Introduction

Wheat is unique among cereals in that its flour can form dough when mixed with water. In addition, wheat dough retains the gas produced during fermentation or by chemical leavening and thus gives a leavened product. These two characteristics of wheat flour dough are responsible for the popularity of wheat products. The most popular yeast leavened product by far is bread [1]. The majority of the bread produced around the world is white bread. However, over the last few decades, the popularity of non-white breads such as bran supplemented and whole wheat bread has been rapidly increasing [2]. Whole wheat flour consists of the original composition of bran, germ, and endosperm of wheat. Germ and bran fractions consist of healthy components and they are removed during the grain-refining process.

Whole wheat products are relatively rich in essential minerals, vitamins, dietary fiber and phytochemicals that have been linked to disease prevention. Increased consumption of whole wheat products has been associated with the reduced risk of major chronic diseases including cardiovascular disease, type II diabetes, obesity and cancers. Given the unique features including dough rheology and bread-baking, wheat with a global production of over 700 million tonnes is the most abundant source of protein (20%) and calories (30%) in the diet of human populations [3]. However, the use of whole wheat flour is limited because of the poor processing and end-product quality. The presence of fiber in the dough system had a positive effect on water absorption and gassing power while a negative influence on other dough and baking quality characteristics including farinograph stability and bread loaf volume [4]. Several enzymes are used in the baking industry in order to improve dough handling, machinability, stability of bread quality, loaf volume and shelf life of bread. Xylanases find widespread use in breadmaking applications [5]. Xylanase is a hemicellulase that can hydrolyse the complex polysaccharides [6]. Xylanase reduces the water absorption of whole wheat flour and increases loaf volume and crumb softness by hydrolyzing arabinoxylans [7]. The inclusion of xylanases reduces the farinograph absorption, and improves the gas retention capacity of the doughs and the volume of the breads, as well as reducing their hardness and increasing water retention during baking [8].

In the previous studies of the authors, total and water-extractable arabinoxylan, total and soluble dietary fiber, phytic acid, total phenolic, protein and mineral contents, mineral bioavailabilities, bile acid binding capacities, gluten and gliadin levels of modern and old bread wheat varieties were determined to investigate whether there have been substantial changes in these properties over the course of around one century (since 1930s) [9, 10]. Besides these nutritional and functional properties investigated, the effect of xylanase addition on water-extractable arabinoxylan contents of whole wheat breads was investigated [9]. Therefore, the main aim of the current study was to determine and compare the quality characteristics, dietary fiber contents and glycemic index of whole wheat breads produced by using modern and old bread wheat varieties. The second major aim was to investigate the effect of an enzyme (xylanase) on the quality, staling and dietary fiber content of whole wheat bread samples.

Materials and methods

Materials

Six modern bread wheat and nine old bread wheat varieties were used. They were provided by Field Crops Central Research Institute, Ankara, Turkey. The old bread wheat varieties were registered in the years from 1931 to 1970. The names of varieties Bayraktar 2000, Demir 2000, Ikizce 96, Kirac 66, Bolal 2973, Bezostaja 1, Yektay 406, Surak 1593/51, Ankara 093/44, Kose 220/39, Sivas 111/33 and Ak 702 were shortened as Bayraktar, Demir, Ikizce, Kirac, Bolal, Bezostaja, Yektay, Surak, Ankara, Kose, Sivas, Ak in the text for convenience.

Xylanase (fungal origin) was supplied by Arti Gida Inc. Co. Maximum recommended amounts of enzymes were used in the study (3.0 mg enzyme/100 g flour).

The wheat samples were milled according to AACCI Method 26-21 (AACC International, 2000) by using a Buhler MLU 202 pneumatic Laboratory Mill (Uzwil, Sweden). Bran samples were ground by using Perten 3100 Laboratory Mill equipped with 500 μm sieve. Bran (ground) and flour of each sample were mixed homogenously to get whole wheat flour. Whole wheat flours were allowed to rest for 2 weeks prior to use.

Methods

Quality evaluation of whole what flours

Moisture contents were determined using the AACCI Method 44-15A [11]. The Zeleny sedimentation values of the flour samples were obtained according to ICC (International Association for Cereal Science and Technology) Method 116/1 [12]. Modified sedimentation analysis is based on the remeasuring of the sedimentation value 2 h after the addition of bromophenol blue solution [13]. Farinograph properties of the flour samples were determined according to AACCI Method 54-21 [11] using a Farinograph (Brabender Farinograph-AT, Duisburg, Germany) equipped with a 50-g bowl. From the Farinograph curve, the development time (min.), water absorption (14% moisture basis), stability (min.), softening degree (BU, 12 min. after max.) and Farinograph quality number are obtained.

Whole wheat bread making and quality evaluation

Whole wheat breads were produced by using the modified AACCI Method 10-10B [11]. 100 g of whole wheat flour (14% mb), yeast solution (25 ml, 8.0%), salt solution (25 ml, 6.0%, non-iodized) and water (according to the Farinograph water absorption value) were used in the formula. Xylanase was dissolved in water and then the xylanase solution was added to the whole wheat flour during bread production. Doughs were mixed by using a National Mfg pin mixer (Lincoln, NE, USA). Doughs were left for fermentation and then punched, rounded and molded. After the second fermentation (30 min), the dough was sheeted, molded and panned. The final proof was 45 min. The whole wheat breads were baked in the rotary oven (Despatch, Minneapolis, MN, USA) at 230 °C for 25 min. The baking tests were performed in duplicate and mean values were reported. Preliminary studies on whole wheat breads indicated that longer proof times caused dough collapse and crust deformation, resulting in lower loaf volume. Hence, the final proof was decreased to 45 min to obtain whole wheat breads with better quality.

The bread volumes were determined by the rapeseed displacement method using a loaf volumeter (National Mfg, Lincoln, NE, USA) after cooling the loaves at room temperature for 2 h. In addition, the firmness of whole wheat breads at day 0 was determined according to AACCI Method 74-09 [11]. Other whole wheat breads (duplicate samples) were placed in plastic bags and stored for 1 and 3 days at room temperature for the determination of firmness. A texture analyzer (Stable Microsysytems, TA-XT plus, Godalming, Surrey, England) equipped with a 50 kg load cell and a 36 mm cylinder probe was used for texture analysis. The force (firmness, g) required to compress 40% of two slices (1.25 cm each) was determined at 1.7 mm/sec test speed. Color L*, a*, b* (D65, 10º) values of whole wheat flour samples were measured using a spectrophotometer (MiniScan XE PLUS Associates Laboratory Inc, Reston, VA, ABD) [14]. Sensory characteristics (symmetry, crust color, crumb cell structure, crumb color, softness) of whole wheat breads were evaluated by a panel of three experts experienced in bread quality evaluation. The ages of experts ranged from 27 to 50. Whole wheat breads were provided in duplicate under normal white light at room temperature. Two panelists were female and all were nonsmokers. The symmetry and crust color of the breads were scored using the 5-point numerical rating scale (1: poor and 5: very good) while crumb cell structure, crumb color and softness of the breads were scored using the 10-point numerical rating scale (1: poor and 10: very good). The final score was reported as the mean of scores given by all panelists.

Dietary fiber analysis

The total and soluble dietary fiber contents of whole wheat breads were analyzed according to the method of AOAC Method no: 991.43 [15].

Estimation of in vitro glycemic index value

Whole wheat bread samples were firstly digested according to the method of Englyst et al. [16]. Whole wheat bread samples (100 mg) were weighed into tubes with 10 glass beads (4 mm diameter) added to each tube. Two milliliters of 0.05 M hydrochloric acid (HCl) were added and the tubes were incubated at 37 °C in a shaking water bath for 30 min. Then, sodium acetate buffer (4 ml, 0.5 M) was added to each tube. Aliquots (100 μl) were taken at 0 and 90 min and mixed with 1 ml of absolute ethanol. One millilitre of freshly prepared enzyme solution was added to each tube and the tubes were incubated vertically at 37 °C in a shaking water bath. These solutions were centrifuged at 800×g for 10 min, and the glucose content of the supernatant was measured with glucose oxidase–peroxidase (GOPOD) reagent by using a spectrophotometer at 510 nm wavelength. The hydrolysis index (HI) was calculated by dividing the area under the curve of the sample to the area under the curve of the reference sample (white bread). The in vitro GI was calculated by using the equation of Goni et al. [17] as GI = 39.71 + 0.549 * HI.

Statistical analysis

All experiments were performed in duplicates and the mean values were recorded. Results were analyzed using a one-way analysis of variance (ANOVA) and statistical analysis was performed with the software JMP (Version 11.0.0, SAS Institute Inc., 2013). When significant (p < 0.05) differences were found, the least significant difference (LSD) and t-test were used to determine the differences among means.

Results and discussion

Quality evaluation of whole wheat flours

Zeleny sedimentation and modified Zeleny sedimentation values of whole wheat flours obtained from modern varieties and old varieties are presented in Table 1. There were significant differences between genotypes in terms of sedimentation and modified sedimentation values (p < 0.05). Sedimentation and modified sedimentation values of the whole wheat flour samples ranged from 10 to 20 ml and from 15 to 33 ml, respectively. Bezostaja and Sivas had the highest sedimentation (20 and 20 ml), while Bezostaja, Esperia and Demir had the highest modified sedimentation (33, 32 and 32 ml) values among all samples. Sedimentation values of whole wheat flours are generally lower than the white flours obtained from the respective wheat samples because the bran particles negatively affect the swelling of flour particles during the test which results in lower sedimentation values [18, 19]. In addition, the range of sedimentation values of the whole wheat flours was narrower as compared to that of modified sedimentation values. Therefore, the modified sedimentation value might be a better parameter for evaluating the gluten quality of whole wheat flours as compared to the regular sedimentation value.

Table 1 Zeleny and modified Zeleny sedimentation values
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Farinograph results (development time, water absorption, stability, softening degree and Farinograph quality number) of whole wheat flours obtained from modern varieties and old varieties are given in Table 2. There were significant differences between genotypes in terms of dough development time, water absorption, stability, softening degree and quality number values (p < 0.05). Farinograph development time, water absorption, stability, softening degree and quality number values of whole wheat flour samples ranged from 1.5 to 13.8 min, 57.3 to 67.0%, 1.8 to 19.7 min, 26 to 98 min and 27 to 268, respectively. Farinograph properties of the whole wheat flours obtained from the modern wheat varieties were better than those of the old varieties. Modern wheats generally had higher dough development time, water absorption, stability, quality number and lower softening degree values as compared to the old wheat varieties. Esperia had the highest Farinograph development time, stability, quality number and lowest softening degree values (13.8 min, 19.7 min, 268, and 26 BU, respectively) among all samples obtained from both modern and old wheat varieties. The differences between mean dough development time, stability, softening degree and quality number values of modern and old wheats were significant (p < 0.01).

Table 2 Farinograph results of whole wheat flours
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Farinograph water absorption is mainly influenced by the properties of the main flour components: gluten and starch. To be properly interpreted, it must be considered together with the other Farinograph parameters. Thus, high-water absorption, combined with a low degree of softening indicates good quality flour, whereas high-water absorption combined with a high degree of softening indicates poor quality flour. In practice, higher stability and a lower degree of softening indicate that dough will be more able to sustain long mechanical processing treatments [20]. While high water absorption is a desirable characteristic in good bread making flour, too long mixing time is not preferred by bakers due to loss of time and energy. On the other hand, the qualities of flours with very short mixing times are generally not good.

Quality evaluation of whole wheat breads and the effect of xylanase on bread quality

Volume values of control and enzyme (xylanase) added whole wheat bread samples are presented in Table 3. The loaf volumes of control and enzyme (xylanase) added whole wheat bread samples produced by using modern bread wheat varieties were in the range of 293–402 ml and 327–430 ml, respectively. The loaf volumes of control whole wheat bread samples produced by using old bread wheat varieties were in the range of 237–337 ml. There were significant differences between volume values of the control whole wheat breads in both modern and old bread wheat varieties (p < 0.05). There were also significant differences between volume values of enzyme added whole wheat breads of modern varieties (p < 0.05).

Table 3 Volumes of whole wheat bread samples
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Control and enzyme added whole wheat breads of modern variety Esperia (398 and 423 ml) and Tosunbey (402 and 430 ml) had the highest loaf volumes while control whole wheat bread of old variety Ak (237 ml) had the lowest volume value among all whole wheat breads. The differences between the mean volume values of whole wheat breads produced by using modern (356 ml) and old (262 ml) varieties were statistically significant (p < 0.01). The xylanase added whole wheat breads had higher volumes as compared to those of control breads. The differences between volumes of control and respective xylanase added whole wheat breads were statistically significant (p < 0.05) except whole wheat bread produced by using Demir variety.

In the study of Shah et al. [5], partially purified xylanase was used and the effects of xylanase addition on the fermentation and the final bread quality were analyzed. Similar to the present study, Shah et al. [5] stated that improvements in bread volumes were significant with the addition of xylanase. Tebben et al. [21] determined the effects of α-amylase, cellulase, glucose oxidase, maltogenic α-amylase and xylanase on whole wheat bread properties, particularly on loaf volume and they concluded that the greatest whole wheat bread loaf volume was observed for xylanase at the medium and high utilization levels. Indrani et al. [22] investigated the effect of enzymes (fungal-amylase, glucose oxidase, protease and xylanase) on bread quality and they stated that the bread quality was the best with xylanase, as it produced the highest improvement in specific loaf volume and overall quality score. Jiang et al. [23] also reported that the utilization of xylanase in the dough led to improvements in the breadmaking quality characteristics, especially specific volume and crumb structure.

Crumb and crust L*, a*, b* color values of control and enzyme (xylanase) added whole wheat bread samples are presented in Tables 4 and 5, respectively. Significant differences existed between the control whole wheat breads of some modern and old bread wheat varieties in terms of L*, a* and b* crumb color values (p < 0.05). However, the differences between the mean L*, a* and b* values of the whole wheat breads produced from modern and old varieties were not statistically significant. In whole wheat breads produced from modern varieties, xylanase addition did not have a significant effect on crumb L*, a*, b* color values.

Table 4 Crumb color values of whole wheat bread samples
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Table 5 Crust color values of whole wheat bread samples
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There were significant differences between the control whole wheat breads of some modern and old bread wheat varieties in terms of crust L*, a*, and b* color values (p < 0.05). The differences between the mean crust L* and a* values of the whole wheat breads produced from modern and old varieties were statistically significant (p < 0.01). Xylanase addition caused significant decreases in the crust L* and b* values of only one of the cultivars (Esperia) and none of the crust color values of the other cultivars were affected by the xylanase addition. Hence, the overall effect of xylanase addition on crust color was not significant. Whole wheat breads of old varieties (57.5 and 8.57) had higher L* and lower a* crust color values as compared to those of modern varieties (51.8 and 10.14).

Quality evaluation characteristics of the control and xylanase added whole wheat bread samples are presented in Table 6. Sensory characteristics such as symmetry, crust color, crumb cell structure, crumb color and softness were evaluated. The differences between the mean symmetry, crumb cell structure and softness values of whole wheat breads produced by using modern and old varieties were statistically significant (p < 0.05). Whole wheat breads of modern varieties had better mean symmetry, crumb cell structure and softness values as compared old varieties. In modern varieties, xylanase addition resulted in improved crumb cell structure and softness characteristics.

Table 6 Quality evaluation of whole wheat bread samples
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Staling of whole wheat breads and effect of xylanase on staling

Texture analysis was carried out on the control and xylanase added whole wheat bread samples of varieties. Firmness values were determined on the day of production (day 0), and one day and three days (day 1 and day 3) after the production to evaluate staling properties. The crumb firmness results of the control and xylanase added whole wheat bread samples on day 0, day 1 and day 3 are presented in Table 7. Firmness values of control whole wheat bread samples of modern bread wheat varieties on the day 0, day 1 and day 3 ranged between 631–2552, 1564–4669 and 2303–6101 g (g), respectively. Firmness values of xylanase added whole wheat bread samples of modern bread wheat varieties on day 0, day 1 and day 3 ranged between 478–2401, 1165–4546 and 2038–6440 g, respectively. Firmness values of control whole wheat bread samples of old bread wheat varieties on day 0, day 1 and day 3 ranged between 1971–4451, 3769–7435 and 4216–11289 g, respectively. The firmness values of whole wheat breads of different varieties were significantly different on days 0, 1 and 3 (p < 0.05). Furthermore, there were generally significant differences between the firmness values of control and respective xylanase added whole wheat breads (p < 0.05).

Table 7 Staling properties of whole wheat bread samples
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Considering the day 0 firmness values of control whole wheat bread samples produced by using modern and old varieties, the lowest value was determined in the modern variety Tosunbey (631 g) while the highest value was observed in the old variety Ak (4451 g) (p < 0.05). As the mean firmness values of the whole wheat breads compared at 0, 1 and 3 day after production, the old varieties (3598, 6125, 9305 g) had significantly higher values than those of modern varieties (1448, 2818, 3797 g).

The firmness values of the whole wheat breads of each variety increased from day 0 to day 3 indicating increased staling. As the effect of xylanase on staling is evaluated on day 0, day 1, and day 3 the xylanase added whole wheat breads had lower or equivalent firmness values compared to those of control whole wheat breads as they are compared on the same day. Shah et al. [5] investigated the effects of xylanase addition on the bread texture and stated that firmness was decreased with the addition of xylanase. Jiang et al. [23] also stated that xylanase retarded the bread staling by reducing the initial crumb firmness and the firming process during storage. Tebben et al. [21] determined the effects of xylanase on whole wheat bread properties of bread texture, and staling. The greatest reduction in hardness of fresh bread was obtained for the high level of xylanase and xylanase reduced the rate of bread firming over 7 days. Results of these studies were in line with the present study that the application of xylanase in bread production had an increasing effect on loaf volume, decreasing effect on crumb firmness and improving effect on bread staling.

Dietary fiber content of whole wheat breads and the effect of xylanase on dietary fiber

Dietary fiber analysis was carried out on the control and xylanase added whole wheat bread samples. The total, soluble and insoluble dietary fiber results of control and xylanase added whole wheat bread samples are presented in Table 8. There were generally statistically significant differences between whole wheat breads in terms of the total, soluble and insoluble dietary fiber contents (p < 0.05). The total and soluble dietary fiber contents of control whole wheat bread samples produced using modern bread wheat varieties ranged between 15.97–18.72% and 2.95–4.03%, respectively. The total and soluble dietary fiber contents of xylanase added whole wheat bread samples produced by using modern bread wheat varieties ranged between 15.95–18.68% and 3.14–4.28%, respectively. The highest total dietary fiber contents were observed in Esperia, Demir and Tosunbey varieties in both control (18.72, 18.61, 18.40%) and xylanase added whole wheat bread samples (18.39, 18.68, 18.19%), respectively while the highest soluble dietary fiber contents were observed in Demir variety, in both control (4.03%) and xylanase added (4.28%) whole wheat breads among the modern varieties. Among the whole wheat breads of modern bread wheat varieties, Kenanbey variety had the lowest total and soluble dietary fiber contents in both control (15.97% and 2.95%) and xylanase added samples (15.95% and 3.14%), respectively.

Table 8 Total, soluble, insoluble dietary fiber contents (%, dmb) of whole wheat bread samples
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The total and soluble dietary fiber contents of whole wheat bread samples produced by using old bread wheat varieties ranged between 17.01–19.69% and 3.14–4.45%, respectively. The highest total dietary fiber content was observed in whole wheat bread samples produced using Bolal (19.69%) while the lowest value was observed in Sivas (17.01%) among old bread wheat varieties.

Considering the effect of xylanase on the soluble dietary content of whole wheat breads, it was observed that xylanase addition had an increasing effect on soluble fractions. Soluble dietary fiber contents of whole wheat breads produced from Kenanbey, Tosunbey, Bayraktar and Demir significantly increased with the addition of xylanase. There were also significant differences between the mean soluble dietary fiber contents of control and xylanase added whole wheat breads (p < 0.01). The xylanase added whole wheat breads had higher mean soluble dietary fiber content (3.65%) as compared to control whole wheat breads (3.45%).

There were significant differences between the mean soluble dietary fiber contents of control whole wheat breads produced using modern and old varieties (p < 0.01). The whole wheat breads of old varieties had higher mean soluble dietary fiber content (4.03%) as compared to modern varieties (3.45%). Consuming dietary fiber at recommended levels is an essential component of balanced nutrition, and it has various metabolic and physiological effects. However, the daily consumption of dietary fiber is generally below the required levels [24]. The adequate total dietary fiber intake is reported as 14 g per 1,000 kcal, or 25 g for adult women and 38 g for adult men, based on the research demonstrating protection against coronary heart disease [25]. World Health Organization (WHO) pointed out that overweight and obesity, as well as the related noncommunicable diseases are largely preventable by the choice of healthier foods such as whole grains and adequate physical activity [26]. The selection of wheats with higher dietary fiber content might be a good option for the production of whole wheat breads to increase the daily intake of dietary fiber.

Glycemic index of whole wheat breads

In vitro glycemic index values of whole wheat breads produced by using modern varieties and old varieties are presented in Table 9. There were generally statistically significant differences between whole wheat breads in terms of in vitro glycemic index values (p < 0.05). The in vitro glycemic index values of whole wheat breads produced by using modern varieties and old varieties ranged from 77.0 to 82.3 and from 77.6 to 82.5, respectively.

Table 9 Estimated glycemic index values (in vitro) of whole wheat bread samples
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Atkinson et al. [27] reviewed the literature and listed the glycemic index of over 2,480 individual food items by using 205 articles published between the year of 1981 and 2007. They listed the glycemic index values of whole wheat breads as 74 ± 2 and 75 ± 2. In the present study, the mean glycemic index values of whole wheat breads produced by using modern and old varieties were determined as 79.6 and 80.4, respectively. These results were slightly higher than the glycemic index values of listed in Atkinson et al. [27]. Scazzina et al. [28] determined glycemic index values of the 141 commercial foods in Italy by in vivo methods using blood glucose concentrations of subjects. They identified that the glycemic index of whole wheat breads was in the range of 36–84. Whole wheat flour breads generally have lower glycemic index values as compared to refined wheat flour breads. Olagunju et al. [29] stated that whole grains and products could induce decreased blood glucose concentration and insulin secretion which is attributable to their richness in fiber and micronutrients. In the present study, correlation analysis were performed between the total dietary fiber contents and in vitro glycemic index values. Correlation analysis indicated that there was a significant negative correlation (p < 0.05) between total dietary fiber contents and in vitro glycemic index values (r = − 0.869) for modern varieties. For old varieties, there was a negative correlation (r = − 0.256) between them. Evaluating both modern and old varieties showed that there was a significant negative correlation (p < 0.05) between total dietary fiber contents and in vitro glycemic index values (r = − 0.523). Correlation analysis revealed that glycemic index decreased with the increasing level of dietary fiber. Further investigations are needed in this manner.

Kirac, Ak, Ankara, Kose (old varieties) and Bayraktar (modern variety) had the highest (82.5, 82.3, 81.6, 81.5 and 82.3) while Tosunbey and Esperia (modern varieties) had the lowest (77.1 and 77.0) in vitro glycemic index values among all modern and old bread wheat varieties. In terms of mean glycemic index values, the modern varieties had a lower value (79.6) than the old varieties (80.4). However, the differences between the mean glycemic index values of modern and old varieties were not statistically significant. Xylanase addition is not expected to have a major effect on the glycemic index of whole wheat breads; therefore, it was not investigated in this study. Lappi et al. [30] also stated that intake of the xylanase added whole wheat bread did not improve the postprandial glucose response.

Conclusion

In the present study, a set of old bread wheat varieties registered as early as the 1930s and modern bread wheat varieties registered during the last three decades were analyzed to determine and compare the quality characteristics, dietary fiber contents and in vitro glycemic index values of their whole wheat breads. The effects of xylanase on the quality, staling and dietary fiber content of whole wheat breads were also investigated. Overall, the results of the present study indicate that modern varieties had better whole wheat bread making quality as compared to old varieties and xylanase had an improving effect on soluble dietary fiber content, bread volume and various bread characteristics such as crumb cell structure and softness. In addition, there were no significant differences between the whole wheat breads produced from modern and old varieties as their mean in vitro glycemic index values were compared.