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Wheat porridge with soy protein isolate and skimmed milk powder: Rheological, pasting and sensory characteristics
Journal of Food Engineering 103 (2011) 1–8
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Wheat porridge with soy protein isolate and skimmed milk powder: Rheological, pasting and sensory characteristics R. Sai Manohar a,⇑, G.R. Urmila Devi a, Suvendu Bhattacharya b, G. Venkateswara Rao a a b
Flour Milling, Baking and Confectionery Technology Department, Central Food Technological Research Institute (Council of Scientific and Industrial Research), Mysore 570 020, India Food Engineering Department, Central Food Technological Research Institute (Council of Scientific and Industrial Research), Mysore 570 020, India
a r t i c l e
i n f o
Article history: Received 15 April 2010 Received in revised form 11 August 2010 Accepted 20 September 2010 Available online 25 September 2010 Keywords: Porridge Pasting Rheology Wheat grits
a b s t r a c t Porridges made from raw and roasted wheat grits (dalia) with added soy protein isolate (SPI) and skimmed milk powder (SMP) were investigated by employing rheological, pasting and sensory tests in order to develop nutritious products. All porridge samples, irrespective of the presence of SPI/SMP and the application of roasting process, exhibited shear-thinning behaviour, fitted well with Cross equation (r P 0.972, p 6 0.01) compared to power law and Casson models. The power indices for porridges prepared from raw grits were higher (0.643–0.764) compared to roasted samples (0.582–0.659). Pasting characteristics like peak viscosity, hot paste viscosity, cold paste viscosity, break down viscosity and total set back viscosity decreased due to incorporation of SPI/SMP. Roasted wheat grits exhibited increased pasting temperature but decreased other pasting characteristics. Pasting temperature increased from 64.4 to a maximum of 74.2 °C due to incorporation of SPI/SMP up to 10%. Porridges made from raw grits along with SPI/SMP had excellent sensory characteristics although roasted grit samples were thinner in consistency and had a better flavour. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Wheat porridge is a common breakfast food in several countries. It is conventionally prepared by boiling wheat grits in water, known as dalia in South East Asian countries, frequently with added milk and sugar for enhancing acceptability. Further, it is also consumed as a savoury dish after cooking wheat grits with water, salt, spices and vegetable pieces. Coarse grinding of either polished or unpolished wheat is practiced in a plate mill followed by cleaning and sieving to obtain grits of particle sizes between 300 and 850 lm (Anon, 1984); these grits are used for preparing dalia. Wheat porridges vary widely in their consistency based on the method of preparation. They are either too watery to possess a low energy density or sometimes too thick; in both cases, it is difficult to consume in sufficient quantities. The increase in consistency during heating of wheat grits can be attributed mainly to gelatinisation of starch. The degree of gelatinisation and consequently the viscosity is influenced by the ratio of water-to-starch in the slurry, added ingredients, processing conditions, nature of starch granules and the presence of other fractions in the cereal grits. Swelling, loss of birefringence and increase in paste viscosity are the indices of the onset of gelatinisation. Several ingredients and additives are of-
⇑ Corresponding author. Tel.: +91 821 2517730; fax: +91 821 2517233. E-mail address: [email protected] (R. Sai Manohar). 0260-8774/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2010.09.006
ten used to modify the pasting characteristics of starches; sugar is added to increase the gelatinisation or pasting temperature and the paste viscosity by decreasing the availability of water in the system (Evans and Haisman, 1982; Spices and Hoseney, 1982; Kim and Walker, 1992). The other additives whose role has been studied include several surfactants and emulsifiers which promote a reduction of starch swelling and paste consistency (Roach and Hoseney, 1995; Stampfli and Nerste, 1995). Cereal grits, supplemented with proteins, can be a good approach to combat protein-energy malnutrition. Soy protein isolate (SPI) and skimmed milk powder (SMP) are commonly used for such fortification due to easy availability, low cost and being a good source of essential amino acids. On the other hand, roasting improves functional properties and the nutritional status of products (Rufian-Henares et al., 2009). Further, determining the pasting characteristics of porridge gives an idea of the extent of cooking in addition to an insight into the phenomenon like gelatinisation and retrogradation. Hence, there is a scope to incorporate SPI and SMP in wheat porridges, and study the pasting characteristics. The rheological characteristics of foods are of importance in processing, quality control, product development, process design, and consumer acceptance. Studies on the rheological characterization of wheat porridge are limited. The rheological characteristics of wheat porridges are influenced by particle size, ratio of solid to water, ingredients and cooking time and temperature of gruel (Sai Manohar et al., 1998; Gujral and Sodhi, 2002). These studies
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have indicated that wheat porridges exhibit shear-thinning behaviour and the flow behaviour is affected by degree of polishing of wheat, type of grit used, sugar concentration and temperature. However, the studies on the effect of SPI and SMP on the pasting and rheological characteristics of wheat porridge are scarce while such information is particularly important for development of products having nutritional advantages. Therefore, the objectives of the present investigation are to determine the pasting characteristics and rheological behaviour of wheat porridges made from raw and roasted wheat grits (dalia) with added SPI and SMP at different levels and examine the sensory acceptability of the developed products. 2. Materials and methods 2.1. Materials Medium hard wheat, obtained from a local supermarket, was manually cleaned and milled into coarse grits (dalia) by passing through a vertical plate (disc) mill. Wheat grits of particle sizes from 300 to 850 lm were separated by sieving; fines and oversized grits were discarded. Skimmed milk powder (Sagar brand, Gujarat Co-operative Milk Marketing Federation, Anand, Gujarat, India) was obtained from a local supermarket while soy protein isolate was obtained from Protein Technologies International, New Delhi, India. Roasting of wheat grits was carried out at 140 °C for 10 min in a laboratory type drum roaster with baffles. 2.2. Methods 2.2.1. Preparation of wheat porridge Based on the observations of preliminary studies, raw and roasted wheat grits were cooked separately for 18 min in boiling water with a grit-to-water ratio of 1:10 (w/w) to obtain wheat porridges. Cooking for 18 min allowed complete starch gelatinisation; no birefringent light was observed under polarised light. Wheat porridges were also prepared by incorporating 5% and 10% of SPI and SMP in place of grit. 2.2.2. Proximate composition The proximate composition of wheat grits was determined by following the AACC (2000) methods on triplicate samples; the AACC methods used for the determination of moisture, ash, protein, fat and crude fibre were 44-15A, 08-12, 46-13, 30-10 and 32-10, respectively; carbohydrate content was obtained by difference. 2.2.3. Pasting characteristics and amylogram parameters The pasting/cooking characteristics of wheat grits were determined by a micro-viscoamylograph (Model # 803201, Brabender, Duisberg, Germany). Wheat grits were ground in a laboratory model grinder fitted with a sieve having apertures of 130 lm. SPI and SMP were added at 5% and 10% (w/w) levels on flour basis. The amylographic procedure was in accordance with the method (# 22-10) of AACC (2000) with modification. A total mass of 115 g consisting of 15 g of powdered dalia sample on 14% moisture basis and remaining water was heated from an initial temperature of 30 °C to a final temperature of 92 °C at a heating rate of 7.5 °C/ min, and holding at 92 °C for 3 min followed by cooling to 50 °C at the rate of 7.5 °C/min. Finally, the cooked mass was allowed to remain at 50 °C for 2 min. The speed of the mixing blade was 250 rpm while the torque measuring range employed was 300 cmg. The peak viscosity (PV), hot paste viscosity (HPV), cold paste viscosity (CPV), break down viscosity (BDV) and total set back viscosity (SBV) were determined as detailed in Fig. 1.
Fig. 1. A sample pasting curve showing different pasting parameters.
2.2.4. Rheological characteristics Porridges with different formulations, as described earlier in Section 2.2.1, were prepared and the strained liquid was subjected to rheological measurements employing a controlled stress rheometer (Model # RT10 Rotovisco, Haake, Karlsruhe, Germany). The strained liquid was immediately cooled in a water bath maintained at a temperature of 30 ± 0.1 °C and subjected to rheological testing by employing the coaxial cylinder attachment. The shear rate was progressively increased linearly from 0.1 s1 up to 1000 s1 over a span of 120 s; a total of 50 data sets were obtained consisting of shear rate, shear stress and apparent viscosity. A temperature controlled circulatory water bath was used to determine the flow properties at a constant temperature of 30 ± 0.1 °C. All rheological measurements were conducted on triplicate samples. The time-independent flow behaviour of dispersions was subjected to common rheological models like power law, Casson and Cross models. The Cross equation (Eq. (1)) (Cross, 1965) comprises a relaxation parameter (k).
g ¼ g1 þ
go g1 1 þ ðkc_ Þm
ð1Þ
Here, g is the apparent viscosity (Pas), go is asymptotic value of viscosity at a very low shear rate (Pas), g1 is asymptotic value of viscosity at a very high shear rate (Pas), c_ is shear rate (s1) and m is the dimensionless power index of Cross model. The apparent viscosity of the liquids was obtained as the ratio of shear stress and shear rate when the latter was taken as 100 s1. The value of g1 can be obtained from the experimental results corresponding to the equilibrium viscosity obtained at the end of shearing. For situation with go g1 and g g1, we can assume go g1 ’ go and g1 ? 0. Thus, Eq. (1) can be simplified to
g¼
go 1 þ ðkc_ Þm
ð2Þ
The statistical analyses of the samples included fitting of the shearrate/shear-stress data to rheological models like power law, Casson and Cross equations using non-linear analysis. The constants of these equations were obtained by employing the software (Software version VI:X, Rotovisco, RT 10) provided by the rheometer manufacturer. The goodness of fit to Cross models was judged by determining the correlation coefficient (r) and its statistical significance was judged at a probability of 0.01. 2.2.5. Sensory evaluation of porridges A 9-point hedonic scale, where 9 was the highest score and 1 the lowest, was used to test sensory attributes like appearance, flavour, mouth feel and overall acceptability of the porridges (Nnam, 2001). The extent to which a product was liked was expressed as like extremely (9 points), like very much (8 points), like moderately (7 points), like slightly (6 points), neither like nor dislike (5 points), dislike slightly (4 points), dislike moderately (3 points), dislike very much (2 points), and dislike extremely (1 point). Ten experienced panellists evaluated the products. Each panellist eval-
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uated five porridge samples at a time. The product temperature during evaluation was maintained at about 30 °C. 2.2.6. Statistics All the experiments were performed thrice. The pasting and rheological characteristics and sensory parameters were statistically analysed by using one way analysis of variance (ANOVA) and examined their statistical significance at p 6 0.05. The calculated standard deviation was less than 3% and therefore the corresponding bars are not reported on the figures. The experiments were laid out on a randomised design and the treatment or group differences were compared using Duncan’s new multiple range test (Steel and Torrie, 1980) at p 6 0.05. 3. Results and discussion
Pasting temperature (°C)
The proximate composition of control wheat grits (dalia) was 9.9% moisture, 1.6% ash, 11.6% protein, 1.6% fat, 1.8% crude fibre
and 73.5% carbohydrate (by difference), while that of nutritious dalia was 9.2%, 2.4%, 21.8%, 1.8%, 2.2% and 62.6%, respectively. The starch contents of these samples are 62.5% and 52.8%, respectively. Mineral and protein contents of nutritious dalia was higher because of the addition of 10% each of SPI and SMP so that the protein content was twice of the control sample. 3.1. Pasting characteristics The pasting characteristics of raw and roasted wheat grits (dalia) are shown in Fig. 2 and 3. Pasting or gelatinisation temperature (GT) of roasted dalia was significantly (at p 6 0.05) higher than raw dalia (67.7 and 64.4 °C, respectively). PV, HPV and CPV were 375, 360 and 662 BU, and 668, 470 and 845 BU for roasted and raw dalia, respectively, indicating markedly lower apparent viscosities for roasted dalia due to heat treatment. In addition, BDV and SBV were also significantly lower for roasted dalia. Roasting is process of cooking in a dry medium. Several changes are associated with roasting process that includes physical and
90
80
70
60 Control
05 SPI
10 SPI 05 SMP 10 SMP 10 SPI Control + 10 SMP
05 SPI
Control
10 SPI 05 SMP 10 SMP 10 SPI + 10 SMP
Roasted
Peak viscosity (BU)
800
600
400
200 Control
05 SPI
10 SPI
05 SMP
control
10 SMP
10 SPI + 10 SMP
Control
05 SPI
10 SPI
05 SMP
10 SMP
10 SPI + 10 SMP
Roasted
Hot paste viscosity (BU)
500
400
300
200 Control 05 SPI 10 SPI 05 SMP 10 SMP 10 SPI Control 05 SPI 10 SPI 05 SMP 10 SMP 10 SPI + 10 + 10 SMP SMP Control
Roasted
Fig. 2. Effect of soy protein isolate (SPI) and skimmed milk powder (SMP) on the pasting temperature, peak viscosity and hot paste viscosity of raw and roasted wheat grits.
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Cold paste viscosity (BU)
900
600
300 Control
05 SPI
10 SPI
05 SMP
10 SMP
10 SPI + 10 SMP
Breakdown viscosity (BU)
Control
05 SPI
10 SPI
05 SMP
10 SMP
10 SPI + 10 SMP
10 SMP
10 SPI + 10 SMP
10 SMP
10 SPI + 10 SMP
Roasted
200 150 100 50 0 Control 05 SPI 10 SPI
05 SMP
10 SMP
10 SPI Control 05 SPI 10 SPI + 10 SMP
Control
Total setback viscosity (BU)
Control
05 SMP
Roasted
400
300
200
100 Control 05 SPI 10 SPI
05 SMP
10 SMP
10 SPI Control 05 SPI 10 SPI + 10 SMP
Control
05 SMP
Roasted
Fig. 3. Effect of soy protein isolate (SPI) and skimmed milk powder (SMP) on the cold paste viscosity, breakdown viscosity and total setback viscosity of raw and roasted wheat grits.
chemical changes; the latter may be associated with the formation of resistant starch, and starch based complex like starch–protein. Hence, the water binding and holding capacities of starch are modified that affects the pasting and rheological characteristics of the product. Ingbian and Adegoke (2007) observed lower breakdown viscosities of the roasted maize without any pretreatment like soaking and cooking. Germination, roasting and pregelatinization for tigernut resulted in marked decrease in peak viscosity of the samples (Ade-Omowaye et al., 2009). SPI marginally increased the pasting temperature from 64.4 to 65.6 °C, and from 67.7 to 69.1 °C due to 10% replacement for raw and roasted wheat grits, respectively (Fig 2); both these increase were significant at p 6 0.05. Li et al. (2007) reported that incorpo-
ration of soy protein concentrate (SPC) increased the pasting temperature of corn starch; this was attributed to the reduction of available water for starch due to the presence of SPC. Apparent viscosity increased with an increase in temperature due to pasting but the extent of increase was less when SPI was added. Further, increase in temperature resulted in enhanced break down, and hence, viscosity decreased. BDV was marginally less (but was significant at p 6 0.05) for SPI-dalia dispersions compared to control dalia. On cooling, the paste viscosity increased from 435 to 771 BU and from 397 to 708 BU, respectively for 5% and 10% SPI incorporated samples. Set back viscosities of SPI-dalia were slightly lower than that of control sample. Similarly the PV, HPV, CPV, BDV and SBV of SPI-dalia were lower than control dalia; these changes,
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60
50 Shear stress (Pa) 40
Power law
30
Casson
20
10 Cross
0
0
200
400
600
800
1000
-1
Shear rate (s ) Fig. 4. Sample rheogram for porridge made with raw grits showing fitting to power law, Casson and Cross models.
though less in magnitude, were statistically significant at p 6 0.05. Much higher reductions in these viscosities (significant at p 6 0.05) were observed with roasted dalia and SPI-dalia. Addition of SMP increased significantly (at p 6 0.05) the pasting temperature from 64.4 to 66.5 °C and from 67.7 to 73.1 °C due to 10% replacement of raw and roasted wheat grits, respectively (Fig. 2). Noisuwan et al. (2007, 2008) reported that addition of SMP increased pasting temperature of rice starch gels; viscosity increased with an increase in temperature and the extent of increase
was much less when SMP was added. PV values were 570 and 500 BU for 5% and 10% SMP samples, respectively. HPV were 401 and 356 BU for 5% and 10% SMP samples, respectively. On cooling, the paste viscosity increased to 730 and 647 BU on incorporation of 5% and 10% SMP, respectively. BDV was much lesser for SMP-dalia dispersions than that of control dalia (Fig. 3). SBV values of SMP were lower than corresponding control samples. The PV, HPV, CPV, BDV and SBV of SMP were much lower than the control dalia; higher extent of reduction in viscosity parameters were observed with roasted dalia and SMP; these changes were statistically significant at p 6 0.05. Addition of SPI and SMP (in combination) to raw grits markedly increased pasting temperature to 74.2 °C whereas decreased PV, HPV, CPV, BDV and SBV to 426, 308, 524, 118 and 216 BU, respectively; these changes were statistically significant at p 6 0.05. This trend was also observed for roasted grits (Figs. 2 and 3). Minimum BDV was observed for SPI, SMP along with roasted dalia. Earlier studies on the functionality of protein isolate or concentrate (Liu et al., 1999; Luck et al., 2002; Roesch and Corredig, 2002) indicated that the compact tertiary structure and quaternary structure of protein components resulted in high emulsifying properties due to the formation of a continuous network through hydrophobic, covalent and/or intermolecular hydrogen bonds. The functionality of an individual biopolymer in foods is also affected by its interactions with other food components such as polysaccharides, proteins, lipids, sugars, salts and emulsifier (Chantrapornchai and McClements, 2002; Richardson et al., 2003; Tolstoguzov, 2003). The reduction in Brabender viscosity parameters was due to dilution of starch in the dispersion resulting from the incorporation of SPI and/or SMP. The variations in pasting behaviour can be attributed to the differences in the degree of modification of
Apparent viscosity (m.Pas)
200
100
0 Control
05 SPI
10 SPI
05 SMP 10 SMP
10 SPI+10 SMP
Control
05 SPI
10 SPI
05 SMP 10 SMP
10 SPI+10 SMP
Roasted
Control
Zero shear viscosity (m.Pas)
300
200
100
0 Control
05 SPI
10 SPI
05 SMP 10 SMP
Control
10 SPI+10 SMP
Control
05 SPI
10 SPI
05 SMP 10 SMP
10 SPI+10 SMP
Roasted
Fig. 5. Apparent viscosity and zero shear viscosity of porridges as affected by soy protein isolate (SPI) and skimmed milk powder (SMP).
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starches in presence of other bipolar compounds. Marco and Rosell (2008) reported that different protein isolates reduced different viscosity parameters when incorporated in rice dough systems due to dilution effect and protein starch interactions. Starch that shows no or less breakdown would make a better paste against heat and/or shearing process during product making. Sample that showed smaller setback would be more stable, and hence, might exhibit less changes in viscosity during storage. 3.2. Rheological characteristics Porridges with different formulations of dalia with SPI and SMP were subjected to rheological measurements and fitting to three common rheological models such as power law, Casson and Cross equations. Among these models, the Cross model provided the best fit with correlation coefficient (r) values between 0.972 and 0.991 followed by power law (0.839 6 r 6 0.952) and Casson (0.748 6 r 6 0.827). The sample rheogram (Fig. 4) also shows the good fit for Cross and power law models. The relative increase in shear stress was rather higher at low shear rates while it gradually decreased as the shear rate was increased. The pattern of curve as depicted in Fig. 4 indicated the shear-thinning behaviour which is common for many food dispersions. The Cross model parameters like zero shear viscosity, apparent viscosity, relaxation time and Cross power constant were computed. The standard deviation (SD) for these four parameters was up to 6%. Apparent viscosity and zero shear viscosity of porridges as affected by addition of SPI and SMP are shown in Fig. 5. Appar-
ent viscosity decreased from 199.9 to 137.6 mPas, and from 199.9 to 160.8 mPas with replacement of raw dalia with 10% SPI and SMP, respectively, in the porridge, and these changes were statistically significant at p 6 0.05. Apparent viscosity decreased to 120.2 mPas when both SPI and SMP were added to the formulation. Zero shear viscosity also decreased from 0.67 to 0.32 mPas, and from 0.67 to 0.47 mPas on replacement of raw dalia with 10% SPI and SMP, respectively; it decreased to 0.29 mPas when both SPI and SMP were added. Apparent viscosities were between 61.2 and 76.9 mPas for porridges made with a formulation containing roasted dalia, SPI and SMP. Zero shear viscosity values were also low and ranged from 0.13 to 0.19 mPas for porridges made with roasted dalia, SPI and SMP. The decrease in viscosity was due to dilution of leached out starch on cooking either with SPI or SMP. Relaxation time (k) and Cross power constant (m) of wheat porridge, as affected by the addition of SPI and SMP, are shown in Fig. 6. Relaxation time increased from 28.2 to 126.6 s, and to 73.2 s due to replacement of raw dalia with 5% SPI and SMP, respectively. Low relaxation time indicates a predominantly liquid-like sample; addition SPI and/or SMP increased relaxation time to shift towards the desirable solid-like feel; this change was statistically significant at p 6 0.05. Similar trends were also observed for porridges made with roasted dalia with added SPI and SMP. Cross power constant (m) values ranged from 0.64 to 0.76 for porridges made with raw dalia with added SPI and SMP; these values ranged from 0.58 to 0.66 for porridges made with roasted dalia having SPI and SMP. These re-
Cross power constant (m)
0.800
0.600
0.400 Control 5% SPI 10 SPI 05 SMP 10 SMP 10 SPI + 10 SMP
Control 05 SPI
10 SPI 05 SMP 10 SMP 10 SPI+ 10 SMP
Roasted
Control
Relaxation time (λ)
140
100
60
20 Control 5% SPI
10 SPI
05 SMP
Control
10 SMP
10 SPI + 10 SMP
Control
05 SPI
10 SPI
05 SMP
10 SMP
10 SPI+ 10 SMP
Roasted
Fig. 6. Cross model parameters (power constant and relaxation time) of porridges as affected by soy protein isolate (SPI) and skimmed milk powder (SMP).
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sults indicated that all porridge samples exhibited shear-thinning with or without roasting or with the addition of either SPI or SMP or both. Porridges made with roasted dalia were thinner as reflected by their apparent viscosities (Fig. 5). Shear-thinning behaviour for Akamu – a semi-liquid food based on maize, millet and sorghum was observed by Sopade and Kassum (1992). Dispersions of gelatinized wheat, corn and tapioca starches also exhibited shear-thinning behaviour (Bagley and Christianson, 1982; Christianson and Bagley, 1983, 1984; Doublier, 1981; Evans and Haisman, 1979; Wong and Lelievre, 1982; Sai Manohar et al., 1998; Gujral and Sodhi, 2002). Marginally higher values (significant at p 6 0.05) for Cross power constants could be attributed to strained liquids that were thinner than the porridges in the present study; the decrease in apparent viscosity is also evident from Eq. (2). The usefulness of time-independent flow behaviour studies of food dispersions includes a relationship between flow and structure, and correlates with physical and sensory parameters. These shear-thinning dispersions exhibit an increase in apparent viscosity with an increase in concentration of solid but decreases with an increase in temperature. This behaviour is expected for food systems wherein true solutions are difficult to identify (Roopa and Bhattacharya, 2009). The relaxation time of Cross model depends on the nature of material forming the discontinuous phase; it also increases due to an increase in concentration. Further, an increase in SPI and/or SMP decreases the starch content that gelatinizes during wet heating. On the other hand, the protein content increases by addition of SPI and SMP wherein the nature of hydration changes. Hence, a complex situation arises when part of wheat grit is supplemented with SPI or SMP. 3.3. Sensory assessment The sensory attributes of porridges made from raw and roasted dalia along with SPI and SMP are presented in Tables 1 and 2. The appearance of the finished product changed from white to creamish white when SPI was incorporated which is a desirable quality attribute for porridge. The improvement in appearance was better with SMP (significant at p 6 0.05); the combined use of both SPI and SMP further improved the appearance. Significant improvement in flavour was noticed in porridges prepared with SMP or SPI compared to control samples. The interaction of sugar and protein present in SMP might have an important role here to enhance the flavour. Mouth feel increased significantly (at p 6 0.05) as a result of incorporation of either SPI or SMP. The taste of the samples was improved when the porridges were prepared along with milk solids. Addition of SPI and SMP together further enhanced the mouth feel. The combined effect of SPI and SMP were also reflected in the higher scores for overall quality. However, porridges prepared Table 1 Effect of soy protein isolate (SPI) and skimmed milk powder (SMP) on the sensory characteristicsA of wheat porridge made from raw dalia.
Control 5% SPI 10% SPI 5% SMP 10% SMP 10% SPI + 10% SMP SEMB (±) A
Appearance
Flavour
Mouth feel
Overall acceptability
7.5c 8.0b 8.2b 8.2b 8.4ab 8.6a
7.0d 8.0c 8.1c 8.2bc 8.5a 8.4ab
7.0d 7.2cd 7.9b 7.5c 8.2b 8.6a
7.2c 7.4c 8.0b 8.0b 8.4a 8.6a
0.09
0.08
0.07
0.12
Means in the same column followed by different superscripts differ significantly (p < 0.05) according to Duncan’s multiple range test (DMRT). B Standard error of mean (SEM) at 54 degrees of freedom.
Table 2 Effect of soy protein isolate (SPI) and skimmed milk powder (SMP) on the sensory characteristicsA of wheat porridge made from roasted dalia.
Control 5% SPI 10% SPI 5% SMP 10% SMP 10% SPI + 10% SMP SEMB (±)
Appearance
Flavour
Mouth feel
Overall acceptability
6.3d 7.0c 7.5bc 7.2bc 7.9ab 8.1a
8.2c 8.2c 8.4bc 8.3bc 8.6a 8.7a
6.0d 6.5c 6.8bc 6.6bc 7.2a 7.4a
6.8d 7.0cd 7.2bc 7.1c 7.5b 7.9a
0.08
0.09
0.08
0.08
A
Means in the same column followed by different superscripts differ significantly (p < 0.05) according to Duncan’s multiple range test (DMRT). B Standard error of mean (SEM) at 54 degrees of freedom.
from roasted dalia scored lower sensory values compared to those from unroasted dalia; only flavour scores improved for former samples possibly due to Maillard reaction (Rufian-Henares et al., 2009). Mouth feel was better for porridges made from unroasted dalia compared to samples having roasted dalia even after the incorporation of SPI and SMP. This might be due to the lower viscosity of the porridges prepared from roasted dalia. The overall quality of porridges prepared with roasted dalia after the incorporation of SPI and SMP were lower compared to corresponding wheat porridges samples made with unroasted dalia. 4. Conclusions Porridges made from wheat grits (dalia) with the addition of SPI and SMP significantly altered the rheological, pasting and sensory attributes in addition to increasing the protein content compared to that made with wheat grits alone. All porridges exhibited shear-thinning characteristics irrespective of the added ingredients. Pasting temperature increased with the incorporation SPI or SMP. Roasting of dalia markedly influenced pasting temperature and apparent viscosity of the paste. Porridges made from roasted dalia were thinner in consistency but had better flavour characteristics compared to products made with raw dalia. Porridges made from raw dalia along with SPI and SMP had excellent sensory characteristics. References AACC, 2000. Approved Methods of American Association of Cereal Chemists, 10th ed. AACC International, St. Paul, MN, USA. Ade-Omowaye, B.I.O., Adegbite, A.M., Adetunji, B.R., 2009. Evaluation of some chemical and physicochemical properties of meals from pretreated tigernut seeds (Cyperus esculentus). Nutrition and Food Science 39, 142–150. Anon, 1984. Indian Standard Specification for Wheat Porridges. IS:10769-1984, Reaffirmed in 2005. Indian Standards Institution, New Delhi, India. Bagley, E.B., Christianson, D.D., 1982. Swelling capacity of starch and its relationship to suspension viscosity: effect of cooking time, temperature and concentration. Journal of Texture Studies 13, 115–126. Chantrapornchai, W., McClements, D.J., 2002. Influence of NaCl on optical properties, large-strain rheology and water holding capacity of heat-induced whey protein isolate gels. Food Hydrocolloids 16, 467–476. Christianson, D.D., Bagley, E.B., 1983. Apparent viscosities of dispersions of swollen cornstarch granules. Cereal Chemistry 60, 116–121. Christianson, D.D., Bagley, E.B., 1984. Yield stresses in dispersions of swollen, deformable cornstarch granules. Cereal Chemistry 61, 500–503. Cross, M.M., 1965. Rheology of non-Newtonian fluids: a new flow equation for pseudoplastic systems. Journal of Colloid Science 20, 417–437. Doublier, J.L., 1981. Rheological studies on starch – flow behaviour of wheat starch pastes. Starch 33, 415–420. Evans, I.D., Haisman, D., 1979. Rheology of gelatinised starch suspensions. Journal of Texture Studies 10, 347–370. Evans, I.D., Haisman, D.R., 1982. The effect of solutes on the gelatinization temperature range of potato starch. Starch 34, 224–231. Gujral, H.S., Sodhi, N.S., 2002. Back extrusion properties of wheat porridge (Dalia). Journal of Food Engineering 52, 53–56.
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Ingbian, E.K., Adegoke, G.O., 2007. Proximate compositions, pasting and rheological properties of mumu – a roasted maize meal. International Journal of Food Science and Technology 42, 762–767. Kim, C.S., Walker, C.E., 1992. Changes in starch pasting properties due to sugars and emulsifiers as determined by viscosity measurement. Journal of Food Science 57, 1009–1013. Li, J.-Y., Yeh, A.-I., Fan, K.-L., 2007. Gelation characteristics and morphology of corn starch/soy protein concentrate composites during heating. Journal of Food Engineering 78, 1240–1247. Liu, M., Lee, D.S., Damodaran, S., 1999. Emulsifying properties of acidic subunits of soy 11s globulin. Journal of Agricultural and Food Chemistry 47, 4970–4975. Luck, P.J., Lanier, T.C., Daubert, C.R., Kwanyuen, P., 2002. Viscoelastic behaviour of commercially processed soy isolate pastes during heating and cooling. Journal of Food Science 67, 1379–1382. Marco, C., Rosell, C.M., 2008. Effect of different protein isolates and transglutaminase on rice flour properties. Journal of Food Engineering 84, 132–139. Nnam, N.M., 2001. Chemical sensory and rheological properties of porridges from processed sorghum (Sorghum bicolor), bambara groundnut (Vigna subterranean L. Verdc) and sweet potato (Lpomoea batatas) flours. Plant Food for Human Nutrition 56, 251–264. Noisuwan, A., Bronlund, J., Wilkinson, B., Hemar, Y., 2008. Effect of milk protein products on the rheological and thermal (DSC) properties of normal rice starch and waxy rice starch. Food Hydrocolloids 22, 174–183. Noisuwan, A., Hemar, Y., Bronlund, J.E., Wilkinson, B., Williams, M.A.K., 2007. Viscosity, swelling and starch leaching during the early stages of pasting of normal and waxy rice starch suspensions containing different milk protein ingredients. Starch 59, 379–387. Richardson, G., Langton, M., Bark, A., Hermansson, A.M., 2003. Wheat starch gelatinization – the effect of sucrose, emulsifier and the physical state of the emulsifier. Starch 55, 150–161.
Roach, R.R., Hoseney, R.C., 1995. Effect of certain surfactants on the swelling, solubility, and amylograph consistency of starch. Cereal Chemistry 72, 571– 577. Roesch, R.R., Corredig, M., 2002. Characterization of oil-in-water emulsions prepared with commercial soy protein concentrate. Journal of Food Science 67, 2837–2842. Roopa, B.S., Bhattacharya, S., 2009. Characterization and modeling of timeindependent and time-dependent flow behaviour of sodium alginate dispersions. International Journal of Food Science and Technology 44, 2583– 2589. Rufian-Henares, J.A., Delgado-Andrade, C., Morales, F.J., 2009. Assessing the Maillard reaction development during the toasting process of common flours employed by the cereal products industry. Food Chemistry 114, 93–99. Sai Manohar, R., Manohar, B., Haridas Rao, P., 1998. Rheological characteristics of wheat porridge (cooked dalia), a semi-liquid breakfast food. Journal of Cereal Science 27, 103–108. Sopade, P.A., Kassum, A.L., 1992. Rheological characterisation of Akamu, a semiliquid food made from maize, millet and sorghum. Journal of Cereal Science 15, 193–202. Spices, D., Hoseney, R.C., 1982. Effect of sugars on starch gelatinisation. Cereal Chemistry 58, 128–131. Stampfli, L., Nerste, B., 1995. Emulsifiers in bread making. Food Chemistry 52, 353– 360. Steel, R.G.D., Torrie, J.H., 1980. Principles and Procedures of Statistics. McGraw-Hill, New York. Tolstoguzov, V., 2003. Some thermodynamic considerations in food formulation. Food Hydrocolloids 17, 1–23. Wong, R.B.K., Lelievre, J.L., 1982. Rheological characteristics of wheat starch pastes measured under steady shear conditions. Journal of Applied Polymer Science 27, 1433–1440.
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Wheat porridge with soy protein isolate and skimmed milk powder: Rheological, pasting and sensory characteristics R. Sai Manohar a,⇑, G.R. Urmila Devi a, Suvendu Bhattacharya b, G. Venkateswara Rao a a b
Flour Milling, Baking and Confectionery Technology Department, Central Food Technological Research Institute (Council of Scientific and Industrial Research), Mysore 570 020, India Food Engineering Department, Central Food Technological Research Institute (Council of Scientific and Industrial Research), Mysore 570 020, India
a r t i c l e
i n f o
Article history: Received 15 April 2010 Received in revised form 11 August 2010 Accepted 20 September 2010 Available online 25 September 2010 Keywords: Porridge Pasting Rheology Wheat grits
a b s t r a c t Porridges made from raw and roasted wheat grits (dalia) with added soy protein isolate (SPI) and skimmed milk powder (SMP) were investigated by employing rheological, pasting and sensory tests in order to develop nutritious products. All porridge samples, irrespective of the presence of SPI/SMP and the application of roasting process, exhibited shear-thinning behaviour, fitted well with Cross equation (r P 0.972, p 6 0.01) compared to power law and Casson models. The power indices for porridges prepared from raw grits were higher (0.643–0.764) compared to roasted samples (0.582–0.659). Pasting characteristics like peak viscosity, hot paste viscosity, cold paste viscosity, break down viscosity and total set back viscosity decreased due to incorporation of SPI/SMP. Roasted wheat grits exhibited increased pasting temperature but decreased other pasting characteristics. Pasting temperature increased from 64.4 to a maximum of 74.2 °C due to incorporation of SPI/SMP up to 10%. Porridges made from raw grits along with SPI/SMP had excellent sensory characteristics although roasted grit samples were thinner in consistency and had a better flavour. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Wheat porridge is a common breakfast food in several countries. It is conventionally prepared by boiling wheat grits in water, known as dalia in South East Asian countries, frequently with added milk and sugar for enhancing acceptability. Further, it is also consumed as a savoury dish after cooking wheat grits with water, salt, spices and vegetable pieces. Coarse grinding of either polished or unpolished wheat is practiced in a plate mill followed by cleaning and sieving to obtain grits of particle sizes between 300 and 850 lm (Anon, 1984); these grits are used for preparing dalia. Wheat porridges vary widely in their consistency based on the method of preparation. They are either too watery to possess a low energy density or sometimes too thick; in both cases, it is difficult to consume in sufficient quantities. The increase in consistency during heating of wheat grits can be attributed mainly to gelatinisation of starch. The degree of gelatinisation and consequently the viscosity is influenced by the ratio of water-to-starch in the slurry, added ingredients, processing conditions, nature of starch granules and the presence of other fractions in the cereal grits. Swelling, loss of birefringence and increase in paste viscosity are the indices of the onset of gelatinisation. Several ingredients and additives are of-
⇑ Corresponding author. Tel.: +91 821 2517730; fax: +91 821 2517233. E-mail address: [email protected] (R. Sai Manohar). 0260-8774/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2010.09.006
ten used to modify the pasting characteristics of starches; sugar is added to increase the gelatinisation or pasting temperature and the paste viscosity by decreasing the availability of water in the system (Evans and Haisman, 1982; Spices and Hoseney, 1982; Kim and Walker, 1992). The other additives whose role has been studied include several surfactants and emulsifiers which promote a reduction of starch swelling and paste consistency (Roach and Hoseney, 1995; Stampfli and Nerste, 1995). Cereal grits, supplemented with proteins, can be a good approach to combat protein-energy malnutrition. Soy protein isolate (SPI) and skimmed milk powder (SMP) are commonly used for such fortification due to easy availability, low cost and being a good source of essential amino acids. On the other hand, roasting improves functional properties and the nutritional status of products (Rufian-Henares et al., 2009). Further, determining the pasting characteristics of porridge gives an idea of the extent of cooking in addition to an insight into the phenomenon like gelatinisation and retrogradation. Hence, there is a scope to incorporate SPI and SMP in wheat porridges, and study the pasting characteristics. The rheological characteristics of foods are of importance in processing, quality control, product development, process design, and consumer acceptance. Studies on the rheological characterization of wheat porridge are limited. The rheological characteristics of wheat porridges are influenced by particle size, ratio of solid to water, ingredients and cooking time and temperature of gruel (Sai Manohar et al., 1998; Gujral and Sodhi, 2002). These studies
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have indicated that wheat porridges exhibit shear-thinning behaviour and the flow behaviour is affected by degree of polishing of wheat, type of grit used, sugar concentration and temperature. However, the studies on the effect of SPI and SMP on the pasting and rheological characteristics of wheat porridge are scarce while such information is particularly important for development of products having nutritional advantages. Therefore, the objectives of the present investigation are to determine the pasting characteristics and rheological behaviour of wheat porridges made from raw and roasted wheat grits (dalia) with added SPI and SMP at different levels and examine the sensory acceptability of the developed products. 2. Materials and methods 2.1. Materials Medium hard wheat, obtained from a local supermarket, was manually cleaned and milled into coarse grits (dalia) by passing through a vertical plate (disc) mill. Wheat grits of particle sizes from 300 to 850 lm were separated by sieving; fines and oversized grits were discarded. Skimmed milk powder (Sagar brand, Gujarat Co-operative Milk Marketing Federation, Anand, Gujarat, India) was obtained from a local supermarket while soy protein isolate was obtained from Protein Technologies International, New Delhi, India. Roasting of wheat grits was carried out at 140 °C for 10 min in a laboratory type drum roaster with baffles. 2.2. Methods 2.2.1. Preparation of wheat porridge Based on the observations of preliminary studies, raw and roasted wheat grits were cooked separately for 18 min in boiling water with a grit-to-water ratio of 1:10 (w/w) to obtain wheat porridges. Cooking for 18 min allowed complete starch gelatinisation; no birefringent light was observed under polarised light. Wheat porridges were also prepared by incorporating 5% and 10% of SPI and SMP in place of grit. 2.2.2. Proximate composition The proximate composition of wheat grits was determined by following the AACC (2000) methods on triplicate samples; the AACC methods used for the determination of moisture, ash, protein, fat and crude fibre were 44-15A, 08-12, 46-13, 30-10 and 32-10, respectively; carbohydrate content was obtained by difference. 2.2.3. Pasting characteristics and amylogram parameters The pasting/cooking characteristics of wheat grits were determined by a micro-viscoamylograph (Model # 803201, Brabender, Duisberg, Germany). Wheat grits were ground in a laboratory model grinder fitted with a sieve having apertures of 130 lm. SPI and SMP were added at 5% and 10% (w/w) levels on flour basis. The amylographic procedure was in accordance with the method (# 22-10) of AACC (2000) with modification. A total mass of 115 g consisting of 15 g of powdered dalia sample on 14% moisture basis and remaining water was heated from an initial temperature of 30 °C to a final temperature of 92 °C at a heating rate of 7.5 °C/ min, and holding at 92 °C for 3 min followed by cooling to 50 °C at the rate of 7.5 °C/min. Finally, the cooked mass was allowed to remain at 50 °C for 2 min. The speed of the mixing blade was 250 rpm while the torque measuring range employed was 300 cmg. The peak viscosity (PV), hot paste viscosity (HPV), cold paste viscosity (CPV), break down viscosity (BDV) and total set back viscosity (SBV) were determined as detailed in Fig. 1.
Fig. 1. A sample pasting curve showing different pasting parameters.
2.2.4. Rheological characteristics Porridges with different formulations, as described earlier in Section 2.2.1, were prepared and the strained liquid was subjected to rheological measurements employing a controlled stress rheometer (Model # RT10 Rotovisco, Haake, Karlsruhe, Germany). The strained liquid was immediately cooled in a water bath maintained at a temperature of 30 ± 0.1 °C and subjected to rheological testing by employing the coaxial cylinder attachment. The shear rate was progressively increased linearly from 0.1 s1 up to 1000 s1 over a span of 120 s; a total of 50 data sets were obtained consisting of shear rate, shear stress and apparent viscosity. A temperature controlled circulatory water bath was used to determine the flow properties at a constant temperature of 30 ± 0.1 °C. All rheological measurements were conducted on triplicate samples. The time-independent flow behaviour of dispersions was subjected to common rheological models like power law, Casson and Cross models. The Cross equation (Eq. (1)) (Cross, 1965) comprises a relaxation parameter (k).
g ¼ g1 þ
go g1 1 þ ðkc_ Þm
ð1Þ
Here, g is the apparent viscosity (Pas), go is asymptotic value of viscosity at a very low shear rate (Pas), g1 is asymptotic value of viscosity at a very high shear rate (Pas), c_ is shear rate (s1) and m is the dimensionless power index of Cross model. The apparent viscosity of the liquids was obtained as the ratio of shear stress and shear rate when the latter was taken as 100 s1. The value of g1 can be obtained from the experimental results corresponding to the equilibrium viscosity obtained at the end of shearing. For situation with go g1 and g g1, we can assume go g1 ’ go and g1 ? 0. Thus, Eq. (1) can be simplified to
g¼
go 1 þ ðkc_ Þm
ð2Þ
The statistical analyses of the samples included fitting of the shearrate/shear-stress data to rheological models like power law, Casson and Cross equations using non-linear analysis. The constants of these equations were obtained by employing the software (Software version VI:X, Rotovisco, RT 10) provided by the rheometer manufacturer. The goodness of fit to Cross models was judged by determining the correlation coefficient (r) and its statistical significance was judged at a probability of 0.01. 2.2.5. Sensory evaluation of porridges A 9-point hedonic scale, where 9 was the highest score and 1 the lowest, was used to test sensory attributes like appearance, flavour, mouth feel and overall acceptability of the porridges (Nnam, 2001). The extent to which a product was liked was expressed as like extremely (9 points), like very much (8 points), like moderately (7 points), like slightly (6 points), neither like nor dislike (5 points), dislike slightly (4 points), dislike moderately (3 points), dislike very much (2 points), and dislike extremely (1 point). Ten experienced panellists evaluated the products. Each panellist eval-
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uated five porridge samples at a time. The product temperature during evaluation was maintained at about 30 °C. 2.2.6. Statistics All the experiments were performed thrice. The pasting and rheological characteristics and sensory parameters were statistically analysed by using one way analysis of variance (ANOVA) and examined their statistical significance at p 6 0.05. The calculated standard deviation was less than 3% and therefore the corresponding bars are not reported on the figures. The experiments were laid out on a randomised design and the treatment or group differences were compared using Duncan’s new multiple range test (Steel and Torrie, 1980) at p 6 0.05. 3. Results and discussion
Pasting temperature (°C)
The proximate composition of control wheat grits (dalia) was 9.9% moisture, 1.6% ash, 11.6% protein, 1.6% fat, 1.8% crude fibre
and 73.5% carbohydrate (by difference), while that of nutritious dalia was 9.2%, 2.4%, 21.8%, 1.8%, 2.2% and 62.6%, respectively. The starch contents of these samples are 62.5% and 52.8%, respectively. Mineral and protein contents of nutritious dalia was higher because of the addition of 10% each of SPI and SMP so that the protein content was twice of the control sample. 3.1. Pasting characteristics The pasting characteristics of raw and roasted wheat grits (dalia) are shown in Fig. 2 and 3. Pasting or gelatinisation temperature (GT) of roasted dalia was significantly (at p 6 0.05) higher than raw dalia (67.7 and 64.4 °C, respectively). PV, HPV and CPV were 375, 360 and 662 BU, and 668, 470 and 845 BU for roasted and raw dalia, respectively, indicating markedly lower apparent viscosities for roasted dalia due to heat treatment. In addition, BDV and SBV were also significantly lower for roasted dalia. Roasting is process of cooking in a dry medium. Several changes are associated with roasting process that includes physical and
90
80
70
60 Control
05 SPI
10 SPI 05 SMP 10 SMP 10 SPI Control + 10 SMP
05 SPI
Control
10 SPI 05 SMP 10 SMP 10 SPI + 10 SMP
Roasted
Peak viscosity (BU)
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600
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200 Control
05 SPI
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05 SMP
control
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05 SPI
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Roasted
Hot paste viscosity (BU)
500
400
300
200 Control 05 SPI 10 SPI 05 SMP 10 SMP 10 SPI Control 05 SPI 10 SPI 05 SMP 10 SMP 10 SPI + 10 + 10 SMP SMP Control
Roasted
Fig. 2. Effect of soy protein isolate (SPI) and skimmed milk powder (SMP) on the pasting temperature, peak viscosity and hot paste viscosity of raw and roasted wheat grits.
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Cold paste viscosity (BU)
900
600
300 Control
05 SPI
10 SPI
05 SMP
10 SMP
10 SPI + 10 SMP
Breakdown viscosity (BU)
Control
05 SPI
10 SPI
05 SMP
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10 SPI + 10 SMP
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10 SPI + 10 SMP
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200 150 100 50 0 Control 05 SPI 10 SPI
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10 SPI Control 05 SPI 10 SPI + 10 SMP
Control
Total setback viscosity (BU)
Control
05 SMP
Roasted
400
300
200
100 Control 05 SPI 10 SPI
05 SMP
10 SMP
10 SPI Control 05 SPI 10 SPI + 10 SMP
Control
05 SMP
Roasted
Fig. 3. Effect of soy protein isolate (SPI) and skimmed milk powder (SMP) on the cold paste viscosity, breakdown viscosity and total setback viscosity of raw and roasted wheat grits.
chemical changes; the latter may be associated with the formation of resistant starch, and starch based complex like starch–protein. Hence, the water binding and holding capacities of starch are modified that affects the pasting and rheological characteristics of the product. Ingbian and Adegoke (2007) observed lower breakdown viscosities of the roasted maize without any pretreatment like soaking and cooking. Germination, roasting and pregelatinization for tigernut resulted in marked decrease in peak viscosity of the samples (Ade-Omowaye et al., 2009). SPI marginally increased the pasting temperature from 64.4 to 65.6 °C, and from 67.7 to 69.1 °C due to 10% replacement for raw and roasted wheat grits, respectively (Fig 2); both these increase were significant at p 6 0.05. Li et al. (2007) reported that incorpo-
ration of soy protein concentrate (SPC) increased the pasting temperature of corn starch; this was attributed to the reduction of available water for starch due to the presence of SPC. Apparent viscosity increased with an increase in temperature due to pasting but the extent of increase was less when SPI was added. Further, increase in temperature resulted in enhanced break down, and hence, viscosity decreased. BDV was marginally less (but was significant at p 6 0.05) for SPI-dalia dispersions compared to control dalia. On cooling, the paste viscosity increased from 435 to 771 BU and from 397 to 708 BU, respectively for 5% and 10% SPI incorporated samples. Set back viscosities of SPI-dalia were slightly lower than that of control sample. Similarly the PV, HPV, CPV, BDV and SBV of SPI-dalia were lower than control dalia; these changes,
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60
50 Shear stress (Pa) 40
Power law
30
Casson
20
10 Cross
0
0
200
400
600
800
1000
-1
Shear rate (s ) Fig. 4. Sample rheogram for porridge made with raw grits showing fitting to power law, Casson and Cross models.
though less in magnitude, were statistically significant at p 6 0.05. Much higher reductions in these viscosities (significant at p 6 0.05) were observed with roasted dalia and SPI-dalia. Addition of SMP increased significantly (at p 6 0.05) the pasting temperature from 64.4 to 66.5 °C and from 67.7 to 73.1 °C due to 10% replacement of raw and roasted wheat grits, respectively (Fig. 2). Noisuwan et al. (2007, 2008) reported that addition of SMP increased pasting temperature of rice starch gels; viscosity increased with an increase in temperature and the extent of increase
was much less when SMP was added. PV values were 570 and 500 BU for 5% and 10% SMP samples, respectively. HPV were 401 and 356 BU for 5% and 10% SMP samples, respectively. On cooling, the paste viscosity increased to 730 and 647 BU on incorporation of 5% and 10% SMP, respectively. BDV was much lesser for SMP-dalia dispersions than that of control dalia (Fig. 3). SBV values of SMP were lower than corresponding control samples. The PV, HPV, CPV, BDV and SBV of SMP were much lower than the control dalia; higher extent of reduction in viscosity parameters were observed with roasted dalia and SMP; these changes were statistically significant at p 6 0.05. Addition of SPI and SMP (in combination) to raw grits markedly increased pasting temperature to 74.2 °C whereas decreased PV, HPV, CPV, BDV and SBV to 426, 308, 524, 118 and 216 BU, respectively; these changes were statistically significant at p 6 0.05. This trend was also observed for roasted grits (Figs. 2 and 3). Minimum BDV was observed for SPI, SMP along with roasted dalia. Earlier studies on the functionality of protein isolate or concentrate (Liu et al., 1999; Luck et al., 2002; Roesch and Corredig, 2002) indicated that the compact tertiary structure and quaternary structure of protein components resulted in high emulsifying properties due to the formation of a continuous network through hydrophobic, covalent and/or intermolecular hydrogen bonds. The functionality of an individual biopolymer in foods is also affected by its interactions with other food components such as polysaccharides, proteins, lipids, sugars, salts and emulsifier (Chantrapornchai and McClements, 2002; Richardson et al., 2003; Tolstoguzov, 2003). The reduction in Brabender viscosity parameters was due to dilution of starch in the dispersion resulting from the incorporation of SPI and/or SMP. The variations in pasting behaviour can be attributed to the differences in the degree of modification of
Apparent viscosity (m.Pas)
200
100
0 Control
05 SPI
10 SPI
05 SMP 10 SMP
10 SPI+10 SMP
Control
05 SPI
10 SPI
05 SMP 10 SMP
10 SPI+10 SMP
Roasted
Control
Zero shear viscosity (m.Pas)
300
200
100
0 Control
05 SPI
10 SPI
05 SMP 10 SMP
Control
10 SPI+10 SMP
Control
05 SPI
10 SPI
05 SMP 10 SMP
10 SPI+10 SMP
Roasted
Fig. 5. Apparent viscosity and zero shear viscosity of porridges as affected by soy protein isolate (SPI) and skimmed milk powder (SMP).
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starches in presence of other bipolar compounds. Marco and Rosell (2008) reported that different protein isolates reduced different viscosity parameters when incorporated in rice dough systems due to dilution effect and protein starch interactions. Starch that shows no or less breakdown would make a better paste against heat and/or shearing process during product making. Sample that showed smaller setback would be more stable, and hence, might exhibit less changes in viscosity during storage. 3.2. Rheological characteristics Porridges with different formulations of dalia with SPI and SMP were subjected to rheological measurements and fitting to three common rheological models such as power law, Casson and Cross equations. Among these models, the Cross model provided the best fit with correlation coefficient (r) values between 0.972 and 0.991 followed by power law (0.839 6 r 6 0.952) and Casson (0.748 6 r 6 0.827). The sample rheogram (Fig. 4) also shows the good fit for Cross and power law models. The relative increase in shear stress was rather higher at low shear rates while it gradually decreased as the shear rate was increased. The pattern of curve as depicted in Fig. 4 indicated the shear-thinning behaviour which is common for many food dispersions. The Cross model parameters like zero shear viscosity, apparent viscosity, relaxation time and Cross power constant were computed. The standard deviation (SD) for these four parameters was up to 6%. Apparent viscosity and zero shear viscosity of porridges as affected by addition of SPI and SMP are shown in Fig. 5. Appar-
ent viscosity decreased from 199.9 to 137.6 mPas, and from 199.9 to 160.8 mPas with replacement of raw dalia with 10% SPI and SMP, respectively, in the porridge, and these changes were statistically significant at p 6 0.05. Apparent viscosity decreased to 120.2 mPas when both SPI and SMP were added to the formulation. Zero shear viscosity also decreased from 0.67 to 0.32 mPas, and from 0.67 to 0.47 mPas on replacement of raw dalia with 10% SPI and SMP, respectively; it decreased to 0.29 mPas when both SPI and SMP were added. Apparent viscosities were between 61.2 and 76.9 mPas for porridges made with a formulation containing roasted dalia, SPI and SMP. Zero shear viscosity values were also low and ranged from 0.13 to 0.19 mPas for porridges made with roasted dalia, SPI and SMP. The decrease in viscosity was due to dilution of leached out starch on cooking either with SPI or SMP. Relaxation time (k) and Cross power constant (m) of wheat porridge, as affected by the addition of SPI and SMP, are shown in Fig. 6. Relaxation time increased from 28.2 to 126.6 s, and to 73.2 s due to replacement of raw dalia with 5% SPI and SMP, respectively. Low relaxation time indicates a predominantly liquid-like sample; addition SPI and/or SMP increased relaxation time to shift towards the desirable solid-like feel; this change was statistically significant at p 6 0.05. Similar trends were also observed for porridges made with roasted dalia with added SPI and SMP. Cross power constant (m) values ranged from 0.64 to 0.76 for porridges made with raw dalia with added SPI and SMP; these values ranged from 0.58 to 0.66 for porridges made with roasted dalia having SPI and SMP. These re-
Cross power constant (m)
0.800
0.600
0.400 Control 5% SPI 10 SPI 05 SMP 10 SMP 10 SPI + 10 SMP
Control 05 SPI
10 SPI 05 SMP 10 SMP 10 SPI+ 10 SMP
Roasted
Control
Relaxation time (λ)
140
100
60
20 Control 5% SPI
10 SPI
05 SMP
Control
10 SMP
10 SPI + 10 SMP
Control
05 SPI
10 SPI
05 SMP
10 SMP
10 SPI+ 10 SMP
Roasted
Fig. 6. Cross model parameters (power constant and relaxation time) of porridges as affected by soy protein isolate (SPI) and skimmed milk powder (SMP).
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sults indicated that all porridge samples exhibited shear-thinning with or without roasting or with the addition of either SPI or SMP or both. Porridges made with roasted dalia were thinner as reflected by their apparent viscosities (Fig. 5). Shear-thinning behaviour for Akamu – a semi-liquid food based on maize, millet and sorghum was observed by Sopade and Kassum (1992). Dispersions of gelatinized wheat, corn and tapioca starches also exhibited shear-thinning behaviour (Bagley and Christianson, 1982; Christianson and Bagley, 1983, 1984; Doublier, 1981; Evans and Haisman, 1979; Wong and Lelievre, 1982; Sai Manohar et al., 1998; Gujral and Sodhi, 2002). Marginally higher values (significant at p 6 0.05) for Cross power constants could be attributed to strained liquids that were thinner than the porridges in the present study; the decrease in apparent viscosity is also evident from Eq. (2). The usefulness of time-independent flow behaviour studies of food dispersions includes a relationship between flow and structure, and correlates with physical and sensory parameters. These shear-thinning dispersions exhibit an increase in apparent viscosity with an increase in concentration of solid but decreases with an increase in temperature. This behaviour is expected for food systems wherein true solutions are difficult to identify (Roopa and Bhattacharya, 2009). The relaxation time of Cross model depends on the nature of material forming the discontinuous phase; it also increases due to an increase in concentration. Further, an increase in SPI and/or SMP decreases the starch content that gelatinizes during wet heating. On the other hand, the protein content increases by addition of SPI and SMP wherein the nature of hydration changes. Hence, a complex situation arises when part of wheat grit is supplemented with SPI or SMP. 3.3. Sensory assessment The sensory attributes of porridges made from raw and roasted dalia along with SPI and SMP are presented in Tables 1 and 2. The appearance of the finished product changed from white to creamish white when SPI was incorporated which is a desirable quality attribute for porridge. The improvement in appearance was better with SMP (significant at p 6 0.05); the combined use of both SPI and SMP further improved the appearance. Significant improvement in flavour was noticed in porridges prepared with SMP or SPI compared to control samples. The interaction of sugar and protein present in SMP might have an important role here to enhance the flavour. Mouth feel increased significantly (at p 6 0.05) as a result of incorporation of either SPI or SMP. The taste of the samples was improved when the porridges were prepared along with milk solids. Addition of SPI and SMP together further enhanced the mouth feel. The combined effect of SPI and SMP were also reflected in the higher scores for overall quality. However, porridges prepared Table 1 Effect of soy protein isolate (SPI) and skimmed milk powder (SMP) on the sensory characteristicsA of wheat porridge made from raw dalia.
Control 5% SPI 10% SPI 5% SMP 10% SMP 10% SPI + 10% SMP SEMB (±) A
Appearance
Flavour
Mouth feel
Overall acceptability
7.5c 8.0b 8.2b 8.2b 8.4ab 8.6a
7.0d 8.0c 8.1c 8.2bc 8.5a 8.4ab
7.0d 7.2cd 7.9b 7.5c 8.2b 8.6a
7.2c 7.4c 8.0b 8.0b 8.4a 8.6a
0.09
0.08
0.07
0.12
Means in the same column followed by different superscripts differ significantly (p < 0.05) according to Duncan’s multiple range test (DMRT). B Standard error of mean (SEM) at 54 degrees of freedom.
Table 2 Effect of soy protein isolate (SPI) and skimmed milk powder (SMP) on the sensory characteristicsA of wheat porridge made from roasted dalia.
Control 5% SPI 10% SPI 5% SMP 10% SMP 10% SPI + 10% SMP SEMB (±)
Appearance
Flavour
Mouth feel
Overall acceptability
6.3d 7.0c 7.5bc 7.2bc 7.9ab 8.1a
8.2c 8.2c 8.4bc 8.3bc 8.6a 8.7a
6.0d 6.5c 6.8bc 6.6bc 7.2a 7.4a
6.8d 7.0cd 7.2bc 7.1c 7.5b 7.9a
0.08
0.09
0.08
0.08
A
Means in the same column followed by different superscripts differ significantly (p < 0.05) according to Duncan’s multiple range test (DMRT). B Standard error of mean (SEM) at 54 degrees of freedom.
from roasted dalia scored lower sensory values compared to those from unroasted dalia; only flavour scores improved for former samples possibly due to Maillard reaction (Rufian-Henares et al., 2009). Mouth feel was better for porridges made from unroasted dalia compared to samples having roasted dalia even after the incorporation of SPI and SMP. This might be due to the lower viscosity of the porridges prepared from roasted dalia. The overall quality of porridges prepared with roasted dalia after the incorporation of SPI and SMP were lower compared to corresponding wheat porridges samples made with unroasted dalia. 4. Conclusions Porridges made from wheat grits (dalia) with the addition of SPI and SMP significantly altered the rheological, pasting and sensory attributes in addition to increasing the protein content compared to that made with wheat grits alone. All porridges exhibited shear-thinning characteristics irrespective of the added ingredients. Pasting temperature increased with the incorporation SPI or SMP. Roasting of dalia markedly influenced pasting temperature and apparent viscosity of the paste. Porridges made from roasted dalia were thinner in consistency but had better flavour characteristics compared to products made with raw dalia. Porridges made from raw dalia along with SPI and SMP had excellent sensory characteristics. References AACC, 2000. Approved Methods of American Association of Cereal Chemists, 10th ed. AACC International, St. Paul, MN, USA. Ade-Omowaye, B.I.O., Adegbite, A.M., Adetunji, B.R., 2009. Evaluation of some chemical and physicochemical properties of meals from pretreated tigernut seeds (Cyperus esculentus). Nutrition and Food Science 39, 142–150. Anon, 1984. Indian Standard Specification for Wheat Porridges. IS:10769-1984, Reaffirmed in 2005. Indian Standards Institution, New Delhi, India. Bagley, E.B., Christianson, D.D., 1982. Swelling capacity of starch and its relationship to suspension viscosity: effect of cooking time, temperature and concentration. Journal of Texture Studies 13, 115–126. Chantrapornchai, W., McClements, D.J., 2002. Influence of NaCl on optical properties, large-strain rheology and water holding capacity of heat-induced whey protein isolate gels. Food Hydrocolloids 16, 467–476. Christianson, D.D., Bagley, E.B., 1983. Apparent viscosities of dispersions of swollen cornstarch granules. Cereal Chemistry 60, 116–121. Christianson, D.D., Bagley, E.B., 1984. Yield stresses in dispersions of swollen, deformable cornstarch granules. Cereal Chemistry 61, 500–503. Cross, M.M., 1965. Rheology of non-Newtonian fluids: a new flow equation for pseudoplastic systems. Journal of Colloid Science 20, 417–437. Doublier, J.L., 1981. Rheological studies on starch – flow behaviour of wheat starch pastes. Starch 33, 415–420. Evans, I.D., Haisman, D., 1979. Rheology of gelatinised starch suspensions. Journal of Texture Studies 10, 347–370. Evans, I.D., Haisman, D.R., 1982. The effect of solutes on the gelatinization temperature range of potato starch. Starch 34, 224–231. Gujral, H.S., Sodhi, N.S., 2002. Back extrusion properties of wheat porridge (Dalia). Journal of Food Engineering 52, 53–56.
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