Effects of fungal α-amylase on chemically leavened wheat flour doughs

Journal of Cereal Science 56 (2012) 644e651

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Effects of fungal a-amylase on chemically leavened wheat flour doughs M.J. Patel a, b, J.H.Y. Ng a, b, W.E. Hawkins a, c, K.F. Pitts c, d, S. Chakrabarti-Bell a, b, * a

Centre for Grain Food Innovation, 26 Dick Perry Avenue, Kensington, WA 6151, Australia CSIRO Food Futures National Research Flagship, GPO Box 1600, Canberra, ACT 2601, Australia c Department of Agriculture and Food Western Australia, 3 Baron-Hay Court, South Perth, WA 6151, Australia d CSIRO Food and Nutritional Sciences, 671 Sneydes Road, Werribee, VIC 3030, Australia b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 March 2012 Received in revised form 30 July 2012 Accepted 4 August 2012

Chemical leaveners are used in doughs to generate carbon dioxide, as an alternative to yeast, in making a range of bakery products. In this study, the effects of fungal a-amylase and ascorbic acid on chemically leavened doughs were followed by measuring dough extensibility, true rheological properties, the amount of free liquid in doughs following ultracentrifugation and the quality of baked products. As with yeasted doughs, the bake qualities of chemically leavened doughs also improved in the presence of fungal a-amylases. The bake qualities were not affected when the equivalent amount of ascorbic acid was added. The differences in dough formulations were detected from measurements of true rheological properties, not from extensibilities of doughs. The amount of free liquid was larger and of lower viscosity in doughs containing a-amylases. The properties of the continuous liquid phase were found to be important in defining the rheological and baking qualities of doughs. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.

Keywords: Dough improver Fungal a-amylase Chemically leavened breads Ultracentrifuged dough liquor

1. Introduction Chemical leaveners are used in frozen doughs, refrigerated doughs as well as fresh doughs for making a wide range of bakery products ranging from breads and pizza crusts to cakes and muffins (Holcomb and Rayas-Duarte, 2012; Kulp et al., 1995). These leaveners are mixtures of certain acidic and alkaline carbonate compounds that react readily to produce carbon dioxide, the same gas as produced by yeast during proofing of doughs. The alkaline component is almost always sodium bicarbonate (baking soda). The leavening acids include sodium pyrophosphate (SAPP), sodium aluminium phosphate (SALP), glucono-deltalactone (GDL) and citric acid. Rapid production of carbon dioxide from the leavening reaction helps to reduce and/or eliminate proofing time and thereby reduce product preparation time (Heidolph et al., 2000). Due to this unique method of production of gas, chemical leaveners have gained popularity for use with frozen doughs as with yeasted frozen doughs, bake qualities are compromised when yeast

Abbreviations: Am, a-amylase; As, ascorbic acid; CSIRO, Commonwealth Scientific and Industrial Research Organisation, Australia; D, control dough; DI, dough improver; BSV, baked specific volume. * Corresponding author. CSIRO Food Futures National Research Flagship, GPO Box 1600, Canberra, ACT 2601, Australia. Tel.: þ61 8 6436 8558. E-mail address: [email protected] (S. Chakrabarti-Bell).

dies during freezing (Miller, 2006). Refrigerated yeasted doughs require special packaging as continued production of gas from yeast fermentation can lead to bursting of packages during storage. The convenience of ‘ready to bake’ frozen and refrigerated doughs is also popular with consumers as well as food service organisations and have promoted the use of chemical leaveners in the baking industry. Note that although product quality is acceptable, the loaf volumes, texture and eating qualities are not as high as commonly obtained with yeasted fresh doughs. Further research is required in this area (Heidolph et al., 2000) for developing chemically leavened doughs with superior finished product qualities. Dough improvers are a class of additives that are also widely used in the baking industry due to their beneficial effects on dough handling and end product quality even when added in minute amounts. Examples of dough improvers include ascorbic acid (added also as a nutritional supplement), mono-and di-glycerides, enzymes like fungal and bacterial alpha (a)-amylases, and calcium salts such as calcium iodate. Whilst many of these improvers have been used in chemically leavened doughs (Domingues and Lonergan, 2007), there is little information about the use of a-amylases in these doughs despite their widespread use for yeasted doughs. Understanding of the functionalities of these improvers, especially ascorbic acid and amylases, has been gained by studying yeasted doughs only.

0733-5210/$ e see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jcs.2012.08.002

M.J. Patel et al. / Journal of Cereal Science 56 (2012) 644e651

1.1. Ascorbic acid in doughs Loaf volumes increase when doughs contain ascorbic acid. It is acknowledged that ascorbic acid strengthens the gluten matrix by increasing the density of di-sulphide bonds in gluten (Aamodt et al., 2003; Koehler, 2003) and stronger gluten leads to higher loaf volumes. In line with this, higher shear moduli for doughs containing ascorbic acid have been reported (Larsson and Eliasson, 1996). They also report that doughs containing ascorbic acid had less free liquid (dough liquor). There are no set rules for the dosage of these improvers. Increases of up to 20% in baked specific volume of yeasted breads have been reported with ascorbic acid added at as low concentration as 30 mg/kg flour (Grosch and Wieser, 1999; Joye et al., 2009). In chemically leavened doughs, addition of ascorbic acid in the range of 100 ppm to 10,000 ppm has been reported by Domingues and Lonergan (2007). 1.2. a-amylases Larger loaves with softer texture are obtained when dough contains either fungal or bacterial a-amylases. Fungal a-amylases facilitate proofing under ambient conditions while bacterial a-amylases act at higher temperatures and enhance loaf expansion during baking. The fungal a-amylases have been used in amounts of w300-400 mg per 100 g flour (Kim et al., 2006; Sahlstrom and Brathen, 1997). Due to the ease of use and benefits observed on finished product qualities, mixtures of ascorbic acid and fungal a-amylases are widely used in bakeries in Australia as a way to boost the bread-making qualities of local flours (discussion with Mr. Robert Millard, Bakery Training Instructor, Polytechnic West, Perth, Australia). It is thought that fungal a-amylases depolymerise damaged starch and reduce its ability to bind moisture, thus allowing more moisture to be available for gluten hydration (Martinez-Anaya and Jimenez, 1997). Microscopic observation has shown that starch granules disintegrate in doughs mixed with a-amylase, possibly due to extensive hydration and swelling (Blaszczak et al., 2004). Depolymerisation also facilitates the production of dextrin or fermentable sugars, which in turn facilitates the production of carbon dioxide by yeast (Kragh, 2003; Linko et al., 1997). Thus, more gas is produced and the loaves are larger. Beneficial effects of a-amylases on quality (texture) have also been reported for chapattis (Hemalatha et al., 2010). However, this cannot be explained by greater gas production as chapattis are not made with yeast. Hence it is not clear if the observed improvement in finished product qualities arising from addition of a-amylases occurs due to production of excess gas during fermentation or from changes in dough strength resulting from improved gluten hydration. It is not simple to design studies incorporating yeasted doughs that decouple these two potential mechanisms. Such an opportunity is available when analysing the effects of fungal a-amylases on chemically leavened doughs, since the gas production is independent of production of simple sugars resulting from degradation of starch by a-amylases. This study was carried out to determine the effects of fungal a-amylase and ascorbic acid on chemically leavened doughs and thereby to gain a better understanding of their mechanisms of action on doughs and breads. Doughs were mixed with either each improver individually or commercially available dough improver, which is a mix of a-amylase and ascorbic acid. Control doughs were also mixed with no improver. Baked products were analysed for baked specific volume and crumb strength. Doughs were ultracentrifuged to extract dough liquor and the liquor volumes were measured. Dough elasticity and strain-hardening properties were measured using lubricated compression tests (Chakrabarti-Bell et al., 2010). The amounts

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of simple sugars and damaged starches in doughs were also measured. Results are presented. 2. Materials and methods 2.1. Sample preparation Commercial bakers’ flour was sourced from Western Australia. The protein content, damaged starch content and optimal water absorption was 12.3% at a 14% moisture basis, 7.4% by flour weight, and 65.2%, respectively. However, all doughs were mixed at 62.5% moisture as used in commercial bakeries for this particular flour. The chemical leavener was a mixture of sodium bicarbonate (supplied by IMCD Australia) added at 32.3% w/w dry leavener, GDL (IMCD Australia) added at 47.7%, and SALP (also known as LevinLiteÒ and supplied by Fibrisol, Australia) at 20%. While GDL reacts with soda to produce carbon dioxide at any temperature, SALP reacts with soda at higher temperatures and produces carbon dioxide during baking. This mix of leaveners was included at 6.5% of dough weight for all doughs. At this leavener concentration, there was no gas production in doughs at room temperature as evidenced by a lack of change in dough density for at least 20 min (data not shown for brevity). Commercially available Bakels Advance 1000 Improver (supplied by Springer Foods, Myaree, Australia) was used as the dough improver (‘DI’), and contrasted with the effect of pure a-amylase (FungamylÒ BG, sourced from Bakels; ‘Am’), pure ascorbic acid (‘As’), and a mixture of the latter two improvers (‘Am þ As’). The enzyme activities of pure a-amylase and DI were measured using the Amylazyme assay procedure used for the measurement of cereal and microbial a-amylase activity (AACC Method 22.05 and RACI Standard Methods), and were 1650 and 300 Ceralpha U/ml, respectively. These values represent fully active enzymes. The lower value for the commercial improver is due to the smaller amount of a-amylase present in the commercial mix compared to a sample of pure a-amylase. Note that DI was added to doughs at a rate of 100 ppm by flour weight as previous work identified that higher addition rates resulted in no further improvement in bake volume (data not shown for brevity). Deionised water was used to mix all doughs. The control dough featuring no DI, pure a-amylase or ascorbic acid used dextrose as filler (supplied by IMCD Australia). Both pure a-amylase and ascorbic acid were used at 100 ppm level. Further details are presented in Table 1. 2.2. Methods The following were measured for all doughs: (i) dough mixing energies, (ii) amounts of damaged starch in flour and doughs, (iii) amounts of free sugars in doughs, (iv) amounts of liquid released from dough during ultracentrifugation, (v) dough extensibility using the Brabender Extensograph, (vi) dough rheological properties in compression, (vii) specific volume of crumbs after baking and (viii) strength of crumbs in compression. 2.2.1. Dough mixing and bread baking All doughs were mixed in 400 g batches using a six pin mixer running at 100 rpm. All were mixed at a constant flour-to-addedwater ratio of 1.6, giving an added moisture content of 62.5%. The leavener and dough improver contents were held constant at 6.5% and 1% of dough weight, respectively, with the only exception being the control dough which employed dextrose as a filler for improver. In total, five different dough formulations were made, namely control dough with no improver (D), dough with commercial improver (D þ DI), dough with a-amylase (D þ Am), dough with

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Table 1 Summary of results for all dough formulations, including abbreviations, dough properties, and baked product properties. Parameter

Mixing energy (W h/kg) Damaged starch in doughs (% dough weight) Free sugar (% dough weight) Dough properties Power Law flow consistency K (Pa sn) Power Law flow index, n % free liquid (g/g) Baking results % moisture loss (g/g) BSV (ml/g) Young’s modulus of breads (kPa)

Treatment Dough with ascorbic acid

Control dough

Dough with a-amylase and ascorbic acid

Dough with a-amylase

Dough with commercial improver

D þ As

D

D þ Am þ As

D þ Am

D þ DI

6.43  0.3a 7.4  0.2

5.74  0.6a 7.6  0.2

6.70  0.3b 7.5  0.2

6.45  0.6a 7.6  0.2

7.1  0.6b 6.4  0.2

2.4  0.05

2.4  0.05

2.6  0.05

2.7  0.05

1.6  0.05

2.92  0.13a 0.39 12.1  0.47a

2.32  0.06a 0.42 11.5  0.83a

2.03  0.12a 0.41 21.0  0.33b

1.73  0.02b 0.42 20.0  0.27b

1.92  0.06b 0.44 19.8  0.76b

13.3  0.2a 2.88  0.02a 14.8  2.65a

14.1  0.2a 2.97  0.03a 13.5  4.14a

13.7  0.3a 3.08  0.02b 11.1  2.16b

14.5  0.1a 3.20  0.04b 12.2  4.50b

14.5  0.3a 3.34  0.03b 13.5  4.14b

The values are expressed as mean  standard error of the mean. Values with the same superscript in each row are not significantly different (p > 0.05).

2.2.2. Damaged starch and free sugar Pieces of fresh, leavened doughs were collected, freeze-dried and then ground using a mortar and pestle. The samples were

analysed for damaged starch using the enzymatic and colourimetric digestion process described in AACC Method 76-31/ICC Method No 164 (Megazyme kitÒ). The free sugar content of these samples was determined by extraction into hot 80% ethanol (Ebell, 1969; MacRae et al., 1974; Rose et al., 1991) to selectively remove mono- and disaccharides. Colloidal proteins and pigments were precipitated using Carrez reagents and soluble sugars reacted with anthrone in 75% sulphuric acid. The colourimetric endpoint was read using a UVeVis spectrophotometer at 625 nm. Results were determined against D-glucose standards and as such are expressed in glucose equivalent values. 2.2.3. Ultracentrifugation All tests were conducted using duplicate batches. A Beckman Coulter Optimal L-90 ultracentrifuge was used to spin all doughs,

20

0.12

15 0.08

10

0.06 0.04

5

Mixing energy (W hr/kg)

0.10 Mixer power (kW)

ascorbic acid (D þ As), and dough with both latter improvers (D þ Am þ As); see Table 1. Dough batches were mixed to constant mixing energy. To accomplish this, the mixer was fitted with a power load sensor that output a DC voltage in direct proportion to the power drawn by the motor. The DC voltage was sampled by a high speed digital analogue converter and logged via USB into a LabviewÒ datafile. The power signal was integrated with respect to time in real time by Labview to obtain the current mixing energy. This was kept constant to ensure batch-to-batch replicability. Note that the mixer power was ‘tared’ with no dough in the bowl and the tare value subtracted to obtain only the energy used to mix dough. The temperatures of the flour and water were held consistent before mixing; this ensured a consistent dough temperature between batches following mixing. When mixed to ‘optimal’, i.e. until a peak was observed in the mixer power profile, dough was too sticky to handle during subsequent experiments. Therefore mixing was stopped before the peak was reached. The location of the end of mixing (160 s) in relation to the peak mixing time is presented in Fig. 1. All doughs were mixed for 100 s in a preliminary stage, paused for 10 s to add the leaveners, and then remixed for a further 60 s. All dough characterisation tests were carried out in duplicate within a batch and between batches. Doughs used for rheological and ultracentrifuge tests were mixed without the leavener. Following mixing, leavened doughs were divided into four pieces of similar weight and minimally manipulated by hand into uniform sized rectangular pads. These pieces were placed in baking pans and rested for 5 min before baking at 220  C for 20 min. Dough samples were collected for freeze-drying and subsequent testing for damaged starch and free sugar content. The baked breads were left to cool to lab temperature (controlled at 20  C) and weighed to estimate the moisture lost during baking. A single rectangular-shaped block of crumb was cut from each loaf using an electric knife, weighed and its dimensions measured using a digital calliper. These data allow the calculation of crumb sample volumes which along with the sample mass yields the baked specific volume of the breads (BSV, ml/g). With duplicate dough batches and four breads from each mix, an average BSV was calculated for each dough using eight samples of the resulting breads.

0.02

0

0.00 0

100

200 300 Time (s)

400

500

Fig. 1. Profile of mixer power and mixing energy for the control dough during mixing to peak power. The dashed lines indicate when mixing was stopped during preparation of the doughs used in this study.

M.J. Patel et al. / Journal of Cereal Science 56 (2012) 644e651

employing a rotational speed of 41,000 rpm (131,000 g) for 1 h under ambient temperature (23  C). The free liquid, gel, starch and extracted dough phases were clearly visible and photos are presented later to illustrate how their relative amounts varied between different doughs. The liquid phase was slowly poured out of the tubes and weighed. However, a viscometer was not available and liquid viscosities were not obtained. This is an area for future experimentation. 2.3. Rheological measurements 2.3.1. Brabender Extensograph Doughs for Extensograph testing were mixed in the same manner as all other dough batches (in the six pin mixer) with the exception that leaveners were excluded. Sample preparation followed AACC method 54-10, which requires that samples are rested in a temperature controlled chamber at 30  C for 45 min prior to extension. Dough thicknesses appeared different between formulations after the sample preparation process and photos of doughs immediately prior to extension are presented later for illustration. This test measures both dough strength (resistance to extension) and extensibility. The former is given by the maximum height of the force-extension (or load-stroke) profile from the machine. The extensibility is the distance travelled by the hook before dough strands break and is given by the width of the profile. 2.3.2. Compression testing of doughs The protocol employed is very similar to that used elsewhere (Chakrabarti-Bell et al., 2010; Charalambides et al., 2005) and only essential details are reproduced here. Cylindrical samples of dough were produced by hand by pressing freshly mixed dough into a cylindrical polytetrafluorethylene mould. Excess material was removed from the top and bottom with a sharp blade. The sample and former were then transported onto a loading platen fitted to the base of a servo-hydraulic Instron 8872 Fatigue Testing System (Instron Ltd, Buckinghamshire, UK). The sample was carefully ejected from the mould by hand; this process was aided by lubricating the inner surface of the mould. However, while the internal dimensions of the mould were 22 mm in both diameter and height, the dough samples exhibited some elastic ‘rebound’ such that the diameter after ejection from the mould was >22 mm. This necessitated that the sample diameters be measured (for later data analysis) before compression, which was performed nonintrusively using a camera and image analysis software. Doughs were compressed at three true strain rates (5/s, 0.5/s, and 0.05/s), and for each rate, tests were performed in duplicates both within a batch and between batches. Both the upper (mobile) and lower compression platens were lubricated to eliminate known friction effects on the compression force. The compression force (F) and the corresponding deformation (d) were recorded using the Wavematrix software supplied with the Instron. While dough is an aerated material, it has been shown that for most practical purposes, dough can be assumed to be incompressible (Wang et al., 2006). Thus the following equations can be used to estimate true stress and true strains. True stresss ¼ Fh=ðpR2 HÞ and true strainε ¼ lnðh=HÞ, where H indicates the initial height, R the initial radius, and h the current height such that h(t) ¼ H  d(t). 2.3.3. Compression testing of breads Eight loaves were baked from each of the doughs studied here, i.e. four loaves from each of two batches (Section 2.2.1). Of these, two were randomly selected from each batch for compression testing using the Instron apparatus. The crusts were carefully cut away and compression was conducted at a constant true strain rate

647

of 0.1/s. As before, the forceedisplacement data were used to calculate the true stress and true strain although a different equation was required for the true stress, namely s ¼ F=ðcross  sectional areaÞ. This is because breads are compressible and the cross-sectional area of the bread samples remained constant during each test. Using methods described elsewhere (Wang et al., 2011), Young’s moduli and yield strengths of breads were estimated from averaged plots of true stress-true strain data. The Young’s modulus was obtained from the linear range of the first derivative of stress with respect to strain. The yield stress, yield strain, collapse stress and collapse strain were obtained from the second derivative of the stress-strain data. This was necessary as the transition from linear to plateau to densification (Gibson and Ashby, 1988) was gradual and single points for the transitions could not be identified. 2.4. Statistical analysis The IBM Statistical Package for the Social Sciences (SPSS v.17) was used to conduct all statistical analyses. Values were obtained as the average  the standard error of the mean. Analysis of variance was performed using Fisher’s least significant difference test to compare the by-treatment means. Significance of results was determined by p-value for the different factors using a 95% confidence level. 3. Results 3.1. Mixing energies The mixing energy of the doughs varied with formulation (Table 1). The doughs containing ascorbic acid (D þ DI and D þ Am þ As) had slightly higher mixing energies than the other doughs, although this difference was not statistically significant. A high level of reproducibility in rheological data was observed both within and between batches, which confirms that the dough mixing protocol used here (constant temperature ingredients and constant mixing energy target) produces dough batches of consistent quality. 3.2. Baking results Statistically significant differences were observed between breads containing a-amylases and not containing a-amylase with respect to all parameters measured except for the amount of moisture lost during baking, which was similar between treatments (Table 1). Furthermore, the data split into two camps: low (but similar) BSVs for the control dough and dough with ascorbic acid, and higher (but similar) BSVs for doughs containing a-amylases. Thus, a-amylases have the same effect on chemically leavened doughs as for yeasted doughs, producing bigger loaves with softer crumbs; see also Section 3.5.3. 3.3. Damaged starch and free glucose The results from the Megazyme tests show little difference in damaged starch content between the doughs, with the exception of D þ DI which featured a smaller value (Table 1). However all values were similar to that for the flour (7.4%). Little difference was observed in the amount of free sugar between doughs except for a slightly lower value for the D þ DI dough. Data for damaged starch in doughs is unavailable in the literature to the authors’ knowledge, and it is unclear at this time if the resolution of the tests is high enough to determine the effects of a-amylases on dough.

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M.J. Patel et al. / Journal of Cereal Science 56 (2012) 644e651

3.4. Ultracentrifuge In line with previous results (Larsson and Eliasson, 1996), doughs separated into free liquid, gel, gluten, unseparated dough and free starch phases. Fig. 2 presents the liquid extracts from the doughs and the matter remaining in the tubes following spinning. Starch was deposited at the bottom of the tubes only from doughs that contained a-amylase. Liquor obtained from doughs without a-amylase (D and D þ Am) was low in volume, highly viscous and pale yellow in colour. Low liquor volume for doughs containing ascorbic acid was reported previously (Larsson and Eliasson, 1996). By contrast, doughs containing a-amylase produced a higher volume of much lower viscosity liquor which was brownish in colour. Doughs containing higher quantities of the liquor produced larger loaves and those with smaller amounts produced smaller loaves. It will be shown later that the liquor quantities also tracked with dough consistencies. While a correlation between the amount of free liquid following centrifugation and the specific volumes of breads has been

reported previously (MacRitchie, 1976; Seguchi et al., 2003), dough liquor viscosity data were unavailable in the literature. Given that the leavener concentrations were similar between all doughs, it can be envisaged that more steam was produced from the thin and plentiful dough liquor which facilitated the expansion of the loaves during baking. Note that the above mechanism could also explain the formation of larger loaves with a-amylase containing yeasted doughs. Further research is required to gain a better understanding of the contribution of steam and carbon dioxide in defining the baking qualities of doughs. 3.5. Rheology tests 3.5.1. Brabender Extensograph There was little effect of dough formulation on extensibilities of doughs as measured by the Extensograph. The photographs of doughs resting in dough holders showed differences between doughs; the control dough and doughs containing a-amylases sagged appreciably more than those containing ascorbic acid (Fig. 3) indicating differences in the way the doughs had relaxed.

Fig. 2. (a) Liquids extracted from doughs after centrifugation, and (b) starch deposited at the bottom of the centrifuge tubes for doughs containing a-amylase. Note the absence of similar deposits for the control dough and dough prepared with ascorbic acid.

M.J. Patel et al. / Journal of Cereal Science 56 (2012) 644e651

Higher values were obtained for Rmax (dough strength) when ascorbic acid was present. Note that the presence of ascorbic acid in doughs was noticed only from values of Rmax, although no effects were seen in the baking results. Note also that the thicknesses of the strands of ascorbic acid containing doughs were larger than other doughs, and this would rise to higher stretching forces. Further research is required to understand the effects of thickness of strands on measurement of stretching forces and Rmax. The lack of ability of Rmax values to consistently differentiate between dough formulations and baking qualities has been reported elsewhere (Cavanagh et al., 2010; Mann et al., 2009; Stojceska and Butler, 2012). 3.5.2. Dough in compression A sample plot of true stress vs. true strain is presented in Fig. 4(a) for the dough with ascorbic acid. Duplicates within a batch and between batches exhibited tight grouping. This and all other

649

doughs strain-harden as seen from the (non-linear) increase in stress with strain. A simple analysis was used to quantify the ratedependency of doughs. The stress at a strain of 0.5 was plotted against rate and a Power Law relationship fitted to the data. The R2 values of the fits were very high for all doughs, and the resulting parameters are commonly termed the ‘flow consistency’ (K e preexponential factor) and the ‘flow index’ (n e exponent from the fit); see Table 1. High K values indicate doughs to be more viscous and readily strain-hardening. Thus the control dough and the dough mixed with ascorbic acid were more viscous and more strainhardening than the a-amylase doughs. Little difference was observed in flow index between doughs. A final point is that the diameters of the compression samples increased upon ejection from the sample mould, i.e. immediately before the compression phase of each experiment. This is very likely a measure of the elasticity of the doughs, which must ‘springback’ from the stresses imposed on them during sample forming. The

Fig. 3. Results and photographs from Brabender Extensograph tests.

M.J. Patel et al. / Journal of Cereal Science 56 (2012) 644e651

a

4

3.4

-7

Rate 5/s

-6

3.3

3.2

-4 -3

BSV (ml/g)

True stress (kPa)

3

-5

Rate 0.5/s

-2

2

3.1

3.0

Rate 0.05/s

-1

1 2.9

0 0.0

-0.2

-0.4 -0.6 True strain (-)

-0.8

-1.0 5

-14

-10

2.88 ml/g 2.97 ml/g 3.08 ml/g 3.20 ml/g 3.34ml/g

-8 -6 -4 -2 0

0.0

-0.2

-0.4 -0.6 True strain (-)

10

15

20

25

% free liquid (g/g)

D+As D D+Am+As D+Am D+DI

-12

True stress (kPa)

0

2.8

b

Power Law flow consistency K (Pa sn )

650

-0.8

-1.0

Fig. 4. (a) Sample stress-strain plots in compression at three true strain rates. Data correspond to the dough with ascorbic acid (D þ As) and show reproducibility within and between batches. (b) True stress-true strain plots of breads in compression at true strain rate of 0.1/s. The BSVs are ranked (in ascending order) D þ As (B); D (>); D þ Am þ As (); D þ Am (6); D þ DI (,).

doughs split into three groups based on this data, namely D þ As (22 mm, 12 samples) > D w D þ Am þ As (23 mm, 24 samples) > D þ Am w D þ DI (24 mm, 24 samples). These results indicate that doughs containing a-amylases were more elastic (greater expansion in diameter) than doughs containing ascorbic acid. Further measurements of dough elasticity are merited in future. Fig. 5 presents the variation of BSV and flow consistency with percentage of liquid extracted during centrifuging. Flow consistency decreases while BSV increases with liquid extracted. Only the a-amylase containing doughs featured significantly more liquid and this was also lower in viscosity. This suggests that the gluten (or dough matrix) is suspended in this low viscosity liquid, which likely explains the lower flow consistency (and possibly easier rebound when released from a stretch, i.e. higher elasticity) for these doughs. In summary, there were only two different quality suspending media, and both rheology and baking qualities separated based on its characteristics.

Fig. 5. BSV (B) and dough flow consistency K (D) vs. the percentage of free liquid extracted from doughs. Data separate between formulations based on the amount of liquid extracted, i.e. box ‘A’ incorporates doughs not containing a-amylase and box ‘B’ incorporates those containing a-amylase.

Note that the strain-hardening characteristics of doughs showed more consistent difference between dough formulations than dough strengths obtained from Extensograph testing. 3.5.3. Compression testing of breads Bread is a foam and the true stress-true strain plots of bread follow the behaviour of foams in compression; see Fig. 4(b). These curves are averaged from four samples including duplicates within and between batches. While a linear range is observed at small strain, the plateau formation (partial breakdown of foam structure) leading to collapse conditions (complete breakdown of foam structure) was gradual unlike simple honeycomb foams (Gibson and Ashby, 1988). Given this gradual rise in stress with increase in compression, attempts were not made to derive the yield and collapse conditions. The Young’s moduli for these breads were derived from the linear range and are presented in Table 1. The variation in Young’s modulus followed the same trend as shown by Wang et al. (2011) in that higher modulus corresponded with lower BSV. Statistically, there were only two groups of products: breads with and without a-amylases with the former having lower Young’s moduli (softer than control and ascorbic acid breads). Note that stresses at large strains remained lower for the a-amylase breads, indicating that they would be perceived as softer during mastication as well. 3.6. Conclusions The addition of fungal a-amylases to chemically leavened doughs leads to the formation of bigger loaves with softer crumbs. Ascorbic acid, when added in similar amount as a-amylases, has little effect on baking qualities. The differences in quantity and quality of dough liquors provide an explanation for the effects of these additives on the baking and rheological properties of doughs. Results indicate that doughs are suspensions of solid phase dough in a continuous aqueous medium, and that the quality of the continuous medium is a key driver for

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rheological and baking qualities of doughs. With large quantities of low viscosity fluid, doughs are softer in consistency, provide less resistance to strain-hardening and exhibit greater elastic springback. The reverse is true when the mobility of dough is restricted by a viscous suspending medium of limited amount. Furthermore, it is likely that more steam is produced from a larger volume of lower viscosity liquor which, given the favourable strain-hardening properties of these doughs, facilitates expansion of dough during baking. The above mechanism would also explain the beneficial effects of a-amylases observed with both yeasted breads and chapattis. The measurements of true rheological properties along with the volume of free liquid present in doughs are important as these data provide insights into a-amylase action on doughs. Further research is required to obtain a better understanding of the effects of dough improvers on liquor properties, the production of steam and carbon dioxide, and thus on baking qualities. Acknowledgements The authors would like the thank Dr. Marcus Newbury (CSIRO) and Dr. Crispin Howitt (CSIRO) for helpful discussions, Mr. Michael Watson (Deltagen Australia Pty Ltd.) for measuring enzyme activities, Dr. Ken Dods (Chemistry Centre, Perth, Western Australia) for measuring free sugars and damaged starches and Mr. Stephen Brown (State Agricultural Biotechnology Centre, Murdoch University, Australia) for assistance with the ultracentrifuge tests. References Aamodt, A., Magnus, E.M., Faergestad, E.M., 2003. Effect of flour quality, ascorbic acid, and DATEM on dough rheological parameters and hearth loaves characteristics. Journal of Food Science: Food Chemistry and Toxicology 68 (7), 2201e2210. Blaszczak, W., Sadowska, J., Rosell, C.M., Fornal, J., 2004. Structural changes in the wheat dough and bread with the addition of alpha-amylases. European Food Research and Technology 219 (4), 348e354. Cavanagh, C.R., Taylor, J., Larroque, O., Coombes, N., Verbyla, A.P., Nath, Z., Kutty, I., Rampling, L., Butow, B., Ral, J.-P., Tomoskozi, S., Balazs, G., Békés, F., Mann, G., Quail, K.J., Southan, M., Morell, M.K., Newberry, M., 2010. Sponge and dough bread making: genetic and phenotypic relationships with wheat quality traits. Theoretical and Applied Genetics 121 (5), 815e828. Chakrabarti-Bell, S., Bergström, J.S., Lindskog, E., Sridhar, T., 2010. Computational modeling of dough sheeting and physical interpretation of the non-linear rheological behavior of wheat flour dough. Journal of Food Engineering 100 (2), 278e288. Charalambides, M.N., Goh, S.M., Wanigasooriya, L., Williams, J.G., Xiao, W., 2005. Effect of friction on uniaxial compression of bread dough. Journal of Materials Science 40 (13), 3375e3381. Domingues, D.J., Lonergan, D.A., 2007. Refrigerated, Chemically-leavened Dough in Low Pressure Package. U.S.P. Office, USA. Ebell, L.F., 1969. Specific total starch determinations in conifer tissues with glucose oxidase. Phytochemistry 8 (1), 25e36. Gibson, L.J., Ashby, M.F., 1988. Cellular Solids: Structure and Properties. Pergamon Press, Oxford. Grosch, W., Wieser, H., 1999. Redox reactions in wheat dough as affected by ascorbic acid. Journal of Cereal Science 29 (1), 1e16.

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