- Email: [email protected]
Rheology of corn dough with gum arabic: Stress relaxation and two-cycle compression testing and their relationship with sensory attributes
Journal of Food Engineering 74 (2006) 89–95 www.elsevier.com/locate/jfoodeng
Rheology of corn dough with gum arabic: Stress relaxation and two-cycle compression testing and their relationship with sensory attributes Sila Bhattacharya *, H.V. Narasimha, Suvendu Bhattacharya Central Food Technological Research Institute, Mysore 570 020, India Received 6 July 2004; accepted 12 February 2005 Available online 18 April 2005
Abstract Rheology of corn flour doughs was studied employing stress relaxation and two-cycle compression tests performed at different moisture (48–62%) and gum Arabic (0–2%) contents of the dough. The parameters determined from small-deformation (5% strain) stress relaxation experiments were the empirical constants k1 and k2, while two-compression cycle tests gave textural parameters like hardness, cohesiveness, springiness and adhesiveness of the doughs. Corn doughs were sensitive to moisture contents that were reflected by the values of the relaxation constants (k1 and k2) and textural parameters. Addition of gum markedly affected sensory adhesiveness and cohesiveness. These values were interrelated with sensory attributes using principal component analysis (PCA) to obtain suitable condition for handling of dough. 2005 Elsevier Ltd. All rights reserved. Keywords: Corn dough; Rheology; Gum Arabic; Stress relaxation; Two-cycle compression; Sensory attributes
1. Introduction Cereal flours or starches are widely used because of their desirable effect on the acceptability of the product, particularly the texture or more specifically, the crispness of the finished product. Addition of a hydrocolloid substance to a system that is rich in starch which itself is a thickening agent, strongly influences the rheological behaviour of the system, may it be a paste or slurry (Alloncle & Doubilier, 1991) or a dough (Singh Sidhu & Bawa, 2002). Food gums, though added at low levels upto 5% (Burdock, 1997), enable strong absorption at the oil interface and promote emulsion stability (Williams, Philips, & Randall, 1990) apart from increasing the overall viscosity of the system. Gum Arabic (Acacia senegal), *
Corresponding author. Tel.: +91 821 2513910; fax: +91 821 2517233. E-mail address: [email protected] (S. Bhattacharya). 0260-8774/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.02.006
despite its relatively high molecular weight (400,000) yields dispersions of surprisingly low viscosity. Such behaviour is uncharacteristic of polysaccharides in general (Williams et al., 1990). The consequence of this phenomenon is the wide application of gum Arabic in several foods and pet foods (Kravtchenko & Merlat, 2000). The rheological characteristics of a dough can predict about their expected behaviour under various processing conditions of stress such as flattening, sheeting, rolling, extrusion, etc., that in turn may help to select suitable raw materials and their proportions, and to decide the appropriate process equipment. Consequently, the quality of the finished product including texture, and hence, consumer acceptability is affected. The viscoelastic behaviour of dough can be measured by determining the stress–strain relationship (Bhattacharya, Bhattacharya, & Narasimha, 1999) or by the application of step changes in stress or strain; the
90
S. Bhattacharya et al. / Journal of Food Engineering 74 (2006) 89–95
Nomenclature Y(t) F0 F(t) t
decaying parameter (dimensionless) initial force at time t = 0 (N) force at relaxation time t (N) time of relaxation (s)
relation between stress and strain is time-dependent, and can be expressed in terms of creep compliance (constant stress) or the stress relaxation function (constant strain) (Hamann & McDonald, 1992). The former type of test needs a costly and sophisticated controlled stress rheometer whereas a conventional medium-cost texture-measuring device is suitable for the latter test. Determination of the rheological properties of a dough using the principle of stress relaxation methodology is one of the most common methods (Navickis, 1987; Safari-Ardi & Phan-Thien, 1998; Larsson, Eliasson, Johansson, & Svensson, 2000) for its characterisation because of its simplicity in conducting the experiments and adequate repeatability of experimental results. The stress decay data during relaxation experiment can be utilised to determine a few relaxation indices, and may be fitted to empirical equations (Bhattacharya & Narasimha, 1997). But fitting of stress relaxation curves may sometimes cause problems since food doughs may exhibit non-linear viscoelastic behaviour when subjected to large deformation (Budiman, Stroshine, & Campanella, 2000). The problem can be solved by the method of normalisation, as suggested by Peleg and Normand (1983). In a different approach, the application of imitative method comprising two-cycle compression or two-bite test can provide useful information about the rheological behaviour of dough that may correlate well with sensory attributes (Hamann & McDonald, 1992) concerning acceptance of the finished product. The products that can be made using corn flour dough include extrusion-shaped cereal-based chips that are fried/baked/microwaved to obtain a crisp snack. The role of the gum, added to dough, thus can serve as an agent that can reduce the oil uptake (Pszczola, 1999; Bhattacharya, Narasimha, & Bhattacharya, 2003). It is, therefore, necessary to investigate the rheological behaviour of corn dough with gums and relating these properties with the sensory attributes of the product. The role of water content in this situation plays an important role as it acts as a plasticiser that affect the rheological behaviour markedly (Hibberd & Parker, 1978; Larsson et al., 2000). In an earlier research, the present authors (Bhattacharya et al., 2003) have reported that a three element spring-dashpot model can predict the stress relaxation data obtained from lowstrain experiments of corn dough while the addition of
k 1, k 2 p r
constants in Eq. (2) probability level (dimensionless) correlation coefficient (dimensionless)
gum Arabic reduces the oil content in the finished fried product. It may be mentioned here that relationship between low-deformation stress relaxation data with largedeformation compression data are scarce particularly for corn doughs with added gums. The objectives of the present research are thus (a) to determine the rheological behaviour of corn doughs at different moisture and gum contents employing lowstrain stress relaxation and large-strain two-cycle compression testing and (b) to correlate these objective results with sensory measurements.
2. Materials and methods 2.1. Material Corn kernels were procured from the local market of Mysore, India; after cleaning, they were debranned and degermed in a laboratory model McGill polisher (Bhattacharya & Bhattacharya, 1994). The kernels were ground in a laboratory model vertical grinder to obtain corn flour that passed through a 200 mesh British Standard (BS) sieve (75 lm opening); the temperature of the flour was kept below 40 C during grinding. Gum Arabic (acacia) powder was obtained from Nice Chemicals, Cochin, India. 2.2. Dough preparation Varying quantities of water were added to attain the desired dough moisture content (48, 53, 57 and 62%, wet basis), and mixed for 2 min in a Hobart mixer at a low speed to obtain homogeneous doughs. Cylindrical samples (35 mm diameter and 20 mm height) were prepared (Bhattacharya & Narasimha, 1997) and the process was replicated twice. Dough samples with gum at 0, 1 and 2% (based on the weight of corn flour) were also made maintaining the above mentioned moisture levels. 2.3. Stress relaxation Stress relaxation characteristics of the lubricated (using paraffin oil) cylindrical sample were determined by compressing the sample up to a strain of 0.05 at a compression rate of 200 mm min1. When the compression was achieved at the desired level, the cross head
S. Bhattacharya et al. / Journal of Food Engineering 74 (2006) 89–95
surface (50 mm in diameter) was stopped, and the dough was allowed to relax for 480 s. The force at different relaxation times was continuously monitored. If a stress is applied quickly on a viscoelastic material and the strain is kept constant later after attaining the desired strain, the material shows a decaying stress as a function of time. The relaxation curve has been modelled by (Eq. (1)) and later normalised (Eq. (2)) by Peleg and Normand (1983), and Purkayastha and Peleg (1986) and was applied by Bhattacharya and Narasimha (1997) on blackgram dough to calculate the decay parameters. Y ðtÞ ¼ F 0 =ðF 0 F ðtÞÞ
ð1Þ
F 0 t=½F 0 F ðtÞ ¼ k 1 þ k 2 t
ð2Þ
Here, 1/k1 is the initial decay rate and 1/k2 denotes the asymptotic value of the relaxed portion of the stress. The constants (k1 and k2) were obtained from the intercept and slope, respectively from the relaxation curve according to Eq. (2). The reported results are the mean of five observations and the whole experiment was replicated twice. 2.4. Two-cycle compression The cylindrical samples as mentioned earlier, were compressed up to a strain of 0.25 for two-times in succession at a speed of 10 mm min1 using a Universal Testing machine (Model # 5R, Lloyd, UK). The software provided by the manufacturer was used to calculate hardness, cohesiveness, elasticity or springiness and adhesiveness. Five samples were examined each time and the experiment was repeated twice. 2.5. Sensory assessment The corn dough samples, about 10 mm thick, were assessed by 10 untrained panelists by hands and fingers to determine the sensory attributes such as hardness, cohesiveness, elasticity or springiness and adhesiveness as suggested by Kalviainen, Roininen, and Tuorila (2000) with suitable modifications according to Table 1. Later, the data were analyzed using principal component analysis (PCA) to correlate the different parameters including objective and subjective data (Lawless & Heymann, 1998) using the software Statistica (Statsoft,
91
Tulsa, OH, USA). The anchors were between 0 and 15 cm to indicate the lowest and highest levels, respectively of the sensory attributes. 2.6. Microstructure The dough samples were dehydrated, mounted on aluminium stubs with conductive adhesive, coated with gold in a sputter coater and were examined with a scanning electron microscope (Model 435 VP, Leo Electron Microscopy Ltd., Cambridge, UK) as mentioned earlier (Bhat & Bhattacharya, 2001), and representative photomicrographs of doughs with different level of gum were obtained. 2.7. Data analysis The independent variables were the moisture content (48.0, 52.7, 57.4 and 62.1%) and gum content (0, 1 and 2%) such that a total of 12 combinations were possible to generate 36 data sets due to repetition experiments. The correlation coefficients, presented in Table 1, were based on 36 data points. The significance of the correlation coefficients was judged at a probability of 0.01. The contour graphs were generated by fitting data to a second order polynomial against the independent variables of moisture and gum contents.
3. Results and discussion Three different set of results, such as, parameters of stress relaxation (k1 and k2), two-cycle compression parameters (hardness, cohesiveness, springiness and adhesiveness) and sensory attributes (hardness, cohesiveness, springiness and adhesiveness) have been cited and discussed for corn dough. 3.1. Stress relaxation The parameter [F0t/(F0 F(t))] when plotted against relaxation time yielded straight lines (r P 0.998, p 6 0.01) for corn doughs at all combinations of moisture content (48–62%) and gum content (0–2%); corn doughs without gum exhibited higher slopes indicating
Table 1 Different sensory attributes and their method of evaluation Sensory attribute
Method of determination
Anchors
Hardness
Force experienced to touch the fore-finger and thumb when the dough sample (10 mm thick, 35 mm diameter) was placed between the two fingers Pressing the sample by the tip of the forefinger to about 5 mm thickness and feeling the recovery while releasing the finger Pulling the dough sample to separate completely Extent of adhesion to fingers while releasing the fingers at the end of hardness measurement
Soft–Hard
Springiness Cohesiveness Adhesiveness
Non-springy–Springy Non-cohesive–Cohesive Non-adhesive–Adhesive
92
S. Bhattacharya et al. / Journal of Food Engineering 74 (2006) 89–95
pronounced solid behaviour than the gum containing doughs (data not presented). At higher moisture contents, the protein in the dough was probably more swollen and consequently more deformable (Visser, 1991), and the protein concentration was lower. Thus, the protein–protein interactions within the dough network may have decreased lowering the stress. The addition of gum in the dough shows less F(t) values during relaxation such that they tend to be a poor elastic solid that can be flattened or rolled easily. The reason behind such behaviour may be due to the ability of gum to hold high volume of water and thereby increasing the plasticising effect of the dough. 3.2. Relaxation parameters (k1 and k2) The reciprocal of the constant k1 indicates the initial decay rate. Thus, a high k1 value is associated with a dough that possess a low-decay rate indicating pronounced elastic behaviour. The k1 values were in the range of 1.5 and 1.9 (Fig. 1) that are very low compared to blackgram dough (about 10–18, Bhattacharya & Narasimha, 1997) due to higher moisture levels (48– 62% in the present case compared to 32–40% for blackgram) and material characteristics. An increase in moisture content decreased k1 values to attain a minimum of 1.5 at about 55% moisture. The k2 values were between 25.2 and 38.6 s1(Fig. 2) and higher values were observed for doughs without gum. An increase in moisture and/or gum usually reduced k2 values to attain a minimum level at about 55% moisture and 1.4% gum contents. As k2 value for a liquid is low in magnitude (Bhattacharya & Narasimha, 1997), a decrease in k2 is expected by shifting the dough from a viscoelastic solid to a viscoelastic liquid; this tendency was observed in most of the cases except for doughs with very high moisture contents accompanied by high level of gum. Possibly, a high level
Fig. 1. Effect of moisture and gum content on relaxation parameter k1.
Fig. 2. Variation in relaxation parameter k2 with moisture and gum content of dough.
of gum (2%) may bind the water as well as the flour in such a manner such that the lubricating effect of water is arrested. 3.3. Two-cycle compression test The hardness values of the dough were between 3 and 40 N, and showed a continuous decreasing trend with an increase in moisture content while gum had a marginal effect (Fig. 3). As hardness is a direct measure of the resistance of the dough towards the compressive stress, it is expected that high level of moisture (such as 62%) produces a dough with meager hardness (less than 5 N) which also indicates the easy flattening characteristics. The springiness or elasticity of the dough samples were between 2 and 3 mm (i.e., relaxing only to the extent of 40–60%), and decreased both with moisture and gum contents (Fig. 4) in a curvilinear manner.
Fig. 3. Contour plot for hardness at different moisture and gum content of dough.
S. Bhattacharya et al. / Journal of Food Engineering 74 (2006) 89–95
93
Fig. 4. Springiness as a function of moisture and gum content of dough.
Fig. 5. Cohesiveness as a function of moisture and gum content of dough.
Fig. 7. Microstructure of corn dough (A) without gum and with (B) 1% gum and (C) 2% gum.
Fig. 6. Effect of moisture and gum content on adhesiveness of dough.
A high springiness value is undesirable for flattening purpose to obtain the flattened sheets because the thickness of the sheets would increase during post-flattening
resting. In such a situation, the incorporation of gum at low levels like 1.0–1.5% may be helpful. The cohesiveness, on the other hand, increased in a near linear mode with both of these variables (Fig. 5) while moisture content had a dominating effect. A dough with cohesiveness less than 0.2 resulted with low-moisture content in absence of gum; these doughs, if compressed to a high level of strain (>0.4) such as in the case of flattening, fails to retain their integrity due to inadequate cohesive
94
S. Bhattacharya et al. / Journal of Food Engineering 74 (2006) 89–95
characteristics. On the contrary, the adhesiveness of the dough (0.13–1.44) was highly affected by moisture content showing an increase of even above 10 folds when moisture is elevated to 62% (Fig. 6). The effect of gum depends on the moisture content of dough; at a lowmoisture content, such as 48%, the addition of gum increased adhesiveness whereas at a high moisture content such as 62%, gum played a marginal role. As high adhesiveness of dough is undesirable for handling of dough like transportation, sheeting, flattening and rolling, it is better to avoid doughs that are highly adhesive in nature; an adhesiveness less than 1.0 appears suitable for flattening when moisture content is less than 60%. 3.4. Microstructure The photomicrographs (Fig. 7) of corn doughs (at magnifications of 1500·) indicated loosely bound particles that were partially adhered to other flour particles possibly by soluble flour fraction or leached out materials. Significant differences in dough microstructure (Fig. 7A–C) were not observed when gums up to 2% level were incorporated.
in moisture content markedly decreased hardness and springiness while opposite trend was true for cohesiveness and adhesiveness. The last two attributes also increased with an increase in gum content. Table 3 shows the correlations between the sensory attributes and objective measurements. Good correlation (r P j0.90j, p 6 0.01) were observed between objective adhesiveness and sensory hardness, and with springiness while both were negatively related. Objective cohesiveness also correlated well (r P j0.93j, p 6 0.01) with sensory hardness and adhesiveness. The stress relaxation parameters (k1 and k2) had no-significant effect on sensory attributes. PCA analysis was conducted with respect to four sensory attributes such as hardness, cohesiveness, elasticity and adhesiveness to establish the relationship between attributes and samples. The principal component (PC1) on the X-axis could explain 84% (Fig. 8) of the variance in data while PC2 accounted for only 12%; thus, in total 96% of data were suitable for the application of PCA methodology. Hardness and springiness/ elasticity formed a group on the positive side of X-axis with high loadings indicating high association or corre-
3.5. Sensory assessment and interrelations The results of the sensory assessment of the textural attributes (hardness, cohesiveness, springiness/elasticity and adhesiveness) are shown in Table 2. An increase
Table 2 Sensory assessment of dough Moisture content of dough (%)
Gum content (%)
Hardness
Cohesiveness
Springiness
Adhesiveness
48 53 57 62 48 53 57 62 48 52 57 62
0 0 0 0 1 1 1 1 2 2 2 2
13.8 ± 0.6 10.2 ± 0.7 7.6 ± 0.4 3.7 ± 0.4 12.7 ± 0.5 12.3 ± 0.6 6.3 ± 0.2 3.0 ± 0.3 12.2 ± 0.8 6.5 ± 0.6 4.2 ± 0.5 1.5 ± 0.2
3.7 ± 0.3 4.5 ± 0.7 7.3 ± 0.4 7.9 ± 0.2 6.3 ± 0.7 7.8 ± 0.6 9.9 ± 0.9 13.9 ± 0.7 12.5 ± 0.3 11.9 ± 0.9 14.2 ± 0.5 14.0 ± 0.4
12.1 ± 0.8 10.9 ± 0.6 4.5 ± 0.6 1.9 ± 0.1 13.2 ± 0.6 7.7 ± 0.6 2.3 ± 0.6 1.4 ± 0.6 12.8 ± 0.3 2.3 ± 0.4 1.4 ± 0.3 1.5 ± 0.6
2.1 ± 0.1 2.8 ± 0.3 7.0 ± 0.8 9.7 ± 0.3 5.4 ± 0.2 8.6 ± 0.6 12.9 ± 0.4 14.0 ± 0.3 8.7 ± 0.7 10.4 ± 0.5 12.2 ± 0.4 14.0 ± 0.6
Fig. 8. Principal component analysis (PCA) of corn flour dough. Symbols 53/1 indicates for corn dough having 53% moisture and 1% gum content. Other samples are defined in a similar manner.
Table 3 Correlation coefficient (r) matrix between the sensory attributes and objective parameters Sensory attributes
Rheological (objective) parameters Hardness
Hardness Cohesiveness Springiness Adhesiveness
Cohesiveness
0.902*
0.944*
0.602 0.931* 0.818*
0.820* 0.889* 0.933*
Springiness
Adhesiveness
k1
k2
0.665 0.914* 0.663 0.903*
0.896*
0.112 0.415 0.334 0.445
0.314 0.679 0.383 0.668
0.729* 0.895* 0.912*
S. Bhattacharya et al. / Journal of Food Engineering 74 (2006) 89–95
lation with the sample 53/1 followed by 48/2 and 48/1; the values are expressed here as moisture content/gum content. This means that low-moisture samples are associated with high values of hardness and elasticity. On the other hand, high moisture sample (62/0) without gum exhibited low magnitudes of these two attributes. Samples like 57/2, 62/1 and 53/2 had loaded more with cohesiveness and adhesiveness which was indicated by their presence on the negative side of X-axis. Samples without gum with 48, 53 and 57% moisture contents exhibited poor cohesiveness and adhesiveness. As sheeting/flattening/forming needs enough cohesiveness with low values for hardness, elasticity and adhesiveness, the suitable conditions may be 52–57% moisture with 1–2% gum.
References Alloncle, M., & Doubilier, J. L. (1991). Viscoelastic properties of maize starch/hydrocolloid pastes and gels. Food Hydrocolloids, 5, 455–467. Bhat, K. K., & Bhattacharya, S. (2001). Deep fat frying characteristics of chickpea flour suspensions. International Journal of Food Science and Technology, 35, 499–507. Bhattacharya, S., & Bhattacharya, S. (1994). Flow behaviour of cooked maize flour suspensions and applicability of mathematical models. Journal of Food Process Engineering, 17, 263–278. Bhattacharya, S., & Narasimha, H. V. (1997). Puncture and stress relaxation behaviour of blackgram (Phaseolus mungo) flour-based papad dough. Journal of Food Process Engineering, 20, 301– 316. Bhattacharya, S., Bhattacharya, S., & Narasimha, H. V. (1999). Uniaxial compressibility of blackgram doughs blended with cereal flours. Journal of Texture Studies, 30, 659–675. Bhattacharya, S., Narasimha, H. V., & Bhattacharya, S. (2003). Effect of gum Arabic on the rheology of corn flour doughs and fried product quality. Journal of Texture Studies, 34, 421–436. Budiman, M., Stroshine, R. L., & Campanella, O. H. (2000). Stress relaxation and low field proton magnetic resonance studies of cheese analog. Journal of Texture Studies, 31, 477–498. Burdock, G. A. (1997). Encyclopedia of food and colour additives (vol. II). Boca Raton, FL, USA: CRC Press.
95
Hamann, D. D., & McDonald, G. A. (1992). Rheology and texture properties of Surimi based food. In T. C. Lanier & C. M. Lee (Eds.), Surimi technology (pp. 429–500). New York: Marcel Dekker. Hibberd, G. E., & Parker, N. S. (1978). A parallel plate rheometer for measuring the viscoelastic properties of wheat flour dough. Cereal Chemistry, 55, 102–110. Kalviainen, N., Roininen, K., & Tuorila, H. (2000). Sensory characterisation of high viscosity gels made with different thickeners. Journal of Texture Studies, 31, 407–420. Kravtchenko, T., & Merlat, L. (2000). Soluble and natural acacia fibre for foods. LVT-Lebensmittel-Verfahrens-und-Verpackungste-chnik, 45, 115–118. Larsson, H., Eliasson, A. C., Johansson, E., & Svensson, G. (2000). Influence of added starch on mixing of dough made with three wheat flours differing in high molecular weight sub unit composition: Rheological behaviour. Cereal Chemistry, 77, 633–639. Lawless, H. T., & Heymann, H. (1998). Sensory evaluation of food: Principles and practices. New York: Chapman & Hall, pp. 606–608. Navickis, L. L. (1987). Corn flour addition to wheat flour doughs. Effect on rheological properties. Cereal Chemistry, 64, 307–310. Peleg, M., & Normand, M. D. (1983). Comparison of two methods for stress relaxation data presentation of solid foods. Rheologica Acta, 22, 108–113. Pszczola, D. E. (1999). Starches and gums more beyond fat replacement. Food Technology, 53(8), 74–76, 78, 80. Purkayastha, S., & Peleg, M. (1986). Comparison between projected mechanical equilibrium conditions of selected food materials in stress relaxation and creep. Journal of Texture Studies, 17, 433– 444. Safari-Ardi, M., & Phan-Thien, N. (1998). Stress relaxation and oscillatory tests to distinguish between dough prepared from wheat flours of different varietal origin. Cereal Chemistry, 75, 80–84. Singh Sidhu, J. P., & Bawa, A. S. (2002). Dough characteristics and baking studies of wheat flour fortified with Xantham gum. International Journal of Food Properties, 5, 1–11. Visser, J. (1991). Factors affecting the rheological and fracture properties of hard and semihard cheese. Bulletin of International Dairy Federation, 268, 49–61. Williams, P. A., Philips, G. O., & Randall, R. C. (1990). Structure– function relationship of gum Arabic. In G. O. Phillips, P. A. Williams, & D. J. Wedlock (Eds.). Gums and stabilizers for the food industry (vol. 5, pp. 25–36). Oxford: DIRL Press.
Rheology of corn dough with gum arabic: Stress relaxation and two-cycle compression testing and their relationship with sensory attributes Sila Bhattacharya *, H.V. Narasimha, Suvendu Bhattacharya Central Food Technological Research Institute, Mysore 570 020, India Received 6 July 2004; accepted 12 February 2005 Available online 18 April 2005
Abstract Rheology of corn flour doughs was studied employing stress relaxation and two-cycle compression tests performed at different moisture (48–62%) and gum Arabic (0–2%) contents of the dough. The parameters determined from small-deformation (5% strain) stress relaxation experiments were the empirical constants k1 and k2, while two-compression cycle tests gave textural parameters like hardness, cohesiveness, springiness and adhesiveness of the doughs. Corn doughs were sensitive to moisture contents that were reflected by the values of the relaxation constants (k1 and k2) and textural parameters. Addition of gum markedly affected sensory adhesiveness and cohesiveness. These values were interrelated with sensory attributes using principal component analysis (PCA) to obtain suitable condition for handling of dough. 2005 Elsevier Ltd. All rights reserved. Keywords: Corn dough; Rheology; Gum Arabic; Stress relaxation; Two-cycle compression; Sensory attributes
1. Introduction Cereal flours or starches are widely used because of their desirable effect on the acceptability of the product, particularly the texture or more specifically, the crispness of the finished product. Addition of a hydrocolloid substance to a system that is rich in starch which itself is a thickening agent, strongly influences the rheological behaviour of the system, may it be a paste or slurry (Alloncle & Doubilier, 1991) or a dough (Singh Sidhu & Bawa, 2002). Food gums, though added at low levels upto 5% (Burdock, 1997), enable strong absorption at the oil interface and promote emulsion stability (Williams, Philips, & Randall, 1990) apart from increasing the overall viscosity of the system. Gum Arabic (Acacia senegal), *
Corresponding author. Tel.: +91 821 2513910; fax: +91 821 2517233. E-mail address: [email protected] (S. Bhattacharya). 0260-8774/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.02.006
despite its relatively high molecular weight (400,000) yields dispersions of surprisingly low viscosity. Such behaviour is uncharacteristic of polysaccharides in general (Williams et al., 1990). The consequence of this phenomenon is the wide application of gum Arabic in several foods and pet foods (Kravtchenko & Merlat, 2000). The rheological characteristics of a dough can predict about their expected behaviour under various processing conditions of stress such as flattening, sheeting, rolling, extrusion, etc., that in turn may help to select suitable raw materials and their proportions, and to decide the appropriate process equipment. Consequently, the quality of the finished product including texture, and hence, consumer acceptability is affected. The viscoelastic behaviour of dough can be measured by determining the stress–strain relationship (Bhattacharya, Bhattacharya, & Narasimha, 1999) or by the application of step changes in stress or strain; the
90
S. Bhattacharya et al. / Journal of Food Engineering 74 (2006) 89–95
Nomenclature Y(t) F0 F(t) t
decaying parameter (dimensionless) initial force at time t = 0 (N) force at relaxation time t (N) time of relaxation (s)
relation between stress and strain is time-dependent, and can be expressed in terms of creep compliance (constant stress) or the stress relaxation function (constant strain) (Hamann & McDonald, 1992). The former type of test needs a costly and sophisticated controlled stress rheometer whereas a conventional medium-cost texture-measuring device is suitable for the latter test. Determination of the rheological properties of a dough using the principle of stress relaxation methodology is one of the most common methods (Navickis, 1987; Safari-Ardi & Phan-Thien, 1998; Larsson, Eliasson, Johansson, & Svensson, 2000) for its characterisation because of its simplicity in conducting the experiments and adequate repeatability of experimental results. The stress decay data during relaxation experiment can be utilised to determine a few relaxation indices, and may be fitted to empirical equations (Bhattacharya & Narasimha, 1997). But fitting of stress relaxation curves may sometimes cause problems since food doughs may exhibit non-linear viscoelastic behaviour when subjected to large deformation (Budiman, Stroshine, & Campanella, 2000). The problem can be solved by the method of normalisation, as suggested by Peleg and Normand (1983). In a different approach, the application of imitative method comprising two-cycle compression or two-bite test can provide useful information about the rheological behaviour of dough that may correlate well with sensory attributes (Hamann & McDonald, 1992) concerning acceptance of the finished product. The products that can be made using corn flour dough include extrusion-shaped cereal-based chips that are fried/baked/microwaved to obtain a crisp snack. The role of the gum, added to dough, thus can serve as an agent that can reduce the oil uptake (Pszczola, 1999; Bhattacharya, Narasimha, & Bhattacharya, 2003). It is, therefore, necessary to investigate the rheological behaviour of corn dough with gums and relating these properties with the sensory attributes of the product. The role of water content in this situation plays an important role as it acts as a plasticiser that affect the rheological behaviour markedly (Hibberd & Parker, 1978; Larsson et al., 2000). In an earlier research, the present authors (Bhattacharya et al., 2003) have reported that a three element spring-dashpot model can predict the stress relaxation data obtained from lowstrain experiments of corn dough while the addition of
k 1, k 2 p r
constants in Eq. (2) probability level (dimensionless) correlation coefficient (dimensionless)
gum Arabic reduces the oil content in the finished fried product. It may be mentioned here that relationship between low-deformation stress relaxation data with largedeformation compression data are scarce particularly for corn doughs with added gums. The objectives of the present research are thus (a) to determine the rheological behaviour of corn doughs at different moisture and gum contents employing lowstrain stress relaxation and large-strain two-cycle compression testing and (b) to correlate these objective results with sensory measurements.
2. Materials and methods 2.1. Material Corn kernels were procured from the local market of Mysore, India; after cleaning, they were debranned and degermed in a laboratory model McGill polisher (Bhattacharya & Bhattacharya, 1994). The kernels were ground in a laboratory model vertical grinder to obtain corn flour that passed through a 200 mesh British Standard (BS) sieve (75 lm opening); the temperature of the flour was kept below 40 C during grinding. Gum Arabic (acacia) powder was obtained from Nice Chemicals, Cochin, India. 2.2. Dough preparation Varying quantities of water were added to attain the desired dough moisture content (48, 53, 57 and 62%, wet basis), and mixed for 2 min in a Hobart mixer at a low speed to obtain homogeneous doughs. Cylindrical samples (35 mm diameter and 20 mm height) were prepared (Bhattacharya & Narasimha, 1997) and the process was replicated twice. Dough samples with gum at 0, 1 and 2% (based on the weight of corn flour) were also made maintaining the above mentioned moisture levels. 2.3. Stress relaxation Stress relaxation characteristics of the lubricated (using paraffin oil) cylindrical sample were determined by compressing the sample up to a strain of 0.05 at a compression rate of 200 mm min1. When the compression was achieved at the desired level, the cross head
S. Bhattacharya et al. / Journal of Food Engineering 74 (2006) 89–95
surface (50 mm in diameter) was stopped, and the dough was allowed to relax for 480 s. The force at different relaxation times was continuously monitored. If a stress is applied quickly on a viscoelastic material and the strain is kept constant later after attaining the desired strain, the material shows a decaying stress as a function of time. The relaxation curve has been modelled by (Eq. (1)) and later normalised (Eq. (2)) by Peleg and Normand (1983), and Purkayastha and Peleg (1986) and was applied by Bhattacharya and Narasimha (1997) on blackgram dough to calculate the decay parameters. Y ðtÞ ¼ F 0 =ðF 0 F ðtÞÞ
ð1Þ
F 0 t=½F 0 F ðtÞ ¼ k 1 þ k 2 t
ð2Þ
Here, 1/k1 is the initial decay rate and 1/k2 denotes the asymptotic value of the relaxed portion of the stress. The constants (k1 and k2) were obtained from the intercept and slope, respectively from the relaxation curve according to Eq. (2). The reported results are the mean of five observations and the whole experiment was replicated twice. 2.4. Two-cycle compression The cylindrical samples as mentioned earlier, were compressed up to a strain of 0.25 for two-times in succession at a speed of 10 mm min1 using a Universal Testing machine (Model # 5R, Lloyd, UK). The software provided by the manufacturer was used to calculate hardness, cohesiveness, elasticity or springiness and adhesiveness. Five samples were examined each time and the experiment was repeated twice. 2.5. Sensory assessment The corn dough samples, about 10 mm thick, were assessed by 10 untrained panelists by hands and fingers to determine the sensory attributes such as hardness, cohesiveness, elasticity or springiness and adhesiveness as suggested by Kalviainen, Roininen, and Tuorila (2000) with suitable modifications according to Table 1. Later, the data were analyzed using principal component analysis (PCA) to correlate the different parameters including objective and subjective data (Lawless & Heymann, 1998) using the software Statistica (Statsoft,
91
Tulsa, OH, USA). The anchors were between 0 and 15 cm to indicate the lowest and highest levels, respectively of the sensory attributes. 2.6. Microstructure The dough samples were dehydrated, mounted on aluminium stubs with conductive adhesive, coated with gold in a sputter coater and were examined with a scanning electron microscope (Model 435 VP, Leo Electron Microscopy Ltd., Cambridge, UK) as mentioned earlier (Bhat & Bhattacharya, 2001), and representative photomicrographs of doughs with different level of gum were obtained. 2.7. Data analysis The independent variables were the moisture content (48.0, 52.7, 57.4 and 62.1%) and gum content (0, 1 and 2%) such that a total of 12 combinations were possible to generate 36 data sets due to repetition experiments. The correlation coefficients, presented in Table 1, were based on 36 data points. The significance of the correlation coefficients was judged at a probability of 0.01. The contour graphs were generated by fitting data to a second order polynomial against the independent variables of moisture and gum contents.
3. Results and discussion Three different set of results, such as, parameters of stress relaxation (k1 and k2), two-cycle compression parameters (hardness, cohesiveness, springiness and adhesiveness) and sensory attributes (hardness, cohesiveness, springiness and adhesiveness) have been cited and discussed for corn dough. 3.1. Stress relaxation The parameter [F0t/(F0 F(t))] when plotted against relaxation time yielded straight lines (r P 0.998, p 6 0.01) for corn doughs at all combinations of moisture content (48–62%) and gum content (0–2%); corn doughs without gum exhibited higher slopes indicating
Table 1 Different sensory attributes and their method of evaluation Sensory attribute
Method of determination
Anchors
Hardness
Force experienced to touch the fore-finger and thumb when the dough sample (10 mm thick, 35 mm diameter) was placed between the two fingers Pressing the sample by the tip of the forefinger to about 5 mm thickness and feeling the recovery while releasing the finger Pulling the dough sample to separate completely Extent of adhesion to fingers while releasing the fingers at the end of hardness measurement
Soft–Hard
Springiness Cohesiveness Adhesiveness
Non-springy–Springy Non-cohesive–Cohesive Non-adhesive–Adhesive
92
S. Bhattacharya et al. / Journal of Food Engineering 74 (2006) 89–95
pronounced solid behaviour than the gum containing doughs (data not presented). At higher moisture contents, the protein in the dough was probably more swollen and consequently more deformable (Visser, 1991), and the protein concentration was lower. Thus, the protein–protein interactions within the dough network may have decreased lowering the stress. The addition of gum in the dough shows less F(t) values during relaxation such that they tend to be a poor elastic solid that can be flattened or rolled easily. The reason behind such behaviour may be due to the ability of gum to hold high volume of water and thereby increasing the plasticising effect of the dough. 3.2. Relaxation parameters (k1 and k2) The reciprocal of the constant k1 indicates the initial decay rate. Thus, a high k1 value is associated with a dough that possess a low-decay rate indicating pronounced elastic behaviour. The k1 values were in the range of 1.5 and 1.9 (Fig. 1) that are very low compared to blackgram dough (about 10–18, Bhattacharya & Narasimha, 1997) due to higher moisture levels (48– 62% in the present case compared to 32–40% for blackgram) and material characteristics. An increase in moisture content decreased k1 values to attain a minimum of 1.5 at about 55% moisture. The k2 values were between 25.2 and 38.6 s1(Fig. 2) and higher values were observed for doughs without gum. An increase in moisture and/or gum usually reduced k2 values to attain a minimum level at about 55% moisture and 1.4% gum contents. As k2 value for a liquid is low in magnitude (Bhattacharya & Narasimha, 1997), a decrease in k2 is expected by shifting the dough from a viscoelastic solid to a viscoelastic liquid; this tendency was observed in most of the cases except for doughs with very high moisture contents accompanied by high level of gum. Possibly, a high level
Fig. 1. Effect of moisture and gum content on relaxation parameter k1.
Fig. 2. Variation in relaxation parameter k2 with moisture and gum content of dough.
of gum (2%) may bind the water as well as the flour in such a manner such that the lubricating effect of water is arrested. 3.3. Two-cycle compression test The hardness values of the dough were between 3 and 40 N, and showed a continuous decreasing trend with an increase in moisture content while gum had a marginal effect (Fig. 3). As hardness is a direct measure of the resistance of the dough towards the compressive stress, it is expected that high level of moisture (such as 62%) produces a dough with meager hardness (less than 5 N) which also indicates the easy flattening characteristics. The springiness or elasticity of the dough samples were between 2 and 3 mm (i.e., relaxing only to the extent of 40–60%), and decreased both with moisture and gum contents (Fig. 4) in a curvilinear manner.
Fig. 3. Contour plot for hardness at different moisture and gum content of dough.
S. Bhattacharya et al. / Journal of Food Engineering 74 (2006) 89–95
93
Fig. 4. Springiness as a function of moisture and gum content of dough.
Fig. 5. Cohesiveness as a function of moisture and gum content of dough.
Fig. 7. Microstructure of corn dough (A) without gum and with (B) 1% gum and (C) 2% gum.
Fig. 6. Effect of moisture and gum content on adhesiveness of dough.
A high springiness value is undesirable for flattening purpose to obtain the flattened sheets because the thickness of the sheets would increase during post-flattening
resting. In such a situation, the incorporation of gum at low levels like 1.0–1.5% may be helpful. The cohesiveness, on the other hand, increased in a near linear mode with both of these variables (Fig. 5) while moisture content had a dominating effect. A dough with cohesiveness less than 0.2 resulted with low-moisture content in absence of gum; these doughs, if compressed to a high level of strain (>0.4) such as in the case of flattening, fails to retain their integrity due to inadequate cohesive
94
S. Bhattacharya et al. / Journal of Food Engineering 74 (2006) 89–95
characteristics. On the contrary, the adhesiveness of the dough (0.13–1.44) was highly affected by moisture content showing an increase of even above 10 folds when moisture is elevated to 62% (Fig. 6). The effect of gum depends on the moisture content of dough; at a lowmoisture content, such as 48%, the addition of gum increased adhesiveness whereas at a high moisture content such as 62%, gum played a marginal role. As high adhesiveness of dough is undesirable for handling of dough like transportation, sheeting, flattening and rolling, it is better to avoid doughs that are highly adhesive in nature; an adhesiveness less than 1.0 appears suitable for flattening when moisture content is less than 60%. 3.4. Microstructure The photomicrographs (Fig. 7) of corn doughs (at magnifications of 1500·) indicated loosely bound particles that were partially adhered to other flour particles possibly by soluble flour fraction or leached out materials. Significant differences in dough microstructure (Fig. 7A–C) were not observed when gums up to 2% level were incorporated.
in moisture content markedly decreased hardness and springiness while opposite trend was true for cohesiveness and adhesiveness. The last two attributes also increased with an increase in gum content. Table 3 shows the correlations between the sensory attributes and objective measurements. Good correlation (r P j0.90j, p 6 0.01) were observed between objective adhesiveness and sensory hardness, and with springiness while both were negatively related. Objective cohesiveness also correlated well (r P j0.93j, p 6 0.01) with sensory hardness and adhesiveness. The stress relaxation parameters (k1 and k2) had no-significant effect on sensory attributes. PCA analysis was conducted with respect to four sensory attributes such as hardness, cohesiveness, elasticity and adhesiveness to establish the relationship between attributes and samples. The principal component (PC1) on the X-axis could explain 84% (Fig. 8) of the variance in data while PC2 accounted for only 12%; thus, in total 96% of data were suitable for the application of PCA methodology. Hardness and springiness/ elasticity formed a group on the positive side of X-axis with high loadings indicating high association or corre-
3.5. Sensory assessment and interrelations The results of the sensory assessment of the textural attributes (hardness, cohesiveness, springiness/elasticity and adhesiveness) are shown in Table 2. An increase
Table 2 Sensory assessment of dough Moisture content of dough (%)
Gum content (%)
Hardness
Cohesiveness
Springiness
Adhesiveness
48 53 57 62 48 53 57 62 48 52 57 62
0 0 0 0 1 1 1 1 2 2 2 2
13.8 ± 0.6 10.2 ± 0.7 7.6 ± 0.4 3.7 ± 0.4 12.7 ± 0.5 12.3 ± 0.6 6.3 ± 0.2 3.0 ± 0.3 12.2 ± 0.8 6.5 ± 0.6 4.2 ± 0.5 1.5 ± 0.2
3.7 ± 0.3 4.5 ± 0.7 7.3 ± 0.4 7.9 ± 0.2 6.3 ± 0.7 7.8 ± 0.6 9.9 ± 0.9 13.9 ± 0.7 12.5 ± 0.3 11.9 ± 0.9 14.2 ± 0.5 14.0 ± 0.4
12.1 ± 0.8 10.9 ± 0.6 4.5 ± 0.6 1.9 ± 0.1 13.2 ± 0.6 7.7 ± 0.6 2.3 ± 0.6 1.4 ± 0.6 12.8 ± 0.3 2.3 ± 0.4 1.4 ± 0.3 1.5 ± 0.6
2.1 ± 0.1 2.8 ± 0.3 7.0 ± 0.8 9.7 ± 0.3 5.4 ± 0.2 8.6 ± 0.6 12.9 ± 0.4 14.0 ± 0.3 8.7 ± 0.7 10.4 ± 0.5 12.2 ± 0.4 14.0 ± 0.6
Fig. 8. Principal component analysis (PCA) of corn flour dough. Symbols 53/1 indicates for corn dough having 53% moisture and 1% gum content. Other samples are defined in a similar manner.
Table 3 Correlation coefficient (r) matrix between the sensory attributes and objective parameters Sensory attributes
Rheological (objective) parameters Hardness
Hardness Cohesiveness Springiness Adhesiveness
Cohesiveness
0.902*
0.944*
0.602 0.931* 0.818*
0.820* 0.889* 0.933*
Springiness
Adhesiveness
k1
k2
0.665 0.914* 0.663 0.903*
0.896*
0.112 0.415 0.334 0.445
0.314 0.679 0.383 0.668
0.729* 0.895* 0.912*
S. Bhattacharya et al. / Journal of Food Engineering 74 (2006) 89–95
lation with the sample 53/1 followed by 48/2 and 48/1; the values are expressed here as moisture content/gum content. This means that low-moisture samples are associated with high values of hardness and elasticity. On the other hand, high moisture sample (62/0) without gum exhibited low magnitudes of these two attributes. Samples like 57/2, 62/1 and 53/2 had loaded more with cohesiveness and adhesiveness which was indicated by their presence on the negative side of X-axis. Samples without gum with 48, 53 and 57% moisture contents exhibited poor cohesiveness and adhesiveness. As sheeting/flattening/forming needs enough cohesiveness with low values for hardness, elasticity and adhesiveness, the suitable conditions may be 52–57% moisture with 1–2% gum.
References Alloncle, M., & Doubilier, J. L. (1991). Viscoelastic properties of maize starch/hydrocolloid pastes and gels. Food Hydrocolloids, 5, 455–467. Bhat, K. K., & Bhattacharya, S. (2001). Deep fat frying characteristics of chickpea flour suspensions. International Journal of Food Science and Technology, 35, 499–507. Bhattacharya, S., & Bhattacharya, S. (1994). Flow behaviour of cooked maize flour suspensions and applicability of mathematical models. Journal of Food Process Engineering, 17, 263–278. Bhattacharya, S., & Narasimha, H. V. (1997). Puncture and stress relaxation behaviour of blackgram (Phaseolus mungo) flour-based papad dough. Journal of Food Process Engineering, 20, 301– 316. Bhattacharya, S., Bhattacharya, S., & Narasimha, H. V. (1999). Uniaxial compressibility of blackgram doughs blended with cereal flours. Journal of Texture Studies, 30, 659–675. Bhattacharya, S., Narasimha, H. V., & Bhattacharya, S. (2003). Effect of gum Arabic on the rheology of corn flour doughs and fried product quality. Journal of Texture Studies, 34, 421–436. Budiman, M., Stroshine, R. L., & Campanella, O. H. (2000). Stress relaxation and low field proton magnetic resonance studies of cheese analog. Journal of Texture Studies, 31, 477–498. Burdock, G. A. (1997). Encyclopedia of food and colour additives (vol. II). Boca Raton, FL, USA: CRC Press.
95
Hamann, D. D., & McDonald, G. A. (1992). Rheology and texture properties of Surimi based food. In T. C. Lanier & C. M. Lee (Eds.), Surimi technology (pp. 429–500). New York: Marcel Dekker. Hibberd, G. E., & Parker, N. S. (1978). A parallel plate rheometer for measuring the viscoelastic properties of wheat flour dough. Cereal Chemistry, 55, 102–110. Kalviainen, N., Roininen, K., & Tuorila, H. (2000). Sensory characterisation of high viscosity gels made with different thickeners. Journal of Texture Studies, 31, 407–420. Kravtchenko, T., & Merlat, L. (2000). Soluble and natural acacia fibre for foods. LVT-Lebensmittel-Verfahrens-und-Verpackungste-chnik, 45, 115–118. Larsson, H., Eliasson, A. C., Johansson, E., & Svensson, G. (2000). Influence of added starch on mixing of dough made with three wheat flours differing in high molecular weight sub unit composition: Rheological behaviour. Cereal Chemistry, 77, 633–639. Lawless, H. T., & Heymann, H. (1998). Sensory evaluation of food: Principles and practices. New York: Chapman & Hall, pp. 606–608. Navickis, L. L. (1987). Corn flour addition to wheat flour doughs. Effect on rheological properties. Cereal Chemistry, 64, 307–310. Peleg, M., & Normand, M. D. (1983). Comparison of two methods for stress relaxation data presentation of solid foods. Rheologica Acta, 22, 108–113. Pszczola, D. E. (1999). Starches and gums more beyond fat replacement. Food Technology, 53(8), 74–76, 78, 80. Purkayastha, S., & Peleg, M. (1986). Comparison between projected mechanical equilibrium conditions of selected food materials in stress relaxation and creep. Journal of Texture Studies, 17, 433– 444. Safari-Ardi, M., & Phan-Thien, N. (1998). Stress relaxation and oscillatory tests to distinguish between dough prepared from wheat flours of different varietal origin. Cereal Chemistry, 75, 80–84. Singh Sidhu, J. P., & Bawa, A. S. (2002). Dough characteristics and baking studies of wheat flour fortified with Xantham gum. International Journal of Food Properties, 5, 1–11. Visser, J. (1991). Factors affecting the rheological and fracture properties of hard and semihard cheese. Bulletin of International Dairy Federation, 268, 49–61. Williams, P. A., Philips, G. O., & Randall, R. C. (1990). Structure– function relationship of gum Arabic. In G. O. Phillips, P. A. Williams, & D. J. Wedlock (Eds.). Gums and stabilizers for the food industry (vol. 5, pp. 25–36). Oxford: DIRL Press.