Textural and sensory quality enhancement of sorghum tuwo

Abstract

The study was focused on the improvement of textural and sensory qualities of sorghum tuwo by the use of hydrothermal treatment in modifying the physicochemical properties of sorghum grains. Flour was obtained from sorghum grains subjected to hydrothermal treatments at temperatures ranging between 65 and 95 °C for varying time intervals of 30, 45 and 60 min respectively. Flours obtained through the hydrothermal processes were found to be greatly affected in both pasting and functional properties as well as in flour yield when compared with flour obtained through the traditional processing methods. The sensory evaluation on sorghum tuwo prepared showed that a particular tuwo sample, D₁ (from flour whose grains were hydrothermally treated at 95 ± 1 °C for 30 min), was rated highest in comparison with that from traditionally produced flour in terms of taste, texture (mouldability) and overall acceptability.

Introduction

Sorghum (Sorghum bicolor L. Moench) is a cereal grain widely used for human consumption in the semi-arid tropics (SAT) and it serves as a source of staple diet for millions of people in this region providing essentially their calorie needs. In Nigeria, sorghum grains are being processed into several traditional food products, which include ogi, kunu, eko and tuwo (Obilana, 1982). In fact, sorghum utilisation as human food accounts for almost three-quarters of total utilisation particularly in Africa (FAO, 1995).

The use of sorghum grains for the production of sorghum tuwo is one of its food uses in Nigeria. Tuwo is a food gel usually prepared from unfermented sorghum flour obtained through dry milling of sorghum grains. The consumption of sorghum tuwo is popular in the Nigerian savannah regions where sorghum production is highest. The preparation procedures for sorghum tuwo exhibit little variations across different places (Obilana, 1982) and the food product is usually consumed for lunch and/or dinner together with common vegetable soups in the region.

One major quality problem associated with sorghum tuwo is that of textural quality. Soon after its preparation, the food product becomes stiff and does not give room for much hand-moulding, which is the preferred mode of consumption. The food product has a scattering tendency while moulding with hand and this serves as a psychological problem for the traditional consumers and therefore reduces the expected satisfaction derivable during consumption. It is very important to note that the general acceptance of a food product by the consumers is dependent not only on its nutritional status but also on other factors such as aesthetic appeal, cultural disposition of people and rheological properties of the product, among others (Matz, 1962; Ihekoronye & Ngoddy, 1985).

Traditionally, one approach of circumventing the textural quality inadequacy of sorghum tuwo is to consume the food product while hot/warm to avoid the stiffness that might result if the product is allowed to cool. It has, however, been observed that the quality and acceptability of a cereal product are usually influenced by the physical and chemical properties of the cereal from which it is produced (Akingbala et al., 1995) and these properties may be modified through chemical, physical and enzymic processes so as to obtain desired functional characteristics (Galliard & Bowler, 1987).

Over the years, efforts are being made by the plant breeders to develop sorghum hybrids of high grain quality in terms of nutritional characteristics while still maintaining the yield and yield stability breeding focus (Doggett, 1982). The essence of these efforts is to develop specific sorghum hybrids that might be useful in the production of diverse traditional sorghum-based food products with improved quality attributes. Similarly, some of the previous studies (Akingbala & Rooney, 1990) on sorghum tuwo were focused on the factors responsible for the quality attributes of the product while little efforts had been made on the improvement possibilities on such quality attributes.

This study therefore was aimed at reducing the rheological problem associated with sorghum tuwo with a view to enhancing its textural and sensory qualities thereby making the product more acceptable to the traditional and non-traditional consumers alike within Nigeria and beyond.

Materials and methods

Materials

Sorghum grain (white variety) was obtained from Bodija market in Ibadan, Nigeria. The white variety was chosen because it is the one commonly used for tuwo preparation because of the resultant white/creamy colour of the food product that is preferred by the people.

Methods

Preparation of sorghum flour using an improved traditional method.

A 5-kg batch of cleaned sorghum grains (white variety) was first tempered with water using a quantity of 4% (v/w) followed by decortication of the grains on a Grantex dehulling machine. The machine removed the hulls and germs of the grains. The decorticated grains (grits) were then ground into flour using a disc attrition mill (Agrico Model 2A, New Delhi, India). The sorghum flour eventually obtained was then sieved using a sieve size of 425 μm and the sieved flour sealed in airtight polythene bags and kept for subsequent uses. A total of four 5-kg batches were used for the experiment.

Preparation of flour from hydrothermally treated sorghum grains

Three batches (8 kg each) of dry and wholesome sorghum grains were first soaked separately in cold water (submerged) for 1 h after which each was drained and then subjected to hydrothermal treatments carried out at 65 ± 1 °C for varying time intervals of 30, 45 and 60 min respectively. Hydrothermal treatment is essentially heating of grain sample in hot water (submerged) at a specified temperature and time.

Other batches (8 kg each) were similarly prepared and subjected to hydrothermal treatments at 75 ± 1, 85 ± 1 and 95 ± 1 °C for the same aforementioned varying time intervals respectively. At the end of the treatment, each batch was drained and then dried in an air draught oven at 55 °C and the drying was normally terminated when the moisture content of the drying grains dropped to an estimated level of 10 ± 1%. This was accomplished by periodically checking the dropping weight of the drying grains whose initial moisture level (immediately after hydrothermal treatment) had been determined using an air draught oven at 105 °C for 3–4 h. The dried grains eventually obtained were ground into flour and sieved using a sieve size of 425 μm and subsequently kept in an airtight polythene bag until required.

Evaluation of material balance of flour production

The basic principle of the law of conservation of mass (Earle, 1983) was used in evaluating the material balance of flour production both from the improved traditional method and that of the hydrothermal treatment. Mathematically, this can be expressed as follows: sg = sf + hg + ss + ot + lm (where sg = weight of sorghum grains; sf = weight of sieved flour; hg = weight of hulls and germs; ss = weight of soluble solids in the water used for hydrothermal treatment; ot = weight of overtails from flour sieving operation; and lm = weight of other lost materials incurred during processing).

Particle size analysis of the flour

The particle size analysis of sorghum flour obtained through the improved traditional processing method and that of hydrothermal treatments was carried out using Endecotts Test Sieve Shaker. Different sieves with varying apertures (i.e. 850, 425, 300, 150 and 63 μm) were arranged in decreasing order of aperture. The sieves were then fastened into a rigid position using a fastening screw after a definite quantity (i.e. 250 g) of the flour sample was placed inside the topmost sieve. The sieve shaker was then switched on for 15 min after which the quantity of flour retained on each sieve was collected for analysis.

Proximate chemical composition of sorghum grains and flour

The proximate chemical composition of sorghum grains and that of the flour obtained through the improved traditional processing method was carried out using the official methods (AOAC, 1990). Carbohydrate content was determined by difference method (i.e. by subtracting the percent crude protein, crude fibre, crude fat and ash from 100% dry matter). All analyses were carried out in triplicates.

Assessment of pasting properties of sorghum flour samples

The pasting properties of sorghum flour obtained through the improved traditional processing method and that from various hydrothermally treated sorghum grains were determined using Brabender viscoamylograph (Mazurs et al., 1957). Flour slurry of 10% (w/v, dry basis) was gradually heated up from 30 to 95 °C at the rate of 2.5 °C min⁻¹, held at a temperature of 95 °C for 30 min, cooled down to 50 °C under running tap water and finally held at this low temperature for another 30 min.

Analysis of functional properties of sorghum flour

The bulk density of sorghum flour was determined according to the method of Akpapunam & Markakis (1981). The flour of known weight was transferred into a 250-mL graduated glass cylinder and the volume determined. The bulk density of packed flour was calculated after tapping the cylinder until the flour stopped settling, after about 2 min. The results were expressed in g cm⁻³.

The determination of water and oil absorption capacities of sorghum flour was carried out according to the modified method of Prinyawiwatkul et al. (1997). Each flour sample (5.0 g) was thoroughly mixed with 25 mL of deionised water or oil in 50-mL centrifuge tubes. Suspensions were stirred intermittently over a 30-min period at room temperature (i.e. 30 ± 2 °C) and then centrifuged at 12 000 × g for 30 min. The volume of the supernatant was measured and the water or oil absorption capacity was calculated. The results were expressed in percentages.

The foaming capacity and stability of the flour were determined according to the method of Coffman & Garcia (1977). The percentage volume increase immediately after whipping was used as the capacity index while that of stability was 2 h after, using 2% (w/v) flour concentration.

Preparation of sorghum tuwo

Sorghum tuwo was prepared using the procedure of Bolade et al. (2002). The overall ratio of water to flour used in sorghum tuwo preparation was 3.5:1 (v/w). Sorghum tuwo was prepared from flour produced through the traditional processing technique and that produced from four selected hydrothermally treated sorghum grains. Cold slurry of sorghum flour was first prepared by mixing 25% of the required quantity of water with 20% of the required quantity of the flour. This was followed by bringing 60% of the total water required into boiling and the cold slurry initially prepared was added to this boiling water with vigorous stirring to form a pap-like consistency. The remaining quantity of the flour (80%) was then added gradually to the boiling pap-like paste with continuous stirring so as to prevent lump formation and to ensure smooth gel formation. The remaining quantity of water (15%) was finally added to the formed gel, covered properly without stirring, and allowed to cook for about 3–5 min after which it was stirred properly to ensure smoothness of the gel. The product so obtained is called sorghum tuwo.

Sensory quality evaluation of sorghum ‘tuwo’

The five sorghum tuwo samples prepared from the various forms of sorghum flour were subjected to sensory evaluation, after cooling under ambient temperature, so as to ascertain their consumers’ acceptability in terms of colour, taste, aroma, texture (mouldability) and overall acceptability using a multiple comparison test (Larmond, 1977; IFT, 1981). A fifteen-member taste panel was used to carry out the ratings of the tuwo samples. The panelists were asked to rate the samples on the basis of colour, taste, aroma, texture (mouldabililty) and overall acceptability using a nine-point scale (i.e. 9 = like extremely; 5 = neither like nor dislike; 1 = dislike extremely). The panelists were all familiar with the traditional product and they were also instructed on the use of sensory evaluation procedures. The scores from the ratings were then analysed using standard procedure of analysis of variance (Anova) and differences in mean values determined at P < 0.05.

Evaluation of softness index of sorghum tuwo

The softness index of sorghum tuwo was evaluated using Universal Penetrometer (Model HU1200-00, Geneq Inc., Montreal, Canada). The food product was first packed inside a circular cup placed perpendicular to the penetration needle of the penetrometer. The needle was then allowed to fall freely onto the product under the influence of gravity after which the degree of needle penetration was measured from the penetrometer indicator dial. The results were expressed in millimetres (mm).

Statistical analyses

All determinations or measurements reported in this study were carried out in triplicates. In each case, a mean value and standard derivation were calculated. Anova was also performed and differences in mean values determined using Duncan's test at P < 0.05 by employing Anova and Duncan procedures of statistical analytical systems (SAS, 1990). In the case of two-group samples, an independent samples t-test was used in determining the statistical level of significance, P < 0.05.

Results and discussion

The material balance in the production of sorghum flour was generally the same between both traditionally processed and hydrothermally treated grains (Table 1). No significant differences were seen between the levels of hull and germ, sieved flour and processing losses between the two methods of production. However, overtails from sieving were greater in the hydrothermally treated samples. This significant difference may be as a result of heating and cooling.

The particle size distribution of sorghum flour from the traditional processing methods and some selected hydrothermal treatments is presented in Table 2. In spite of the fact that a sieve aperture of 425 μm was used for sieving the original flour, certain quantities were retained on the bigger sieve aperture such as 850 μm as well as on the 425-μm aperture. This may be because of possible irregularity in the shape of the flour particles (Donelson & Yamazaki, 1972) or the flour might have clumped together because of water movement and absorption within the flour particles particularly after the initial sieving. It was also observed from Table 2 that the particle size distribution of sorghum flour exhibited a variation with respect to the traditional processing method and hydrothermal treatment method and with respect to hydrothermal treatment of low temperature to that of high-temperature treatment. The proportion of flour retained by a combination of 150- and 300-μm apertures was 64.31% from the traditional processing method, while the proportion was 65.61% from hydrothermal treatment at 65 ± 1 °C for 45 min and there was generally a progressive increase in the flour proportion up to 70.04% from hydrothermal treatment at 95 ± 1 °C for 45 min. The increase in the temperature of hydrothermal treatment seemed to influence an increase in uniformity of the flour particle size between 150 and 300 μm.

The particle size distribution of flour from cereals is of technological significance as the particle size can influence the ease of dispersion in water, extent of surface area availability for a reaction, and the quality of products derivable from such flour (Chaudhary et al., 1981; Earle, 1983; Wang & Flores, 2000; Peterson & Fulcher, 2001).

The proximate chemical composition of sorghum grains and the flour traditionally produced from it is presented in Table 3. The values obtained for sorghum grains showed that there were considerable variations in the grain composition in comparison with findings from other researchers (Hulse et al., 1980; Rooney & Serna-Saldivar, 1991). These variations can be attributed to factors such as location at which the crop is grown, density of plant population, season, water and stress, and agronomic practices including fertiliser application (Deyoe & Shellenberger, 1965; Deosthale & Belavady, 1978; FAO, 1995). The sorghum flour obtained from the traditional processing method showed that there were significant differences (P < 0.05), when compared with whole grains, in the values for crude fat, crude fibre, ash and carbohydrate, on dry weight basis; as well as for moisture content. The sorghum flour generally had lower values in crude fat (0.95%), crude fibre (0.87%) and ash (0.79%) than that of whole grains, while higher values were recorded for the moisture content (10.56%) and carbohydrate (87.66%). The appreciable increase in the flour moisture content revealed that bulk of the moisture in the sorghum grains was concentrated within the endosperm fraction of the kernel and was not evenly distributed among the kernel's fractions. Similarly, the increase in the carbohydrate value in the flour also revealed that the endosperm fraction from which the flour was milled was a major contributor to the kernel's total carbohydrate (FAO, 1995).

The only chemical component that was not significantly different (P < 0.05) in both sorghum flour and grain was the crude protein having values of 9.73% and 10.22% respectively. The non-significant difference in the values of crude protein may be because of the concentration of protein within the endosperm of the grain from which the flour was obtained (FAO, 1995), while loss of protein together with the removed components (i.e. hull and germ) was highly minimal.

The pasting properties of sorghum flour obtained from the traditional processing method and various hydrothermal treatments of sorghum grains are presented in Table 4. The apparent gelatinisation temperature of sorghum flour traditionally produced (C₀) was 75 °C and this value was significantly different (P < 0.05) from that of hydrothermal treatment sources. The apparent gelatinisation temperature of all the flours ranged between 72.5 and 87.5 °C, while the flour–source combinations with no significant differences in their apparent gelatinisation temperature were A₃ and B₃; A₂, C₂ and D₁; and B₂ and C₁. It was generally observed, however, that higher temperatures of hydrothermal treatments seemed to favour an increase in the apparent gelatinisation temperature of the sorghum flour.

The peak viscosity of C₀ was 440 BU, while that of A₁ (480 BU) and A₂ (500 BU) were significantly higher than that of C₀. The peak viscosities of flours from all other sources were significantly lower than that of C₀, while the flour–source combinations in which there were no significant differences (P < 0.05) in their peak viscosities were A₃ and C1; B₂ and C₂; and B₁, B₃ and D₁. The peak viscosities of all the flours from hydrothermal treatment sources were reached after attaining a temperature of 95 °C, while that of C₀ was reached before attaining a temperature of 95 °C. The time to reach peak viscosity by the traditionally produced flour (C₀) was 25 min, while flours from various hydrothermal treatments exhibited longer time in reaching respective peak viscosities ranging between 27 and 41 min. However, the time to reach peak viscosity by flours obtained from high-temperature hydrothermal treatments (i.e. 75–95 °C) was not significantly different from each other, while significant differences existed only among A₁, A₂ and A₃ from relatively low-temperature hydrothermal treatments (i.e. 65 °C).

The viscosity of C₀ when the temperature was cooled to 50 °C was 560 BU, while this value was not significantly different (P < 0.05) from that of B₁, B₃ and D₁. The viscosity at 50 °C for D₃ was significantly lower (530 BU) than that of C₀, while viscosities at 50 °C for others were significantly higher than that of C₀. However, some of the flour–source combinations were not significantly different from each other in their viscosities at 50 °C and these were A₁ and C₁; C₁ and D₂; and B₂ and C₂.

The breakdown/stability of the paste during cooking showed that C₀ exhibited a breakdown having a negative value (−25 BU), while majority of the flours from various hydrothermal treatments were stable having zero values with the exception of A₁ and A₂ (flours whose grains were hydrothermally treated at 65 ± 1 °C for 45 and 60 min) that also exhibited a breakdown (−50 and −20 BU respectively). The reason for this occurrence particularly for C₀, A₁ and A₂ that thinned down (broke down) during cooking is most probably because of a progressive fragmentation and solubilisation of the swollen granules, while other flours that were relatively stable during cooking seemed to dissolve in hot water to give molecularly dispersed solutions rather than pastes of swollen granules (Mazurs et al., 1957).

The setback value of the flour, which is a measure of the difference between the viscosity of the paste when cooled to 50 °C and the peak viscosity, showed that C₀ had the minimum setback value (+120 BU), while other flours (flours obtained through hydrothermal treatments) had higher setback values which ranged between +190 and + 620 BU. The setback value of the paste during cooking is a reflection of the retrogradation tendency of such starch product: the higher the setback value, the higher the retrogradation tendency (Mazurs et al., 1957).

The percent stability index (PSI) of the cooked paste in actual use for the various flours revealed that only C₀ (traditionally produced flour), A₂, B₃, C₂ and D₁ (flour whose grains were hydrothermally treated at 65 ± 1, 75 ± 1, 85 ± 1 and 95 ± 1 °C for 45, 60, 45 and 30 min respectively) had relatively low values (i.e. +27.3%, −8.0%, +27.3%, +18.4% and −15.2% respectively), while others had higher PSI value ranging between +40% and +173.7%. The PSI of the cooked paste in actual use is essentially the ratio of the difference between the viscosity of the paste after 30 min at 50 °C and the peak viscosity to the peak viscosity itself. When the PSI value (as calculated from Rasper, 1969) of cassava (Manihot utilissima cv Ankra) starch at approximately 8% starch concentration (w/v) was compared with that of sorghum flour, it showed that cassava starch could have as low as −60.3%. Therefore, it was on the basis of relative low PSI values that the five sorghum flour samples (i.e. C₀, A₂, B₃, C₂ and D₁) were selected for subsequent analysis with respect to their functional properties, sensory evaluation of sorghum tuwo prepared from them and their respective rheological assessment.

Table 5 shows the functional properties of selected sorghum flour samples. The bulk density of C₀ was 0.87 g cm⁻³, while those of other flours obtained through hydrothermal treatments were marginally higher (0.89–0.97 g cm⁻³) and were not significantly different from that of C₀ at P < 0.05 except B₃. The results generally indicated that the hydrothermal treatment did increase the bulk density of sorghum flour marginally. The bulk density essentially offers packaging advantage to the flour that has higher value (Fagbemi, 1999) as more quantity can be packaged within a specific volume.

The water absorption capacity of C₀ was 225.06%, while those of other flours obtained through hydrothermal treatments were generally of higher values (260.13–295.27%). The values were significantly different from that of C₀ at P < 0.05, while those from hydrothermal treatments were also significantly different from each other and it was observed that the increase in value was directly related to increase in the temperature of hydrothermal treatments. The higher values observed with flours from hydrothermal treatments was most probably because of the partial gelatinisation of the starch component and partial swelling of the crude fibre component (Padmashree et al., 1987).

The oil absorption capacity of C₀ was 139.60%, while those of other flours from hydrothermal sources were significantly different (P < 0.05) from C₀ and from each other. Samples A₂ and B₃ had higher values (204.64% and 182.30% respectively) of oil absorption capacity than C₀, while C₂ and D₁ had lower values (130.23% and 111.40% respectively). The relatively low temperature of hydrothermal treatment seemed to favour the significant increase in oil absorption capacity, while the relatively high-temperature treatment also favoured a significant decrease in oil absorption capacity. Oil absorption capacity is essentially related to the ability of the flour to entrap oil and this phenomenon is better appreciated in foods where oil acts as a flavour retainer thereby increasing the mouth feel of such food products (Kinsella, 1976).

The foaming capacity of C₀ was 4.35%, while there was a significant decrease in the values for flour from hydrothermal treatments (3.22–4.12%) except A₂ which had a non-significant lower value (4.12%). Food materials with high partially denatured protein were observed to give better foaming properties (Tamsama et al., 1969; Richest et al., 1974). However, low foaming capacity value as observed generally in sorghum flour (Elkhalifa et al., 2005) may be attributed to its relative low protein content coupled with its reduced protein solubility as a result of hydrothermal treatments (Tagodoe & Nip, 1994).

The foaming stability values of all the flour samples, after 2 h, were generally poor. No foaming stability was observed in A₂, B₃, C₂ and D₁, while C₀ only had about 0.80%.

Hydrothermal treatment of sorghum grains might have adversely affected the foaming stability of the flour.

Table 6 shows the sensory evaluation ratings and softness index of sorghum tuwo. Tuwo sample from C₀ was rated the highest in terms of colour and aroma. The colour rating of tuwo from C₀ was significantly different (P < 0.05) from that of others, while there was no significant difference in the colour ratings of tuwo from A₂, C₂ and D₁, and B₃ and C₂. The aroma rating of tuwo from C₀ was not significantly different from that of A₂ and D₁, while a significant difference existed with tuwo from B₃ and C₂. The highest rating of both colour and aroma in tuwo from C₀ revealed a possible adverse effect of hydrothermal treatment on the colour and aroma of flour obtained from such treatment.

Tuwo from D₁ was rated highest in terms of taste, texture (mouldability) and overall acceptability. However, the taste and mouldability ratings of tuwo from D₁ were not significantly different (P < 0.05) from those of A₂ but significantly differently from those of others. Similarly, the overall acceptability of tuwo from D₁ was significantly higher than that of the other samples.

The softness index (penetrometer reading) of tuwo from C₀ was 18.1 mm, while that from D₁ (tuwo sample with the highest rating in overall acceptability) was 24.1 mm. The softness indexes of all tuwo samples were significantly different from each other, while it was also observed that tuwo prepared from hydrothermal sources generally had higher penetrometer reading (19.2–24.1 mm) as against tuwo prepared from traditionally produced flour (18.1 mm).

In conclusion, this study has revealed that the use of hydrothermally treated sorghum grains in the production of flour meant for sorghum tuwo preparation will go a long way in predisposing the flour towards having a tuwo product of enhanced textural and sensory qualities. The hydrothermal treatment to which sorghum grains were subjected was found to be temperature and time specific, while the penetrometer readings of tuwo products were also found to be influenced by the hydrothermal treatment.

References