The efficacy of sorghum flour addition on dough rheological properties and bread quality:A short review

Nhlanhla Mtelisi Dube,Fei Xu*,Renyong Zhao

College of Food Science and Technology,Henan University of Technology,Zhengzhou 450001,China

ABSTRACT The baking quality of sorghum-wheat bread is equally reliant on the quantity and the quality of protein existent in the composite flours.A lot of procedures have been proposed for expanding the use of sorghum either as a composite flour blended with wheat or potentially as a straight flour.In any case,the amount of flour that can be subbed is up to a specific degree of sorghum flour to deliver sensory attractive products.The manipulation of the chemical composition,textural properties of the sorghum kernel,and selection of a proper hybrid could proffer desirable value addition results in bread making.The quality of the flour and the adherence to acceptable flour size standards is the key to producing good quality bread.In this paper,the effects of adding sorghum flour on dough rheological properties and the quality of bread were discussed in detail.

Keywords: Bread quality Sorghum-wheat bread Composite flour Dough rheological properties

1.Introduction

Bread is a bakery product whose main ingredients are comprised of water,flour,salt,yeast,sugar,fat,which are mixed and fermented to form a viscoelastic dough before being baked[1].There are numerous types of bread differentiated by either flavor constituents,textural properties,size,shape,and baking conditions.Bread consumption has expanded consistently in many parts of the developing world because of changing dietary patterns and consistently developing population.In any case, a greater amount of wheat flour required for making bread must be imported,which is costly.Non-wheat grains are deficient of gluten proteins (gliadins and glutenin), hence the produced dough mixtures are characterized by poor viscoelastic and textural properties[2].The viscoelastic network serves as a prerequisite during dough mixing for improved resistance to mechanical stress and gas retention produced during fermentation[3].

Sorghum (Sorghum bicolor L.Moench) ranks 5th after maize,rice,wheat,and barley amongst the top important cereal crops in the world with its origin believed to be from Africa,Indian Ocean,Australia and the Pacific Ocean[4].It has been utilized mainly as human food in developing countries and as a major source of fodder in some developed countries[5-7].In some parts of the aforementioned areas,water availability is a major limiting factor for crop production characterized by reoccurring and extensive droughts[8].Be that as it may,sorghum's deficiency of gluten proteins makes it a desirable substitute for those with celiac disease[8].Composite flour innovation alludes to a meticulous procedure of blending various flours to create nutritious superior products while increasing profit margins.While the formulation of sorghum-wheat composite flour is crucial for value-added nutritionally improved products,the products produced should be characterized by acceptable consumer sensory attributes[9].Rheological assessments are an ideal methodology mostly utilized for estimating indicators of quality and texture for bakery products such as bread.This is of specific significance concerning product development and quality control[10].This review will discuss the efficacy of sorghum-wheat utilization in dough rheological properties and bread quality.

2.Proximate composition of flours

Having in-depth knowledge about sorghum grain quality characteristics is very important as it has got a large bearing on the quality of the end product.With issues such as tannins,grain color,biofortification,grain hardness,amylase activity potential being areas of concern[6].This is because each hybrid has different grain quality features that may inform end product quality and application[11].

The results shown in Table 1 refer to the determination of the major constituents of sorghum-wheat flour and the outcomes are utilized to analyze if the flour is within its expected compositional parameters.This is of paramount importance in product formulation to enforce compliance to set food standards,laws,and product safety to consumers.The interactions between gluten proteins, wheat starch,and lipids are of importance to the formation of a viscoelastic dough in bread making [12].Moisture content is a function of post-harvest handling and storage conditions that affect the physical and chemical aspects of food hence the substitution of sorghum resulted in either an increase or decrease in the moisture content of composite flours[13-15].The low moisture content lowers the likelihood of microbial growth in the product because of the low water activity[16].

The increase in sorghum proportion reported resulted in a significant decrease in protein content,suggesting that the sorghum grain had less protein content as compared to the wheat flours that were utilized[13].Primarily protein content (glutenin and gliadins) is a key parameter in bakery products such as bread.They give the dough viscoelastic properties and improved gas retention during fermentation,which affected bread volume and textural properties consequently having a bearing on consumer acceptance.Carson et al.[12]concluded that at set total protein content,as sorghum flour increased, peak height and water absorption decreased while dough development time increased notably.The difference in water absorption between sorghum flour and wheat flour was attributed to the gluten proteins.In essence,the increase in sorghum flour weakened the protein network ultimately,resulting in longer development time and lower dough strength[12].

Starch is a significant source of stored energy found in plants and its importance in bread making is identified with its water absorption property,gelatinization,pasting behavior,crystallization,and retrogradation behavior during dough development and baking [17].The carbohydrates content of sorghum-wheat composite flours shown in Table 1 ranged from 72%-79%, wherein starch accounted for around 90%.Sorghum flour has comparable amounts of starch compared to wheat flour, however,with a significantly lesser α-amylase(40%-50%)and amylolytic(10%)activity contrasted to that of wheat flour[10].Amylase is also responsible for gas retention during fermentation in bread and it is attributed to the interaction between starch and gluten in dough making which creates a stable network that hinders the collapse of bread,thereby inhibiting Maillard browning and caramelization reactions responsible for the intensification of flavors and crust color during the process of baking[10,18].Therefore,an increase in sorghum addition resulted in adverse effects of the attributes of starch content with respect to dough rheological and bread quality characteristics.

The higher values of fat,fiber,and ash in the sorghum flour may be attributed to the fact that sorghum was not de-germed and dehulled before milling,the germ removal leads to the reduction of the lipid content of the flour.The fat content insulates the proteins from the water so as to slow the process of formation of gluten.Hence the reason why an increase in sorghum addition resulted in reduced dough extensibility and low bread volume.Regarding the low ash content reported by Sibanda et al.[13] despite them using whole sorghum flour could be attributed to the 0.2 mm sieve they used.Whereas Amir et al.[14]used a 20 mm mesh size resulting in high fiber and ash content.Ash content may adversely affect the color of the noodles with higher the ash content likely to result in darkernoodles [21].It can, therefore, be insinuated that an increased ash content influenced the darker color of sorghum-wheat bread above 25%substitution[22,23].

Table 1 Proximate analysis of composite sorghum-wheat flour.

3.Flour particle size

When flour is fine, it hydrates more rapidly and more evenly, the surface of fresh noodles is smoother and less streaky,and cooking quality is better.Therefore,flour particle size notably affects non-wheat noodle quality.The finer the flour the better the quality of non-wheat noodles[22,24].Trappey et al.[23]also concluded that sorghum flour composition and particle size have a bearing on the quality of gluten-free bread.They also noted that flours with lower amounts of fiber and smaller particle size(60%extraction flour)produced bread with more acceptable characteristics such as volume, crumb structure,color,and texture.Reducing particle size can have substantial effects on the physicochemical changes,generated by an increased contact surface of particles,and this can affect food quality[25].Yousif et al.[26]stated that a significant higher(P ＜0.05)flour volume-weighted particle size for both white (151.60 μm) and red (203.10 μm) sorghum flour as compared to the wheat control(85.40 μm).They attributed the larger particle size of the sorghum flours to the reduction of in vitro starch digestibility of the sorghumwheat at all substitution levels aided by the presence of sorghum polyphenols and starch-protein interactions which are linked to disulfide cross-linkages in sorghum proteins[26].Liu et al.[27]noted that a harder kernel and larger kernel size was linked with high starch damage in their sorghum noodles study.Hatcher et al.[28]reported that noodles prepared from flour with fine particle size ranging from 85 to 110 μm showed improved textural characteristics than those prepared from flour with coarse particle sizes.Hugo et al.[29] conveyed in their study that sorghum flour exhibited coarser flour particles as compared to wheat.As a result,for the bread sample with 30%sorghum addition, the specific volume reduced (sorghum sample 4.8 cm3,wheat control 3.50 cm3)and crumb firmness increased(sorghum sample 11.50 g,wheat control 24.10 g).This could be attributed to the poor gas holding capacity.However,within the same study,fermented and dried sorghum group resulted in an improved bread specific volume(3.8 cm3)comparing to untreated sorghum group(3.5 cm3)[29].

4.Effect of dough rheological properties on bread

4.1.Water absorption

The relative quantity of the water absorbed by the key constituents of flour is hypothesized as principal parameters adding the rheological properties of dough and flour-quality traits as far as baking is concerned.As shown in Table 2,Sibanda et al.[13]reported a significant decrease in the water absorption ability of the flour from 57.73%for 100%wheat flour to 53.03%for the 30%composite flour.This could be related to the low water absorption capacities of millet and sorghum flour[30].Whereas lower water absorption is not desirable because it has been identified with quicker staling in bread[31].Interestingly,Akajiaku et al.[19]reported that water absorption of composite flour blends increased with an increase in sorghum substitution ranging from 2.85 g/cm3for 100%wheat flour to 3.28 g/cm3for 80%sorghum-wheat flour.They attributed their findings to a higher 100%sorghum flour protein(14.27%)and carbohydrate(70.57%)content while the control wheat recording the lowest protein(11.23%)and carbohydrate(17.05%)content.This,therefore, suggests that choosing a high protein and carbohydrate content hybrid could improve the water absorption capacity of composite flours.Torres et al.[31]discovered that dough containing up 30% sorghum replacement of finely ground flour samples had higher water absorption than those containing coarser flour which is desirable in a bread flour.

4.2.Dough development time,stability and extensibility

Table 2 showed that dough development time and dough stability decreased with an increase in sorghum substitution of the sorghum-wheat composite flour.The rise in the dough development time combined with the decrease of dough stability is suggestive of the weakening of the gluten network structure during the process of kneading [13].However, in other associated findings, they noticed that the addition of extruded sorghum increased the water absorption,dough development time,and decreased the stability of wheat-sorghum composite dough.Starch gelatinization was thought to be responsible for the increase of the water absorption index of the sorghum flour through extrusion cooking.Consequently,the inclusion of extruded sorghum flour has the propensity to increase the water absorption of sorghum-wheat composite dough.Increasing the water absorption may decrease the watergluten interactions,and therefore the dough development time of such composite dough increases [32,33].The dough with a more extensible character is particularly essential for improved gas retention during the process of baking which results in a good loaf volume[34].Hussein et al.[15]reported that the extensibility of 100%wheat control dough progressively decreased from 156 mm to 77 mm of 40%sorghum-wheat dough,while Sibanda et al.[13]also recorded similar results of a decrease in extensibility from 132 mm for 100%wheat control dough to 36 mm for 30%composite flour dough.The findings of Sibanda et al.[13]and Hussein et al.[15]on dough extensibility had negative effects on the volume of bread samples as they both recorded a volume reduction from the control wheat to the last degree of sorghum substitution.These changes possibly resulted from an increase in hydrophobic sorghum flour kafirin proteins which are not functional as far as giving the necessary viscoelastic qualities to the dough.Whereas dough extensibility exhibited an increase in both maximum tensile stress and loaf volume with increasing quantities of added zein[34].

Table 2 Characteristics of rheological properties of sorghum-wheat flours.

5.Effect of starch properties on bread quality

The progressions that starch experiences during gelatinization and retrogradation are significant determinants of its functional properties for food processing.These properties have got a bearing on the quality,acceptability,dietary benefit,and shelf-life of the completed products[32].Starch retrogradation is regularly considered to have a negative impact in light of its significant contribution to the staling of bread and other starch-rich foods,which can cause decreased shelf life and consumer acceptability leading to critical waste, and resulting in a significant loss to the processors[33].Dahir et al.[36]noted that pasting properties generally up to sorghum 10%addition were comparable to the wheat control with the decline in peak viscosity attributed to the increase of ruptured sorghum starch granules which weakened the gluten network,trough viscosity decreased due to the rupturing of granules due to shearing,while a decrease and low setback values were suggestive of a tendency to retrograde and other parameters followed the same trajectory suggesting that the addition of sorghum had a negative effect on pasting properties.It was discovered that sorghum fermentation caused a decrease in viscosity while increasing pasting time and pasting temperature.These properties have effects on bread making as a higher pasting temperature needs a higher temperature for baking.This lower viscosity consequently indicated the higher pasting temperature of flour[37].Starch damage is a key component in the bread quality.Damaged starch is prone to amylase degradation,resulting in an increasing amount of sugars for yeast fermentation and thus more gas produced by yeast.It should be noted that a higher starch degradation most likely weakens the starch gel which is key in the bread formation in the absence of a gluten network [38].High values of firmness and tensile strength were noted in fine flour particle size and high starch damage flour that came from harder kernels in sorghum noodles[28].

6.Effect of proteins on bread quality

Sorghum proteins, similar to those of all grains aside from wheat, rye and triticale, are not able to form a gasholding,viscoelastic dough on account of the poor viscoelastic properties of hydrophobic kafirin dough contrasted with that produced using wheat gluten(glutenins and gliadins) shown in Fig.1.The protein content of composite bread decreased gradually ranging from 11.55% for the control 100% wheat flour bread to 11.26% for the 25%sorghum-wheat flour bread.They attributed the decrease to lower protein content in sorghum flour as compared to wheat [9].Hussein et al.[15] reported a similar trend with a decrease from the control of wheat bread protein content 12.96%to 11.38%for the 40%sorghum substitution.This can be attributed to lower levels of gluten network in the dough and consequently less ability of the dough to rise;due to the weaker cell wall structure hence producing bread with low specific volume as compared to wheat bread [39].The poor hydration of kafirins Fig.2 may likewise be connected to their α-helical structure predominantly,as opposed to high-molecular-weight glutenin subunits of wheat, which have a significant level of β-sheet and β-turn structure[40].The generally high hydrophobicity of sorghum kafirin proteins is because they are encapsulated in protein bodies in the endosperm of sorghum kernels,which makes them inaccessible for participation in dough fibril formations[34].Therefore,it can be concluded that the reduction in peak viscosity, dough extensibility,water absorption capacity,and stability with the increased sorghum addition is because of the reduction of the protein quality and quantity which adversely affects the dough rheological properties.Composite flours characterized by a higher protein quality resulted in stronger dough and bread comparable to that of wheat flour with sensory acceptable traits ranging from 10%-20%sorghum addition [12,16,32].Hence, the use of additives by food technologists to improve the gas holding capacity of products made from or including sorghum has thus been sought over the years.

Fig.1.Graphic presentation of a sorghum protein body.

7.Additives used to improve sorghum-wheat flour

It is especially certain that without the advantage of wheat's gluten proteins and also the gas-holding properties they furnish, bread made exclusively from sorghum required an alternate bread-production innovation and also the approach of hydrocolloids[7].Their thickening capacity has prompted their utilization as suspension and emulsion stabilizers where they work by hindering molecule sedimentation [41].Hydrocolloid and emulsifier use in composite sorghum-wheat dough systems is still very limited.However,Dahir et al.[36]examined the influence of three additives specifically carboxymethylcellulose,xanthan gum, and sodium stearoyl lactylate on rheofermentation and pasting properties of composite sorghum-wheat flour.They discovered that the increased sorghum flour substitution weakened the rheofermentation and pasting properties of composite flours.A desirable interplay between the diverse composite dough additives during the stage of fermentation may be very essential to achieve the dough with the desired fermentation properties.Amongst the three hydrocolloids, they finally noted that sodium stearoyl lactylate addition exhibited a more positive influence on both the fermentation and pasting properties of composite sorghum-wheat flour.They accredited this to its nature of positive interaction with flour constituents particularly starch[36].Dough formation as well as production of gas and retention together affect the aerated structure of bread during fermentation [42].Sorghum-wheat composite dough containing nonextruded and extruded sorghum flour was reported to have similar elasticity after adding 0.5%-1% xanthan gum.The hardness of bread increased when extruded sorghum flour and xanthan gum were added also it resulted in a significant decrease in a specific volume and increment of the crumb hardness[43].Carson et al.[12]used 30%-80% of sorghum flour as a part replacement for wheat flour while boosting protein content at all levels by 12%,15% and 18% from wheat flour and vital wheat gluten.They concluded that composite flour having 50%sorghum plus 15%total protein produced bread volume with about 60% superior to sorghum flour plus exogenous protein only[12].

8.Sensory attributes of bread

8.1.Texture

Fig.2.Wheat protein components.

Jafari et al.[44]in their study noted that the addition of sorghum flour extrudate increased the composite bread hardness,cohesiveness,springiness,chewiness,and gumminess compared to non-extruded sorghum flour.They linked it to the fractional loss of starch integrity which could affect the bread hardness through the formation of the weak gluten-starch structure.Consequently,increasing extruded sorghum-wheat composite bread hardness could be associated with the decrease of sorghum starch granules integrity after extrusion cooking [45].Similarly Khating et al.[9]recorded a significant decrease in bread texture hedonic scores ranging from 8.60%for 100%wheat control bread to 3.60%for 25%sorghum-wheat bread while attributing the textural decrease to the rough texture sorghum flour characteristics.Other scholars also noted that when the quantity of sorghum flour increased in bread,the softness and mouth feel of bread sensory scores declined considerably.These findings were more evident in the samples which were constituted with whole sorghum rather than decorticated sorghum flour levels [39].In the same study, they went on to submit that up to 5%substitution with whole and decorticated sorghum flours had no significant difference(P ≥0.001)observed in terms of hardness[39].This could mean that small quantities of substitution with sorghum flour could make bread that is sensory desirable.With the increase in sorghum substitution proportion up to 40%,it was also reported that wheat-sorghum bread crumb was compact and the texture was coarse.These variations were due to the low qualities and quantities of sorghum flour protein [15].Siddeeg et al.[46] noticed that composite bread produced at 50%sorghum to wheat flour proportion produced loaves of bread that had low textural scores hence, the bread was not acceptable to the consumers.Sibanda et al.[13]concluded that incorporation of sorghum up to 30%appeared to affect the physicochemical composition of the flour and the dough although it did not have a significant effect on the texture of the bread compared to the control 100%wheat.The varied differences in textural scores could be as a result of different territorial and regional taste preferences.

8.2.Color

Apparently,at equivalent degrees of substitution,bread containing the white sorghum variety scored higher as far as consumer acceptance is concerned than those containing the brown-colored variety.The darkbrown color granted by the brown-colored sorghum variety contributed essentially to the lower acceptance for the products since it wasn't the same as the usual light brown appetizing bread[47].The crumb color of the bread subbed with 5%and 20%sorghum flour was fundamentally equivalent to the control;though at more elevated levels of substitution, samples were essentially darker (P ＜0.05) [46].Yousif et al.[26] reported L*C*h color values which are similar to L*a*b*,however,the method defined color differently employing cylindrical coordinates as opposed to rectangular coordinates.Where,L* indicates lightness, C* represents chroma,and h is the hue angle.According to the report of Ranasalva et al.[48],the replacement up to 25% of millet flour to the bread formulation culminated in such a dark inner and outer bread color.Contrasted with the control (100% wheat flour),flatbread with 50%white sorghum flour exhibited a lower C* value [26].Be that as it may, the L* value of 100%wheat flour flatbread was like that of the white sorghum flour flatbread.In the same study, inverse patterns were found in the correlation of the color values for the control(100%wheat flour)flatbread and the red flour sorghum containing flatbreads;the red flour sorghum flatbread had comparable C*values however much lower L*values than the control(100%wheat flour)flatbread.This demonstrated that the red flour sorghum flatbreads were darker than the control,conceivably an immediate result of the presence of more elevated levels of colored polyphenolics,for example,anthocyanins in the red sorghum flour[26].The anthocyanins are the significant class of flavonoids concentrated in sorghum and could be potential food colorants.In general,this group of compounds contributes to the blues,purples,and reds in plants[49].Pigmentation of sorghum might be attractive in certain food products,for example,customary African opaque beer,yet can be unwanted in flour and porridges[50].This could imply that decision of sorghum hybrid to be utilized to make bread affects the color of the final product desired.

8.3.Flavor and taste

Wheat flour tortillas were produced from composite flours containing up to 30% decorticated sorghum flour.The less desirable aroma and decreased overall preference were recorded for tortillas containing 25%or 30%sorghum flour.Be that as it may,the sensory properties of tortillas containing under 20%sorghum proportion were not significant (P ＞0.05) from those of the control tortilla [31].These outcomes were in concurrence with the findings of Khating et al.[9] who likewise reported a taste decline with the expanded degree of sorghum flour substitution above 20%.Taste scores decreased significantly as the level of sorghum flour increased characterized by a slightly bitter taste at a 10%or a greater added level of whole sorghum flour may be due to the phenolic compounds and tannins found in the seed-coat[39].This suggested that tannin free sorghum hybrids could be ideal for bread making.However, Sibanda et al.[13] detailed that there was no huge distinction(P ＞0.05)with sorghum substitution up to 30%on aroma,flavor,and taste,this could be because of various territorial inclinations with regards to the flavor and taste of food.Moreover,Carson et al.[51]noted that 50%composite dough produced bread that was characterized by an acceptable taste with an average 9-point hedonic score of 6.9.However,in the same study,they also reported that sorghum composite bread had high astringency and sourness scores compared to rye bread.

9.Conclusion

Sorghum-wheat flour can be used to make loaves of bread that are commercially accepted and comparable to that of wheat.It is clear from the literature reviewed that an increased sorghum addition to wheat flour has a negative impact on the rheological qualities of the bread.There is scope for increased utilization of sorghum either through the incorporation of additives such as hydrocolloids or through the addition of sorghum in small proportions up to 20%-30%to wheat to compensate for the lack of gluten proteins in sorghum to make sensory desirable products.A comprehensive approach to sorghum-wheat bread which entails a thorough control of all key physiochemical properties from the milling process up to the final product is ideal for producing bread that has superior rheological properties.This information is good for the food manufacturers to view sorghum as a viable commercial product that can be coopted to increase their profit margins if adherence to quality parameters is upheld.In addition,work needs to be performed on the optimization of the physio-chemical properties of sorghum flour for an improved percentage of replacement in bread making.In addition, adding essential of vital gluten powder to sorghum bread to boost the viscoelasticity of sorghum dough and understanding the effect of high fat content of sorghum on storage-based bread quality characteristics over a period of time could be ideal for further study.

Conflicts of interest

The authors declare that there are no conflicts of interest.

Acknowledgments

This study was supported by the Doctoral Scientific Research Start-up Foundation from Henan University of Technology(2019BS022).