Effect of soluble dietary fibre addition on rheological and breadmaking properties of wheat doughs

Journal of Cereal Science 49 (2009) 190–201

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Effect of soluble dietary fibre addition on rheological and breadmaking properties of wheat doughs Donatella Peressini*, Alessandro Sensidoni Department of Food Science, University of Udine, via Sondrio 2, 33100 Udine, Italy

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 April 2008 Received in revised form 1 August 2008 Accepted 5 September 2008

The aim of this experimental work was to evaluate the effect of inulin addition on the rheological properties of common wheat doughs and bread quality. Three commercial fructan products of different number average degree of polymerisation (DPn) were used (DPn ¼ 10 for inulin ST; DPn ¼ 23 for inulin HP and HP-gel). Inulin contents from 2.5 to 7.5% on dry matter (wheat flour plus inulin) were used. Dough rheological properties were investigated using farinograph and dynamic rheological measurements. Upon addition of dietary fibre (DF), significant increase in mixing time and stability, and decrease in water absorption were recorded. Inulin ST exerted greater effect on water absorption than HP products. Inulin with high DP determined large changes in linear viscoelastic properties of dough. The storage modulus (G0 ) gradually increased and tan d decreased with increasing levels of inulin HP and HP-gel, which contribute to the overall dough elasticity and strength. The increase in solid-like behaviour with DF content prevented expansion of wheat dough during the fermentation stage. No significant differences were observed between sample HP and HP-gel. Enrichment with inulin ST led to lower changes in linear viscoelastic properties of dough at farinograph water absorption than inulin HP. Bread volume was significantly reduced and crumb hardness was enhanced by inulin HP level in the range 5–7.5%. When inulin ST was added to a flour suitable for breadmaking, a trend of increasing bread volume with the increase of DF content was found. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: Dietary fibre Inulin Fructan Wheat dough Rheology Breadmaking quality

1. Introduction The relationship between food and health has an increasing impact on food innovation due to the popularity of the concept of functional food. The practise of using nutrition knowledge at food product level to improve the health of the consumer forms the general concept of functional foods. According to the world market analysis for functional foods and beverages, three segments dominate the market: functional bakery products and snacks (18%), diary products (23%) and functional drinks (59%) (Schaafsma and Kok, 2005). In recent years, dietary fibre (DF) has received increasing attention from researchers and industry due to the likely beneficial effects on the reduction of coronary heart-related diseases, diabetes incidence and gut neoplasia (Brighenti, 1999). DF intake Abbreviations: tan d, loss tangent; G0 , storage modulus; G00 , loss modulus; DE*, total colour difference; DF, dietary fibre; DP, degree of polymerisation; DPn, number average degree of polymerisation; DPw, weight average degree of polymerisation; d.b., dry basis; CSLM, confocal scanning laser microscopy; MS, moderately strong; W, weak; PU, Promylograph units; RH, relative humidity; WA, water absorption; VDmax, maximum dough expansion volume; Vs, specific volume. * Corresponding author. Tel.: þ39 0432 558157; fax: þ39 0432 558130. E-mail address: [email protected] (D. Peressini). 0733-5210/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jcs.2008.09.007

should range from 20 to 35 g/day for optimal benefits but usual intake in developed countries such as the United States is only about half this level (Dreher, 2001). The most common form of DF is insoluble fibre (cellulose, lignin and some hemicellulose), which reduces constipation and is being studied for its potential to reduce the risk of colon/rectal cancer. Although soluble fibre is less common in foods than insoluble fibre, it is believed to have important effects in the digestive and absorptive processes (Dreher, 2001). Fibre-rich foods are produced by adding functional fibre or using basic ingredients with high-DF content, e.g. wholemeal bread and breakfast cereals containing whole or partially processed grain. Despite its ability to supply considerable amounts of DF, the market share of bread made with wholemeal flour is smaller than that of bread baked with white flour. Consequently, there is much interest in the development of white bread with elevated DF content. Recently, research attention has focussed on the use of other sources of DF in bread including soluble DFs such as inulin-type fructans (O’Brien et al., 2003; Praznik et al., 2002; Wang et al., 2002). Chemically, inulin-type fructans are a linear polydisperse carbohydrate material consisting mainly of D-fructose joined by b-(2 / 1) linkages (Roberfroid, 2005). The last fructose may be linked with a glucose by an a-(1 / 2) bond as in sucrose.

D. Peressini, A. Sensidoni / Journal of Cereal Science 49 (2009) 190–201

The main sources of inulin that are used in the food industry are chicory and Jerusalem artichoke (Bornet, 2001). Native chicory inulin is a non-fractionated inulin extracted from fresh roots that always contains glucose, fructose, sucrose and small oligosaccharides. The degree of polymerisation (DP) of chicory fructans varies from 2 to 60 (average DP ¼ 12) and about 10% of the fructan chains in native DF have a DP ranging between 2 and 5 (Roberfroid, 2005). The long-chain inulin (DP: 10–60; average DP ¼ 25) is produced by applying physical separation techniques to eliminate all oligomers with DP < 10. Any inulin-type fructan containing only molecules with a DP less than 10 is referred to as oligofructose. Native inulin was found to be a semi-crystalline material, which was present in the glassy state at 25  C and water activity lower than 0.75 (Zimeri and Kokini, 2002). Recent studies have identified several beneficial attributes of oligofructose and inulin. They include stimulation of colonic bifidobacteria and lactobacilli (prebiotic activity), improvement of bowel function, increased calcium absorption, positive effects on glucose and lipid metabolism, and stimulation of immune system (Biedrzycka and Bielecka, 2004; Rao, 2001; Roberfroid, 1993, 2005). Most prebiotics are oligosaccharides. Inulin with DP from 10 to 60 is an exception. Low-chain fructans act more intensively in proximal colon and inulin in more distal colonic regions (Biedrzycka and Bielecka, 2004). The only basis for limiting use of such fibre in the human diet relates to gastrointestinal tolerance. A series of clinical studies showed that up to 20 g/day of inulin and/or oligofructose is well tolerated (Carabin and Flamm, 1999). Despite the large amount of information available on the nutritional and physiological properties of fructans, very little information is available on their effects on dough and bread quality. Manufacturing high-DF products has its challenges regarding technological changes and maintenance of desired sensory properties. The main problem of DF supplementation in baking is the detrimental effects on dough handling and on consumer acceptance, due to changes in dough rheological properties, loaf volume reduction, increase of crumb hardness and unsuitable taste and mouthfeel (Go´mez et al., 2003; Pomeranz et al., 1977; Wang et al., 2002). The deleterious effect of various fibres on dough and bread quality depends greatly on DF properties and opposite effects have ¨ zboy and been frequently observed (Izydorczyk et al., 2001; O Ko¨ksel, 1997; Praznik et al., 2002). The aim of this experimental work was to evaluate systematically the potential use of inulin as fibre enriching ingredient in breadmaking. The effects of various commercial inulin products on rheological properties of dough prepared from wheat flours with diverse gluten quality were evaluated using farinograph and dynamic rheological measurements. Dough proofing behaviour and final quality of the supplemented bread were also determined.

2. Materials and methods 2.1. Materials Two commercial common wheat flours, flour MS (moderately strong, W ¼ 270, P/L ¼ 0.60 and 12% d.b. protein) and flour W (weak, W ¼ 130, P/L ¼ 0.30 and 10.5% d.b. protein) were used. Three inulin products from chicory were donated by Orafti Food Ingredients (Belgium): RaftilineÒ HP (inulin HP), RaftilineÒ HP-gel (inulin HP-gel) and RaftilineÒ ST (inulin ST). Table 1 shows composition and characteristics of different inulin samples (Orafti Food Ingredients). Inulin HP and HP-gel have the same composition, but HP-gel exhibits better wettability and disperses more easily in water due to different drying conditions (patent pending, Orafti Food Ingredients).

191

Table 1 Composition and characteristics of different inulin samples (Orafti Food Ingredients).

DPna DPwb DP range DP 2–9 (% d.b.) DP >10 (% d.b.) Glucose, fructose, sucrose (% d.b.) Water solubility at 25  C (g/L) Water wettability Density (g/L) a b

Inulin ST

Inulin HP

Inulin HP-gel

10 13 2–60 25 67 12 120 Fair 600

23 26.5 2–60 2 98 0.5 <5 Good 490

23 26.5 2–60 2 98 0.5 <5 Very good 430

Number average degree of polymerisation. Weight average degree of polymerisation.

2.2. Wheat flour–inulin blends Inulin contents from 2.5 to 7.5% on dry matter (wheat flour plus inulin) were used. Wheat flour–inulin was mixed for 20 min to ensure uniform blending (Hobart N50). 2.3. Farinograph test Farinograph test was performed according to AACC Approved Method 54-21 (AACC, 2000) (T6, Promylograph, Max Egger, Austria). Farinograph water absorption was defined as the amount of water (%, on 14% moisture flour basis) required to reach a dough consistency of 500 PU (Promylograph units). 2.4. Dynamic rheological measurement Rheological measurements were carried out using a controlledstress rheometer (SR5, Rheometric Scientific, Germany) equipped with a serrated parallel plate geometry (25 mm diameter, 2 mm gap). Dough was mixed at 30  C until maximum development in the 100-g farinograph, at optimum water absorption and constant water absorption (54.2%). A sample of dough was immediately removed from the bowl and placed between the plates of the rheometer. Excess dough was carefully trimmed and the exposed edge coated with mineral oil to prevent drying. Sample was rested 10 min after loading before testing. This resting time was sufficient for the dough to relax. Frequency sweep test was performed at 25  C from 0.1 to 10 Hz within the linear viscoelastic range. Data obtained were storage modulus (G0 ), loss modulus (G00 ) and tan d (G00 /G0 ), although only G0 and tan d were discussed. Statistical comparisons were made at 1 Hz. Results are the average of three replicates, where each replicate represents a separately mixed dough. 2.5. Yeasted dough expansion Dough prepared for breadmaking (100 g) was placed in a 500 mL glass cylinder and inserted in a cabinet at 37  C and 75% relative humidity (RH) for 150–250 min. Dough volume (mL) was recorded every 30 min until a decrease in volume occurred. Measurements were performed in triplicate. 2.6. Baking procedure Pan bread formula contained flour–inulin (100 g), sucrose (4 g), salt (2.4 g), fat (3 g) yeast, (3 g) and variable water on the basis of farinograph water absorption. Bread doughs were mixed until farinograph peak consistency (mixing time) (Promylograph T6), rested for 15 min prior to rounding and sheeting by hand, fermented at

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37  C and 75% (RH) for 70 min and baked at 203  C for 30 min. Bread quality was evaluated 1 h after baking.

observations. FITC stains starch granules (green) and Rhodamine B visualises proteins (red).

2.7. Loaf volume

2.12. Sensory evaluation

Loaf volume (mL) was obtained by rapeseed displacement according to Approved Method 10-05 (AACC, 2000). Data are reported as the mean of two measurements, which were performed on two loaves from different experiments.

A nine point hedonic rating scale was used to determine acceptability of inulin-enriched bread. The panel consisted of ten untrained assessors, who evaluated bread overall acceptability. A score of 1 represents ‘dislike extremely’ and a score of nine represents ‘like extremely’. Samples were randomly coded and served individually.

2.8. Moisture content of bread crumb Moisture content of the bread crumb (3 g) was determined by oven drying for 12 h at 105  C. Bread slices were cut from the central portion of the loaf and one cylindrical test piece was taken from the centre of each slice. Data are reported as the mean of six measurements, which were performed on two loaves from different experiments.

2.13. Statistical analysis All experiments were performed in a completely randomized design. Statistical differences in dough and bread properties were determined by one-way analysis of variance (ANOVA) and Duncan’s multiple range test (P ¼ 0.05) (Statistica software version 5, 1997). 3. Results and discussion

2.9. Hardness of bread crumb 3.1. Mixing properties Four slices were cut from the central portion of bread loaf and one cylindrical test piece (20 mm height and 40 mm diameter) was taken from each slice. Uniaxial compression test was performed using an Instron Universal Testing Machine (mod. 4301, UK) at ambient conditions (23  2 C, ambient humidity). Test piece was compressed to a final height of 5 mm (Cauchy strain ¼ 0.75) between two parallel plates at a cross-head speed of 50 mm/min. Crumb hardness (N) is defined as the maximum force. Data are reported as the mean of eight measurements, which were performed on two loaves from different experiments. 2.10. Colour of crust Colour measurements of bread crust were performed with a tristimulus colour analyser (Minolta Chroma Meter CR200, Minolta Camera Co., Japan). The instrument was equipped with a CR200 measuring head connected to a microcomputer. The illuminant C (CIE, standard, 6774 K) was used and the instrument was calibrated using the standard white tile (L* ¼ 98.03, a* ¼ 0.23, b* ¼ 0.25). Results were expressed as total colour difference (DE*) between fibre-free bread and supplemented bread according to the following equation (Go´mez et al., 2003; Zanoni et al., 1995):

DE* ¼

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DL*2 þ Da*2 þ Db*2

where DL* is brightness difference, Da* is redness difference and Db* is the yellowness difference. Values are means of fourteen measurements, which were performed on two loaves from different experiments. 2.11. Confocal scanning laser microscopy (CSLM) Dough microstructure was visualised using confocal scanning laser microscopy technique according to the procedure described by van de Velde et al. (2003) (LEICA TCS NT microscope, Leica Microsystems, Heildelberg, Germany). After mixing, dough was immediately frozen in liquid nitrogen and kept at 18  C for CSLM analysis. Frozen flour MS dough and flour enriched doughs with 5% inulin ST and 5% inulin HP at farinograph water absorption were prepared as described by Peighambardoust et al. (2006). A solution of fluorescein isothiocyanate (FITC, 1% w/v) and Rhodamine B (0.1% w/v) in dimethylformamide was used for non-covalent labelling of starch and proteins, respectively. Stained frozen doughs were defrosted at ambient temperature at least 1 h before CSLM

The farinograph results of dietary fibre-supplemented dough and the reference dough (without fibre addition) are shown in Figs. 1, 2 and Table 2. Water absorption was decreased by inulin addition in agreement with Wang et al. (2002) and O’Brien et al. (2003). The extent of the decrease varied widely with the inulin type. The lowest water absorption was observed with inulin ST, which decreased water absorption linearly from 52.8 to 55% in the reference to 40.9–45.4% in the sample with 7.5% fibre (r2 ¼ 0.92– 0.99). No relationship between water absorption and fibre content was established for inulin HP. Inulin HP and HP-gel showed similar behaviour. The influence on water absorption was higher for shortchain than long-chain inulin probably due to a lubricating effect of sugars and oligosaccharides (Rouille´ et al., 2005). Time required for dough development, or to reach a dough consistency equivalent to 500 PU, increased as a consequence of inulin HP and HP-gel addition at levels of 5% and 7.5%. Inulin ST promoted an increase in mixing time only when added at the maximum dose to flour MS. Flours MS and W gave farinograph curves consistent with moderate strong and weak doughs, respectively, according to alveograph data. The strength is evident from tolerance to overmixing, which was higher for sample MS than sample W. Upon addition of fibre, significant increase in mixing stability was recorded in both flours in agreement with Wang et al. (2002). The addition of inulin to wheat flours improved the strength of the respective doughs and the effect was especially prominent in the case of weaker flour. These results indicate that inulin may have a potential to replace a portion of wheat flour without causing detrimental consequences for dough quality. Similar effect on dough strength was observed by Izydorczyk et al. (2001) when bglucans or arabinoxylans were added to wheat flours. Inulin has high affinity for water (Gennaro et al., 2000) and is thought to compete with other flour constituents for available moisture. However, water absorption of fibre-enriched dough decreased because commercial inulin contains low molecular weight sugars and oligosaccharides, which reduce dough consistency (Rouille´ et al., 2005). Solutions of inulin exhibit viscoelastic properties, which depend on polymer DP and concentration (Zimeri and Kokini, 2003). Therefore, inulin might influence dough properties in various ways: 1) it may affect dough consistency (Rouille´ et al., 2005); 2) formation of elastic networks (Zimeri and Kokini, 2003), which contribute to the overall dough elasticity and strength; 3) interaction with gluten (Wang et al., 2002). Work

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193

Fig. 1. Farinograph curves of flour MS (a) and flour enriched with inulin HP at 2.5% (b), 5% (c) and 7.5% (d), inulin HP-gel at 2.5% (e), 5% (f) and 7.5% (g) and inulin ST at 2.5% (h), 5% (i) and 7.5% (l).

Fig. 2. Farinograph curves of flour W (a) and flour enriched with inulin HP at 2.5% (b), 5% (c) and 7.5% (d), inulin HP-gel at 2.5% (e), 5% (f) and 7.5% (g) and inulin ST at 2.5% (h), 5% (i) and 7.5% (l).

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Table 2 Effect of inulin addition on farinograph water absorption (WA), maximum volume of yeasted dough (Vmax) and viscoelastic properties (G0 , tan d at 1 Hz) of doughs at farinograph WA and constant WA. Inulin (%)

WAa (%)

Vmaxb (cm3)

0 2.5 5.0 7.5

54.2 51.6 51.2 51.4

Inulin HP-gel

0 2.5 5.0 7.5

Inulin ST

Farinograph WA

Constant WAc

G (kPa)

tan d (–)

G0 (kPa)

tan d (–)

323 a 310 a 283 b 260 c

13.7 a 19.0 b 25.6 c 39.1 d

0.38 0.36 0.33 0.28

13.7 a 17.5 b 21.6 c 28.8 d

0.38 a 0.37 a 0.31 b 0.29 b

54.2 50.6 50.4 51.4

323 a 300 b 290 b 268 c

13.7 a 18.2 b 27.8 c 41.0 d

0.38 a 0.36 a 0.32 b 0.27 b

– – – –

– – – –

0 2.5 5.0 7.5

54.2 50.5 46.7 44.5

323 a 330 a 342 b 323 a

13.7 a 14.5 a 17.3 b 22.2 c

0.38 a 0.37 a 0.35 ab 0.34 b

13.7 a 10.4 b 8.2 c 7.4 d

0.38 a 0.38 a 0.38 a 0.37 a

0 2.5 5.0 7.5

52.8 47.5 46.6 47.0

298 a 285 b 258 c 230 d

11.0 a 16.6 b 27.8 c 42.8 d

0.40 a 0.38 ab 0.35 b 0.31 c

– – – –

– – – –

Inulin HP-gel

0 2.5 5.0 7.5

52.8 46.8 45.9 46.0

298 a 288 a 263 b 240 c

11.0 a 15.7 b 27.6 c 42.3 d

0.40 0.38 0.34 0.30

a a b c

– – – –

– – – –

Inulin ST

0 2.5 5.0 7.5

52.8 45.9 42.8 40.9

298 310 308 300

11.0 a 12.0 b 13.9 c 20.1 d

0.40 a 0.40 a 0.39 a 0.37 b

– – – –

– – – –

Flour MS Inulin HP

Flour W Inulin HP

a a a a

0

a a b b

For each inulin-type values within a column followed by the same letter are not significantly different (P > 0.05). a Water absorption. Values are farinograph water absorption. b Maximum volume of dough during fermentation. c Water absorption of 54.2%.

conducted on galactomannan-enriched bread showed the presence of DF in close association with starch, but it is not clear if this implies a physical interaction between polysaccharides or, simply, viscous micro-pools of hydrated galactomannan in intimate contact with starch granules (Brennan et al., 1996). Recent studies on pasta provided evidence of non-starch polysaccharide matrix encasing starch granules when inulin was added (Brennan et al., 2004;  et al., 2002). Tudorica The influence of fibre on dough microstructure was investigated using CSLM. For this purpose, flour MS and supplemented samples containing 5% inulin were mixed at optimal water absorption and mixing time (4 min for reference and 5% inulin ST; 10 min for 5% inulin HP). Fig. 3 shows CSLM images of doughs at two magnifications to allow a better interpretation of changes in dough structure. Free-fibre dough is reported in Fig. 3a1 and a2. The presence of a dispersed protein phase (red) with interspersed starch granules (green) and protein rich spots at high magnifications were observed as previously reported by Peighambardoust et al. (2006). A mixing time of 4 min for flour MS dough led to the appearance of coarse protein domains. Enriched dough with inulin ST showed a higher concentration of protein phase than reference sample probably due to the lower water absorption, but gluten structure seems to be not influenced by supplementation (Fig. 3b1 and b2). Mixing of dough containing inulin HP until farinograph peak consistency led to the formation of a fine gluten structure surrounding starch granules with a higher homogeneity than only flour dough (Fig. 3c1 and c2). Amend and Belitz (1991) and Sutton et al. (2003) reported that stretching of glutenin particles to a more extended configuration induced unfolding and breakdown

favouring the formation of intermolecular cross-links and development of a continuous protein network. For inulin HP enriched dough, the fine gluten structure implies a fully developed dough. 3.2. Dough viscoelastic properties The effect of inulin on the viscoelastic properties of wheat dough was assessed by dynamic, small deformation tests. This testing procedure was performed for two objectives: firstly to determine the effect of inulin on rheological properties of doughs for breadmaking, secondly to understand the contribution of different inulin samples to dough elasticity. For the first objective, doughs were prepared with various water contents in order to obtain constant value of dough consistency (optimum for breadmaking). Figs. 4 and 5 show frequency sweep results for doughs containing flour MS-inulin blends, prepared using farinograph water absorption and optimum mixing time. Similar trends were observed for flour W-inulin blends (data not shown). The storage modulus and tan d at a constant frequency (1 Hz) were used to compare samples (Table 2). The dough from flour MS (moderate strong) showed higher G0 and lower tan d values than for flour W (weak), as expected from alveograph and farinograph testing. The addition of inulin with high DP to either flours imparted consistent changes in linear viscoelastic properties of dough. The storage modulus increased and tan d decreased with increasing levels of inulin HP and HP-gel. Addition of 7.5% DF resulted in G0 values that were about 3 and 4 times that of the base flours MS and W, respectively. Rheological properties for inulin HP-gel enriched doughs were essentially the same as that of inulin HP. Storage modulus increase and tan d decrease were strongly correlated with increasing inulin HP and HP-gel content (r2 ¼ 0.95–0.98). Generally, a higher G0 and a lower tan d indicate a more elastic and solid-like material. Enrichment with inulin ST led to smaller changes in dough rheological properties than inulin HP. The storage modulus slightly increased with inulin content. Addition of 7.5% DF resulted in G0 values that were about 1.6 and 1.8 times that of the base flours MS and W, respectively. Tan d was not affected by inulin ST below 7.5%. Janssen et al. (1996) found no direct relationship for either dynamic moduli or tan d to loaf volume. Nevertheless, low effect of DF on dough viscoelastic behaviour is desirable, because it may indicate minor changes in breadmaking performance. The fibre effect on dough viscoelastic properties could be direct or indirect. The indirect effect is related to variation of water-flour ratio (absorption), while the direct effect means a contribution of inulin to dough elasticity. It was well established that dynamic moduli (G0 , G00 ) are very sensitive to water content, increasing as water content decreases (Dreese et al., 1988b; Edwards et al., 1999; Navickis et al., 1982). Rheological measurements of doughs at farinograph water absorption do not allow to decouple the effects of hydration and fibre. For this purpose, the linear viscoelastic properties of doughs at constant water absorption were evaluated. Results for flour MS-inulin blends at 54.2% water absorption are reported in Table 2. The storage modulus of inulin ST samples at 54.2% water absorption was lower than the control and a trend of decreasing G0 with increase of DF content was found. Our results are in accordance with Rouille´ et al. (2005), who reported an appreciable decrease in linear steady-state creep compliance of doughs at fixed water addition as low molecular weight content increased. No significant differences in tan d were observed between flour MS and inulin ST enriched samples (P > 0.05). Constant tan d values suggest that fibre addition did not induce fundamental changes in structure of optimum developed doughs. Taking into account these results, we assume a probable diluent action of inulin ST into the dough, which has the advantage to induce only slight changes in

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195

Fig. 3. CSLM images for flour MS dough (a1, a2) and flour enriched doughs with 5% inulin ST (b1, b2) and 5% inulin HP (c1, c2) at different magnifications. Image size of 1  1 mm (a1–c1); image size of 0.5  0.5 mm (a2–c2). Green, starch granules; red, protein. Doughs at farinograph water absorption and maximum development.

the viscoelastic properties of bread doughs (at farinograph water absorption). For inulin HP doughs at 54.2% water absorption, the trend was an increase in G0 and a decrease in tan d with increase in fibre content. At any given moisture level, inulin HP gave higher elasticity and solid-like behaviour than control sample. It is evident that inulin with high DP is able to contribute to the overall dough elasticity and strength. Wang et al. (2002) proposed interactions between inulin and gluten to explain the fibre effect on dough strength. In addition, we suggest the formation of elastic networks due to inulin–inulin interaction. Kim et al. (2001) have found that

stirred suspensions of inulin give sol–gel transition due to polymer association. In addition to the minimum concentration, the system also needs a minimum length of inulin chain for gel formation. Smaller chains with relatively low DP would remain in the liquid portion without association with the gel structure (Kim et al., 2001). 3.3. Expansion of yeasted bread doughs Changes in yeasted dough volume as a function of fermentation time are illustrated in Fig. 6. Dough volume reached a plateau during the first 60–90 min and then remained constant until

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5.0

0.8

a

a

0.6

tan delta (-)

Log G' (Pa)

4.5

0.4

4.0

0.2

3.5

0.0 -1.5

-0.5

0.5

1.5

-1.5

Log frequency (Hz)

-0.5

0.5

1.5

Log frequency (Hz)

5.0

0.8

b

b

0.6

tan delta (-)

Log G' (Pa)

4.5

0.4

4.0

0.2

3.5

0.0 -1.5

-0.5

0.5

1.5

-1.5

Log frequency (Hz) 5.0

-0.5

0.5

1.5

Log frequency (Hz) 0.8

c

c

0.6

tan delta (-)

Log G' (Pa)

4.5

0.4

4.0

0.2

3.5

0.0 -1.5

-0.5

0.5

1.5

Log frequency (Hz)

-1.5

-0.5

0.5

1.5

Log frequency (Hz)

Fig. 4. Storage modulus vs. frequency for flour MS dough and flour enriched with inulin at farinograph water absorption. Inulin HP (a), HP-gel (b) and ST (c) at levels of 0 (-), 2.5% (,), 5% (:) and 7.5% (6).

Fig. 5. Tan d vs. frequency for flour MS dough and flour enriched with inulin at farinograph water absorption. Inulin HP (a), HP-gel (b) and ST (c) at levels of 0 (-), 2.5% (,), 5% (:) and 7.5% (6).

150–240 min depending on the sample. Maximum expansion volume (VDmax) is reported in Table 2. Dough expansion gradually decreased with increasing levels of inulin HP and HP-gel. Addition of 7.5% DF resulted in dough volumes that were about 17–20% and 19–23% lower than those of the base flours MS and W, respectively.

Wang et al. (2002) did not observe changes in the percentage of gas retention due to inulin addition, but we cannot rule out loss of gas during proofing completely. Interestingly, VDmax was strongly related to tan d of bread doughs when supplemented with inulin HP and HP-gel (r2 ¼ 0.91 for flour MS and 0.94 for flour W). Probably,

D. Peressini, A. Sensidoni / Journal of Cereal Science 49 (2009) 190–201

the increase in solid-like behaviour with DF content prevents expansion of wheat dough during the fermentation stage as was previously observed (Wang et al., 2002). This effect is due to contribution of inulin HP to dough elasticity. In addition, inulin improved dough characteristics allowing generally longer fermentation times than the control. Enrichment with inulin ST did not change significantly VDmax with the exception of 5% DF-flour MS, which gave slightly higher volume than the control (P ¼ 0.05).

a

Volume (cm3)

350

250

3.4. Bread quality

150

50 0

100

200

Time (min)

b 350

Volume (cm3)

197

250

150

50 0

100

200

The impact of different levels of inulin HP and ST on bread properties is presented in Table 3. Inulin HP has brought about similar effects on the specific volume (Vs) and crumb hardness of loaves made from flours MS and W. In this case, Vs was significantly reduced and crumb hardness was enhanced by high fibre content in accordance with the decrease in dough expansion (P < 0.05). This detrimental effect was observed at 7.5% inulin for flour MS and at 5% and 7.5% for flour W. For inulin ST-flour MS samples, bread volume appeared to be significantly higher and hardness lower than the control (P < 0.05), and a trend of increasing Vs with the increase of DF content was found (Table 3). Flour W exhibited slightly higher Vs value when 2.5% inulin ST was added, whereas a significantly lower value was observed at 7.5% (P < 0.05). Since dough rheological properties (Table 2) and expansion during fermentation (Fig. 6) are not sufficient to explain this behaviour, it appears that inulin ST may influence thermo-mechanical properties of dough during baking. The rheological changes in heated dough are essentially due to changes in the starch fraction (starch gelatinisation), which dramatically increase dough viscosity and solid-like behaviour (Peressini et al., 1999). Glucose, fructose and sucrose represent 12% of inulin ST product. The capability of small solutes such as sugars and salt to delay starch gelatinisation has been recognised by several authors (Dreese et al., 1988a; Ghiasi et al., 1982; Hoover and Senanayake, 1996; Spies and Hoseney, 1982). On the other hand, inulin ST strongly reduced dough water absorption (Table 2), which is the main factor determining starch gelatinisation during heating (Rolee and LeMeste, 1999; Wootton and Bamunuarachchi, 1979).

Time (min) Table 3 Effect of inulin on bread properties.

c 350

Volume (cm3)

Flour MS Inulin HP

Inulin (%)

Vsa (cm3/g)

Hardnessb (N)

Moistureb (%)

0 2.5 5.0 7.5

4.69 4.69 4.64 4.29

a a a b

13.6 a 13.6 a 15.1 a 32.5 b

40.68 a 39.75 b 39.10 c 39.82 bc

0 2.5 5.0 7.5

4.69 a 4.97 b 5.19 c 5.67 d

13.6 a 11.2 b 11.1 b 9.2 b

40.68 a 39.60 b 38.42 c 37.12 d

0 3.87 a 11.76 b 20.67 c

0 2.5 5.0 7.5

4.50 a 4.50 a 4.12 b 3.09 c

17.9 a 19.4 a 31.4 b 73.6 c

40.71 a 39.00 b 38.67 bc 37.64 c

0 3.68 a 5.58 b 10.89 c

0 2.5 5.0 7.5

4.50 a 4.75 b 4.53 a 4.15 c

17.9 a 14.1 a 16.2 ab 27.2 c

40.71 a 38.52 b 37.15 c 37.30 c

0 4.43 a 11.45 b 18.50 c

DE*c () 0 3.92 a 8.94 b 8.67 b

250 Inulin ST

150

Flour W Inulin HP

50

Inulin ST

0

100

200

Time (min) Fig. 6. Changes in volume vs. fermentation time for flour MS dough and flour enriched with inulin at farinograph water absorption. Inulin HP (a), HP-gel (b) and ST (c) at levels of 0 (-), 2.5% (,), 5% (:) and 7.5% (6).

For each inulin-type values within a column followed by the same letter are not significantly different (P > 0.05). a Specific volume of bread. b Values for bread crumb. c Values for bread crust.

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For enriched bread with inulin ST, volume increase could be due to gelatinisation delay, which may promote higher dough expansion during the baking stage. Obviously, dough rheological properties play an important role in maintaining stability against premature

rupture during baking. In the case of flour W supplemented with 7.5% inulin ST, dough strain hardening probably was not sufficiently high to prevent failure and loss of gas retention during higher expansion in the oven (Table 3).

Fig. 7. Slices of MS bread with and without inulin (A). HP: 2.5% (B), 5% (C) and 7.5% (D). ST: 2.5% (E), 5% (F) and 7.5% (G).

D. Peressini, A. Sensidoni / Journal of Cereal Science 49 (2009) 190–201

Figs. 7 and 8 show slices of MS and W breads, respectively. W bread without fibre exhibited a coarser crumb grain than MS sample (Figs. 7A and 8A). This result is consistent with the strength of the two flours. A higher degree of gas cell coalescence had

199

occurred in W dough during processing, because weak dough exhibits low strain hardening and poor gas cell stability (Dobraszczyk, 2004; Zghal et al., 2001). Inulin HP at 2.5% and 5% did not induce large changes in crumb structure of MS bread, while

Fig. 8. Slices of W bread with and without inulin (A). HP: 2.5% (B), 5% (C) and 7.5% (D). ST: 2.5% (E), 5% (F) and 7.5% (G).

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a slight decrease in size of gas cells was observed at 7.5% (Fig. 7). Inulin ST addition to MS flour gave finer crumb texture (smaller gas cells) than the reference and HP samples (Fig. 7). This result might be explained by gelatinisation delay due to inulin ST addition. The consequence of this delay would be faster bubble inflation, which gives greater strain hardening and higher proportion of finer cells (Dobraszczyk and Roberts, 1994). Contrary to this, Rouille´ et al. (2005) observed that low molecular weight sugars led to a decrease in crumb fineness. This different behaviour could be due mainly to preparation of doughs at constant hydration, which influences dough consistency and starch gelatinisation. For W bread (Fig. 8), inulin ST addition at 2.5% and 5% gave finer crumb texture than the reference, while a higher proportion of coarse cells were observed at 7.5% content, confirming low bubble stability, which induced failure and loss of gas retention during higher expansion in the oven (Table 3). When inulin HP was added to weak flour at 7.5%, a dense crumb was observed suggesting excessive limitation of bubble growth during processing (Fig. 8D). The moisture content of control bread crumbs from the two base flours was found to be slightly higher than those supplemented with inulin (Table 3). This obviously reflects the lower water absorption values of inulin-fortified flours (Table 2). The effect of fibre addition on bread crust colour is summarised in Table 3. The colour difference (DE*) increased with the increase in fibre. Generally, crust browning was higher for bread with inulin ST as substitute than inulin HP at the same level. Addition of 7.5% inulin ST resulted in DE* values that were about twice that of inulin HP. It is an expected result since inulin ST contains higher content of reducing sugars, which are involved in Maillard reaction during baking (Rada-Mendoza et al., 2004). Further hydrolysis of low molecular weight fructan to fructose during the baking process may favour non-enzymatic browning (Praznik et al., 2002). Bread crumb colour of reference was similar to the colour of all supplemented breads (data not shown). Sensory evaluation was performed on bread prepared with flour suitable for breadmaking (MS), while biscuit flour (W) was not tested. Table 4 shows the results of sensory evaluation of breads performed on the day of baking. High bread acceptance was produced from wheat flour in which 2.5 or 5% was replaced with inulin ST and HP. Addition of 7.5% inulin ST resulted in the lowest score because of too sweet taste. In the case of 7.5% inulin HP, assessors assigned a lower score than the reference sample due to higher crumb hardness. Our results suggest a level of 5% inulin HP and ST to produce a functional bread of high sensory acceptance. Roberfroid et al. (1998) reported that a daily intake of 4 g of inulin or oligofructose is sufficient to enhance bifidobacteria, that are generally recognised as being beneficial for health. From a nutritional point of view DP is particularly important since fructans with different DP have distinct physiological properties (Van Loo, 2004). Fructans with a low DP are rapidly fermented in the colon and they modulate the

Table 4 Sensory evaluation of MS bread.

Flour MS Inulin HP

Inulin ST

Inulin (%)

Scores

0 2.5 5.0 7.5

7.0 a 7.2 a 6.7 a 6.1 b

0 2.5 5.0 7.5

7.0 a 7.3 a 6.8 a 5.0 b

For each inulin-type values within a column followed by the same letter are not significantly different (P > 0.05).

intestinal flora (prebiotic effect) in the proximal part of the large intestine. Longer-chain fructans are more slowly fermented and reach more distal parts of the colon, where they maintain a beneficial metabolic activity. 4. Conclusions Flour replacement at different levels (from 2.5 up to 7.5%) by inulin change dough machinability, viscoelasticity and breadmaking performances. The trend and the extent of effects of fibre on the breadmaking process (mixing, proofing, baking) depend on inulin type in the blend and on the extent of flour substitution. Addition of inulin to both weak and moderately strong flours resulted in a strengthening effect. Caution should be paid to inulin HP because of the adverse increase in solid-like properties of the dough, which may reduce expansion during fermentation and baking. Enrichment with inulin ST led to lower changes in linear viscoelastic properties of dough than inulin HP and had no negative effects on crumb hardness and volume of bread prepared with flour suitable for breadmaking (MS). Nevertheless, addition of inulin ST over 5% is not recommended because of sweet taste. The present study has indicated that breads made with about 5% inulin ST and HP had high sensory acceptance. Therefore, the addition of inulin could be an effective way to produce functional white flour bread without changing negatively its desirable physical properties. Further studies are needed to evaluate changes in fibre-enriched bread characteristics during storage. Acknowledgements The authors gratefully acknowledge Luisa Gracco for technical assistance, Hadi Peighambardoust for CSLM images and Mrs. Veerle De Bondt from Orafti for providing inulin samples and their characterisation. References Amend, T., Belitz, H.D., 1991. Microstructural studies of gluten and a hypothesis on dough formation. Food Structure 10, 277–288. American Association of Cereal Chemists, 2000. Approved Methods of the AACC, 10th ed. The Association, S. Paul, MN. Biedrzycka, E., Bielecka, M., 2004. Prebiotic effectiveness of fructans of different degrees of polymerization. Trends in Food Science and Technology 15, 170–175. Bornet, F.R.J., 2001. Fructo-oligosaccharides and other fructans: chemistry, structure and nutritional effects. In: McCleary, B.V., Prosky, L. (Eds.), Advanced Dietary Fibre Technology. Blackwell Science, Oxford, UK, pp. 480–493. Brennan, C.S., Blake, D.E., Ellis, P.R., Schofield, J.D., 1996. Effects of guar galactomannan on wheat bread microstructure and on the in vitro and in vivo digestibility of starch in bread. Journal of Cereal Science 24, 151–160. Brennan, C.S., Kuri, V., Tudoric a, C.M., 2004. Inulin-enriched pasta: effects on textural properties and starch degradation. Food Chemistry 86, 189–193. Brighenti, F., 1999. Carboidrati e fibra. In: Costantini, A.M., Cannella, C., Tomassi, G. (Eds.), Fondamenti di nutrizione. Il Pensiero Scientifico Editore, Rome, pp. 197–222. Carabin, I., Flamm, W.G., 1999. Evaluation of safety of inulin and oligofructose as dietary fiber. Regulatory Toxicology and Pharmacology 30, 268–282. Dobraszczyk, B.J., 2004. The physics of baking: rheological and polymer molecular structure–function relationships in breadmaking. Journal of Non-Newtonian Fluid Mechanics 124, 61–69. Dobraszczyk, B.J., Roberts, C.A., 1994. Strain hardening and dough gas cell-wall failure in biaxial extension. Journal of Cereal Science 20, 265–274. Dreese, P.C., Faubion, J.M., Hoseney, R.C., 1988a. Dynamic rheological properties of flour, gluten and gluten-starch doughs. I. Temperature-dependent changes during heating. Cereal Chemistry 65, 348–353. Dreese, P.C., Faubion, J.M., Hoseney, R.C., 1988b. Dynamic rheological properties of flour, gluten and gluten-starch doughs. II. Effect of various processing and ingredient changes. Cereal Chemistry 65, 354–359. Dreher, M.L., 2001. Dietary fiber overview. In: McCleary, B.V., Prosky, L. (Eds.), Advanced Dietary Fibre Technology. Blackwell Science, Oxford, UK, pp. 1–16. Edwards, N.M., Dexter, J.E., Scanlon, M.G., Cenkowski, S., 1999. Relationship of creep-recovery and dynamic oscillatory measurements to durum wheat physical dough properties. Cereal Chemistry 76, 638–645. Gennaro, S., Birch, G.G., Parke, S.A., Stancher, B., 2000. Studies on the physicochemical properties of inulin and inulin oligomers. Food Chemistry 68, 179–183.

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