Use of durum wheat clear flour in vital gluten and bioethanol production

Journal of Cereal Science 80 (2018) 50e56

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Use of durum wheat clear flour in vital gluten and bioethanol production Abdulvahit Sayaslan a, *, Mehmet Koyuncu a, Selman Türker b, Yavuz Irklı c, Abdullah Serin c, Fatma Güls¸ah Orhan c
lu Mehmetbey University, Department of Food Engineering, Karaman, Turkey Karamanog Necmettin Erbakan University, Department of Food Engineering, Konya, Turkey c Kombassan Komgıda A.S¸., Karaman, Turkey a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 October 2017 Received in revised form 11 January 2018 Accepted 25 January 2018

During milling of durum wheat (Triticum durum) into semolina used for pasta processing, certain amount of clear flour (5e15%) with low economic value is obtained. This study aimed at determining the suitability of durum clear flours for vital gluten and bioethanol productions. The durum clear flours were wet-processed into vital gluten by three wet-milling methods, namely dough-washing, dough-water dispersion and flour-water dispersion. Vital glutens with acceptable purities (71.0e82.1% protein, Nx5.7, dm), yields (9.8e14.3%, dm) and recoveries (48.7e76.8%) were achieved by the dough-water and flourwater dispersion methods. However, vital gluten by the dough-washing method could not be isolated satisfactorily. The dough mixing and breadmaking qualities of vital glutens from the clear flours were found comparable to the commercial vital gluten. The carbohydrate-rich remnants of the clear flours upon isolation of glutens were subjected to enzymatic hydrolysis and yeast fermentation, leading to ethanol yields and conversion efficiencies of 32.2e33.5% (g/g, based on clear flour solids) and 80.5 e87.6%, respectively. In conclusion, except for the dough-washing method, vital gluten and bioethanol with acceptable purities, yields, recoveries and qualities can be produced by the dough-water and flourwater dispersion methods. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Durum clear flour Vital gluten Bioethanol Quality

1. Introduction When durum wheat (Triticum durum) is milled into semolina that is used for pasta production, a substantial amount of clear flour (5e15%) with low economic value is obtained (Sissons et al., 2008;
cı and Go €g
üs¸, 2008). Durum clear flour is mostly used for feed Yag because it is not suitable for breadmaking due to its weaker gluten, yellowish color, excessive starch damage and improper granulation €
lu and Ünal, 2001). Search has been (Ozen et al., 1986; Kemahlıog underway to discover new areas of value-added usage for this byproduct. In this respect, possible uses of durum clear flours at relatively low levels (5e20%) were reported in extruded snack
cı and Go €g
üs¸, 2009a; 2009b, 2009c), various breads foods (Yag € (Ozen et al., 1986; Kılıç, 1999), sausage fillers, low-quality pasta or noodles (Sissons et al., 2008). Processing of durum clear flour into vital gluten and bioethanol also seems a promising approach,

* Corresponding author. E-mail address: [email protected] (A. Sayaslan). https://doi.org/10.1016/j.jcs.2018.01.014 0733-5210/© 2018 Elsevier Ltd. All rights reserved.

which constituted the goal of this study. Vital wheat gluten, along with starch, is produced commonly from coarsely ground wheat flour by wet-milling technology (Sayaslan, 2004; Van Der Borght et al., 2005; Wronkowska, 2016) and finds applications in food, feed and other industries (Magnuson, 1985; Maningat et al., 1994; Ortolan and Steel, 2017). In the food industry, vital gluten is mostly used in the bakery products to increase their protein contents and/or quality (Maningat et al., 1994; Marchetti et al., 2012; Ortolan and Steel, 2017). More than 15 processes were developed for wet-milling of wheat; however, only four of them starting with flour instead of kernel had industrial application, namely the Martin, hydrocyclone, Alfa-Laval/Raisio and high-pressure disintegration processes (Sayaslan, 2004; Van Der Borght et al., 2005; Wronkowska, 2016). Being the oldest one, the Martin wet-milling process is heavily dependent on gluten agglomeration properties and uses large amount of water, whereas the other three are relatively newer wetmilling processes are less dependent on gluten agglomeration properties with a lesser amount of water requirement.

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Laboratory-scale methods imitating the industrial processes were designed by Godon et al. (1983), Weegels et al. (1988), Meuser et al. (1989), Bergthaller et al. (1998) and Sayaslan et al. (2012). Among the laboratory-scale wet-milling methods, the doughwashing represents the industrial Martin process, the doughwater dispersion imitates the industrial hydrocyclone process, while the flour-water dispersion symbolizes the industrial AlfaLaval/Raisio and high-pressure disintegration processes (Sayaslan, 2004; Van Der Borght et al., 2005; Sayaslan et al., 2012). In this study, these laboratory wet-milling methods were used to isolate vital gluten from durum clear flours and the remnants rich in starch were subjected to enzymatic hydrolysis and yeast fermentation to produce ethanol. Due to decreasing fossil fuels and their environmental risks, search for alternative bioenergy sources has increased over the last decades (Guo et al., 2015). Among the renewable bioenergy sources, productions of biodiesel from fat/oil and bioethanol from carbohydrates have been steadily growing with enormous economic value (Demirbas¸, 2008; Ho et al., 2014). In many countries, such as the USA and the EU, legislations and new targets were put into action to increase the shares of renewable bioenergy sources (Shrestha and Gerpen, 2010; Walker, 2010; Guo et al., 2015). Corn and sugar cane are the largely used sources for bioethanol, while rapeseed, soybean and palm oils for the biodiesel production in the world (Demirbas¸, 2008; Sanchez and Cardona, 2008). However, as corn and sugar cane are main food and feed sources, utilization of lignocellulosic materials, food wastes and by-products has been under investigation as alternatives for bioethanol production (Kim et al., 2011; Kiran et al., 2014; Yang et al., 2014; Karmee, 2016). The purpose of this study was to investigate the suitability of durum clear flour, a by-product of semolina production, for the coproduction of vital gluten and bioethanol through different wetmilling methods. 2. Materials and methods 2.1. Materials The durum clear flours were obtained from three different semolina mills in Turkey (Komgıda, Karaman; Selva, Konya; Nuh, Ankara) and they were randomly coded as A, B and C. A relatively weak bread flour (base or control flour) and commercial vital wheat gluten, which were used in comparing the qualities of isolated glutens from the clear flours, were supplied by Komgıda A.S¸. (Karaman, Turkey). The enzymes (thermostable a-amylase, amyloglucosidase, cellulase, hemicellulase) and chemicals used in ethanol production were purchased from Biasis (Turkey) and Sigma-Aldrich (Germany). 2.2. Methods 2.2.1. Vital gluten isolation from durum clear flours by doughwashing method Vital gluten isolation from the durum clear flours was carried out by the laboratory dough-washing wet-milling method described by Sayaslan et al. (2010, 2012) with some modifications. In brief, clear flour (50 g, 14% mb) was mixed to a dough at optimum water absorption level for optimum time in the Chopin-mixolab device (Chopin, France). The optimally kneaded dough was placed in a beaker containing 75 ml of distilled water and rested for 30 min at room temperature. The rested dough was first manually washed under running water (50 ml/min) for 3 min over a 63-mm screen. Partially washed gluten was then divided into two equal portions and further washed on the Glutomatic system (Erkaya, Turkey) at 50 ml/min water rate for 2 min. The isolated wet gluten

51

was frozen, freeze-dried, milled to pass 0.3-mm screen and saved for quality assessment. The aqueous phases obtained throughout the gluten isolation process, which contains mainly starch and water-soluble solids, were collected and saved for ethanol production. 2.2.2. Vital gluten isolation from durum clear flours by doughdispersion method Vital gluten isolation from the durum clear flours through the dough-water dispersion approach was performed by the laboratory wet-milling method described by Sayaslan et al. (2010, 2012) with some modifications. Briefly, clear flour (50 g, 14% mb) was mixed to a dough at optimum water absorption level for optimum time in the mixolab instrument (Chopin, France). The dough, together with 75 ml of distilled water, was placed in 300-ml cup of Waring blender and rested at room temperature for 30 min. The rested dough was dispersed in the blender at full speed for 1 min and then spinned in a swinging-bucket centrifuge at 2500
g for 15 min. Four fractions from top to bottom were separated, namely supernatant fraction containing water-soluble solids, damaged starch plus hemicellulose, wet gluten and starch fractions. Of these fractions, the wet gluten was washed by the Glutomatic system at a water rate of 50 ml/min for 2 min, freeze-dried and milled as previously described for quality evaluation. The other three fractions and the liquid phases obtained throughout the gluten isolation were combined and saved for ethanol production. 2.2.3. Vital gluten isolation from durum clear flours by flour-water dispersion method Vital gluten isolation from the durum clear flours through the flour-water dispersion method was carried out by the laboratory wet-milling method of Sayaslan et al. (2010, 2012) with some modifications. In summary, clear flour (50 g, 14% mb) plus 75 ml of distilled water at 35  C were placed in a 300-ml capacity centrifuge tube and homogenized at 6000 rpm for 2 min using a rotor-stator with 37 mm in diameter. The dry matter content of the dispersion was adjusted to about 27% with additional water and centrifuged at 2500
g for 15 min in a swinging-bucket centrifuge. As in the dough-dispersion method, four fractions were separated. Of these fractions, the wet gluten was placed in a beaker containing 75 ml of water and rested for 20 min to mature gluten. Finally, the wet gluten was washed by the Glutomatic system at a water rate of 50 ml/min for 2 min, freeze-dried and milled for quality evaluation. The other three fractions and the liquid phases obtained throughout the gluten isolation process were combined and saved for ethanol production. 2.2.4. Ethanol production from starch-rich remnants of vital gluten isolation The starch-rich aqueous remnants, which were collected and saved during vital gluten isolation from the clear flours, were subjected to enzymatic hydrolysis and yeast fermentation for ethanol production. The dry matter contents of the aqueous remnants were first adjusted to 6.5% (about 5% fermentable carbohydrates) with additional water and then converted to ethanol following the procedure of Zhao et al. (2009a, 2009b) with some adjustments to wet-milling process. For this purpose, the starchrich aqueous remnant (about 700 ml) was first placed in a 1-L erlenmeyer flask. Then, thermostable a-amylase (30 ml, from Bacillus licheniformis, 300 U/g, Sigma-Aldrich, Germany) and KH2PO4 (150 mg) were added. Finally, the erlenmeyer content was first incubated at 95  C for 5 min and then at 86  C for 90 min in a shaking water bath set at 100 rpm. Upon completion of starch gelatinization and a-amylase liquefaction, the erlenmeyer content was cooled down to room temperature and its pH was adjusted to

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4.2e4.3 using 2 M HCl solution. Meanwhile, yeast culture was prepared by dispersing 1 g of baker's instant dry yeast (Saccharomyces cerevisiae) in 19 ml of preculture solution, which contained glucose (20 g/l), peptone (5 g/l), yeast extract (3 g/l), KH2PO4 (1 g/l) and MgSO4.7H2O (0.5 g/l). Prior to inoculation, the yeast culture was incubated at 38  C for 30 min in a shaking water bath set at 300 rpm. For simultaneous saccharification and fermentation, yeast extract (150 ml, 300 g/l), yeast culture (150 ml) and amyloglucosidase (15 ml, from Aspergillus niger, 300 U/ml, Sigma-Aldrich, Germany) were added to the erlenmeyer flask containing the gelatinized and partly hydrolyzed starch. The erlenmeyer flask was fitted with an s-shaped airlock filled partly with mineral oil and its total weight was recorded. The erlenmeyer content was subjected to simultaneous saccharification and fermentation at 30  C for 72 h in a shaking water bath set at 200 rpm. The fermentation process was followed by weight loss of the erlenmeyer flask, resulting from the formation and emission of CO2 gas during fermentation. Ethanol concentration in the erlenmeyer content was calculated based on the ethanol-to-CO2 formation ratio of 1.045 in the fermentation. Ethanol concentrations at the end of the fermentation (72. hour) were also confirmed spectroscopically using enzymatic ethanol analysis kit (Biasis, Turkey). The calculated and ezymatically determined ethanol concentrations were not statistically different (P > 0.05). 2.2.5. Chemical analysis Moisture, protein (Nx5.7), fat and ash contents of the durum clear flours, bread flour, laboratory-isolated glutens, commercial gluten and other wet-milling fractions were determined respectively by the AACC methods 44-15A, 46-30, 30-25 and 08-01 (AACC, 2000). Total starch contents of the clear flours were assayed by the AACC method 76-13 (AACC, 2000) using enzymatic-spectroscopic assay kit (Biasis, Turkey). Damaged starch contents of the durum clear flours were measured on the SDmatic system (Chopin, France) by the ICC method 172 (ICC, 2011). Total carbohydrate and nonstarch carbohydrate contents of the clear flours were calculated by mass difference of their proximate compositions. 2.2.6. Vital gluten quality tests Vital gluten swelling index (WGSI) values of the dry glutens were determined by Hu and Shang (2007). The procedure is based on the solvent absorption capacity of glutens in the presence of lactic acid and sodium dodecyl sulphate (SDS). The color properties (L*, a*, b*) of the dry glutens were measured by the AACC method 14e22 (AACC, 2000) using Hunter colorimeter (ColorFlex, USA). Mixolab instrument (Chopin, France) was used to determine mixing properties of the bread flour and gluten-fortified flours, following the “Chopinþ” protocol, adopted as the ICC method 173 (ICC, 2011). Sedimentation test was conducted by Zeleny (1947). Baking qualities of the isolated glutens, including a commercial vital gluten, were assessed by breadmaking trials. For this purpose, a relatively weak bread flour (10% protein, 14% mb) was used as the base or control flour. The dry glutens were added to the base flour at a level to increase the protein content of the base flour from 10 to about 12%. Breadmaking trials using 100 g of flour with lean formulation (flour, water, yeast and salt) and loaf volume mea€ksel et al. (2000). surements were performed as described by Ko 2.2.7. Statistical analysis The study was conducted using a randomized complete design with three replications. The data were subjected to analysis of variance and the means were compared using Duncan's multiple comparison test.

3. Results and discussion 3.1. Chemical compositions and mixing properties of durum clear flours It is important to know the proximate compositions and mixing properties of the clear flours that will be used in vital gluten and ethanol productions. As listed in Table 1; protein, fat, ash, starch and nonstarch carbohydrate contents of the clear flours were quite high, and they showed significant differences (P < 0.05) by their sources. These differences were likely resulted from the compositions of durum wheats used in semolina production and milling yields of the clear flours. It is expected that general qualities of clear flours are proportional to their yields; the higher the yield, the higher the overall quality due to less contamination of bran and aleurone layers. With relatively high protein (>14%) and starch (>66%) contents, which are comparable values to the previous
lu and Ünal (2001), the clear flours appeared findings of Kemahlıog to be suitable for vital gluten and ethanol production. However, mixing qualities of the clear flours, as compared to the bread flour, were somewhat poor as implied by their shorter mixing times and stabilities (Fig. 1). On the other hand, the damaged starch contents (4.9e6.2%) of the clear flours were reasonable (Table 1) and much
lu lower than a previously reported value of 10.6% by Boyacıog (1992). For wet-milling to produce vital gluten, wheat flour is expected to have high protein content (>11%), proper gluten agglomeration characteristics and low starch damage (<5%) (Sayaslan, 2004; Van Der Borght et al., 2005). In this respect, except for their rather weak dough mixing characteristics, the clear flours seem appropriate for vital gluten processing.

3.2. Vital gluten production from durum clear flours by different wet-milling methods The durum clear flours were wet-milled to isolate vital gluten by three methods, namely the dough-washing, dough-water dispersion and flour-water dispersion. It was found that the clear flours could be wet-processed into vital gluten by the dispersion processes, which are known to be less dependent on gluten agglomeration properties (Sayaslan, 2004; Van Der Borght et al., 2005; Sayaslan et al., 2012). However, being firmly dependent on strong gluten agglomeration characteristics (Sayaslan, 2004; Van Der Borght et al., 2005; Sayaslan et al., 2012), the clear flours could not be wet-processed by the dough-washing method. In fact, when the clear flour doughs were started to get washed under the stream of water in the dough-washing method, they immediately broke

Table 1 Chemical compositions of durum clear flours obtained from three different semolina mills. Component

Content (%)a Durum clear flour A

Moisture 13.8 ± 0.1ab Protein (Nx5.7) 14.7 ± 0.1a Fat 3.7 ± 0.4a Ash 1.72 ± 0.25a Total carbohydrate (calculated value) 79.8 ± 0.2c Total starch 66.7 ± 1.1a Nonstarch carbohydrates 13.1 ± 0.9a (calculated value) Damaged starch 6.2 ± 0.0a a b

B

C

12.5 ± 0.1b 14.9 ± 0.2a 2.4 ± 0.0b 1.45 ± 0.01b 81.3 ± 0.1b 68.7 ± 1.8b 12.6 ± 1.9a

14.1 ± 0.1a 14.4 ± 0.2a 2.1 ± 0.0c 1.28 ± 0.01c 82.2 ± 0.1a 70.5 ± 0.7a 11.6 ± 1.7a

6.2 ± 0.1a

4.9 ± 0.1b

Dry-matter basis. Different letters in the same line indicate significant difference (P < 0.05).

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Fig. 1. Mixolab dough-mixing properties of durum clear flours and control bread flour (Optimum water absorption, mixing time and stability values respectively for clear flours A, B, C and for bread flour were as follows: 63.0%, 3.6 min, 4.4 min; 61.7%, 2.6 min, 4.9 min; 58.6%, 1.5 min, 7.3 min; 60.0%, 3.0 min, 7.2 min).

apart and lost their cohesivity. Although they could get processed to some extent when salted water (2%) was used in the doughmaking and washing stages, this approach is not preferred in the industrial gluten production. Therefore, the dough-washing method was deemed unsuitable for gluten isolation from the durum clear flours and eliminated from the study. The wet-milling properties of the durum clear flours to produce vital gluten by the dispersion methods are given in Table 2. In general, the wet-milling quality parameters of purity, yield and recovery for the clear flours were lower than those for common wheat flours reported by Sayaslan (2004). In the wet-milling of wheat flour, separation of gluten from other flour components with high yield, recovery and purity is crucial and determined by material properties and wet-milling methods (Sayaslan, 2004; Van Der Borght et al., 2005; Sayaslan et al., 2012; Wronkowska, 2016). Being by-products of semolina milling industry, the clear flours were not expected to have similar wet-milling performances to common wheat flours as they were reported to contain elevated levels of


cı and Go €g
üs¸, 2009b) and thus a bran and aleurone layers (Yag decreased share of gluten proteins. Common wheat flour, however, contains highly purified endosperm that is rich in gluten proteins and starch (Hoseney, 1994). Even though protein contents (>14%) of the clear flours were quite high (Table 1), their qualities were determined to be lower (Fig. 1), which accordingly resulted in lower wet-milling performances (Table 2). The wet-milling qualities of the clear flours significantly (P < 0.05) differed by their sources and wet-milling methods (Table 2). The clear flour A was found inferior to the others in wetprocessing parameters. In terms of wet-milling methods, the dough-water dispersion had better wet-milling performance on the clear flours. The flour-water dispersion process, which exerts excessive shear during dispersion stage (Sayaslan et al., 2012), probably caused already weak gluten proteins of the clear flours to get disintegrated, leading to reductions in vital gluten yields and recoveries (Table 2). Taken as a whole, gluten can be isolated from durum clear flours with somewhat lower yields and recoveries by

Table 2 Coproduction of vital gluten and bioethanol from durum clear flours by two wet-milling methods. Wet-milling method

Dough-water dispersion

Flour-water dispersion

a

Durum clear flour

A B C A B C

Vital gutena

Ethanola

Purity (protein content) (%, Nx5.7)b

Yield (%)c

Recovery (%)d

Fermentation time (hour)

Yield (%, g/g)e

Conversion efficiency (%)f

71.2 ± 0.5cg 79.4 ± 0.3b 78.0 ± 1.1b 71.0 ± 1.5c 82.1 ± 1.5a 80.8 ± 1.2a

11.6 ± 0.9b 14.3 ± 0.4a 14.3 ± 0.8a 10.1 ± 0.3c 9.8 ± 1.7c 12.2 ± 0.6b

55.9 ± 3.9c 76.8 ± 2.4a 75.7 ± 0.8a 48.7 ± 2.3d 54.48 ± 7.5c 66.9 ± 4.1b

72 72 72 72 72 72

33.2 ± 1.2a 33.5 ± 0.1a 32.2 ± 1.8a 32.6 ± 0.3a 32.3 ± 1.2a 33.1 ± 3.1a

87.6 ± 3.3a 85.9 ± 0.1a 80.5 ± 4.6b 86.0 ± 0.8a 82.7 ± 3.0 ab 82.7 ± 7.8 ab

Dry-matter basis. Vital gluten purity (%) ¼ Vital gluten protein content (%, Nx5.7, dm). c Vital gluten yield (%) ¼ [Vital gluten amount (g, dm)/Clear flour amount (g, dm)] x 100. d Vital gluten protein recovery (%) ¼ [Vital gluten yield (%) x Vital gluten protein content (%, Nx5.7, dm)]/[Clear flour amount (g, dm) x Clear flour protein content (%, Nx5.7, dm)] x 100. e Ethanol yield (%) ¼ [Ethanol amount (g)/Clear flour amount (g, dm)] x 100. f Ethanol conversion efficiency (%) ¼ [Ethanol yield in practice (%)/Ethanol yield in theory based on starch amount (%)] x 100. g Different letters in the same column indicate significant difference (P < 0.05). b

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A. Sayaslan et al. / Journal of Cereal Science 80 (2018) 50e56

the dispersion methods, especially by the dough-washing method.

on vital gluten quality.

3.3. Quality characteristics of vital glutens isolated from durum clear flours

3.4. Ethanol production from starch-rich remnants of durum clear flours upon vital gluten isolation

As a quality parameter, vital gluten swelling index (VGSI) values of the laboratory-isolated vital glutens were compared with the commercial vital gluten (Table 3). The vital glutens isolated from the clear flours had significantly higher (P < 0.05) VGSI values than the commercial gluten, indicating their vitality and high quality. According to Hu and Shang (2007), VGSI of over 28% is considered as “high quality”, between 24 and 28% as “satisfactory quality” and <24% as “inferior quality”. The clear flour vital glutens were therefore considered as high quality, while the commercial gluten as satisfactory quality. Commercial glutens are dried by flashdrying technology (Maningat et al., 1994), where they are subjected to heat at 50e60  C, whereas the laboratory-isolated glutens were freeze-dried and thus escaped from heat treatment. Furthermore, freeze-drying approach is known to have minimal adverse effects on rehydration capacities of dried products (Krokida and Philippopoulos, 2005). In terms of water absorption capacities, a widely used test for vital gluten quality, the vital glutens from the clear flours showed comparable results to the commercial vital gluten, although significant differences (P < 0.05) were observed by the sources of clear flours (Table 3). When vital glutens were compared by their color properties (Table 3), they were similar in L* (lightness) and a* (redness) values; however, the vital glutens isolated from the clear flours had higher b* (yellowness) values than the commercial gluten isolated from common wheat flour. This is an expected result since durum wheats contain higher levels of yellow-colored pigments than bread wheats (Johnston et al., 1980; Hoseney, 1994). The sedimentation volumes and dough mixing properties of the control bread flour and vital-gluten fortified (about 2 percentage) flours, along with the loaf volumes of the breads produced from them, are given in Table 4. The laboratory-isolated vital glutens and commercial gluten afforded a comparable increase in the Zeleny sedimentation volume of the control flour. The mixing properties, namely mixing time and stability, were also increased similarly by the addition of both the clear flour vital glutens and commercial gluten. Furthermore, loaf volumes of the breads, containing vital glutens isolated from the clear flour A and commercial gluten, were improved significantly (P < 0.05) as compared to the bread from the base flour (Table 4). Based on the results of vital gluten tests, it is clear that the glutens isolated from the durum clear flours are vital and comparable in breadmaking qualities to the commercial gluten isolated from common wheat flours. The results also confirm the findings of Sayaslan at al. (2010) and Van Der Zalm et al. (2011) that high-shear isolation of gluten from flours had no detrimental effect

The starch-rich aqueous fractions of the clear flours remaining after vital gluten isolation were enzymatically hydrolyzed and fermented to ethanol. Ethanol yields and conversion efficiencies are listed in Table 2. The ethanol yields were about 32e33% (g/g) on dry solids of the clear flours and no statistical difference (P > 0.05) was determined by the wet-milling method and clear flour source. In terms of ethanol conversion efficiency, which is the ratio of the practical ethanol yield to the theoretical yield that is based on fermentable carbohydrates including starch, the clear flours differed significantly (P < 0.05), ranging from 80.5 to 87.6%. In the industrial production of ethanol from wheat, a conversion efficiency of over 90% is expected (Zhao et al., 2009b). In this study, ethanol conversion efficiencies from clear flours is somewhat lower (80.5e87.6%) since vital gluten fractions were first isolated from the clear flours and the remaining carbohydrate-rich fractions were used in ethanol production. It is well known that some of the starch (5e10%) remains in the vital gluten fraction (Magnuson, 1985; Maningat et al., 1994; Sayaslan, 2004) and about 8% of sugars is utilized by the yeast during fermentation (Zhao et al., 2009b), which are the possible cause of lower ethanol conversion efficiencies in the study. Compared to the industrial ethanol production from wheat, where only ethanol is produced using frequently low-quality wheats (Zhao et al., 2009b, 2010), the proposed approach of coproducing vital gluten and ethanol from durum clear flours is promising. 3.5. Compositions of residue/feed material and mass recovery in process fractions During the wet-milling and ethanol fermantation of the durum clear flours, most of the proteins were collected in the vital gluten fraction while a great majority of the carbohydrates were converted to ethanol and CO2 as discussed above. The chemical composition of the remaining residue is given in Table 5. This residue fraction, constituting about 15% of the dry mass of the clear flours, can be used as feed material due to its high protein (35.4e41.0%), fat (6.6e8.4%) and carbohydrate (50.7e54.4%) contents. The composition of the residue from the clear flour in this study is comparable with those of the residues from fermantation of several types of wheats (Zhao et al., 2010). Upon conversion of durum clear flours to vital gluten and ethanol, a total of four fractions were obtained, namely vital gluten, ethanol, CO2 and feed residue. The mass distribution (g/g) of the clear flour solids in those fractions were as follows: vital gluten

Table 3 Water absorption capacities and color properties of dried vital glutens isolated from durum clear flours by dough-water and flour-water dispersion methods. Wet-milling method

Dough-water dispersion

Flour-water dispersion

Commercial vital gluten a b c

Vital gluten source

A B C A B C

Vital gluten swelling index (VGSI) (%)a

Water absorption capacity (%)b

Color L*

a*

b*

28.2 ± 1.4bc 28.9 ± 1.3b 30.2 ± 3.4b 27.5 ± 3.9b 29.3 ± 1.6b 38.6 ± 1.0a 24.4 ± 2.9c

233.6 ± 5.0a 243.7 ± 0.9a 228.2 ± 2.5 ab 242.4 ± 3.4a 238.3 ± 7.5a 218.8 ± 9.7b 229.1 ± 7.5 ab

84.2 ± 0.02a 84.0 ± 0.05a 83.6 ± 0.04a 83.0 ± 0.05 ab 82.9 ± 0.03 ab 81.7 ± 0.16b 83.6 ± 0.10a

0.4 ± 0.02d 1.3 ± 0.01b 1.3 ± 0.01b 0.8 ± 0.01c 1.4 ± 0.02b 2.1 ± 0.03a 0.9 ± 0.01c

12.9 ± 0.04f 19.3 ± 0.07c 17.6 ± 0.08d 16.8 ± 0.08d 21.3 ± 0.38b 26.5 ± 0.12a 14.3 ± 0.12e

14% moisture basis. Vital gluten water absorption capacity (%) ¼ [Wet gluten amount (g)/Dry gluten amount (g, dm)] x 100. Different letters in the same column indicate significant difference (P < 0.05).

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Table 4 Sedimentation, mixing and breadmaking properties of a weak bread flour (control) and vital gluten fortified flours. Bread flour (control) or vital-gluten fortified flour Bread flour (control) Dough-water dispersion

Flour-water dispersion

Commercial vital gluten a b c

A B C A B C

Protein content (%, Nx5.7)a

Zeleny sedimentation test

Mixing properties (Mixolab)

Zeleny sedim. (ml)a

Specific Zeleny sedim. (ml)b

Mixing time (min)

Stability (min)

10.0 ± 0.2cc 12.1 ± 0.1a 11.6 ± 0.2b 12.2 ± 0.2a 11.8 ± 0.2 ab 11.6 ± 0.2b 12.1 ± 0.2a 12.0 ± 0.2a

23 ± 0.0c 28 ± 0.7a 24 ± 0.4bc 24 ± 0.0bc 25 ± 0.0b 25 ± 0.8b 26 ± 0.8b 26 ± 0.4b

2.09 ± 0.0a 2.03 ± 0.1 ab 1.88 ± 0.0b 1.80 ± 0.0c 1.72 ± 0.1d 1.98 ± 0.1 ab 1.99 ± 0.0 ab 1.99 ± 0.0 ab

0.99 1.30 1.47 1.42 1.18 1.70 1.27 1.18

4.4 8.8 5.3 6.9 8.0 9.6 7.3 8.2

Loaf volume (ml)

454 ± 16c 506 ± 12b e e 527 ± 29a e e 503 ± 10b

14% moisture-basis. Specific sedimentation volume ¼ Sedimentation volume/Protein content of flour. Different letters in the same column indicate significant difference (P < 0.05).

Table 5 Chemical composition of feed material from durum clear flour An upon vital gluten isolation and ethanol production. Component

Dough-water dispersion (%)a

Flour-water dispersion (%)a

Protein (Nx5.7) Fat Ash Total carbohydrates (calculated value)

41.0 ± 2.0ab 6.6 ± 0.9b 1.72 ± 0.25a 50.7 ± 2.0a

35.4 ± 3.9b 8.4 ± 0.2a 1.84 ± 0.19a 54.4 ± 2.1a

a b

Dry-matter basis. Different letters in the same line indicate significant difference (P < 0.05).

10.1e11.6%, ethanol 32.6e33.2%, CO2 31.2e31.8% and residue/feed 14.3e15.0%. Furthermore, 9.1e11.1% of the clear flour solids were lost during wet-milling and ethanol fermentation processes. This is quite a high value; yet, such losses are inevitable in the laboratoryscale processes, as opposed to trivial losses in the industrial processes.

production was determined to be rich in protein, fat and nonstarch carbohydrates, indicating its suitability for feed material. Overall, it was proved by this laboratory-scale study that durum clear flours can be co-processed into vital gluten and bioethanol to add value to the by-product of semolina/pasta industry. Acknowledgements

4. Conclusions Durum clear flours, a by-product of semolina milling, were determined to be rich in starch (66.7e70.5%), protein (14.4e14.9%), fat (2.1e3.7%), ash (1.28e1.72%) and nonstarch carbohydrates (11.6e13.1%). Despite the high protein contents of the clear flours, their mixing properties and dough strengths were poor as compared to a relatively weak bread flour. Wet-milling of the clear flours by the traditional dough-washing (Martin) method was unsuccessful due to the weak viscoelastic and cohesive properties of the clear flours. However, the clear flours could be wet-milled by the modern wet-milling methods of the dough-water and flourwater dispersion methods, which resemble the industrial hydrocyclone, Alfa-Laval/Raiso and high-pressure disintegration processes that are less dependent on gluten quality and agglomeration characteristics as opposed to the Martin dough-washing method. By the dispersion methods, vital glutens could be isolated with acceptable purities (71.0e82.1% protein content, Nx5.7, dm), somewhat lower yields (9.8e14.3%, dm) and recoveries (48.7e76.8%). The wet-milling qualities of the clear flours differed by the wet-milling method and clear flour source. The glutens isolated from the clear flours were proved to be vital and had comparable breadmaking qualities to the commercial vital gluten isolated from common wheat flour. Upon isolation of the glutens from the clear flours, the remaining carbohydrate-rich fractions were found to be well-suited for ethanol production as indicated by their high ethanol yields (32.2e33.5%, g/g, dm) and conversion efficiencies (80.5e87.6%). The remaining residue of the clear flour after gluten and ethanol

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