The Effect of Oat Fibre Powder Particle Size on the Physical Properties of Wheat Bread Rolls.

In response to the growing interest of modern society in functional food products, this study attempts to develop a bakery product with high dietary fibre content added in the form of an oat fibre powder. Oat fibre powder with particle sizes of 75 µm (OFP1) and 150 µm (OFP2) was used, substituting 4, 8, 12, 16 and 20% of the flour. The physical properties of the dough and the final bakery products were then measured. Results indicated that dough with added fibre had higher elasticity than the control group. The storage modulus values of dough with OFP1 most closely approximated those of the control group. The addition of OFP1 did not affect significantly the colour compared to the other samples. Increasing the proportion of oat fibre powder resulted in increased firmness, which was most prominent in wheat bread rolls with oat fibre powder of smaller particle sizes. The addition of oat fibre powder with smaller particles resulted in a product with the rheological and colour parameters that more closely resembled control sample.


Introduction
The contemporary consumption model, characterized by high ingestion of products rich in sugar and fat, yet low in vitamins, minerals, and dietary fi bre, is responsible for many diseases of affl uence, such as diabetes, asthma, coronary heart disease and obesity (1).Meanwhile, general consumption of bread products decreases as the society's gross domestic product increases (2).However, in developed countries there is also a demand for a greater variety of bread with oat, bran, seeds, etc. (3).In such cases, the most probable solution is to fortify white bread products with dietary fi bre that has many functions (e.g.caloric reduction, available carbohydrate reduction, moisture control, anticaking properties, textural modification and structural enhancement) (4,5).
Dietary fi bre is an essential component in nutrition and health (6,7).Thus, the present study will explore the addition of oat fi bre in wheat roll production in accor-dance with European Food Safety Authority (EFSA) health claims and European Union Commission Regulation No 432/2012 of 16 May 2012 (8,9), which established a list of permitt ed health claims for foods, apart from those referring to the reduction of disease risk and to children's development and health.This health claim states that 'oats increase the faecal bulk' when a product has a high level of dietary fi bre.This means more than 6 g of dietary fi bre in each 100 g of product, therefore complying with European Union Commission Regulation (EU) No 1047/ 2012 of 8 November 2012 amending Regulation (EC) No 1924/2006 (9) regarding the list of nutrition claims.The addition of dietary fi bre to the bread matrix can lead consumers to conclude that a product is healthy.This study was conducted to investigate whether that common belief has merit.
Principally, dietary fi bre is comprised of a non-carbohydrate component called lignin and non-cellulosic polysaccharides, such as pectin substances and hemicellulos-es.The structural components are found mainly in the cell wall of plants (7).Dietary fi bre is primarily a constituent of plant foods and includes materials that have diverse morphological and chemical structure.
Basically, the process involved in the production of bread is a highly complex one that entails a range of parameters that must be regulated.According to Almeida et al. (10), rheological measurements are used in the prediction of both the behaviour of wheat dough during bread production and of the fi nal product quality.Despite the fact that there are diff erent sources of dietary fi bre, Almeida et al. (10) argue that the dietary fi bre used in bread production aff ects both processing and fi nal product quality.
Baking is the most important step in the bread production that transforms the raw dough to the bread using the heating.During baking, the mass loss is observed due to the loss of water that was incorporated into the dough during mixing.As already documented, dietary fi bre may have a negative eff ect on the fi nal bread quality, leading to reduced volume of the bakery product, i.e. loaf volume (11,12).
The addition of large amounts of fi bre to bread results in a change in the bread physical properties, such as increased crumb fi rmness, crumb darkening, or changes in taste (13).These negative eff ects can be reduced by using an appropriate proportion of soluble to insoluble fi bre fractions (14), and the addition of enzymes such as hemicellulose and pentosans (15).Reducing fi bre powder particle size increases water-holding capacity by increasing the surface area.In the bread production, this is ex tremely important, because water is involved in the processes of starch gelatinization, protein denaturation, for mation of fl a vour characteristics and it also signifi cantly aff ects colour (14,15).
The aim of the study is to investigate the eff ects of the addition of oat dietary fi bre powder of two particle sizes on the physicochemical properties of wheat bread rolls during storage and to provide recommendations on the extent to which it is advisable to fortify wheat bread rolls with oat fi bre.

Particle size distribution
Fibre particle size distribution was determined using Morphologi ® G3SE (Malvern Instruments Ltd, Malvern, UK) equipped with a sample dispersion unit for dry samples and a wet cell for hydrated samples.Size distribution was calculated as the relative volume of particles in size bands presented as size distribution curves (Malvern Micro soft ware v. 5.40, Malvern Instruments Ltd.).Particle size distribution parameters included the largest particle size (d (v,0.9) ), mean particle volume (d (v,0.5) ), smallest particle size (d (v,0.1) ), Sauter mean diameter (d (3,2) ), and mean particle diameter (d (4,3) ).Water-holding capacity of the fibre was determined as follows: 10 g of dietary fi bre was soaked in 100 mL of distilled water and left overnight.Then the bulk was fi ltered through a Büchner funnel and the water-holding capacity was calculated as the diff erence between the amount of water used in the soaking process and the amount remaining in the bulk aft er fi ltration.

Bread preparation
Components of the bread dough included (in %): fl our or fl our partly replaced with dietary fi bre 92, pressed yeasts 4, salt 2, sugar 1 and vegetable fat 1. Flour was replaced by dietary fi bre at 4, 8, 12, 16 and 20 % of the fl our wet mass.The water content was calculated based on the water-holding capacity of the dietary fi bre, and it was: 58, 61, 64, 67, 70 and 72 % when the fl our was replaced with 0, 4, 8, 12, 16 and 20 % of dietary fi bre, respectively.The obtained dough had a vis cosity of up to 500 Brabender Units (BU) measured with rheometer Rotovisco RT20 (Haake, Vreden, Germany).All components, apart from fat, were mixed with a spiral mixer (TRQ-42; RM Gastro, Ustron, Poland) at 100 rpm for 4 min.Aft er the addition of oil, the dough was mixed for 6 min at 250 rpm, left to rest for 15 min, and then divided into 60-gram rolls.The divided rolls rested for 15 min at room temperature, and were then put into the proofi ng chamber for 60 min (35 °C, 80 % relative humidity) to double in volume.Bread rolls were then baked at 210 °C for 15 min in a convection oven (CPE 110; Kuppersbuch, Gelsenkirchen, Germany).Aft er cooling, the bread was packaged into paper bags and stored for 3, 24 and 48 h at 20 °C.

Rheology
Rheological measurements were conducted with a Rotovisco RT20 rheometer (Haake).The rheological examination was done using standard dough preparation, but without yeast to avoid the infl uence of fermentation on the results.The creep and recovery test was followed by a fi ve-minute period of relaxation.Next, the examined dough was subjected to the creep test with a constant shear stress of 50 Pa for 60 s, allowing the sample to recover for 180 s aft er the removal of the load.The forced oscillation test was conducted in the plate-plate geometry with a 2-mm gap.The parameters were chosen aft er a set of measurements had been performed to determine the viscoelastic region.The frequency of oscillation was 1 Hz, shear stress 600 Pa, angle rotation sensor 2°, and temperature of the measurement 15 °C.Measurements were per-formed in triplicate and the measured parameters were: G' (storage modulus, in Pa), G" (loss modulus, in Pa) and γ (deformation).

Specifi c volume and moisture
The volume and moisture content of baked bread rolls during storage were determined according to the rapeseed displacement method ( 16).Each roll was weighed and the volume was measured 3, 24 and 48 h after baking.The specifi c volume was reported in cm 3 per g of the loaf.The moisture content of the roll samples was determined by drying them overnight at 105 °C.

Colour
Colour determinations were carried out on bread crumbs using a CR-400 colourimeter (illuminant D65, measurement area ø=8mm, standard observers 2°; Konica Minolta Inc., Tokyo, Japan), and the results were expressed according to the CIELab colour space parameters: L (L=0 (black) and L=100 (white)), a (-a=greenness and +a=redness), and b (-b=blueness and +b=yellowness).The parameters were measured 3, 24 and 48 h aft er baking.Ten shots of the crumb and crust from three diff erent rolls were taken with the colourimeter.The changes in the roll crust colour were estimated according to the browning index (BI) as follows: /1/ where /2/ Firmness Firmness was measured 3, 24 and 48 h aft er baking, using the universal testing machine Instron ® 5965 (Instron Corp., Norwood, MA, USA) with a maximal load of 500 N and 50 % penetration depth using a 40-mm diameter probe.The test speed was 1.7 mm/s and the crumb cube size was 20 mm×20 mm×20 mm.The results were given as the maximum fi rmness in N.

Porosity
Porosity was estimated using image computer analysis.A loaf was cut into slices, each 2.5 cm thick.Photographs of the slices were taken using a digital camera (MicroPublisher 5.0 RTV; QImagining, Surrey BC, Canada) with illumination from lamps (Dulux L 36W/954, daylight; Osram, Munich, Germany) with the colour temperature 5400 K. Images were saved in TIFF format, then analyzed using ImageJ soft ware, v. 1.44 (National Institute of Health, Bethesda, MD, USA).Aft er selection of the central image of the crumb and determining the area, the image was converted into an 8-bit image to obtain a black and white threshold followed by binary segmentation.The results are presented as a percentage of pore area in the total area.The measured parameter was the mean cell area, expressed in mm 2 ; this was calculated by dividing the total area of cells by the total number of cells.

Total dietary fi bre
Total dietary fi bre content in baked bread rolls was measured according to the AOAC method 2009.01-2009(17) modifi ed by the producer using the Fibretec™ 1023 system E (FOSS Inc., Eden Prairie, MN, USA).

Statistical analysis
The statistical calculations, including the analysis of variance and Tukey's test with signifi cance level at α=0.05, and the Pearson's correlation matrix, were performed using STATISTICA v. 10 for Windows (StatSoft Inc., Tulsa, OK, USA).

Particle size
Table 1 presents the results of static automated image analysis of the oat fi bre powder with water-holding capacity used for the wheat roll production.The particle sizes of dietary fi bre suggest that the whole mass of the two dietary fi bres was not homogenous because of a wide range of particle sizes from 14.7 to 132.5 μm of OFP1, and from 20.3 to 333.7 μm of OFP2.With regard to the water--holding capacity of oat fi bre, diff erent outcomes were obtained compared to those achieved by two groups of researchers (16,18) who concluded that decreasing particle size leads to reduced hydration properties.The results confi rm the theory of Chau et al. (19) that a decrease in fibre particle size results in a larger exposure of hydrophilic groups, which is connected with water absorption capacity.

Rheological properties
The creep and recovery test indicated that when the fl our is replaced with oat fi bre powder, the maximum deformation of OFP1 and OFP2 samples obtained aft er 60 s of the creep test was signifi cantly reduced (p≤0.05).The results are shown in Figs.1a and b were higher than of that containing OFP2.The samples of dough containing 4, 8, 12 and 16 % OFP1 recovered their primary form aft er removal of the load more eff ectively than the dough containing OFP2, indicating higher elasticity of this dough.In the case of dough with the addition of 20 % of fi bre powder, this tendency was not observed, which may indicate that the dough was oversaturated with the fi bre powder.Fibre does not form a gluten structure, it creates a parallel viscoelastic system.This tendency conforms to the outcomes of studies by Ahmed et al. (20), who observed that when the fi bre content increases, not only the maximum deformation values decrease, but also elastic recovery is reduced.Dietary fi bre is highly hygroscopic, and therefore it signifi cantly aff ects the rheological properties of dough.It is associated with higher water absorption while kneading, and the phenomenon increases with an increase in the dietary fi bre content in dough (13).The rheological properties signifi cantly aff ect the fi nal quality of the product.The studies by Sabanis et al. (21) indicate that due to higher dough viscosity, small air bubbles formed while kneading showed higher persistance during baking.If viscosity is high, air bubbles may travel upwards during fermentation, but due to their increased amount, the crumbs exhibit bett er porosity.The creep and recovery test is an eff ective way to demonstrate physical properties of bakery products.Van Bockstaele et al. (22) observed that the creep and recovery test showed signifi cant correlations with protein content, Zeleny sedimentation value, farinograph water absorption, alveograph extensibility and bread volume.
The changes in the values of the storage modulus and loss modulus demonstrate that wheat dough with oat fibre is more elastic than viscous (Fig. 2).However, att ention should be paid to the fact that storage modulus values of the OFP1-containing dough were higher than of the OFP2-containing dough.

Specifi c volume and moisture content
The specifi c volume and moisture content of wheat rolls with oat fi bre powder are shown in Table 2.The dietary fi bre of smaller particle sizes in wheat rolls increased the specifi c volume of the fi nal product, but it was smaller than of the control.During storage, the loss of volume was recorded, and the rolls with OFP1 lost from 6.1 to 6.7 % of volume aft er 48 h and those with OFP2 lost between 15.1 and 20.7 % of volume.Reduced loaf volume may also be caused by gluten dilution and physicochemical reactions among fi bre components, water and gluten (21).The moisture content is strongly correlated with the specifi c volume values.This means that the smaller particles take up more water because of the larger surface area.Moisture content is strongly correlated with the water uptake during mixing of the dough (22).Dough with more water-binding substances (such as fi bre) absorbs more water, which can be bound during storage.The moisture of OFP1-containing bread rolls was higher because the smaller particle size and more hydrophilic groups from fi bre were prone to binding water (23).

Colour parameters of wheat rolls
Golden brown crust and creamy white bread crumbs are the most important appealing factors that indicate the quality of a bakery product to consumers (23).Table 2 shows the browning indices of wheat roll crumbs.The rolls containing OFP1 had a lower browning index due to the smaller particle size, which can bind more eff ectively into the starch-gluten matrix and do not infl uence the colour to the same degree as OFP2.It should be noted that the higher the content of dietary fi bre, the browner the crust, as the colour change of the crust of the wheat rolls with high content of dietary fi bre was caused by oxidation reaction and release of sugars, which participate in caramelization during baking (24).It was demonstrated that while storing OFP1-and OFP2-containing samples, a signifi cant change in lightness occurred, but it was not observed in the control sample.OFP1 and OFP2 samples did not diff er in terms of redness (a) values during storage; however, they were statistically signifi cantly diff erent in terms of yellowness (b).The colour of bakery products with added dietary fi bre changes as a result of a caramelization and the Maillard reaction.That is true only if we want to explain the colour of the surface of bakery products directly exposed to high temperatures.In the case of bread crumb, such high temperatures are not reached.Therefore, it is reasonable to perform measurements only for crumbs, the colour of which directly correlates with ingredients used in the dough production (9).Results obtained in this study indi-cate that smaller sizes of dietary fi bre added to bakery products result in fewer diff erences compared to the control sample.However, it is worth noting that the reduced lightness of bakery products may be a result of water binding to the fi bre, creating the environment for reactions between sugars and amino acids (25).

Firmness parameters
Firmness was measured during storage of wheat rolls in order to determine the eff ects of the addition of oat fibre powder on staling (Fig. 3).Staling is one of the most important changes that aff ect bakery products during storage, and it has a signifi cant eff ect on the aroma and fi rmness, perceived by consumers as quality indicators.Staling can be described as increased fi rmness of bakery  products over time caused by retrogradation of amylose and amylopectin.For that reason, increasing interest has been devoted to producing complex compounds with amylopectin and amylose with substances belonging to a dietary fi bre group in order to inhibit retrogradation (26).Fig. 3 shows changes of crumb fi rmness of wheat rolls after the addition of the fi bre powder.It can be seen that the addition of dietary fi bre resulted in increased fi rmness 3 h aft er baking, which is statistically more significant (p≤0.05) in samples with the OFP1 than in those containing OFP2.Firmness increased signifi cantly with the length of storage, especially of the samples containing 16 and 20 % OFP1, which is associated with a high amount of insoluble fi bre in the used fi bre powder (27).Both lack of fi rmness and volume are aff ected by the content of hydrocolloids in fi bre powder.When bread with high mass fraction of dietary fi bre is heated, hydrocolloids with strong bonds are formed, and these bonds cannot be broken by enzymes in fl our due to their low amount (27).Smaller changes in the fi rmness value were observed in the fi bre with larger particles than in the fi bre with smaller particles.However, in this study, it was determined that the addition of both OFP1 and OFP2 increased the fi rmness of bakery products.A correlation analysis demonstrated a signifi cant positive correlation between fi rmness and the storage modulus G' (R=0.77) and loss modulus G" (R=0.78).Strong correlations between rheological parameters and texture indicate that fi rmness depends on the storage modulus.The fact that fi rmness increases with increased fi bre content may be due to the increased water-holding capacity of the fi bre.Therefore, chemical reactions can occur more easily, aff ecting the physical parameters (28).Fibre micronization may also be associated with the destruction of the aleurone layer of cells during grinding.As a result, enzymes, glutathione or phytates are released and they may impair gluten formation due to chemical reactions (29).

Porosity of baked wheat rolls
Visual characteristics of crumbs, such as mean cell area, are among the elements that constitute the quality of the fi nal product (30).As can be seen in Fig. 4 only the average size of bread crumb pores was altered by the addition of dietary fi bre.The crumbs containing OFP1 had less porous structure, which could be explained by lower viscosity of the dough caused by dietary fi bre, resulting in lower carbon dioxide formation during fermentation (31).
Total dietary fi bre Fig. 5 shows the total dietary fi bre content of the tested rolls.An interesting tendency is that bakery products with OFP1 contained more fi bre than bakery products with OFP2.This is associated with the fact that smaller fibre particles have larger surface, and consequently, their water-binding capacity is higher, and water is released faster during baking and storage.Therefore, a lower mass is obtained, and there is a relative increase in the fi bre content in bread rolls (32).

Conclusion
There are only few scientifi c reports focused on the use of oat fi bre as an additive to bakery products.This study has demonstrated that reducing the particle size of fi bre powder aff ects the characteristics of bakery products.Because of greater granulation of bakery products, it is possible to obtain dough with bett er rheological properties and colour parameters than when using products without the addition of fi bre.The problem of increased fi rmness of bakery products accompanying the addition of fi bre may be associated with the high water-binding capacity of dietary fi bre.It is therefore necessary to prepare a formula that optimises the amount of water.

Table 2 .
The specifi c volume (ν), moisture content, and browning index of wheat rolls with oat fi bre powder OFP1 and OFP2=oat fi bre powder with particle sizes of 75 and 150 μm, respectively.Values with diff erent lett ers in superscript diff er signifi cantly (p≤0.05)