Rheological properties of wheat-maize dough and their relationship with the quality of bread treated with ascorbic acid and Malzperle Classic ® bread improver

The aim of this work was to investigate the effect of ascorbic acid and a commercial bread improver on the physical quality of wheat-maize bread, and establish correlations between the physical properties of the bread and rheological properties of the dough. Wheat flour was substituted with 10, 20 or 30% maize flour and the farinograph and extensograph properties of the dough were evaluated. Farinograph water absorption, dough development time, dough stability and farinograph quality number decreased whereas the degree of softening increased with increasing substitution of wheat flour with maize flour. Extensograph dough energy, resistance to extension, extensibility and maximum resistance decreased with increasing substitution of wheat flour with maize flour. Ascorbic acid and commercial bread improver improved bread specific volume and form ratio; decreased crumb firmness, resilience and chewiness; and increased crumb springiness and cohesiveness. Farinograph water absorption and degree of softening; and extensograph energy, extensibility, maximum resistance and ratio number showed the highest number of significant correlations (P ≤ 0.01 or P ≤ 0.05) with the physical properties of wheat-maize bread.


INTRODUCTION
The unique dough-forming and breadmaking property of wheat is ascribed to gluten protein, which is formed when wheat flour is hydrated and subjected to mechanical shear.Substitution of wheat flour with non-wheat flour reduces the bread making potential of wheat flour due to dilution and disruption of the rheological and mechanical properties of gluten (Schoenlechner et al., 2013;Ribotta et al., 2005).Changes in the water absorption capacity and rheological properties of dough made from wheat and non-wheat flours cannot be generalized but seem to be influenced by the botanical origin, physicochemical nature and quantity of non-wheat flour (Shittu et al., *Corresponding author.E-mail: calonyango@yahoo.com. Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License 2009; Ribotta et al., 2005;Hung and Morita, 2005).Instrumentally measured bread characteristics show declining sensory properties such as decreased bread specific volume; increased crumb firmness and chewiness; and decreased crumb cohesiveness and springiness (Charoenthaikij et al., 2012;Mcwatters et al., 2004).
Wheat production in sub-Saharan Africa remains insignificant relative to demand because of the unsuitable production environment (UNECA, 1998).By contrast, maize grows well in diverse agro-ecological zones and is an important source of macro-and micronutrients to millions of people in sub-Saharan Africa.Although maize is principally used to make stiff or thin porridge (Onyango, 2014), potential exists to further increase its utilisation in processed foods such as bread (UNECA, 1998).
Additives and processing aids (also known as bread improvers) are widely used in the breadmaking industry to improve dough handling properties and bread quality.Bread improvers can be added as separate materials during dough preparation although they are also available as ready-to-use mixtures.The premixes commonly consist of a carrier agent, such as enzyme-active soybean flour, to which optimized amounts of several materials such as malt flour, emulsifiers, enzymes, hydrocolloids, vital gluten, oxidizing and reducing agents, sugar and fat have been added (Sluimer, 2005).Amongst these additives, ascorbic acid is the most frequently used in the breadmaking industry because it hinders the detrimental effect of glutathione on gluten proteins (Joye et al., 2009) thereby contributing to improved dough strength, reduced dough stickiness, and improved bread crumb and crust characteristics (Aamodt et al., 2003).
Several bread improvers, such as vital gluten (Mohamed et al., 2010), hydrocolloids (Shittu et al., 2009), emulsifiers (Schoenlechner et al., 2013;Alasino et al., 2011;Yamsaengsung et al., 2010) and enzymes (Schoenlechner et al., 2013) are also recommended for the production of bread from wheat and non-wheat flours.Nonetheless, identifying effective and inexpensive generally recognized as safe (GRAS) additives for the production of bread made from wheat and diverse nonwheat flours remains a challenge.For instance, Mohamed et al. (2010) reported that vital gluten improves the specific volume of wheat-banana bread but Conforti and Davis (2006) reported that it did not improve the specific volume of wheat-soya-flaxseed bread.Furthermore, although Mohamed et al. (2010) reported that 25% vital gluten could make wheat-banana bread with a specific volume that is comparable to wheat bread, practical application of this work may be limited because of the large amount of vital gluten that would be required.Health implications should also be considered in the choice of the bread improver used to make bread from wheat and non-wheat flours.For instance, Alasino et al. (2011) recommended sodium stearoyl lactylate and azodicarbonamide for the production of wheat-pea bread but azodicarbonamide is banned in several countries because it is converted to biurea which is partly transformed to semicarbazide -a compound that has mutagenic and carcinogenic effects (Joye et al., 2009).
The purpose of this work was to evaluate the effect of a commercial bread improver on the physical quality of wheat-maize bread.The commercial bread improver (Malzperle Classic®, IREKS GmbH, Kulmbach, Germany) contained sugars, malt flour, hydrocolloid, ascorbic acid, pH-regulator, enzymes and emulsifier.A further objective was to relate the physical quality parameters of the wheat-maize bread with the rheological properties of the dough.

Raw materials
Wheat flour (
The ingredients and additives were mixed for 1 min and then kneaded for 5 min in a Diosna spiral mixer (Diosna Dierks & Söhne GmbH, Osnabrück, Germany).The dough was allowed to rest for 10 min in a proofing chamber (Manz Backtechnik GmbH, Creglingen, Germany) at 32°C and 80% RH.It was then divided into 750 g pieces and rounded before being allowed to rest for 15 min while covered with a porous plastic sheet.The dough was shaped (Frilado Bäckereimaschinen Fabrik, Dortmund, Germany) and placed in oiled baking tins (290 mm long x 130 mm wide x 100 mm deep).It was proofed for 60 min at 32°C and 80% relative humidity.The tins were loaded into preheated Matador multi-deck oven (Werner und Pfleiderer Lebensmitteltechnik GmbH, Dinkelsbühl, Germany) at 200°C for 40 min with steam injection for ca. 10 s immediately after loading.After baking, the loaves were depanned and left to cool for 2 h.Thereafter, they were packed in unperforated low density polythene bags (BÄKO Marken und Service eG, Bonn, Germany), closed with a twist tie and stored for 22 h at 25°C.The experiments were set-up as single-factor completely randomized designs and the baking tests were made in triplicate The breads were weighed on a 3-decimal digital weighing scale.Bread volume (cm 3 ) was determined by rapeseed displacement and specific volume was calculated from the bread weight and volume.The loaves were sliced into 10 mm thick slices using an electric bread slicer (Wabäma GmbH, Haan, Germany).A slice was taken from the center of the bread and the height (mm) and width (mm) used to calculate the form ratio (height/width) (Aamodt et al., 2003).Two slices were taken from the centre of the bread and 30 mm diameter rings punched out.Texture profile analysis of the bread crumbs (20 mm thick) was measured using a 50 mm diameter aluminum cylinder probe (P/50) attached to a TA-XT2i Texture Analyzer with a 5 kg load cell (Stable Micro Systems, Surrey, UK).The TPA settings were: height calibrated at 30 mm, pre-test speed 1 mms -1 , test speed 5 mms -1 , post-test speed 5 mms -1 , target mode distance, distance 10 mm (50% compression), trigger type auto force 0.05 N, data acquisition rate 200 pps.The waiting time between the first and second compression cycle was 5 s.The texture profile analysis properties (firmness, cohesiveness, springiness, resilience and chewiness) were calculated from the obtained graph using EXPONENT Texture Analysis software version 6.1.5.0 (Stable Micro Systems, Surrey, UK).The measurements were made in triplicate and the results reported as mean ± standard deviation.

Statistical analysis
The data was analyzed using one-way analysis of variance and differences in treatment means evaluated using Tukey's test at 5% with SPSS software v.13.0 (SPSS, Chicago, USA).Pearson correlation coefficients (r) between wheat-maize dough rheological properties and physical properties of the bread were evaluated using SPSS software v.13.0 (SPSS, Chicago, USA).

Rheological properties of wheat-maize dough
The water absorption of dough prepared from 100% wheat flour was not significantly different (P > 0.05) from that of dough prepared from 90% wheat flour and 10% maize flour (Table 1).The water absorption of doughs made from wheat-maize flours tended to decrease with increasing substitution of wheat flour with maize flour.The chemical composition of the non-wheat flour has a strong influence on the water absorption of capacity of the composite dough.Protein-rich flours (Mashayekh et al., 2008;Ribotta et al., 2005) and fibre-rich flours (Mohamed et al., 2010;Mariotti et al., 2006;Koca and Anil, 2007) increase water absorption of dough made from composite flours whereas non-wheat flours with low protein or fibre contents decrease water absorption (Hung and Morita, 2005;Miyazaki and Morita, 2005).Dough development time, dough stability and farinograph quality number decreased by about 40, 60 and 50%, respectively, when wheat flour was substituted with 10% maize flour but did not change significantly (P > 0.05) on further substitution of wheat flour with maize flour (Table 1).The degree of softening tended to increase with increasing substitution of wheat flour with maize flour.These changes in dough rheological properties on increasing substitution of wheat flour with maize flour indicate that the baking quality of the dough was declining due to dilution and disruption of the gluten macromolecular network (Ribotta et al., 2005).Low quality breadmaking flour has a short dough development time and rapidly becomes unstable on prolonged mixing whereas high quality breadmaking flour requires more time to attain maximum consistency and is more stable on prolonged mixing (Sluimer, 2005;Mailhot and Patton, 1988).The decline in dough rheological properties has also been reported when wheat flour was partially substituted with other non-wheat flours such as maize (Miyazaki and Morita, 2005), barley (Finocchiaro et al., 2012), soybean (Mashayekh et al., 2008;Ribotta et al., 2005) and flaxseed (Koca and Anil, 2007).
The dough energy, resistance to extension, extensibility and maximum resistance tended to decrease with increasing substitution of wheat flour with maize flour at   all measurement times (Table 2).The ratio number and maximum ratio number did not show any major changes with increasing wheat flour substitution at all measurement times.Dough rigidity tended to increase with increasing incubation time as was evidenced by the increasing dough energy, resistance to extension, maximum resistance, ratio number and ratio number maximum; and decreasing dough extensibility (Table 2).
The extensograph properties of dough with good breadmaking quality include high resistance to extension, high energy and long extensibility (Sluimer, 2005).The extensograph properties of dough made from wheat and non-wheat flours is dependent on the botanical origin of the non-wheat flour and the modification it has been subjected to.It is for this reason that these results may not fully agree, for instance, with those of Ribotta et al. (2005) or Koca and Anil (2007) who reported on the extensograph properties of wheat-soy and wheatflaxseed dough, respectively.

Physical quality of wheat-maize bread
Table 3 shows the specific volume, form ratio and texture profile analysis of wheat-maize breads prepared from untreated flour, flour treated with ascorbic acid, and flour treated with ascorbic acid and Malzperle Classic® bread improver.The specific volume of breads decreased with increasing substitution of wheat flour with maize flour, irrespective of the treatment.Non-wheat flour reduces the specific volume of bread due to dilution of gluten and disruption of its rheological and mechanical properties (Schoenlechner et al., 2013;Ribotta et al., 2005).
Although ascorbic acid improved the specific volume of wheat-maize breads, a greater effect was achieved when ascorbic acid was supplemented with Malzperle Classic® bread improver.The specific volume of wheat-maize breads treated with ascorbic acid and Malzperle Classic® bread improver were 31-47% higher than for those not containing any additive.By contrast, the specific volumes of wheat-maize breads treated with ascorbic acid only were 5-10% higher than for those not treated with any additive.Hydrocolloids and enzymes present in the Malzperle Classic® bread improver were most likely responsible for the higher specific volume of the wheatmaize breads.Malzperle Classic® bread improver contained guar gum, a hydrocolloid, which improves the specific volume of bread by increasing dough viscosity thereby conferring greater stability to gluten-starch network (Koca and Anil, 2007;Shittu et al., 2009).Polysaccharide-degrading enzymes, proteases and cross-linking enzymes have also been reported to improve the specific volume of bread (Caballero et al., 2007;Schoenlechner et al., 2013).
The form ratio (height to width ratio) is an important quality index that influences consumer acceptance of pan bread.The form ratio of wheat-maize breads tended to decrease with increasing substitution of wheat flour with maize flour, irrespective of the treatment (Table 3).Ascorbic acid did not increase the form ratio of the wheatmaize breads but supplementation of ascorbic acid with Malzperle Classic® bread improver increased the form ratio of wheat-maize breads by 17-33% as compared to those treated with ascorbic acid only or without any additive.The effect of ascorbic acid and Malzperle Classic® bread improver on the form ratio of wheatmaize breads was also evident in their general appearance (Figure 1).The improved form ratio and general  appearance of the wheat-maize breads can be attributed to the action of enzyme(s) in Malzperle Classic® bread improver.Different enzyme combinations, such as transglutaminase and xylanase, amylase and protease or glucose oxidase and protease increase the form ratio of breads (Caballero et al., 2007).
The texture profile analysis property of crumb firmness, which is equivalent to the force of mastication during eating (Guiné and Barroca, 2012), tended to increase with increasing wheat flour substitution, irrespective of the treatment (Table 3).The hydration of glutenin and gliadin fractions in wheat flour enables them to form gluten with three-dimensional viscoelastic network, which stretch and rise as it traps carbon dioxide produced by yeast.When the dough is baked, it yields light, evenly structured bread with a soft crumb.Substitution of wheat flour with non-wheat flour dilutes and disrupts formation of this viscoelastic network and hinders entrapment of carbon dioxide leading to increased crumb firmness.
Crumb firmness of the wheat-maize breads tended to decrease on addition of ascorbic acid and Malzperle Classic® bread improver (Table 3).Crumb firmness ranged between 1.27-7.62N in wheat-maize breads not treated with any additive and declined to 1.29-5.24N in wheat-maize breads treated with ascorbic acid only and further to 0.91-2.20 N in wheat-maize breads treated with ascorbic acid and Malzperle Classic® bread improver.The decrease in crumb firmness can be attributed to the presence of enzymes, emulsifiers and hydrocolloids in Malzperle Classic® bread improver.This finding is consistent with previous results that show the crumbsoftening effects of these additives in breads made from wheat and non-wheat flours (Schoenlechner et al., 2013;Yamsaensung et al., 2010).Emulsifiers enhance crumb softness by forming complexes with amylose and amylopectin, and by reducing starch swelling and solubilisation during gelatinization (Goesaert et al., 2005).Enzymes that are capable of decreasing crumb firmness include amylase-protease (Caballero et al., 2007) and xylanase-transglutaminase mixtures (Schoenlechner et al., 2013).Hydrocolloids are also capable of decreasing crumb firmness by inhibiting starch-gluten interactions or the development of macromolecular entanglements (Shittu et al., 2009;Davidou et al., 1996).
Crumb springiness, resilience and cohesiveness tended to decrease with increasing wheat flour substitution, irrespective of the treatment (Table 3).Crumb springiness and resilience measure recovery of food structure after compression during mastication (Guiné and Barroca, 2012) whereas crumb cohesiveness measures internal cohesion of the material (Bourne, 2002).Thus, decrease in crumb springiness and resilience characterizes loss of crumb elasticity whereas decrease in crumb cohesiveness reflects increased susceptibility of the crumb to crumble.The average increase of crumb springiness of wheat-maize breads was 22% when it was treated with ascorbic acid and 36% when it was treated with ascorbic acid and Malzperle Classic® bread improver.By contrast, crumb resilience decreased marginally by about 2% when wheat-maize bread was treated with ascorbic acid and 8% when it was treated with ascorbic acid and Malzperle Classic® bread improver.Crumb cohesiveness of wheat-maize bread not treated with any additive ranged between 0.638-0.836and increased marginally to 0.667-0.837on addition of ascorbic acid, and further to 0.683-0.834on addition of ascorbic acid and Malzperle Classic® bread improver.The combined interpretation of these three texture profile analysis terms imply that substitution of wheat flour with maize flour decreased the overall bread crumb quality but this was reversed by the addition of ascorbic acid and Malzperle Classic® bread improver.
Crumb chewiness, which is a product of crumb firmness, springiness and cohesiveness (Bourne, 2002), was not significantly affected (P > 0.05) by increasing substitution of wheat flour with maize flour, irrespective of the treatment (Table 3).Wheat-maize breads treated with ascorbic acid and Malzperle Classic® bread improver were less chewy (1.50-2.22N) than those not treated with any additive (2.43-4.64N) or those treated with ascorbic acid only (2.71-3.52 N).

Correlation between wheat-maize dough and bread quality
The rheological quality of dough is an important predictor of its baking performance and the final quality of bread.Correlation coefficients are used to relate dough properties to baking performance but the results cannot be generalized because they are dependent on the type of rheological test, composition of material and sample size (Stojceska and Butler, 2012;Ktenioudaki et al., 2010).The Pearson correlation coefficients (r) between the farinograph and extensograph (at 90 min) properties of wheat-maize dough (70:30 ratio) versus the physical properties of bread are shown in Table 4.Among the extensograph data, we have displayed only the 90 min subset because this incubation time closely corresponded to the total dough resting and proofing time (85 min).Farinograph properties of water absorption and degree of softening showed the highest number of significant correlations (P ≤ 0.01 or P ≤ 0.05) with the physical properties of wheat-maize bread (Table 4).Water absorption was negatively correlated (P ≤ 0.01) with crumb firmness and positively correlated (P ≤ 0.01 or P ≤ 0.05) with most of the other physical properties of wheat-maize bread.The degree of softening was negatively correlated 0.01 or P ≤ 0.05) with almost all physical properties of wheat-maize bread.The dough development time, dough stability and farinograph quality number showed few significant correlations (P ≤ 0.01 or P ≤ 0.05) with the physical properties of wheat-maize bread.Extensograph properties of dough energy, extensibility, maximum resistance and ratio number showed the highest number of significant correlations (P ≤ 0.01 or P ≤ 0.05) with the physical properties of wheat-maize bread.Dough energy and extensibility were negatively correlated (P ≤ 0.01 or P ≤ 0.05) with crumb firm-ness and positively correlated (P ≤ 0.01 or P ≤ 0.05) with almost all the other physical properties of wheat-maize.Maximum resistance was positively correlated (P ≤ 0.01 or P ≤ 0.05) with most of the physical properties of wheat-maize bread while the ratio number was positively correlated (P ≤ 0.01 or P ≤ 0.05) with crumb firmness and negatively correlated (P ≤ 0.01 or P ≤ 0.05) with most of the other physical properties of wheat-maize bread.Resistance to extension and ratio number maximum showed no significant relationships (P > 0.05) with the physical properties of wheat-maize bread, except for the correlation (P ≤ 0.05) between crumb firmness of wheat-maze bread treated and ascorbic acid, and Malzperle Classic® bread improver and resistance to extension.

Conclusion
Substitution of wheat flour with maize flour decreased water absorption and weakened rheological properties of the dough by disrupting and diluting the gluten network.The changes in dough rheology were reflected in the physical properties of the wheat-maize breads, which declined with increasing substitution of wheat flour with maize flour.The physical properties of the wheat-maize breads were improved when ascorbic acid was supplemented with Malzperle Classic® bread improver.Wheatmaize breads treated with ascorbic acid and Malzperle Classic® bread improver were softer, springier, and more cohesive; but less chewy and resilient than breads not treated with any additive or treated with ascorbic acid only.
**Mean ± standard deviation.Means sharing the same letters in each column, for each treatment, are not significantly different from each other (Tukey's HSD test, P ≤ 0.05).

Figure 1 .
Figure 1.Cross-sections of wheat-maize breads.Breads in the first row do not contain any additives; breads in the second row contain ascorbic acid; breads in the third row contain ascorbic acid and Malzperle Classic® bread improver.Letters a, e, and i represent bread made from 100 parts wheat flour; b, f and j represent bread made from 90 parts wheat flour and 10 parts maize flour; c, g and k represent bread made from 80 parts wheat flour and 20 parts maize flour; d, h and l represent bread made from 70 parts wheat flour and 30 parts maize flour.

Table 3 .
Physical properties of wheat-maize bread.

Table 4 .
Pearson correlation coefficients between rheological § quality of wheat-maize § § dough and physical quality of bread.§The extensograph results are for wheat-maize dough incubated for 90 min.