Journal of Food Engineering 116 (2013) 362–368

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Journal of Food Engineering

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Rheological properties of wheat dough supplemented with functionalby-products of food processing: Brewer’s spent grain and apple pomace

Anastasia Ktenioudaki ⇑, Norah O’Shea, Eimear GallagherFood Chemistry and Technology Department, Teagasc Food Research Centre, Ashtown, Dublin 15, Ireland

a r t i c l e i n f o

Article history:Received 5 July 2012Received in revised form 22 October 2012Accepted 1 December 2012Available online 8 December 2012

Keywords:RheologyBy-productsDoughFibrePasting properties

0260-8774/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.jfoodeng.2012.12.005

⇑ Corresponding author.E-mail addresses: anastasia.ktenioudaki@teagasc.

(A. Ktenioudaki).

a b s t r a c t

The physicochemical properties of brewer’s spent grain (BSG) and apple pomace (AP), and their effect ondough rheological properties were investigated. BSG had high protein content and both by-products werefound to be high in dietary fibre. The rheological and the pasting properties of dough were significantlyaltered by the addition of by-products, with the main effects being due to the presence of dietary fibre.The biaxial extensional viscosity was significantly higher for the supplemented doughs; the strain hard-ening index decreased with increasing the levels of flour substitution. The replacement of wheat flourwith the by-products significantly reduced the uniaxial extensibility, while the storage modulus, G00,was found to increase, indicating changes in the structural properties of dough. These changes werereflected in the ability of dough to rise during proofing.

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1. Introduction

For many years, baked products have been a favourable plat-form for delivering healthy compounds to consumers. More re-cently, new sources of functional compounds such as dietaryfibre and bioactive compounds have been sought after, and by-products from the food and drink industry have been examinedfor their potential to increase the nutritional value of cereal prod-ucts. Brewer’s spent grain and apple pomace, are two by-productsgenerated after the production of beer and cider/apple juicerespectively. Researchers have previously assessed their potentialto increase the nutritional value of food products, in particular cer-eal and/or bakery products (Ozturk et al., 2002; Rupasinghe et al.,2008; Stojceska and Ainsworth, 2008; Stojceska et al., 2008; Sudhaet al., 2007a; Ktenioudaki et al., 2012). Both by-products are richsources of dietary fibre (60–71% in brewer’s spent grain; 35–60%in apple pomace) (Mussatto et al., 2006; Ozturk et al., 2002; Bhu-shan et al., 2008). Brewer’s spent grain is particularly rich in pro-tein (Santos et al., 2003) and has a high concentration of tocols(Peterson, 1994). Apple pomace is a rich source of polyphenolswhich contribute to its antioxidant capacity (Garcia et al., 2009).

The significance of rheology in cereal research and the bakeryindustry has been well documented over the years. It is known thatwhen dough rheology characteristics are altered, dough processingand end product quality can be affected. Many studies have dealt

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with the rheological properties of doughs supplemented with fi-bres and/or by-products rich in fibrous materials (Angioloni andCollar, 2009; Anil, 2007; Barros et al., 2010; Correa et al., 2010; Go-mez et al., 2010; Salmenkallio-Marttila et al., 2001; Sullivan et al.,2010). Sudha et al. (2007a) showed that the replacement of wheatflour with apple pomace at levels of up to 30% resulted in stifferdough that exhibited low extensibility and high resistance toextension when tested with an Extensograph. They also found thatthe incorporation of apple pomace altered the mixing properties ofdough by increasing the water absorption and the developmenttime. BSG also has been found to alter the mixing properties ofwheat dough blends (Stojceska and Ainsworth, 2008). Waterabsorption increased from 58% to 61% when wheat flour was re-placed with brewer’s spent grain. Dough development time anddough stability also increased, while a decrease was observed inthe degree of softening (5–25 BU) when the percentage of BSGaddition was increased.

The above results are in general agreement with other docu-mented rheological studies involving the incorporation of fibres indoughs. Biaxial extensibility (measured with the Alveograph) hasbeen found to decrease when cladodes from Opuntia ficus indicawere added to wheat dough (Ayadi et al., 2009); the same was ob-served with the addition of carob and pea fibres (Wang et al., 2002).In both studies, the resistance to deformation increased for the sup-plemented doughs. Addition of cellulose, wheat and oat fibre re-sulted in stiffer doughs, with low tendency to flow as indicated bythe high values of storage and elastic moduli (Gomez et al., 2010).

These effects on the rheological properties of doughs will alsobe reflected by changes in gas retention and dough development,

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and consequently loaf volume, crumb structure and texture. Incor-poration of apple skin powder in the preparation of muffins has re-sulted in reduced volume and harder texture (Rupasinghe et al.,2009). A negative correlation between loaf volume and percentageof brewer’s spent grain added in breads was reported by Stojceskaand Ainsworth (2008).

The majority of the studies mentioned above have providedinformation on such rheological characteristics of dough as themixing and extensional properties, measured mostly with empiri-cal methods. The present study aimed at conducting a comprehen-sive assessment on the inclusion of BSG and AP as ingredients inbakery formulations by assessing the preliminary consequenceson the rheological properties of doughs formulated with theseby-products. Empirical and fundamental rheological techniqueswere employed including both large and small strain rheologicalmethods. A detailed characterisation of flours and the blends wasalso undertaken by examining the physicochemical and the past-ing properties.

2. Materials and methods

2.1. Materials

Commercial wheat flour was purchased locally (Odlums BakersFlour, Odlum group, Alexandra Road, Dublin, Ireland) and was usedfor this study. BSG was obtained from the micro-brewery estab-lishment in University College Cork, Ireland. AP was kindly donatedby Bulmers Ltd of Clonmel, Co. Tipperary, Ireland. The apple pom-ace was freeze dried, vacuum packed and kept at �20 �C until anal-ysis. Both BSG and AP were milled using the Perten Laboratory Mill3100, Perten Instruments AB, Sweden. The particle size distributionwas determined using different size sieves (500, 355, and 250 lm).

2.2. Compositional analysis

Moisture content of the flours was assessed following the ICC110 (ICC, 1976) using a Brabender moisture oven (Brabender, Duis-berg, Germany). The protein content was determined as describedin Alvarez-Jubete et al. (2009) using a nitrogen analyser (FP-328Leco Instrument; Leco Corporation, St Joseph, Michigan, USA). Fatanalysis was performed by acid hydrolysis following the AOACmethod 922.06 (AOAC, 1990) and the following modifications:Ground sample (2 g) were placed in a graduated cylinder, and2 ml of ethanol were added to wet the sample, followed by 15 mlof diluted hydrochloric acid (125 ml concentrated HCl diluted with55 ml distilled water). The cylinders were then placed in a waterbath at 80 �C overnight. The fat in the digested sample was thenextracted with equal volumes (25 ml) of ether and petroleum spiriton three consecutive extractions. The solvents were removed byevaporation on a water bath, and the fat was determinedgravimetrically.

Ash, total starch, and total dietary fibre were measured follow-ing the methods described by Ktenioudaki et al. (2012). For BSGand AP flours insoluble and soluble fibre content was measuredbased on AACC method 32-07.01 (AACC, 1991). All analyses werereplicated three times.

2.3. Hydration properties and oil holding capacity

The hydration properties of the by-products included swelling,water holding capacity and water hydration capacity. The methodswere similar to the methods described by Rosell et al. (2009).Briefly, swelling was measured by mixing 2.5 g of flour with50 ml of water and left overnight before recording the volumeoccupied by the flour. Water holding capacity was determined by

mixing 5 g of flour with 50 ml of water and leaving to hydrateovernight. The next day the excess water was removed and thewater holding capacity was determined as the weight of water re-tained by the flour. Water hydration capacity was measured fol-lowing the AACC 56-30 method (AACC, 1999).

Oil holding capacity was measured following the method de-scribed in Ayadi et al. (2009) with some modifications: flour sam-ple (2.5 g) was mixed with 6 ml of vegetable oil using a glass rod.The mixture was allowed to rest at room temperature for 30 min-utes before it was centrifuged for 15 min at 3000 rpm. The super-natant was carefully removed and the weight of oil retained bythe flour recorded.

2.4. Starch pasting properties

The pasting properties of the flours (wheat, BSG, AP) and blendsof 15%, 25% and 35% of BSG, and 15%, 25% and 35% of AP (replace-ment of wheat flour) were determined with a Rapid Visco Analyser(RVA-4D, Newport Scientific, Sydney, NSW, Australia), as describedby Sullivan et al. (2010). The sample size was 4.00 g and theamount of water added was 25.00 g (corrected for 14% moisturebasis). Each result is the average of two measurements and the testwas replicated three times.

2.5. Dough properties

Dough samples were prepared from blends containing 100%wheat flour (Control), 15%, 25% and 35% BSG, and 15%, 25% and35% AP. Dough mixing properties were investigated using a Brab-ender Farinograph (Brabender OHG, Duisberg, Germany) equippedwith a 300 g mixing bowl. Water absorption, development time,stability and degree of softening were determined for dough con-sistency of 600 BU, following the British Standards method No.4317 20:1999 (British Standards, 1999). The consistency of 600BU has also been used by other researchers (Alaunyte et al.,2012). As the content of BSG and AP increased, the Farinographfailed to record the curve properly. In the case of 35% AP blendthe dough remained on the top of the mixing blades and did notmix properly, therefore the water absorption was estimated basedon test baking trials. Triplicate measurements were carried out andthe results were averaged.

2.6. Rheological assays

Dough samples were prepared by mixing 300 g flour, 6 g salt(table salt purchased locally), 6 g sugar, 3 g emulsified bread fat (Ir-ish Bakels Ltd., Dublin, Ireland) and water as per Farinograph valuein a Kenwood mixer equipped with a dough hook. All experimentswere replicated three times.

2.6.1. Fundamental rheologyRheological measurements were performed on a controlled

stress rheometer (Anton Paar MCR 301, Anton Paar Gmbh, Graz,Austria) fitted with parallel plates consisting of a 25-mm serratedprobe and 25 mm serrated base plate. The method used was as de-scribed by Sullivan et al. (2010) with slight modifications. The sam-ple was placed in an airtight container and allowed to rest for45 min at 30 �C before being loaded on the rheometer. The doughwas compressed between the plates and excess dough was care-fully trimmed and the exposed edges covered with petroleum jelly.The gap was set to 1 mm and the sample was allowed to rest for15 min to allow relaxation of residual stresses. The whole systemwas covered using a peltier hood, with a temperature setting of25 �C. A frequency sweep from 0.1 to 10 Hz was performed witha target strain of 10�3 (0.1%). Preliminary amplitude sweep testsindicated that the strain was within the linear viscoelastic region.

Table 1Physicochemical properties of wheat, apple pomace and brewer’s spent grain flours.

Physicochemical properties AP BSG Wheat flour

Ash (% dw) 1.7 ± 0.6a 3.2 ± 0.0b 0.9 ± 0.2a

Fat (% dw) 2.7 ± 0.3b 4.5 ± 0.3c 0.5 ± 0.1a

Moisture (% dw) 7.1 ± 0.4b 5.6 ± 0.4a 12.7 ± 0.5c

Protein (% dw) 2.4 ± 0.2a 20.8 ± 0.3c 13.1 ± 0.0b

Total starch (% dw) 12.2 ± 0.1b 3.3 ± 0.1a 74.4 ± 0.1c

TDF (% dw) 42.5 ± 0.7b 60.5 ± 1.7c 4.1 ± 0.2a

IDF (% dw) 36.5 ± 1.1a 58.2 ± 0.7b –SDF (% dw) 6.6 ± 0.1b 1.3 ± 0.1a –Water holding capacity (g/g) 6.34 ± 0.23b 5.07 ± 0.24a –Water hydration capacity (ml/g) 2.35 ± 0.05a 2.60 ± 0.10b –Swelling (ml/g) 7.59 ± 0.05b 6.39 ± 0.02a –Oil holding capacity (g/g) 1.33 ± 0.02b 1.21 ± 0.02a –

⁄Different superscripts in the same row denote statistically significant values(P 6 0.001).

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Ten measuring points were recorded. Each result is the average offour measurements.

2.6.2. Biaxial extensionBiaxial extension was performed on the doughs following the

method of Ktenioudaki et al. (2010) using a texture analyser (TA-XT2i, Stable Micro Systems, Surrey, UK) with slight modifications.The samples were allowed to rest for 45 min at 30 �C to allowrelaxation of the stress in the samples (Sliwinski et al., 2004). Dur-ing the test, the samples were compressed to 1.5 mm height. Max-imum force and distance were recorded and biaxial strainhardening (n), biaxial extensional viscosity (g), and biaxial stress(r) were calculated as described in Ktenioudaki et al. (2010).

2.6.3. Uniaxial extensionUniaxial extension was performed on the doughs using the Kief-

fer dough and gluten extensibility rig (Stable Micro Systems, Sur-rey, UK). Samples were placed in the lubricated Teflon mouldand compressed with the lubricated top Teflon plate. The samplewas allowed to rest for 45 min at 30 �C. The displacement speedwas 3.3 mm/s and the maximum force in tension, the displacementand the area under the extension curve were recorded.

2.7. Dough fermentation properties

The fermentation process was monitored using a Chopin rheo-fermentometer F3 (Chopin S.A., Group Tripette and Renaud, Ville-neuve la Garenne, France). The doughs were prepared as for therheological assays but this time 7.5 g fresh yeast (Yeast products,Dublin, Ireland) was included in the formulation. A piece of dough(315 g) was placed in the rheofermentometer basket and a pistonwith 2 kg resistance weight was placed on top of it. The tempera-ture was maintained at 28.5 �C and the duration of the test was 3 h.

2.8. Statistical analysis

ANOVA one way statistical analysis was carried out using Mini-tab (Minitab version 15.1.1.0., Minitab Ltd., UK) to determine sig-nificant differences in the measured properties and to carry outPearson’s correlation analysis between selected parameters.

3. Results and discussion

3.1. Particle size

The particle size of the wheat flour was less than 250 lm. Themajority of the BSG flour (59.5%) consisted of particles sized be-tween 355 and 250 lm, 23.5% consisted of particles of less than250 lm, and 17% included particles larger than 355 lm. AP flourwas finer, with 91.6% consisting of particles less than 250 lm insize. The remaining 8.4% was between 355 and 250 lm.

3.2. Compositional analysis

Table 1 shows the results of the chemical analysis of the BSG, APand the wheat flours. Significant differences were observed be-tween the two by-products. BSG had a high protein content(20.8%), fat content (4.5%) and dietary fibre content (60.5%). APflour also had high fibre (42.5%). These results are in agreementwith other published data. Generally BSG has been reported to con-tain approximately 20% protein and up to 70% fibre (Mussattoet al., 2006). Stojceska and Ainsworth (2008) have reported 20.3%protein and 53% fibre for commercial BSG. AP has been reportedto contain between 10% and 40% fibre content, and 3–5% proteincontent (Vendruscolo et al., 2008). Sudha et al. (2007a) have re-

ported similar values regarding the composition of AP flour (51% fi-bre, 2% protein, and 2.7% fat).

3.3. Hydration properties and oil holding capacity

Fibres are known to have high affinity to water and their hydra-tion properties are of great interest for understanding their behav-iour in a food system such as dough. Results for these propertiesare seen in Table 1. AP appeared to have slightly higher water hold-ing capacity and swelling than the BSG. It has been suggested inthe past that smaller particle size will result in lower water bindingcapacity (Noort et al., 2010; Zhang and Moore, 1997), however ithas also been demonstrated that particle size will not affect thehydration properties (Rosell et al., 2006). Apple pomace hadslightly higher oil holding capacity (1.33 g/g) than BSG (1.21 g/g)(P < 0.05). Both by-products exhibited similar hydration propertiesas those reported for commercial fibres such as cellulose, oat,wheat and apple fibres (Rosell et al., 2009). The results are indica-tive of the high fibre content of the by-products. The hydrationproperties are expected to determine phenomena such as starchgelatinisation and gluten development, hence affecting the rheo-logical and pasting properties of doughs supplemented with BSGand AP flours.

3.4. Mixing properties

AP and BSG had similar water absorptions for the three levelstested, ranging from 70.7% to 73.5% and were significantly higherthan the control (100% wheat flour) (62.3%). The water absorptionincreased with increasing levels of by-product for both AP and BSGand it was highest for the doughs supplemented with 35% AP andBSG. The development time also increased significantly varyingfrom 2.5 min for the control dough to 9 and 7.8 min for the 15%AP and BSG blends respectively, and 15 and 10.5 min for the 25%AP and BSG blends respectively. These results are in agreementwith studies on BSG (Stojceska and Ainsworth, 2008) and AP (Sud-ha et al., 2007a) and with recent studies on the incorporation of fi-bre in dough (Ajila et al., 2008; Penella et al., 2008; Sudha et al.,2007b). Dough stability, which refers to a weakening of the dough,was also found to increase with the addition of the by-products,however the increase was not significant at the levels of 15% or25%. The hydration properties examined in Section 3.3 such as highwater holding capacity and high water binding capacity would beexpected to increase the water absorption of the flours blendedwith the by-products; this was the case for both AP and BSG floursand has been demonstrated before (Stojceska and Ainsworth,2008; Sudha et al., 2007a; Ajila et al., 2008). However such a rela-tionship has not been established in the literature. In many cases,

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high water holding capacity has not led to an increase in waterabsorption. This is believed to be due to the primary role of glutenin a dough system which will mostly affect the water absorption,compared to the hydration properties of a flour which would begoverned by the properties of the fibres (Noort et al., 2010).

3.5. Pasting properties

The presence of the by-products significantly affected the past-ing properties of the flour blends (Table 2). Generally, the pastingproperties peak viscosity, holding strength, breakdown, final vis-cosity, and setback decreased with increasing the level of by-prod-ucts. The decrease was more pronounced for BSG than AP blends.

The peak viscosity of the flour blends was lower than the con-trol, and decreased with increasing levels of by-products. Lowestvalues were observed for the BSG blends; significantly differentto the apple pomace and the control (P = 0). The same was ob-served for the breakdown viscosity, with the exception of the15% apple pomace blend, in which case breakdown (1317 cP),was similar (slightly higher) to the control (1289 cP).

Peak viscosity is associated with starch swelling and breakdownviscosity indicates rupturing of the starch granules. The addition ofby-products in the flour blends generally reduced the amount ofstarch available for gelatinisation (the starch content of the by-products was 3.3% and 12.2% for the BSG and AP respectively). Fur-thermore, due to their high fibre content, they tended to bind morewater, thus making less water available for starch. Hence the re-duced granule swelling as indicated by the low peak viscositiesand the decrease in starch granule rupturing. These results are inagreement with Collar et al. (2006), who also used fibre–flourblends and studied their pasting properties. Sudha et al. (2007a)studied the effect of AP in flour blends using the Amylograph. Theyfound that peak viscosity decreased, whereas breakdown viscosityincreased with increasing the level of AP addition. However themaximum level of apple pomace used in their studies was 15%.At this level, an increase of breakdown viscosity was also observedin the present study, compared to the control dough but only to de-crease again with the increasing levels of apple pomace addition.Pasting temperature for the blends containing AP was approxi-mately 66 �C and similar to the control, whereas for the BSGblends, this parameter was significantly higher (between 83 and86 �C). This indicated restricted starch swelling and amylose leach-ing, as mentioned by Collar et al. (2006) and observed by Mira et al.(2005).

Final viscosity also decreased with increasing levels of the by-products, and it was generally lower than the control. The final vis-cosity is the viscosity of the formed gel during cooling. The re-stricted initial swelling and rupturing of the starch, as well as thepresence of fibre in the by-products could explain this phenome-non. Collar et al. (2007) also observed a decrease in final viscositydue to fibre addition. They attributed this result to a disruption of

Table 2Water absorption and pasting properties of wheat, apple pomace and brewer’s spent grai

Sample Water absorption (%) Peak viscosity (cP) Breakdow

Wheat flour 62.3 ± 0.0a 3339 ± 77i 1289 ± 25AP 789 ± 3b 83 ± 10BSG 18 ± 3a 13 ± 1a

15% AP 70.7 ± 0.0b 3238 ± 23h 1317 ± 1825% AP 71.3 ± 0.5b 2822 ± 33g 1129 ± 7f

35% AP 73.5 ± 0.0c 2601 ± 19f 1009 ± 1315% BSG 70.5 ± 0.0b 1952 ± 5e 1012 ± 8e

25% BSG 72.0 ± 0.0b,c 1352 ± 13d 726 ± 8d

35% BSG 73.0 ± 0.0c 926 ± 11c 499 ± 5c

⁄Different superscripts in the same column denote statistically significant values (P = 0)

the macromolecular network created during cooling by physicalinterference, disruption of secondary forces and steritical hin-drance. Setback (final viscosity minus trough viscosity) has beenassociated with product texture. Increasing the levels of by-products decreased the setback viscosity. In the case of BSG blends,setback was lower than the control. In the case of AP setback washigher than the control for the 15% level, similar to the control atthe 25% level and lower at the 35% level.

There are pronounced differences in the behaviour of the twoby-products during pasting. The BSG had lower values for all thepasting properties than the control and the AP. The main reasonfor the pasting behaviour of the BSG is the low starch content com-bined with the high fibre content. These two conditions diluted thestarch in the blend and restricted the water available for starch gel-atinisation. Although these conditions are also present in the AP,the differences observed with the control were less marked. Thiscould be because a main component of the fibre in AP is pectin.It is known that pectin has the ability to gel and is used as a thick-ening agent in many foods. Min et al. (2010) has showed how iso-lated water soluble pectin enriched fractions from AP, increasedthe pasting properties of flour-pectin blends by possibly interact-ing with the soluble starches present during gelatinization. Thepresence of pectin in the apple pomace–flour blend could explainthe similar values of the breakdown and final viscosity to the con-trol as well as the higher values of pasting properties compared tothe BSG blends.

3.6. Rheological assays

3.6.1. Fundamental rheologyFrequency sweeps were performed on the control, and BSG and

AP supplemented doughs. In all cases the storage modulus (G0) washigher than the loss modulus (G00), indicating the elastic-likebehaviour of doughs. G0 and G00 increased with the addition of bothby-products. Both BSG and AP-containing doughs had significantlyhigher values than the control, indicating that the addition of theseby-products resulted in more solid-like doughs with a lower ten-dency to flow (Fig. 1). Tand decreased as the level of substitutionincreased, also pointing to increased dough elasticity.

3.6.2. Biaxial extensionForce–displacement curves were obtained from the biaxial

extension test. The results were re-plotted into stress-strain curvesand the results analysed as described by Ktenioudaki et al. (2011).Fig. 2a shows the biaxial stress as a function of biaxial strain atconstant strain rate of 0.01 s�1 for control and 15% AP and BSG-containing doughs. All samples exhibited strain hardening proper-ties as the stress increased with the increasing strain. The extent ofstrain hardening varied between the samples; this was expressedby calculating the strain hardening index. The control dough exhib-ited strain hardening index of 1.5, higher than the 15% BSG supple-

n flours and blends.

n (cP) Final viscosity (cP) Setback (cP) Pasting temp. (�C)

g 3413 ± 69g 1363 ± 15g 65.84 ± 0.41a

b 1129 ± 16b 423 ± 23b 69.66 ± 6.40a

18 ± 3a 13 ± 3a –g 3489 ± 22g 1568 ± 53h 66.05 ± 0.22a

3054 ± 54f 1361 ± 30g 65.81 ± 0.41a

e 2789 ± 12e 1197 ± 20f 66.38 ± 085a

1973 ± 11d 1033 ± 6e 85.80 ± 0.89b

1508 ± 28c 883 ± 13d 86.73 ± 0.74b

1134 ± 24b 708 ± 23c 83.33 ± 8.38b

.

Fig. 1. Effect of different levels of BSG and AP addition on the elastic properties (G0)of wheat dough.

Fig. 2. (a) Biaxial stress as a function of biaxial strain at constant strain rate of0.01 s�1. (b) Average values of the biaxial extensional viscosity at constant strainrate of 0.01 s�1 and strain of 0.5.

366 A. Ktenioudaki et al. / Journal of Food Engineering 116 (2013) 362–368

mented dough (1.3) but lower than the 15% AP dough (1.9). Thestrain hardening index decreased with the increase of the level offlour substitution and it was as low as 0.3 for the 35% BSG-contain-ing dough.

The biaxial extensional viscosity was calculated for varyingstrains (0.1, 0.25, 0.5, 0.75, 0.95, 1.15, 1.20, and 1.30) at a strain rateof 0.01 s�1. Significantly higher results were obtained for the sup-plemented doughs when compared to the control. Dough supple-mented with 35% AP had the highest biaxial viscosity (2652 kN s/

m2 for eb = 0.75, e9b = 0.01 s�1), compared to 1252 kN s/m2 for the35% BSG-containing dough, and 186 kN s/m2 for the control dough(Fig. 2b).

Biaxial extensional viscosity has been negatively correlatedwith the ability of the dough to rise during proofing and with finalloaf volume (Ktenioudaki et al., 2010; Rouille et al., 2005). It is be-lieved that high values of biaxial extensional viscosity will hinderdough rising. Both uniaxial extensibility and biaxial extensionalviscosity have been shown to be two critical parameters for highloaf volumes.

3.6.3. Uniaxial extensionThe results from the uniaxial extension test are shown in Fig. 3.

The replacement of wheat flour with the by-products significantlyreduced the extensibility and the area under the curve. This de-crease was higher with increasing levels of flour substitution. Bothextensibility and resistance to extension are required for a doughto be able to expand and retain gas resulting in breads of good vol-ume and texture. The extensibility decreased from 71 mm (controldough) to 9.1 (35% AP-containing dough) and 9.4 mm (35% BSG-containing dough). The area decreased from 16.4 to 2.2 and1.5 mm2 for the 35% AP and BSG doughs respectively (results notshown). Significant differences were also observed between thedifferent levels of by-products. Maximum resistance was not sig-nificantly affected by the addition of BSG However, the doughssupplemented with AP exhibited significantly higher maximumresistance to extension (0.55, 0.57, and 0.42 N for 15%, 25%, and35% AP respectively). These results are in agreement with Sudhaet al. (2007a), who measured the extensional properties supple-mented with apple pomace using the Extensograph.

Results from the rheological tests highlight the changes occur-ring in the dough system with the addition of the by-products.The results from the fundamental rheological test are in accor-dance with the extensional rheological properties, i.e doughs con-taining the by-products became less flexible and more rigid.Statistically significant correlations (P = 0) were found betweentand and biaxial extensional viscosity (r = �0.8), extensibility(r = 0.9) and area under the curve (r = 0.9). The findings from therheological assays are mainly due to the presence of fibre in theby-products. Fibre and fibre components interact with the doughmatrix and gluten development in many ways, causing changesin the rheological properties. For example, it has been shown thatcomponents such as water un-extractable solids (WUSs) present inwheat flour of increased milling extraction have the ability to low-er gluten yield and gluten extensibility, and interfere with glutenparticles (Wang et al., 2003). Fibre components can also interferewith the agglomeration properties of gluten (Noort et al., 2010),present a physical disruption to the gluten matrix (Gan et al.,1992) and restrict the available water for gluten development(Wang et al., 2003; Autio, 2006).

3.7. Rheofermentometer

Table 3 shows the maximum height and the time Tx (time thatporosity occurs) as recorded by the Rheofermentometer. It wasfound that the dough height decreased significantly when wheatflour was substituted with either of the by-products. A decreaseof almost half was observed in the maximum height of doughs con-taining 15% AP or BSG (i.e. dough height decreased from 49.2 mmfor the control dough to 23.6 and 24.1 mm for the AP and BSG-con-taining doughs respectively). The time Tx, was not affected by theaddition of AP, indicating that doughs supplemented with AP canwithstand similar proofing times as the control dough before gasis released. However, the incorporation of BSG reduced Tx to 53,45 and 34 min for the three levels respectively (Table 3). The effecton the dough height is related to the poor extensibility of the

Fig. 3. Maximum force and extensibility of dough samples during uniaxial extension using the Kieffer dough and gluten extensibility rig.

Table 3Max height and time Tx recorded for dough samples using the Rheofermentometer.

Samples Max height (mm) Tx (min)

Control dough 49.2 ± 2.6d 84.5 ± 14.3d

15% AP 23.6 ± 1.6c 82.5 ± 12.3d

25% AP 9.8 ± 1.0b 68.5 ± 6.8c,d

35% AP 4.3 ± 0.4b 67.5 ± 4.0b,c,d

15% BSG 24.2 ± 0.9c 53.0 ± 6.1a,c

25% BSG 11.0 ± 0.7a 45.0 ± 1.5a,b

35% BSG 4.3 ± 0.9a 34.0 ± 3.1a

⁄Different superscripts in the same column denote statistically significant values(P 6 0.001).

A. Ktenioudaki et al. / Journal of Food Engineering 116 (2013) 362–368 367

doughs prepared with the by-products. The low extensibility andhigh biaxial extensional viscosity, poses a limit to the expansionof dough during proofing.

The pasting properties mentioned in Section 3.5 (peak viscosity,holding strength, breakdown, final viscosity, and setback) werehighly correlated with Tx (r = 0.9). It has been shown that starchgelatinisation affects dough rheology (Kim and Cornillon, 2001)by promoting cross linking and acting as filler in the gluten net-work (Dreese et al., 1988), and therefore will influence the proofingability of doughs. The possible interaction of pectin with solublestarch affecting the pasting properties of AP flour blends men-tioned in Section 3.5, may have also influenced the proofing timeTx of doughs with AP.

4. Conclusions

Novel sources of dietary fibre, such as those generated from by-products have received much attention in the literature recently.This study presented information on the physicochemical and rhe-ological, properties of supplemented doughs yielding informationthat is essential for the successful incorporation of these by-prod-ucts into bakery products. The addition of BSG and AP in dough for-mulations had varying effects on the dough properties. Biaxialextensional viscosity and uniaxial extensibility, which are two rhe-ological properties that can affect the baking quality of doughs,were negatively affected by the addition of the two by-products.Similarly the storage and loss moduli of the doughs were increased,indicating a more solid-like behaviour of the supplementeddoughs. The pasting properties peak viscosity, holding strength,breakdown, final viscosity, and setback decreased with increasing

the level of by-products with the decrease being more pronouncedfor BSG-containing doughs than those containing AP. The proofingability of dough was affected in a way that demonstrated thesedoughs during proofing would not achieve high volumes andtherefore breads of low volume and dense structure are to be ex-pected. However, AP supplemented doughs were able to withstandlong proofing times similar to wheat doughs.

This study has demonstrated how food processing by-productscan have good compositional/nutritional properties (e.g. protein,and fibre). However, their inclusion as ingredients in food products(such as baked goods) must be carefully addressed. Comprehensivetrials are initially required, to assess the preliminary consequencesof their inclusion on rheological properties of the newly formulatedproducts. Only when this information is divulged will further re-search and development work be able to proceed. The presentstudy has revealed much important information relating to thefundamental effects of by-products (AP and BSG) inclusions usingbread dough as a model system. It will serve as an essential aidand form the foundation of future bakery optimisation trials.

Acknowledgements

The project was funded by the Irish Department of Agriculture,Food and the Marine under the Food Institutional Research Mea-sure (FIRM). The authors would like to thank Bulmers Ltd of Clon-mel, Tipperary, Ireland for the supply of apple pomace.

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