Rheological Properties of Yoghurt Produced From Blends of Whole Milk and Soybean Powder

Transcription

1 Rheological Properties of Yoghurt Produced From Blends of Whole Milk and Soybean Powder Ome, A.P. 1, Ekwu, F.C. 2 and Ifediba, D. I. 3 1 Department of Food Science and Technology, Michael Okpara University of Agriculture, Umudike, Abia State, Nigeria. 2 Department of Food Science and Technology, Ebonyi State University, Abakaliki, Ebonyi State, Nigeria. 3 Department of Agricultural Technology/Processing, Anambra State Polytechnic, PMB 002 Mgbakwu, Anambra State, Nigeria. Article Received: 29 January 2018 Article Accepted: 30 March 2018 Article Published: 19 May 2018 AB ST R ACT The study was conducted to determine the rheological properties of yoghurts produced from blends of whole milk and soybean powder. Soybean powder was prepared from ground soybean seeds. Four samples of yoghurt product were used, namely; 100% whole milk powder, 30:70, 50:50 and 70:30 (soybean powder: whole milk powder) and analyzed for rheological characteristics. The flow of the soy yoghurt blends was characterized by pseudoplastic behaviour (0.2625<n<0.4748) with shear thinning effect, and increase in shear stress with shear rate. The consistency coefficient, k, increased with increase in soy blends as well as the flow behaviour index (n). The flow behaviour index, n, were not significantly different (P>0.05). Key words: Lactic acid bacteria, yoghurt, starter cultures, flow index, rheology, fermentation. 1. INTRODUCTION Rheology is the study of flowing matter which describes a fluid s internal resistance to flow and may be thought as a measure of fluids friction. Rheology directly affects product handling and flow characteristics the way a material flows. However, controlling rheological properties is essential for polymeric processing and application such as mixing, flow control, dispensing and settling of food materials with high or low viscous nature. For good design of fluid handling equipment in food industry, the knowledge of rheological behaviour is necessary in order to avoid over or undersized pumps and pipes (Ibarz and Barbosa-canovasi, 2003). The rheological behaviour of different foods in Nigeria have been studied. Sopade and Kassum (1992) studied the rheological characteristics of kunun zaki and kunun gyada and observed that kunun gyada exhibited pseudoplastic, while kunun zaki showed Newtonian characteristics. Akanbi (1992) reported that drum dried cowpea slurry exhibited pseudoplastic behaviour when reconstituted in to soup, while Ikegwu and Ekwu (2009) observed that Brachystegia spp, (a traditional soup thickener) dispersions also showed pseudoplastic characteristics with shear thinning effect. Okechukwu (2008) investigated the influence of peanut oil on the flow characteristics of cowpea slurry and found out that the shear responses adequately fitted into the power law equation. Viscosity i.e. resistance to flow, generally indicated by µ is an important rheological property of fluid foods such as beverages and batters and for design of processing lines (Finney, 1973). It is often important for quality control especially for products that are expected to be of a particular consistency in relation to mouth feel (Lewis, 1987; Steffe, 1996). Viscosity is widely used rheological parameter and its relationship with shear rate has been to classify food into Newtonian and non-newtonian with distinguished sub classes such as pseudoplastic, dilatants and thixotropic (Sopade and Kassum, 1992). It has been observed that n=1 in Newtonian fluids, 0<n<1 in pseudoplastic fluids, while in dilatant fluids, 1<n< (Finney, 1973). The main appeal of power law model is its 26 P a g e Website:

2 simplicity and this explains why it is extensively used in both chemical and engineering literatures (Sopade and Kassum, 1992). Yoghurt is defined as a weak gel system and weak gels are unable to keep their structural integrities during high shear (Ozer et al., 1998). However, much of its consumer acceptability is dependent on its sensory properties which in turn seem to be heavily dependent on the method of processing of the material. It should be possible to use rheological data to modify and control the process conditions to give a product with the quality attributes desired by the consumer (Ozer et al., 1998). Having the correct textural properties of yoghurt is critical for consumer s acceptability (Lucey, 2004). According to Lucy (2004), the main factors that determine the rheological properties of yoghurt include: milk heat treatment, incubation temperature, acid development, total solids and type/concentration of stabilizers. Breakdown of gels by shearing is a key part of the manufacture of stirred yoghurt. Yoghurt gels exhibit shear thinning during stirring but rebuild structure to some extent during storage. The breakdown process is poorly understood and this hampers the understanding of factors that control the final viscosity of stirred yoghurt products. Therefore, there is need to investigate more on the rheological behaviour of yoghurt products. 2. MATERIALS AND METHOD 2.1 Sources of samples The soybean seeds were purchased from a local market (Meat Market) located in Abakaliki, Ebonyi State. The milk sample (Whole Milk Powder) and starter culture (Yoghurtment, containing Streptococcus thermophiles, Lactobacillus bulgaricus and Lactobacillus acidophilus) for the production of yoghurt was purchased from Confidence Supermarket, Abakaliki. 2.2 Processing of soybean samples The soybean seeds were thoroughly sorted and cleaned with water to remove damaged seeds, metals, stones, chaffs etc., then boiled for 30mins and soaked for 8h. The tenderized seeds were subjected to dehulling (hand rubbing). Thereafter, were washed with water and oven dried. The dried seeds were milled using Local Attrition Mill and sieved with a wire mesh to achieve a uniform particle size. Then it was stored with a sterile glass bottle at room temperature until use. The flow chart for the processing of soybean powder is shown in figure Mixing of samples and yoghurt production The yoghurt was produced according to the method of Tamime et al, (1989) with slight modification. The mixture had the following blends: Sample A 100% whole milk powder as control, while Sample B (30:70), Sample C (50:50), Sample D (70:30) of soybean and whole milk powder respectively to obtain four samples (A, B, C and D). The mixture was reconstituted with sterile water and homogenized. Then pasteurized at 85oC for 10mins, cooled to 43oC and inoculated with starter culture at the rate of 5g/litre of the solution. The inoculated blends were incubated 27 P a g e Website:

3 at 43oC for 12h and the ph monitored. The blends were further cooled to 25oC after which fermentation was terminated. The resultant yoghurt was stirred and filled into sterile 50cl plastic containers for further analysis. The flow chart for the production of yoghurt is shown in figure 2. Soybean seeds Sorting/cleaning Boiling (30mins) Soaking (8h) Dehulling (hand rubbing) Washing Drying (Oven) Milling (Local Attrition) Sieving (15µ wire mesh) Soybean powder Storage (25 o C) Figure 1: flow chart for the processing of soybean powder 2.4 Viscosity Analysis The viscosity readings of each yoghurt sample was measured using a digital display viscometer (model NDJ-8S) taken at constant time intervals with progressively increasing shear rate (6 60/mins). Viscosity values were obtained by multiplying viscosity readings with appropriate factor supplied by the viscometer manufacturer. 28 P a g e Website:

4 Soybean powder and Whole milk powder Sterile water Basic mix Homogenization Pasteurization of mix (85 o C for 10mins) Cooling (43 o C) Inoculation with starter culture (5g/litre) Incubation (43 o C, 12h, ph 4.5) Cooling (25 o C, termination of fermentation) Stirring and filling into PET bottles Yoghurt Storage (20 o C) Figure 2: flow chart for the production of yoghurt 2.5 Theoretical considerations For Newtonian fluids, the shear rate (γ) of flow is related to the applied shear stress (τ) expressed in a Newtonian law of internal friction according to Ibarz and Barbosa-canovas (2003) as equation 1: τ = µγ P a g e Website:

5 µ = viscosity. Most fluid or semi fluid food materials deviate from the Newtonian behaviour. Assuming no plastic behaviour, the shear stress-shear rate relationship is approximated by power law model (equation 2) as: τ = k γ n 2 k = consistency coefficient or index (N.s/m2), n = flow behaviour index. In terms of apparent viscosity, equation 2 becomes: µ app = k γ n Flow characteristics Flow curves were plotted with the values obtained. From equation 3, log-log plot of µapp against γ, a straight line of slope (n-1) and intercept, k, was obtained from which both the consistency coefficient and flow behaviour index were respectively estimated. 3. RESULTS AND DISCUSSION Rheological Properties of the yoghurt samples The consistency coefficient (k), flow behaviour index (n) and associated correlation coefficient using the power law equation are presented in Table 1: Table 1: Rheological properties of yoghurt samples with different blends of soybean and milk powder Consistency Flow Behaviour Correlation Samples Index (k) N. s/m 2 Index (n) Coefficient (r) A (100%) a a B (30:70) a a C (50:50) b a D (70:30) ab a LSD Results are Means of triplicate determinations. Values in the same column having the same superscripts are not significantly different (P>0.05) A = 100% Whole Milk Powder (control) B = 30% Soybean Powder, 70% Whole Milk Powder blend C = 50% Soybean Powder, 50% Whole Milk Powder blend D = 70% Soybean Powder, 30% Whole Milk Powder blend 30 P a g e Website:

6 The consistency coefficient, k, and flow behaviour index, n, ranged from to , and to respectively for different blends of soybean and whole milk powder blends. The consistency coefficient, k, and the flow behaviour index, n, increased with increase in soybean powder blends. The result is consistent with other findings reported earlier in literature for liquid foods such as kunu zaki and kunu gyada (Sopade and Kassum, 1992). Among all the soybean powder blends studied, the experimental results of the variation of apparent viscosity with shear rate were found to be indicating a shear-thinning pseudoplastic (n<1.0) because of their decrease in apparent viscosity with increase in rate of shear. This is to be expected since the consistency index (k) increased with increase in soybean powder blend (Table 1, Figure 3). Shear thinning has been attributed to a progressive mechanical disentangling of chain within macromolecules as the rate of shearing increases (Launey et al., 1986). Earle (2004) reported that pseudoplastic may occur for a number of different reasons; polymers may align themselves with the flow field, solvent molecules bound to a particle may be removed, or aggregated particles may break down. The result in Figure 3 confirms this where the apparent viscosity decreased with increase in shear rate. The correlation coefficient (r) of log shear stress versus log shear rate data range between and indicates that the power law model adequately described the rheological behaviour of the yoghurt blends. Most Nigerian food dispersions have been described by the power law (Sopade and Kassum, 1992; Ikegwu and Ekwu, 2009). The flow behaviour indices for the yoghurt blends were not significantly different (P>0.05). Figure 4 showed the relationship between shear rate and shear stress. It showed that shear stress increased with increase in rate of shear. The shear stress also increased with increase in soybean powder blend. This is probably due to increase in gelatinization of some of the soybean powder during fermentation after heating at 85oC for 10mins. Okechukwu (2008) reported that cowpea starch exhibits a gelatinization thermal transition range of 66 86oC when monitored with different instruments. Figure 3: The relationship between shear stress and shear rate of the yoghurt samples Key: SBP = Soybean Powder, WMP = Whole Milk Powder 31 P a g e Website:

7 Figure 4: The apparent viscosity of the yoghurt samples at shear rates ( s) at 20oC Key: SBP = Soybean Powder, WMP = Whole Milk Powder The shear stress of sample A (control; 100% whole milk powder) was higher than that of the other samples at any shear rate. For the flow of the blend (30% Soybean, 70% whole milk powder) at 0.1 sec-1 and maximum shear rate of 1.0 sec-1, the shear stress (τ) were and N/m2 respectively while at (70% Soybean, 30% whole milk powder), their values were and N/m2 respectively. This means that the sample blended at (50% Soybean, 50% whole milk powder) requires less stress to flow. This is supported by the fact that apparent viscosity of the sample decreased with increase in shear rate and with increase in soybean powder blend (Figure 4). Ikegwu and Ekwu (2009) made similar observation in their study on rheological characteristics of achi dispersion. The apparent viscosity of the yoghurt samples decreased with increase in shear rate (shear thinning) Figure 4. This observation is similar to the findings of Alkali and Ijabo (2003) for tomato paste, and Sopade and Kassum (1992) for kunu zaki and kunu gyada. The shear thinning effect observed is expected because increase in rotation increases molecular alignment in the direction of flow, reducing resistance to flow and hence the viscosity. Awonorin (1993) reported this phenomenon to be common with hydrocolloidal solutions and food paste with sensitive structure. 4. CONCLUSION The findings obtained from this rheological study showed that the apparent viscosity decreased with increase in shear rate indicating shear-thinning behaviour and characteristics. The soy yoghurt blends studied exhibited pseudoplastic behaviour because their ( n values) were generally below 1 (n<1). It was evident that shear stress also increased with increase in shear rate. The flow behaviour indices of the yoghurt samples were not significantly different (P>0.05) from each other. REFERENCES [1] Akanbi, C. T. (1992). Processing, physical and sensory characteristics of drum dried cowpea bean (gbegiri) soup. Nigerian Food Journal 10: P a g e Website:

8 [2] Alkali, J. S. and Ijabo, O. J. (2003). Effect of processing methods on the viscosity, ph and colour of tomato (Lycoporrsicum escusentum) paste. Journal of Sustainable Tropical Agriculture. Res 5: [3] Awonorin, S. O. (1993). Rheological and mechanical properties of B-vitamin retention and sensory characteristics of sausage made from broiler chicken and guinea fowl. Lebensmittal West-II-Technology 20: [4] Earle, R. L. (2003). Unit operations in Food Processing. Web Edition Publisher; The New Zealand Institute of Food Science and Technology (Inc). [5] Finney, E. E. (1973). Elementary concepts of rheology relevant to food texture studies. In: Texture Measurement of Foods; Kramer, A. and Szczeniak, A. S. Ed., Reidel-Dordrecht-Holland, pp [6] Ibarz, A. and Barbosa-canovas, G. V. (2003). Unit operation in Food Engineering. CRC Press. New York. Pp [7] Ikegwu, O. J. and Ekwu, F. C. (2009). Rheological and functional properties of achi flour as influenced by processing time. Nigerian Food Journal, 27 (2): [8] Launey, B., Doublier, J. L. and Curvelier, G. (1986). Flow properties of aqueous solution and dispersions of polysaccharides. In: Functional properties of food macromolecules. Mitcheel, J.R. and D. A. Ledwward (Eds). Elsevier Applied Science Publication Crown House, England, pp [9] Lewis, M. J. (1987). Physical properties of food and food systems. Chi Chester Ellis Horwood Ltd. Pp [10] Lucey, J. A. (2004). Rheology of yoghurt and fermented milks. Dept. of Food Science, University of Wisconsin, Madison, 1605 Linden Dr., A 19 Babcock Hall, Madison, WI [11] Okechukwu, P. E. (2008). Influence of peanut oil on the flow characteristics of cowpea slurry. Nigerian Food Journal, 26(1): [12] Ozer, B. H., Bell, A. E., Grandison, A. S. and Robinson, R. K. (1998). Rheological properties of concentrated yoghurt labneh. Journal of Texture Studies, 29: [13] Sopade, P. A. and Kassum, A. L. (1992). Rheological characteristics of Nigerian Liquid and semi-liquid foods: kunu zaki and kunu gyada. Nigerian Food Journal, 10:23-33 [14] Steffe, J. F. (1996). Rheological methods in Food Process Engineering. Freeman Press, 2807 Still Valley Dr Easy Lansing, Mi 48823, USA. [15] Tamime, A. Y., Kalab, M. and Davies, G. (1989). Rheology and Microstructure of stained yoghurt (labneh) made from cow s milk by three different methods. Food Microstructure, 8: P a g e Website: