W2h). The value for wheat flour was 22% and was not affected by the addition of RB, which suggests that the heat treatment of RB was intense enough to prevent any proteolytic activity.

To study the effect of RB addition on dough mixing properties, the main farinograph parameters were measured. The results in Table 2 show that addition of RB to the wheat flour did not substantially change the mixing properties of the dough under the experimental conditions outlined above. Water absorption slightly decreased for all the concentrations and types of RB studied, but it was not significant from a functional point of view. Therefore, all bread-making trials were conducted at a fixed water concentration (57% wheat flour basis) indepen- dently of the dough formulas used, and always at the same consistency of the dough. It should be pointed out that dough stability, which indicates the tolerance of dough to kneading, increased with the presence of RB, inversely to the decay value. These results supported the hypothesis that there were no negative interactions between the components of wheat flour and RB. These data were similar to those obtained by KIM et cd. (1997) but not consistent with those of other studies (GHUFRAN-SAEED et ed., 2009; SUDHA et ed., 2007). However in these latter studies the basic conditions, (e.g. wheat flour characteristics, RB composition and concentrations) were different.

The amylogram parameters give information about the viscosity generated by the gelatinization of starch. The most important of these parameters is the peak viscosity shown in Table 2. In the case of the wheat flour used in this study, the peak viscosity value was very high (1204 AU). The addition of RB reduced it slightly, although not significantly from functional point of view. Analyzing the Falling Number, another test measuring the starch viscosity, GUFRAN-SAEED et cd. (2009) made similar findings. The peak viscosity of starch is considerably reduced by a-amylase activity and, in this case, neither RB contributed to any amylase activity. Therefore, for all experimental conditions studied, breadmaking trials were conducted in which the same dose of bread improver was added as the amylase contributor. Throughout the fermentation process the dough behavior was recorded by the rheofermentometer. The influence of RB addition on the fermentation characteristic is shown in Table 3. The maximum dough height (Hm) increased with the addition of RB, regardless of its type and concentration. The time required to reach the maximum dough height was not affected. With respect to gas behavior a backlog in porosity development (Tx) was observed and, except at high concentrations of RB (9%), gas production and retention was higher, and more pronounced in the case of EXT-RB. The only data available for this kind of analysis have been provided by WANG et cd. (2002), who added different fibers to the wheat flour and the results depended on the type of fiber used. In the main, it can be assumed that adding cellulose or bran to wheat flour decreased the gas holding capacity.

From the overall results it can be concluded that supplementing wheat flour with RB, mainly at concentrations of 3 and 6%, did not influence the formulation and mixing properties of the dough and improved the fermentation and breadmaking process of the partially baked wheat bread.

Influence of rice bran on bread quality

Apart from shape and color, the specific volume of bread is also an important element in consumer selection criteria. The specific volume data are shown in Fig. 1. Adding RB-EXT at concentrations of 3 and 6% led to a significant increase in the specific volume and only at the highest concentration (9%) did it slightly decrease, in comparison to the control. On the contrary, the specific volume obtained for STC-RB bread at concentrations of 3 and 6% did not change in comparison to the control, and decreased sharply at 9%. Several authors have reported a progressive decrease in specific volume when RB content is increased (SEKHON et ed., 1997; SHARMA and CHAUAN, 2002; SOARES et ed., 2009). However, other authors have observed an increase in specific volume at low concentrations of RB which decrease at higher concentrations (LIMA et ed., 2002; PARK et od., 2008). LIMA et cd. (2002) also observed differences related to the origin, and therefore the fiber content, of the RB variety: lower fiber contents result in higher volume. When the RB fiber is added directly to the bread, volume decreases (ABDUL-HAMID and LUAN, 2000). When the hemicellulose fraction and insoluble fraction of the RB fiber are added separately, the volume remains unchanged in the first case and decreases in the second (HU et al., 2009). These findings confirm the hypothesis that insoluble RB fiber has an important effect on the specific volume. In the case of wheat bran, the heat treatment counteract the negative effects mentioned above. Wheat bran added to the flour strengthened the gluten matrix (AMIRKAVEEI et al., 2009) and restored the initial bread volume (DE COOK et al., 1999).

The results obtained in our study, consistent with the above, show that a more intensive heat treatment of EXT-RB helps to stabilize the gluten network and, therefore, to retain gas, especially at low bran concentrations (3 and 6%). This is probably due to the oxidation of reducing compounds and is consistent with the higher rate of gas retention in EXT-RB dough. Thus, adding low concentrations of RB that has previously been exposed to high heat treatment acts as a bread improver.

Sensory evaluation

Fig. 2 shows the average sensory evaluation of overall acceptability. The bread without RB (control) was given the highest score, which was significantly different from the rest. The score given to breads with added RB decreased when the RB concentration increased and the lowest scores were given to STC-RB supplemented breads. The difference between the two types of RB becomes significant at concentrations of 6% and 9%. From the qualitative point of view, breads with 9% of RB were described by several panelists as having off-flavors.

Good sensory acceptance is one of the key aspects of using complementary or alternative ingredients in wheat bread. Several studies have been made of RB at different concentrations and in different bread types. The findings are in good agreement with those of our study: that is to say, the highest RB concentration was given the lowest score (SEKHON et al., 1997; SHARMA et al., 2004; SUDHA et al, 2007; GHUFRAN-SAEED et al., 2009). The same thing happened for RB derivatives (ABDUL-HAMID et al., 2000; SADAWARTE et al., 2007; HU et al., 2009). However, no significant differences were detected between 7.5% RB bread and the control by SOARES et al. (2009). PACHECO DE DELAHAYE et al (2009) even showed sensory preferences for 5% RB bread. One possible explanation for this is the places these studies were carried out: the former were from Asia while the latter were from South America, so there might be some variations in the susceptibility to the flavors added by RB to the bread. This is why sensory evaluation results are difficult to extrapolate, because cultural and culinary habits, and the different types of breads consumed need to be taken into account. Our study of a Spanish population indicates that the addition of RB to partially baked wheat bread would be limited to special RB bread with sensory characteristics that are different from those of the traditional bread.

CONCLUSIONS

The present paper studies how substituting wheat flour by two types of RB, thermally treated by two different methods (EXT and STC), at concentrations of 3, 6 and 9%, affects the rhe- lological properties of the dough and the quality of breads obtained in an industrial process of partially baked production. The variables analyzed in the dough showed that RB addition had a very slight or no effect on the wheat flour and even improved gas retention. These findings were more evident at low concentrations of RB, and for EXT-RB addition. The characteristics of the breads obtained depended on the RB type (EXT or STC) and were clearly superior for EXT-RB. These differences were due to the different heat treatment intensities applied to the two types of RB, in which the destruction of heat-sensitive constituents might lead to less protein-fiber interferences during the formation of the gluten network. However, further investigations are required if this hypothesis is to be confirmed.

In conclusion, bread supplemented with low concentrations of EXT-RB may be a way to increase the intake of dietary fiber in the population, as well as of other important compounds present in the lipid fraction of RB. In Spain, however, these breads do not have the same sensorial acceptance as the traditional baguette and should not be treated as a white bread substitute, but as a different specialty with added nutritional value.

ACKNOWLEDGMENTS

This work has been supported by Europas try S.A., Spain.

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Paper received May 15, 2012 Accepted November 30, 2012.