高粱－小麦粉复合面包面团发酵流变学与烘焙特性

Kamal M.ADAM 王 凤 贾春利

黄卫宁1

（1.江南大学食品科学与技术国家重点实验室，江苏 无锡 214122；2.俄克拉荷马州立大学食品与农产品研究中心，美国 斯特尔沃特 74078－6055）

Sorghum （Sorghum bicolor L.Moench）is one of the world’s most important cereal crops.The top five sorghum producers in 2008were the United States，Nigeria，India，Sudan and Australia，with the United States and Australia being the top two exporting countries in 2009（FAO，1995）.In much of Africa and Asia sorghum is an important human food，whereas in countries such as the Unites States and Australia it is used primarily as animal feed （Yousif，Nhepera，＆Johnson，2012）.Grain sorghum ranks third among cereals for human consumption，superseded only by rice and wheat.It is the most important cereal crop in Sudan（Elkhalifa ＆ El－Tinay，2002），where it is used mainly to make traditional food such as Kisra （fermented local thin bread），aceda （fermented thick porridge）and nasha （fermented thin porridge）.Many researches have indicated the possibility of incorporating sorghum in wheat flour at different levels to produce composite bread when wheat flour is not available or it is in short supply （Abdelghafor，Mustafa，Ibrahim，＆ Krishnan，2002）.Bread has always been one of the most popular and appealing food products due to its superior nutritional，sensorial and textural characteristics（Giannou ＆ Tzia，2007）.The most essential ingredients for bread are flour，water，salt，and yeast（Goesaert，Brijs，Veraverbeke，Courtin，Gebruers ＆ Delcour，2005）.The consumption of wheat bread is increasing in Sudan，in both rural and urban areas.However，the only way to produce bread so far is to use imported high gluten wheat which is not suitable to cultivate in the tropical areas like Sudan for climatic reasons（Edema，Sanni，Sanni，2005）.The main problem facing the bakery industry in Sudan is the non－availability of wheat flour，so any effort made to substitute part of the wheat flour by other kinds of available flours，e.g.sorghum，will contribute to lowering of cost.For several decades many works have been done in this area but again nowadays interest is increasing in using sorghum in composite bread particularly in the semi－arid tropic area，due to sorghum’s resilience against the high temperature and drought conditions that may arise due to climate change.However，when sorghum flour is included in composite flour it gives a drier，grittier and a faster firming crumb（Hugo，Rooney，Taylor，2000；Hugo，Rooney，Taylor，2003）.It has been reported that the increased proportion of shortening in the formula improved the quality of bread.Shortening plasticized the dough，making it softer with an enhanced gas retention，and thus，resulting in larger final loaf volume（Inoue，Sapirstein，Bushuk，1995）.It has also been reported that water binding capacity in the batter could possibly be enhanced by a small flour particle size，which may help easy swelling of all components（Schober，Messerschmidt，Bean，Park，＆ Arendt，2005）.Therefore，in this work，two treatments had been done to avoid the adverse effects that attributed to sorghum composite flour.Firstly，the sorghum flour（to produce small particle size）was softened by milling the sorghum grain and screening the flour through one hundred mesh screen，and secondly，the amount of the shortening in the bread formula was increased.The main objective of this work was to optimize the sorghum substitution on the composite pan bread.The effects of fine sorghum flour on composite dough characteristics and pan bread quality were also investigated.

1 Materials and methods

1.1 Materials and equipments

PENGTAI high－gluten wheat flour with moisture，ash，and protein contents of 12.8%，0.58%，and 13.5%determined by AACC International Approved Methods 44－15.02A，08－01.01，and 46－13.01（AACC International，2010），respectively：East Ocean Oils and Grains Industries，Zhangjiagang，China；MEISAN instant dry yeast：Panyu Meishan Mauri Yeast，Guangzhou，China.Shortening，sugar，salt，and grain sorghum were purchased from local markets（Wuxi，China）.

Mixolab，Rheofermentometer F3：Chopin，Villeneuvela－Garenne，France；Mixer（SM－50）、Proof box（SM－32S）、Electric knife （SM－302N）、Deck oven （SM－603T）：Sinmag Machinery Co.，Ltd.，Wuxi，China；Texture analyzer：TA.XT2i，Stable Microsystems，Godalming，U.K.；Hunter lab scan XE colorimeter：lab scan XE，hunter associates laboratory，Reston，USA；Digital scale：FA2004，Hengping Instrument Factory，Shanghai，China.

1.2 Flour preparation

Grain sorghum was milled and screened in our laboratory with one hundred mesh screen.The approximate analysis of its moisture，ash，and protein content were 11.5%，0.85%，and 8.07%respectively，which was determined by AACC International Approved Methods（AACC International，2010）.Composite flour with different substitutions rates 0／100as Control，10／90，20／80，30／70，40／60，50／50 m／m of sorghum to wheat flour respectively were prepared and used for the study measurements.

1.3 Mixing properties of composite dough

The Mixolab was used to study the dough mixing properties of composite flour and control dough（wheat flour）.A certain amount of flour with known moisture content was placed into the Mixolab analyzer bowl and mixed to obtain a dough of 75g.The water required for the dough to produce a torque of 1.1N·m （C1）was added automatically by the Mixolab system.Parameters that were obtained from the Mixolab were water absorption，dough development time，stability amplitude，and C2values which is corresponding to lower protein quality at increased temperature（Anonymous，2005）.In this study，C1torque value was adjusted to 1.1N·m to establish a comparison between the other Mixolab characteristics（Rosell，Collar，＆ Haros，2007；Huang，Li，Wang，Wan，Tilley，Ren，＆ Wu，2010）.

1.4 Fermentation properties of composite dough

A Rheofermentometer F3was used to measure the Gas Production and Dough Development Parameters using the method described by Czuchajowska and Pomeranz（Czuchajowska ＆ Pomeranz，1993）.A dough piece（200g）was placed in a movable basket of the gas meter with a 1 500g cylindrical weight，and the cover of the vat was fitted with an optical sensor.The test was conducted at 30 ℃for 3h.The analyses were done in triplicate.

1.5 Bread－making procedure

A no－time dough procedure was carried out for preparing the bread samples.A basic bread formula，based on flour weight，was used：300g composite flour，1.5%salt，1.5%yeast，3%sugar，5%shortening and variable tap water（based on Mixolab water absorption results）.The dough was optimally mixed in a vertical mixer，rested for 10min，divided into 80g per piece，hand－molded and sheeted，put into tin pans，proofed at 38 ℃ and 85relative humidity in proof box for 90min，then baked in a deck oven set at 180℃top and 210℃ bottom for 25min.The bread quality attributes were evaluated after cooling for one hour at room temperature.

1.6 Baking characteristics of breads

Breads were evaluated for their baking characteristics.The parameters evaluated were：weight，loaf volume，and specific volume.The specific volume of bread was calculated according to the AACC method 10－05.01（AACC International，2010）.Loaf weight was measured by a digital scale and loaf volume was measured by rapeseed displacement（Loaves were placed in a cylinder of known volume into which rapeseeds were run until the cylinder was full.The volume of seeds displaced by the loaf was considered as the loaf volume）.Loaf Specific Volume（LSV），was calculated according to the following：

LSV——Loaf specific volume，mL／g；

V——Loaf volume，mL；

W——Loaf weight，g.

Breads were evaluated for their baking characteristics one hour later after removal from the oven.

1.7 Texture profile analysis（TPA）of composite pan bread

The textural characteristics of the bread were measured with a TA.XT2itexture analyzer equipped with an aluminum 25mm diameter cylindrical probe.Electric knife was used to obtain uniform bread slices of 2cm thickness.Bread slices were taken from the center of each loaf to evaluate the crumb texture.A stack of two slices was prepared and was compressed to 50%of its original thickness.The test conditions were pretest speed，1mm／s；test speed，3mm／s；posttest speed，3mm／s；and trigger force（auto mode），5g.The results recorded for the following parameters，Hardness，Cohesiveness，Springiness，Resilience，Chewiness，and Gumminess.

1.8 Crust and crumb color of composite pan bread

Crust and crumb colors were determined using a Hunter lab scan XE colorimeter.Crust color was measured at the surface of bread and crumb color was measured at the center part of crumb after the bread was cut into slices.Averages of three measurements of L，a，and b values were recorded.

1.9 Sensory evaluations

The sensory analysis was conducted with a group of 20panelists，10males and 10females.Panelists were trained students from School of Food Science and Technology，Jiangnan University.Their ages were from 23to 40 years old.Panelists assessed six breads samples （control 100%Wheat flour）and five different substitutions（10／90，20／80，30／70，40／60，50／50 m／m，sorghum to wheat flour）.An approximately 2.5cm2sample of each bread，labeled with a random three digit code was given to panelists（The order of the samples was randomized.To determine if the observed differences were statistically significant）.Bread was evaluated for surface appearance，texture，mouth feel，aroma and overall acceptability.Panelists used hedonic scale of 9points to evaluate the bread，the 9points were in these order 1－extremely dislike，2－dislike very much，3－dislike moderately，4－dislike slightly，5－neither like nor dislike，6－like slightly，7－like moderately，8－like very much，and 9－extremely like.During sensory evaluation，panelists were provided by water to rinse their mouths after each evaluation.Breads samples were prepared for Sensory evaluation on the same day that sensory evaluation was conducted.Breads were considered acceptable if their mean value for overall acceptability was equal or above 5（neither like nor dislike）.

1.10 Statistical analysis

Analysis of variance （ANOVA）was conducted using the SPSS 16.0General Linear Model procedure（SPSS inc.，USA）.The calculated mean values were compared using Duncan s multiple range tests with significance level of P ≤0.05to analyze the data.

2 Results and discussion

2.1 Mixing properties of composite dough

Adding sorghum substitutions at any level significantly affected the mixolab parameters（P≤ 0.05）.Water absorption was highest for the control（100%Wheat flour）formu－lation（Table 1）.Replacement of wheat flour by sorghum reduced the water absorption value；this result was in good agreement with that reported before（Elkhalifa，et al，2002；Salim－ur－Rehman， Bhatti， Shafique， Mueen－ud－Din， ＆Murtaza，2006）.This result was expected since water absorption is related to the levels of the hydrophilic components，principally proteins and carbohydrates within the formulation.Wheat flour proteins are known for their high water binding properties （Damodaran，2008），whereas sorghum does not contain gluten and the major endosperm proteins（kafirins）are hydrophobic（Belton，Delgadillo，Halford，＆ Shewry，2006）.Dough development times of the formulations containing sorghum flour were highest at minimum sorghum incorporation rate（Table 1）.The development time，was in general reduced when gluten levels were decreased due to the disruption of the continuous gluten network（Sroan，Bean，＆ MacRitchie，2009）；in general the findings of the present study followed this type of trend.This finding was supported by the work of Elkhalifa and El－Tinay who also observed a reduction in dough development time with increased sorghum addition （Elkhalifa，et al，2002）.Increasing the replacement rate of sorghum flour resulted in a decrease in dough stability as previously reported by other researchers（Elkhalifa，et al，2002）.The reduction in dough stability on replacement of wheat flour with sorghum flour observed in the present study was related to the lower levels of gluten in the sorghum formulations（Dendy ＆Dobraszczyk，2001）.C2is attributed to a weakening of protein，the only significant different on C2was observed with the maximum addition of sorghum （50%）which decreased C2from 0.33to 0.27N·m.Amplitude tended to increase gradually with the increasing of sorghum additions.Contrary to the dough development time，stability and C2decreased with increasing the replacement rate of sorghum flour.

Table 1 Mixing dough properties

Table 1 Mixing dough properties

Control：wheat flour，DDT：Dough development time.Sor－：sorghum flour.Means（n ＝ 3）±standard deviation.The results in the same column that have the same letters are not significantly different（P ≤ 0.05）.

Samples Water absorption／% DDT／min Stability／min C2／（N·m）Amplitude／（N·m）Control 62.1±0.5a 1.78±0.11a 9.50±0.34a 0.33±0.03a 0.083±0.005c SOR－10% 61.2±0.6b 1.28±0.24b 8.56±0.39b 0.33±0.01a 0.103±0.015bc SOR－20% 60.8±0.7b 0.89±0.21c 2.58±0.34c 0.32±0.01a 0.106±0.005b SOR－30% 60.7±0.4b 0.65±0.10cd 2.54±0.28c 0.32±0.01a 0.110±0.017bc SOR－40% 60.6±0.3b 0.67±0.06cd 1.58±0.55d 0.30±0.02ab 0.133±0.005a SOR－50% 60.5±0.3b 0.60±0.020d 1.09±0.20d 0.27±0.02b 0.146±0.015a

2.2 Fermentation Properties of composite dough

Fermentation properties of dough investigated by F3 Rheofermentometer were summarized in Table 2.As the sorghum flour percentage increased，the maximum dough fermentation height（Hm）of dough reduced significantly（P≤0.05）.Generally the decrease of dough height could be due to three effects：decreased gas production，excessive tenacity that prevented extension，or lower gluten matrix development that minimized the retention of the CO2formed during fermentation （Gómez，Jiménez，Ruiz，＆ Oliete，2011）.From the results，the decrease of the maximum height was attributed mainly to excessive tenacity that prevented extension and partially due to lower gluten matrix development because the gas production has increased with increasing sorghum flour rate.Time at maximum dough fermentation height（T1）of dough decreased significantly（P≤0.05）only for 40%sorghum substitution as decreased from 2.69to 1.24h.There was no significant difference on H′m（The height of maximum gas formation）between the composite flour and the control.R1（Total gas volume）and R2（Gas retention volume）increased with increasing sorghum flour substitution level.For R3coefficient（Gas retention ratio）there was no significant difference between sorghum substitution levels compared to the control，indicating that the dough prevented gas release efficiently though some other researchers had stated a poor gas－retention of sorghum dough（Hart，Graham，Gee，＆ Morgan，1970）.Therefore，the main reason behind prevented gas release efficiently from the dough was probably due to the high amount of shortening since it has been reported that the increased proportion of shortening（5% ～6%flour basis）in the formula enhanced gas retention of dough，essentially plugging the holes in cell walls（Inoue，et al，1995；Mousia，Campbell，Pandiella，＆Webb，2007）.

Table 2 Fermentation Properties of composite dough

Table 2 Fermentation Properties of composite dough

Control：wheat flour.R1：Total Gas Volume；R2：Gas Retention Volume；R3：Gas Retention Ratio.Sor－：sorghum flour.Means（n ＝ 3）±standard deviation.The results in the same column that have the same letters are not significantly different（P ≤0.05）.

Samples Hm／mm T1／h H′m／mm R1／mL R2／mL R3／%Control 31.70±3.39a 2.69±0.27a 76.9±2.47a 1 489±14.79d 1 460±15.13c 98.1±0.85a Sor－10% 21.56±4.04b 2.22±0.23a 79.9±2.83a 1 561±14.79cd 1 517±24.63bc 97.2±0.79a Sor－20% 16.43±4.58b 2.20±0.09a 77.1±6.42a 1 571±62.88bcd 1 532±56.29bc 97.5±0.45a Sor－30% 8.60±3.50c 2.08±0.95ab 76.8±2.05a 1 614±33.38abc 1 570±35.30ab 97.3±0.60a Sor－40% 7.63±1.60c 1.24±0.05b 80.5±4.11a 1 657±69.61ab 1 595±102.2ab 97.4±0.15a Sor－50% 6.56±1.95c 2.14±0.7ab 74.3±4.34a 1 692±55.71a 1 648±59.35a 96.2±2.16a

2.3 Baking characteristics of breads

The baking characteristics of breads were listed in Table 3.There was an increase in the weight of the bread with increasing percentages of sorghum flour in the blends，the volume of the bread decreased as the level of sorghum increasing.The specific volume of bread was decreased with increasing sorghum substitution levels，due to increased weight as well as decreased volume.These finding is in agreement with those of other research work （Elkhalifa，et al，2002；Aluko ＆ Olugbemi，1989）.The maximum value of bread specific volume was 6.9 （mL／g）which was obtained from the control.While bread made of 50%sorghum flour substitution resulted in the lowest bread specific volume of 3.17（mL／g）.

This result can be attributed to lower levels of gluten network in the dough of 50%sorghum substitution and consequently less ability of the dough to rise；due to the weaker cell－wall structure.

Table 3 Baking characteristics of breads

Table 3 Baking characteristics of breads

Control：wheat flour.Sor－：sorghum flour.Means（n ＝3）±standard deviation.The results in the same row that have the same letters are not significantly different（P ≤ 0.05）.

Parameters Control Sor－10% Sor－20% Sor－30% Sor－40% Sor－50%Weight／g 66.3±1.02c 66.8±0.57bc 67.4±0.49bc 67.6±0.47b 67.5±0.46bc 68.9±0.57a Volume／mL 457.7±2.51a 426.0±5.29b 375.0±5.00c 307.3±4.04d 263.0±12.12e 218.3±5.77f Specific volume／（mL·g－1）6.90±0.13a 6.37±0.03b 5.56±0.09c 4.55±0.08d 3.90±0.18e 3.17±0.10f

2.4 Texture characteristics of composite pan bread

Texture characteristics of composite pan bread were determined using a Texture Analyzer.The parameters measured were hardness，cohesiveness，springiness，gumminess，chewiness，and resilience，as shown in Table 4.It was reported that since wheat flours contain gluten protein which by suitable development gives the bread it’s unique and much desired texture；the inclusion of sorghum flour dilutes wheat gluten，and resulted in weak strength（Taha，2000）.The results showed that，when increasing the rates of sorghum flour in composite bread，the hardness of bread crumb increased gradually from 163.98 （control）to 933.37 （50%sorghum substitution）.It was observed from the results that there was a correlation between hardness and specific volume of bread.As sorghum flour rate increased，the specific volume of breads decreased while the hardness increased，This observation was in agreement with those researchers who was reported that hardness of bread is partially related to the specific volume of bread，the larger the specific volume，the softer the bread（Basman，Kksel，＆ Ng，2002）.Crumb elasticity was described by springiness and resilience（Onyango，Mutungi，Unbehend，＆Lindhauer，2011）.In this work for breads with sorghum flour substitutions up to 20%，no significant difference was observed in the whole TPA parameters except springiness（the percentage recovery of bread）and resilience（the ability of material to return to its original shape after been exhibited to stress）.Chewiness is the most indicative characteristic of bread.In this study，changes in chewiness appeared to be influenced strongly by hardness and cohesiveness rather than springiness.Chewiness increased with the increasing of sorghum substitution level from 123.35（control）to 532.32（50%sorghum substitution）.

Table 4 Texture characteristics of composite pan breads†

2.5 Crumb and crust color of composite pan bread

The color of bread is related to physic－chemical characteristics of the raw dough and chemical reactions that take place during baking which are dependent on operating conditions，such as Millard reactions and caramelization which cause browning of baked products during baking （Tong，Zhang，Wu，Tong，Zhang，＆ Zhang，2010）.The crumb and crust color of the bread samples of different sorghum substitutions were showed in table 5.The results showed that，as the sorghum flour percentage increased，L－values changed significantly from white to gray，a－values changed from green to red，and b－values changed from blue to yellow.Overall，there was no significant difference in L－value between the control and the 10%substitution again there was also no significant difference in L－value between the substitutions 20%and 30%.L－values of the bread crumb decreased from 82.59（control）to 70.41for the 50%sorghum substi－tution，indicating a significant increase in grayish color.The maximuma－value was observed for bread made of 50%sorghum substitution（0.416），the only significant difference in a－values among all the substitution samples was for 50%sorghum substitution；whereas，the lowest a－value（－0.096）was observed for the control made of 100%wheat flour as indicated by a higher intensities of green color.In terms of b－value，the lowest value was associated with bread made from the 30%sorghum substitution（14.2），and bread with 50%sorghum flour substitution had the highest b－value（16.98）.Contrary to the crumb，the crust color resulted in L－values shifting significantly from gray to white as the percentage of sorghum flour replacement increased，a－values shifted from red to green and b－values from yellow to blue.This result indicated that，the breads had obtained undesirable whitish crust color instead of desirable golden brown crust color.

Table 5 Crumb and crust color of composite pan breads†

2.6 Sensory evaluation

The sensory evaluation of composite pan breads made of composite sorghum－wheat flour as well as the 100%wheat bread as control were presented in Table 6.All sensory evaluation properties of surface appearance，texture，mouth feel，aroma and overall acceptability decreased as the substitution of the sorghum level increased except the texture of breads made from 10%sorghum flour substitution，which scored higher than the control.No significant differences were detected in any aspect between breads made of 10%or 20%sorghum flour substitutions compared to control.All tested bread samples were rated as acceptable by the panelists except surface appearance，texture and overall acceptability of the bread that made from 50%sorghum flour substitution.Acceptability studies conducted at the Food Research Centre in Khartoum，Sudan，showed that breads made with composite flour of 30%sorghum and 70%wheat were acceptable（FAO，1995）.However，in this study，bread with up to 40%sorghum substitution was found to be acceptable.This result probably attributed to the addition of 5%shortening to the soft sorghum flour.

Table 6 Sensory evaluation of composite pan breads†

3 Conclusions

The general conclusion which can be derived from the above measurements is that replacement of wheat flour by sorghum reduced the water absorption value，dough development time，and stability particularly when the replacement rate increased，due to the lower levels of gluten.Even though，the only significant difference on C2（weakening of protein）was observed with the maximum replacement of sorghum （50%）.Gas retention ratio presented very high coefficient results probably due to high shortening portion that was added to the formula.Breads crumb and crust color showed no significant difference between bread made of 10%sorghum substitution and the control except on b－value.For breads with sorghum substitutions up to 20%，no significant difference was observed in almost all the TPA parameters except springiness and resilience.In terms of sensory evaluation，no significant differences were detected in any aspect in breads with sorghum flour substitutions of 10%and 20%compared to the control.Our treatments significantly improved the quality of the bread and its acceptability.Our study also established that up to 40%sorghum substitution was acceptable and produced good quality bread compared to the 30%earlier reported.Also further studies on the positive effects of novel ingredients on composite bread quality based on the treatments that have done in this work was recommend.

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