1.1                                                 INTRODUCTION
            Yogurt is produced by a fermentation process during which a weak protein gel develops due to a decrease in the pH of milk (Oroian et al, 2011). The gel structure contributes substantially to the overall texture and organoleptic properties of yogurt and gives rise to shear and time development viscosity (Odoneell and Butter, 2002). Texture is one of the most important attributes used by consumers to assess the quality of yogurt. The most frequent defects related to yogurt texture that may lead to consumer rejection are apparent viscosity variations and the occurrence of Syneresis (Kroger, 1975).
As a flow property, the Knowledge of the rheological properties is important for design and evaluation of the process, process control, consumer acceptability of the product and the optimization of process variables (Brennan et al,1980; Dail and Steffe, 1990, a. b, Harrison and Gunningham, 1985). It is important to establish relationship between structure and flow and to correlate physical parameters (Abu-jdayil, 2003; Resch 2002).
            Yogurt rheological characterization is required for product and process development and to ensure consumer acceptability (Benezech and Mangonnat, 1994). It is also required for effective design of process system.
            Yogurt from animal milk has received a lot of scientific attention by many authors, but studies providing information on the rheological characterization of legume yogurt like product is scanty.
            It is noteworthy, that the dairy sub sector in Africa is thus relegated to the category of subsistence system of production due to minor and peripheral status accorded the sector by various government policies allied with the above, are poor nutrition and genetic constitution of the Africa breeds of ruminant. The above problem leads to insufficient milk availability of 15L of milk. This dramatic decrease in the consumption of milk products stimulated in part the processing of milk from different seeds and nuts.
            Though undervalued in the past, milk from plant source are key ingredients in the diet of African countries. Recently, researchers have shown strong interest in these milk sources due to their high nutritional values and economic potentials. It is worthy repeating that milk sources form plants are seen as a radiating hope as well an ally in the fight against hidden hunger. All the nuts and the seeds of interest in the present study are found on the tropical environment including Nigeria for various purposes.
1.2       OBJECTIVE
1.         The objectives of this study, is to evaluate the rheological characteristics of yogurt blends with tiger nut milk and coconut milk.
2.         To evaluate the physical properties of yogurt blends with tiger nut milk and coconut milk.
3.         To determine the sensory quality of the yogurt blends with tiger nut milk and coconut milk.
            Yogurt is a diary food product, produced by lactic acid bacteria in fermentation of milk. The conversion of lactose into lactic acid gives yogurt its characteristic gel-like texture. (Braing dictionary, 2005; Wikipedia, 2005; Elson and Hass, 2005).
            It is also described as the bacterial curdling of milk, which is produced with the use of specific bacteria (Lactobacillus bulgaricus and streptococcus thermophilus). For the production of yogurt, which has a custard-like consistency (Robins 1980).
            It is believed that yogurt originated in mesopotamia now in Iraq thousands of years ago. Evidence has shown that these people has domestic goals and sheep around 500 B.C. the milk from these animals was stored. In gourds, and in a warm climate, where it naturally formed a curd (Helferich and Westhoff, 1980). Yogurt whose name comes form the Turkish word “yogurt” is the most wisely available fermented milk in the western world today where its popularly derived more from its flavour (Adams and Moss, 1999).
2.2       Types of Yogurt
            There are different types of yogurt, which include:
2.2.1               Pasteurized stirred Yogurt
            This type of yogurt is pasteurized and incubates in a tank and the final coagulum is “broken” by stirring prior to cooling and packaging. It has extended shelf-life.
2.2.2               Strained Yogurt
            This is a type of yogurt which is stained through a paper or cloth filter, traditionally made of muslin to remove the whey. Yogurt once made is refrigerated over night. It is poured in a muslin or cheese cloth bag and hug in the coolest place with a tub placed underneath to collect the dripping whey.
2.2.3   Bio-Yogurt
            This is made with different types of fermentation culture (probiotic culture). Such as lacto bacillus subspcasei, lactobacillus acidophilus, Bifido-bacteria e.t.c and it aids digestion, improves gastrointestinal function and stimulates the immune system.
2.2.4   Organic Yogurt
            This is made with milk form specially fed cows. This type of yogurt is claimed to be more nutrition’s than other yogurts.

2.2.5               Frozen Yogurt
            Frozen yogurts are yogurt that are inoculated and incubated in the same manner as stirred yogurt. In frozen yogurt, cooling is achieved by pumping through a whippier/freezer/ chiller in a fashion similar to ice cream.
2.2.6   Concentrated Yogurt
            These are yogurt that are fermented and inoculated in the same manner as a stirred yogurt. The concentration is done by boiling of some of the water and it is often done under vacuum to reduce the temperature required.
            Yogurt is nutritionally rich in protein. Calcium, riboflavin, vitamin B6, and vitamins B12.
            Yogurt thought to have additional health benefits beyond milk.
            One of the suggested benefits of yogurts is that it acts as a digestive aid. Yogurts encourage the growth of beneficial bacteria In the intestine of the body. This organisms help to digest food more efficiently and protect against other harmful organisms.
            Yogurt is good for people that are lactose intolerant. These people have difficulty digestive milk products, the typically can tolerate yogurt because much of the lactose in the milk is converted to lactic aid by the bacteria culture (Kolars et al; 1984) 
            Yogurt that contain live cultures is sometimes used in an attempt to prevent antibiotic associated diarrhea (Beniwal, et al; 2003).

Table I nutritional value per 100 g of yogurt (full fat)
257kg (6 kcal)
Vitamin A equivi
27ug (3%)
Riboflavin (vit. Biz)
0.14 mg (9%)
121mg (12g)
Lactose content diminishes during storage. Percentages are relative to US recommendation for adults.
Source:           USDA Nutrition database.
2.4       Utilization of Yogurt
            Yogurt is an excellent this by itself. It is also valuable in its many other uses. Yogurt can be used as part of the liquid in cakes, waffles, pancakes and muffins and would help cut down on the amount of baking powder. The thickness of yogurt helps to hold up the baking balter (David, 2003).
            Yogurt is also used as a starter for cheese production and also a starter for yogurt batch processing technique (Rosenthal, 1978: Coyle. 1982).
            Ernest (1996), reported that in the developed countries, yogurt is used as a dessert, between meal, snack, complete lunch, and diet food. Yogurt is useful in the diet for: energy-reduced diets, warring babies on to solid foods, convalescents and alternative to puddings (Tull, 1996).
            Production of yogurt from milk starts with pasteurization and homogenization of the milk. Before pasteurization, stabilizer (such as gelatin or modified food starches) may also be added to improve the body and texture by increasing firmness, mouth feel and also help to keep the flavour uniformly mixed in the yogurt. The milk is then cooled to 430c and inclulated with 1.25% each of lactobacillus bulgaricus and streptococcus thermophilus (Kosikiwski, 1982). During incubation (12 hours), the pH will decreased due to lactic acid production. Yogurt is then stored in the refrigerator at low temperature. The consistency and the flavour of the final product depend on the type of milk used (Namsum, 2008).
            During yogurt production, the micro organism, used help in:
Production of lactic acid which lowers the pH,  makes the yogurt sour in taste, causes the milk protein to thicken and also acts as a preservative.
Milk Mix
Heat treatment  (850c for 15 mins)
Cool to incubation temperature (430c)
Inoculate with starter culture
Incubation (430c), 12 hrs)
Cooling (50c)

Fig 1 flowchart for yogurt production

Source:  Adam’s and moss (1999).
2.1       Fermentation
Fermented foods are those foods which have been subjected to the action of micro-organisms of enzymes so that desirable biochemical changes cause. Significant modification to the food yogurt is perhaps the oldest fermented milk product known and consumed by large segments of our population either as a part of diet or as a refreshing beverage drink. Many fermented milk products which are eaten as they contains living micro-organisms.
            Traditionally, lactic acid bacteria are the most commonly used micro-organism for preservation of food (Anu et. al.,  2010).
2.6.1   History of Tiger-nut
            Tigernut (cyperus esculentus L.) is an underutilized crop which belongs to the division magnoliophyta,classliopsida, order–cryerte and family-cyperaceous (family) and was found to be a cosmopolitan perennial crops of the same genus as the papyrus plant. Other names of the plant are earth almond as well as yellow nut grass (Odoemlan, 2003; Belewn and Belewu, 2007). Tiger-nut also known as Chufe as though to have originated in the Mediterranean area and western Asia but has spread (mainly as a weed) to many parts of the world. It will grow in a very wide range or climatic conditions, and occurs in the tropics subtropics and warm temperate regions and is cultivated in several countries (Kay, 1987).

2.6.2   Varieties of Tiggernut
            Three varieties (yellow, brown and black) are cultivated in Nigeria but only two varieties (yellow and brown) are readily available in the market (Oledele and Aina, 2007). The yellow variety is preferred to all other varieties because of it’s inherent properties like its bigger size attractive colour and fresh body (Belewn and Belewu, 2007), Belewu and Abodunrin, 2008, Umerie et al, 1997). The yellow variety (large and small) also yields more milk upon extraction, contains lower fat and more protein and possesses less anti-nutritional factors especially polyphenoil (Okafor et al, 2003).
2.6.3   Utilization of Tiger nut
            The tiger-nut has small tuberous rhizomes which are eaten raw, baked or roasted, grated to make refreshing beverages and ice creams. (Key 1987; Belewu and Belweay 2007; Oladele and Aina, 2007). Kay, (1987) reported that tiger-nut one used as subsidiary in animal feeding, confectionary, coffee and cocoa adulterant while the secondary products are oil, start, flour, alcohol and leaves.
2.6.4   Chemical Composition of Tiger-Nut
            The high crude lipid and carbohydrate content and its fairly good essential amino acid composition make it a valuable source of food for man. Tiger-nut is rich in energy content, (starch, fat, sugar and protein), mineral (sodium, magnesium, calcium, phosphorous, potassium sine- and traces of copper) and vitamins E and C (Belewu, 2007; Belewu and Abodunrin, 2008).
            Nwaoguikpe. (2010), reported that the proximate composition of two varieties of the best (yellows, large and small size) species of cyperus esculentus on the wet and dry samples showed a higher protein, crude fiber, and lipid in the dried samples.
            Adeyuyitan et al, (2009), reported that tiger nut contain higher essential amino acids than those proposed in the protein standard for satisfying adult needs.
            According to Belewu and Abodurin (2008) and Adejuyitan et al (2009), tiger-nut produces high quality oil, about 25.5% of its contents and the oil was implicated as lauric acid grade oil, non acidic stable and very low instauration.

2.6.5   Health and Nutritional Properties of Tiger-Nut
            According to Masson (2008), tier-nuts have long been recognized for its health benefits as they are high in fiber, protein and natural sugars. They have a high content of soluble glucose and Oleic acid, along with high energy content (Starch, fats, sugar, and proteins) they are rich in minerals such as phosphorus and potassium and in vitamins E and C (Belewu and Elewu, 2007; Belewu and Abodunkin, 2008). Tiger-nut flour has been demonstrated to be a rich source of quality oil and contains moderate amount of protein. The extract from tiger-nut is a product of plant origin with high nutritional and health properties which can be used as a milk substitute (Nwokolo, 1985). The nuts were found to be ideal for children, the elderly and for sports men and women (Martinezi, 2003)
            Tiger-nut (Cyperus esculentus), an under utilized crop was reported to be high in dietary fiber content, which could be effective in the treatment and prevention of many diabetics including colon cancer, coronary diseases, Obesity, diabetics and gastrointestinal (Anderson et al; 1994).
2.7       History of coconut
            The coconut palm is botanically referred to as the “Cocos nucifera”. It is a member of the Arecaceae or palm family. In fact, it is the only member of the genus cocoIs. The palm thrives in the tropical regions and is a major trade component due to its various decorative, Culinary and other non-culinary uses. The palm bears fruits that is light and buoyant and hence, does not rule out the possibility of finding its own course a cross the globe with the help of marine currents (www.buzzle.com)
            It is a large and tall palm that exhibits a height of approximately 30m. The tree has pinnate leaves, each growing to a size of around 6m with pinnate approximately 90cm long (www.buzzle.com.)
2.7.1   Nutritional value of coconut milk
            Coconut milk is the liquid extracted from grounding coconut meat and water. It is the milky white sweet liquid which is obtained by squeezing granted coconut and warm water. It is used to prepare yummy mouth watering descents, sauces, soups and curries. It is packed with vitamins, Minerals, potassium, folate and other vital nutrients. It is included in the list of healthy super foods. One cup of canned coconut milk Contain 445 Calories, where as frozen milk  Contains the maximum number of calories, which are approximately 552.

Table 2:  nutritional value per serving nutritional values present in 100 grams of fresh coconut milk
Serving size
824kj (197kcal)
Vitamin c
Percentage are relative to US recommendation for adults.
Sources: USDA Nutrient databases.
            Fresh coconut milk has a consistency and mildly sweet taste similar to cow’s milk, and it properly prepared should have no coconut odour or at most is very faint odour. It may be consumed raw or use as milk substitute in tea, coffees, and even baking by vegans or people allergic to animals milk. It can also be mixed with fruits to make a yogurt. Substitute (ww.en.wikipedia. org).
2.7.2 Health Benefit of Coconut Milk
            Coconut milk contain a large proportion of lauric acid, as actuated fat that raises blood cholesterol levels by increasing the amount of high density lipoprotein. Cholesterol that is also found in significant amount in breast milk and sebaceous gland secretion (www.drgranny.com)
            The coconut milk helps to maintain blood sugar. Glucose intolerance may cause manganese deficiency in the body and it is a rich source of –manganese. Also it keeps the skin and the blood vessels flexible and elastic.
            Coconut milk helps in building strong bones, and contains phosphorus which is an essential nutrient that the body needs for strengthening bones. It also aids to the preventing of anemia in the body which does not allow the body to develop enough hemoglobin for keeping sufficient oxygen levels in red blood cells (www. Drgranny.com).
            It is rich in magnesium and help in relieving muscle cramps or muscles soreness. It also helps in controlling weight and contains high concentration of dietary fiber (www.digranny.com.)
2. 8.     Rheology
            By definition, rheology is the study of the deformation and flow of matter. It is applicable to many industrial fields such as mining, geology, cosmetics, and polymers. Rheology of fluid foods provides good opportunities of study due to the biological nature of foods. Optimization of product development efforts, processing methodology and quality of food product requires careful investigation of the rheological properties (Rao, 1999; Steffe, 1996).

2.8.1. Significance in Food Industry
Rheological data are essential for several areas in food industry.
          Design of process equipments including heat exchangers,
pipelines, mixers, extruders and pumps;
          Determining the functions of ingredients during product
          Intermediate or final product quality control;
          Shelf life testing;
          Evaluation of food texture and sensory assessment (Pelegrine
et al., 2002; Rao, 1999; Manohar et al., 1998; Steffe, 1996).

2.9       Flow Models for Rheological Properties of Fluids
A flow model is considered to be a mathematical equation that describes rheological data such as shear rate and shear stress in a convenient manner. It is important to quantify how model parameters are affected by state variables such as temperature and concentration (Rao, 1999).
A fluid is distinguished from a solid by its behavior when subjected to a stress (force per unit area of application) or applied force. While an elastic solid deforms by an amount proportional to the applied stress, a fluid continues to deform under the similar applied stress. Shear stress, i, is the stress component applied tangentially to the fluid with units expressed in Pa (N/rn2). Under the applied shear stress, a fluid flows at a velocity which increases with increasing stress. Shear rate, g, is the velocity gradient (rate of deformation) established in a fluid as a result of the applied shear stress. It is expressed in units of reciprocal seconds (s-1). Viscosity is the resistance of the fluid to this stress. It is the property of a fluid which gives rise to forces that resist the relative movement of adjacent layers in the fluid. These viscous forces are caused by the forces existing between the molecules of the fluid (Rao, 1999; Geankoplis, 1993; Bourne, 1982).
For an ideal Newtonian fluid, the shear stress is linear function of the shear rate and the proportionality constant for the relationship, μ, is called the dynamic (or Newtonian) viscosity of the fluid. The relation is given by Newton’s law of viscosity when the flow is laminar (Geankoplis, 1993; Barnes et al., 1989; Van Wazer and Lyons, 1966).
i = μ.g where, i is tangential shear stress, μ is the Newtonian viscosity and g is the shear rate.

2.9.1.              Newtonian Fluids
Fluids that obey Newton’s law of viscosity (Eq. 1) are called Newtonian fluids. For such a fluid, there is a linear relationship between shear stress (i) and the shear rate (g) (Figure 1). This suggests that the viscosity, μ, is constant and it is independent of the rate of shear (Geankoplis, 1993). When shear rate is plotted against shear stress, the slope of the curve, μ, is constant and the plot begins at the origin. Using the units of N for force, m2 for area, m for length, and finally m/s for velocity, gives viscosity as Pa s which is 1000 centipoise (1 Pa s = 1000 cP).
Typical Newtonian fluids contain low molecular weight compounds (e.g. sugars) and that do not include large concentrations of either dissolved polymers (e.g. protein, starch) or insoluble solids. Some examples of Newtonian foods are water, sugar syrups, edible oils, filtered juices, and milk (Rao, 1999). The following examples represent typical Newtonian viscosities at room temperature:
water, lcP; coffee cream 10 cP; vegetable oil, 100 cP; and honey 10000 cP (Steffe, 1996).
2.9.2.        Non-Newtonian fluids
For non-Newtonian fluids, the relation between the shear stress (i) and shear rate (g) is not linear and/or shear stress-shear rate plot does not begin at the origin. The fluid might exhibit time-dependent rheological behavior as a result of structural changes. Typical non-Newtonian materials are dispersions, emulsions, and polymer solutions. The viscosity is not constant but is a function of shear rate and may exhibit one of the two cases. Flow behavior may depend only on shear rate and not duration of shear (time-independent) or may depend on the duration of shear (time-dependent). Thus, non-Newtonian fluids can be divided into two broad categories as time- independent and time-dependent fluids. Various types of time independent behavior have been described in the literature (Rao, 1999; Barnes et al., 1989).
2.10 Time-Independent Fluids
2.10.1 Bingham Plastic Fluids
This category is the simplest since the only difference from Newtonian behavior is that the linear relationship between shear stress and shear rate does not go through the origin (Steffe, 1996).
i = μp1 g+i0                          (2)
Where, i0 is the yield stress and μpl is the plastic viscosity.
A finite stress so called yield stress (i0) is required to achieve flow. Below the yield stress, no flow occurs and the material exhibits solid like characteristics due to the stored energy (Steffe, 1996).
Toothpaste, tomato paste, margarine and chocolate mixtures are some examples of Bingham plastic fluids (Rao, 1999; Worlow, 1992).
2.10.2.     Power-law Fluids
This type of non-Newtonian behavior can be explained by a power-law equation also called Osiwald-de Waele equation. This model has been used extensively to describe the non-Newtonian flow behavior both in theoretical analysis and in practical engineering calculations (Worlow, 1992; Bourne, 1982).
i  =  K . gn                                                                               (3)
where, K is the consistency coefficient (Pa. sn) and n is the flow behavior index, (dimensionless). The consistency coefficient is an indicator of the viscous nature of a fluid.

Apparent viscosity: μa, is the ratio of shear stress to shear rate at a given rate of shear for shear dependent fluids. It represents the viscosity of a Newtonian fluid exhibiting the same resistance to flow at the chosen shear stress or shear rate (Van Wazer and Lyons, 1966).
The apparent viscosity, ia, for power-law fluids (Steffe, 1996) is,

Μ  =   f (g)  = K.gn  = K.gn-1                                                  (4a)

of which the logarithmic form is used to determine the model parameters when experimental data are available as,
lnμa=lnK+(n—1) ln g                                                                                  (4b)

According to the magnitude of the flow behavior index, n, power-law fluids are divided into two categories as shear thinning and shear thickening fluids.         Shear thinning Fluids

The majority of non-Newtonian fluids are covered in this category. With shear thinning (or pseudoplastic) fluids, the shear stress vs. shear rate curve begins at the origin but is concave upward. An increasing shear rate gives a less than a proportional increase in the shear stress. Applesauce, banana puree, orange juice concentrate, and many salad dressings are considered as shear thinning foods. While apparent viscosity is constant with Newtonian materials, it decreases with increasing shear rate in shear thinning fluids. Eq.3 applies to this type of behavior where, the flow behavior index is less than unity (n<1) (Rao, 1999; Steffe, 1996; Bourne, 1982).   Shear thickening Fluids
In shear thickening behavior, the shear stress vs. shear rate curve also go through the origin and it is concave downward; that is, an increasing shear stress gives a less proportional increase in shear rate. Apparent viscosity, the slope of the associated curve, increases with increasing shear rate. This type of flow is observed with gelatinized starch dispersions and corn flour-sugar solutions (Rao, 1999). Power-law model equation (Eq.3) is often applicable with the flow behavior index greater than unity (n> 1). Herschel-Bulkley Fluids
Herschel-Bulkley model is a general relationship to describe the behavior of non-Newtonian fluids (Figure 1).
i = Kγn + io                                                               (5)
It is a very convenient model since it reduces to Newtonian (n = 1) and to power-law behavior (n =1) as special cases (i0 = 0). In addition, the model describes the Bingham Plastic Model where the yield stress is required (Steffe, 1996).
2.11. Time-Dependent Fluids
In some fluids, the apparent viscosity can either increase or decrease with time of shearing at a constant shear rate. Such changes can be reversible or irreversible. Time dependent fluids can be categorized into two classes as thixotropic and rheopectic fluids.
 2.11.1 Thixotropic Fluids
Foods that exhibit time-dependent shear thinning behavior are said to be thixotropic fluids. Most of these fluids possess a heterogeneous system containing a fine dispersed phase. When at rest, particles and molecules in the food are linked together by weak forces. During shear the hydrodynamic forces are sufficiently high to break the interparticle linkages, resulting in a reduction in the size of structural units. Thus, a lower resistance to flow is detected during shear. This type of flow behavior is likely to occur with foods such as salad dressing and soft cheeses where the structural adjustments take place in the food until equilibrium is reached (Rao, 1999). The occurrence of thixotropy implies that the flow history must be taken into account when making predictions about the fluid behavior (Barnes et al., 1989).
2.11.2 Rheopectic Fluids
Rheopexy (or antithixotropy) is associated with time dependent shear thickening behavior. These fluids are quite rare in occurrence. Viscosity of these fluids increases with time at a constant shear rate (Steffe 1996).
2.12.   Variables Affecting Viscosity and Flow Behavior Parameters
It is critical to emphasize the way viscosity depends on variables like shear rate, temperature, pressure, time of shearing, and concentration. Fluids are subjected to high sensitivity due to changes in these variables. Time of shearing and variable shear rates affect viscosity due to the resulting structural changes in the fluid. However, for most practical purposes, the pressure effect is ignored. Temperature and concentration on the other hand, considerably affect rheological parameters (Barnes et al., 1989).
2.12.1. Effect of Temperature
There is usually an inverse relationship between viscosity and temperature. A wide range of temperatures are encountered during processing and storage of fluid foods, so the effect of temperature on rheological parameters is needed to be determined. While the flow behavior index, n, is assumed to be relatively constant with temperature, the effect of temperature on both apparent viscosity, μa and consistency coefficient, K of the power-law model is explained by an Arrhenius type relationship (Rao, 1999) as,
K = K0 exp [- Ea/RT]                                                                                                (Equation 4)
Where K0 is reaction frequency factor, Ea is activation energy of gelatinization (J/mole), R is gas constant (8.314 J/mol K) and T is absolute temperature (K).
The quantity Ea, is the energy barrier that must be overcome before the elementary flow process can occur (Rao, 1999).

2.12.2. Effect of Concentration
Hydrocolloids are polymeric materials that are soluble or dispersible in water; for example: They are usually added to food formulations to increase their viscosity or to obtain a gelled consistency (Lewis, 1987) Kinsella (1976) reported that viscosity is influenced by solubility and swelling properties. Snyder and kwon (1987) reported that the more material there is in solution the higher the viscosity. King (2005) reported that the viscosity of starch granules in suspension increased depending on starch concentration. Lewis (1987) states that viscosity rapidly increases due to concentration increase and there is often a transition from Newtonian to non-Newtonian behavior and the extent of the concentration is governed by the viscosity characteristics of the concentrate.
There is usually a direct nonlinear relationship between concentration of a solute and viscosity at a constant temperature (Bourne, 1982). In most foods, it is often possible to identify the components that play an important role on the rheological properties.
2.12.3 Effect of other Ingredients
Food products are complex mixtures of different ingredients where individual ingredients are mixed together to produce a finished product. In many cases, the individual ingredients consist of mixtures of solid as well as fluid components. Must times, they are not homogeneous, and the properties vary throughout the sample. A change in one of the raw ingredients can also have a dramatic effect on the final product (Herh, et al, 2000).
2.13. Measurement of Flow
The study of the Newtonian and non-Newtonian flow behavior necessitates considerable care and instrumentation. Data from poorly designed instruments can be misleading. A viscometer must be capable of providing readings that are convertible to shear rate (y) and shear stress (i). Further a well designed instrument should provide recording of data in order to study time dependent behavior (Rao, 1999).
For viscometric measurements, the flow in the selected geometry should be steady, laminar, and fully developed. The temperature of the test fluid should be maintained uniform and constant for reliable measurement (Rao, 1999). Viscosity of fluids is highly temperature dependent. For instance, the viscosity of water at 20°C changes 2.5% per 1°C temperature change. Therefore, in all viscosity measurements it is essential that the temperature is closely controlled (Boume, 1982). For Newtonian fluids, viscometers that operate at a single shear rate (eg. glass capillary) are acceptable. For non-Newtonian fluids, data should be obtained at several shear rates. Common viscometric flow geometries for rheological studies on foods are (1) concentric cylinder, (2) cone-plate, (3) parallel plate, (4) capillary/tube/pipe, and (5) slit flow (Rao, 1999). For viscosity measurements laminar flow conditions are desired. Under conditions of turbulent flow of Newtonian fluids, the measured viscosity was higher. However, since non-Newtonian fluids are generally viscous, usually laminar flows are encountered (Rao, 1999).
2.14.               Rotational Viscometers
Traditional rotational viscometers comprise of cone and plate, parallel plate and concentric cylinder units operated under steady shear conditions (Steffe, 1996). The shear rate is derived from the rotational speed of a cylinder or a cone. If the properties of flow behavior are required for the design of processes, it is recommended to use shear rates that cover the range that is expected to be used in the process (Rao, 1999).

2.14.1. Concentric Cylinder Viscometer
The concentric cylinder viscometer is a very common instrument that would operate in a moderate shear rate range. This function makes it a good choice for gathering data used in several engineering calculations (Steffe, 1996). It permits continuous measurements to be made under a given set of conditions and allows time-dependent effects to be studied. This is the most common type of viscometer that is used in the food industry (Boume, 1982). In concentric cylinder geometry, a cylinder (bob) is placed coaxially inside a cup containing the selected volume of the test fluid (Rao, 1999). In Searle system concentric viscometer (Figure 1.4), the bob rotates and the cup is stationary. Couette-type systems are also available where the cup rotates and the bob is stationary (Rao, 1999). In Searle systems, the bob is rotated at a constant speed and the drag of the fluid on the bob is measured by means of a torque sensor. The measured figure is the torque (M) required to maintain a constant velocity of the bob (Q). By changing the rotational speed, thus the shear rate and measuring the resulting shear stress, it is possible to obtain viscosity data over a wide range of shearing conditions (Steffe, 1996).
The following assumptions should be made in order to derive the mathematical relationships for the instrument performance (Steffe, 1996):
          Flow is laminar and steady,
          End effects are negligible,
          Test fluid is incompressible,
          Properties are not a function of pressure,
          Temperature is constant,
          There is no slip at the wall,
          Radial and axial velocity components are zero (Steffe, 1996).

3.0                                                                   MATERIALS AND METHOD
3.1                   SOURCES OF SAMPLES
          Coconut, yellow variety of fresh tiger nut and packaged whole milk powder was purchased from a local market (meat market) in Abakaliki metropolis, Ebonyi state, Nigeria.  While the starter culture was also purchased from a supermarket in Abakaliki metropolis, Ebonyi, state Nigeria.  The lactose and sucrose was purchased from Ogbete main market at Enugu, Enugu State, Nigeria:
3.2                   SAMPLE PREPARATION
      The coconut was broken by using small club while the meaty part was removed using a dull stainless stell knife.  The brown skin was removed with a sharp knife and the meaty part without the brown skin were thoroughly wash and grated using a grater. The grated coconut meaty part were mixed with warm portable water in a bowl allow to stand for 10mins to extract the oil, aromatic compounds and the milk.  The extract were filtered through a muslin cloth to separate the milk from the insoluble chaff to obtain a milk – white emuslsion with a sweet coconut flavour (Belewu, M. A., Belewu, K.Y and Bamidele, R.A, 2010).  The milk was stored at  5oC Inside the refrigerator before use.                                         PREPARATION OF TIGER- NUT MILK
        The method of Belewu and Abodunin (2007) was adoptd.  The preparation of the tiger-nut milk was done by picking out those foreign and bad nuts that could affect the taste and keeping quality of the milk.  The tiger-nut was washed and rinsed with portable water.  One kg of tiger nut was conditioned over night in laities of portable water to soften the fiber.  The total content was blend several times with a blender.  The mash was filtered through a mush in cloth to separate the milk from the insoluble chaff.  It was further stain to obtain a fine consistency and was stored at 180c until use.      YOGURT PREPARATION
            Various yogurt samples was prepared by combining two of the different milk source together.

The experimental treatment was shown in table (3)
Table (3)
T1 = 100% whole milk powder (control)
T2 =100% tiger nut milk yogurt
T3 = 100% coconut milk yogurt
T4 =30% tiger nut milk yogurt + 70% coconut milk yogurt
T5 =50% tiger nut milk yogurt+ 50% coconut milk yogurt
T6 =70% tiger nut milk yogurt + 30% coconut milk yogurt
T7 =  40% tigernut milk yogurt + 40% coconut milk yogurt + 20% whole milk yogurt                                         PREPARATION OF VARIOUS BLENDS YOGURT
The method of Belewu et al; (2005) was used.  The various milk sample mixture in table3 was heated separately to a temperature of 850c for 15mins, then cool rapidly to a temperature of 430c.  Each of the treatment was inoculated at this temperature with 2% of thou this starter culture (streptococcus thermophillus and lactobacillus bulgaricus).  After complete fermentation, 5g of sugar was added to each of the treatment.  The stored yogurt was stored in the refrigerator until for analysis.

        The pH value of the yogurt sample was measured in a 50ml beaker at a temperature of 2o.c using a Digital flt meter (model 152k).            TOTAL TITRATABLE ACIDITY (TTA)
      The total titratable acidity of the yogurt sample was determined as described by Morris (1999). 10ml of yogurt sample was transferred to a proclaim dish with the aid of 10ml pipettle. The pipettle was rinsed with 10ml of water and the rinse was added to the dish 1ml of Phenolphthalein indicator solution was added to the dish and titrated with 0.IN sodium hydroxide (NaOH) solution until a pink colour was obtained. 
The titretable acidity (% lactic acid)= titer value x molarityx 0.09   x   100      
                                                             Volume of sample (ml)              1            TOTAL SOLIDS (TS)
      The total solids was determined by (AOAC,1995) method.  4g of the sample was weighed into a metal dish and keep in a water bath for 30mins and thereafter heated in an over at 1000c for 21/2 hours.  The sample was cooled in a desicator for 30mins and weighed.  The sample was reheated in the over for another one hour, cooled and reweighed.  This was repeated until weight loss between successive weighing become.  Negligible (<0.5g).  the percent total solids was calculated from the formular:
Ts% = Final weight of yogurt sample- weight of dish   x    100 
                            Weight of sample                                     1                                         TOTAL SOLUBILITY (oBRIX)

The total sugar (oBrix) was determined using Refractometer method.  The refractive index of the yogurt was determined at 200c

3.4 Viscosity measurement
      The viscosity readings of each yogurt was measured using a digital display viscometer (model NDJ-85) taken at constant time intervals with progressively increasing shear rate 96-60/mins). Viscosity values was obtained by multiplying viscosity readings with appropriate factor.
      Flow curves was plotted with the values obtained. From equation3, log log plot of Uapp against y, a straight line of slope (n-1) 3.5       SENSORY ANALYSIS
      A twenty-member, semi-trained panel was used to evaluate the various sensory parameters (flavour, appearance/colour, taste/sourness, texture/consistency and overall acceptability). The score was base on a hedonic scale range from 9 representing “like extremely” to1 representing “dislike extremely” (Iwe, 2002).
      All data was subjected to analysis of variance (ANOVA) to determine any significant difference at 5% level (LSD) using Iwe, (2002) method.

Table 1: Physical properties of yogurt and yogurt-like product produced from whole milk, coconut, tigernut and their composites.
Total solid(%)
Men values in the same column having the same superscripts are not significant different (p>0.05).
100% Tigernut milk yogurt
100% Coconut milk yogurt
50% Tigernut milk yogurt + 50%cocnut milk yogurt
70% Tigernut milk yogurt + 30% coconut milk yogurt
30% tigernut milk yogurt + 70%cocunut milk yogurt
40% tigernut milk yogurt + 40% coconut milk yogurt + 20% whole milk yogurt
100% whole milk yogurt (control)
4.1.1   pH
      The pH of coconut tigernut yogurt blend sample is shown in Table 1. The pH of the samples ranged from 4.26(100%whole milk) to 4.62(50% tigernut milk yogurt + 50% coconut milk yogurt), this is within the range reported in literature by Belewu,(2007). There were significant difference (p<0.05) between the samples.
4.1.2   Total titratable acidity (TTA)
      The titratable acidity which is an expression of the percentage Lactic acid content (Lee, 1985) showed no significant difference (p>0.05 in the yogurt sampler.
      An indication of higher acidity of the yogurt samples may confer longer keeping quality on the products. The total titratable acidity of the yogurt ranged from 0.05 (100%whole milk yogurt) to 0.96(40%jiger nut milk yogurt + 40% coconut milk yogurt + 20% whole milk yogurt), this it within the range reported by (Ihekoronye and Nooody; 1985, SON, 1997; Douglas. 1985; Alakali et al, 2005).

4.1.3   Total solid(%)
      The total solid of the yogurt ranged from 13.55 (100% Tigernut milk yogurt) to 26.95 (100% coconut milk yogurt). The total solid of 100% coconut had highest value. All the samples are significantly different from each other accept 40T; 40C: 20m and 100% whole milk, 30% tigernut + 70% coconut and 50% tigernut + 50% coconut.
4.1.4   Total Soluble solid (Brix0).
      Table I also should the total soluble solid (Brixo) of coconut – tigernut yogurt blend sample. The mean values of the total soluble solid of the blend of yogurt ranged from 2.00 (100% tigernut yogurt) to 11.80 (40% tiger + 40% coconut + 10% whole milk). There were significant differences between the samples (p<0.05). The result are however, important as they indicate available substrates for yeast fermentation. On the other hand, the total soluble solid of the test samples could be due to increased activity of amylotytic enzymes which hydrolyze starch and other complex carbohydrates to simple sugar (Kazanes and fields, 1981).

4.2 Rheological  Propertics of Coconut – Tigernut Yogurt blends.
The values of consistency coefficient, K, flow behaviour index(n) and Correclation coefficient (R2) of Coconut – Tigernut yogurt blends are presented in table 5 below
Table 5: Rheological characteristics of coconut tigernut yogurt blends.
Consistency index(k) Ns/m2
Plow behaviour index(n)
Correlation coefficient(R)2
100% T
50% T: 50%
70%T: 30%C
30%T: 70%C
Control = 100% whole milk yogurt
100%T =100% Tigernut yogurt
100%C = 100% Cocount yogurt
50%T: 50% C = 50%Tigernut yogurt + 50% coconut yogurt
70%T: 30% C = 70%Tigernut yogurt + 30% coconut yogurt
30%T: 70% C =30% Tigernut yogurt + 70%cocunt yogurt
40%T: 40%C: 20%M = 40% Tigernut yogurt + 40% coconut yogurt + 20% whole milk yogurt.
Flow Behaviour of the yogurt samples
The values of consistency (k) and flow behaviour (n) indices of the seven yogurt blend samples were studied and are presented in table 5. the apparent viscosity of the yogurt blend samples increased with increase in shear rate, with control having the highest apparent viscosity, followed by 100%C, while the 70%T: 30%C had the lowest at all the share rate studied. This result was expected may be due to the increasing liquid matter content of the samples.
            All the sample showed pseudoplastic behaviour, where viscosity decreased when shear rate increased (Figure 2) and when the yogurt blend increased (table 5). Similar trends were observed by (Awonorin, 1993; Alvarez et al; 2005, Ariahu et al; 2001; Alakali et al; 2003; Sopode and Kassum, 1992; Irtwange et al; 2009; Ikegwu and Ekwu, 2009).
            This pattern is defined as shear thinning, where shear stress increased with increase in shear rate (figure 3) and is expected because an increased in relational speed increases molecular alignment in the direction of flow, reducing resistance to flow and hence viscosity (Rha, 1975).
            However, the curve obtained in figure 2, is a typical of dispersions exhibiting psudoplastic flow and indication that the milk verities had not altered the rheological class of the yogurt dispersion. This phenomenon has been widely reported to be common with hydrocolloid solution and food pastes with sensitive structures (Awonorin, 1993).
            The power mode or the Ostwald de waeles mode (o-kyn) was used in this study to fit this experimental data. All the samples presented a flow behaviour index, n less then I, indicating pseudoplasticity. This is consistent with other pluid food such as tomato paste (Alakali and Ijabo, 2003), palm oil (satimehin et al; 2003) “achi” flour (Ikegwu and Ekwu, 2009; Uzomah and Ahiligwo, 1995), (Kumu Zaki and Kunu Gyada (Sapode and Kassum, 1993). Earle (2004) reported that when n<,I a case of pseudoplesticity is established. Analysis of variance (ANOVA) (P<0.05) showed that flow behaviour index, n and consistency coefficient k, are significantly influenced by the charge of the milk variety used.
            The values of k(table 5) were significantly influenced as the percentage of yogurt blends increased. The lower values of the flow behaviour index(n) can be attributed to higher departure from Newtonian behaviour. Furthermore, the consistency coefficient, increased with the yogurt blends, with very high correlations coefficient, R2, ranging between 0.503 and 0.9552, indicatory that the power law education adequately described the rheolegical pattern of yogurt dispersions.
            The effect of viscosity on the yogurt sample at shear rate of 0.5s-1 as shown in figure 1, indicate that the 100% whole milk yogurt had the highest value of apparent viscosity, followed by 100%C while the least apparent viscosity was 70%T: 30%C.                            
4.3 The result of the sensory analysis of yogurt sample are presented in Table 2
General Acceptance
100% whole Milk
Men values in the same column having the same superscripts are not significant different (p>0.05).
100% whole milk  (control)
100% Tigernut milk yogurt
100% Coconut milk yogurt
50% Tigernut milk yogurt + 50%cocnut milk yogurt
70% Tigernut milk yogurt + 30% coconut milk yogurt
30% tigernut milk yogurt + 70%cocunut milk yogurt
40% tigernut milk yogurt + 40%cocunut milk yogurt + 20% whole milk yogurt.
4.2.1   Appearance
      The appearance of the yogurt refers to the level of visual appeal of the products obtained by fermenting the various milk substrates. From table 2, 1005 whole milk  (control) and 40% tiger nut milk +40% content milk + 20% whole milk are not significantly different from each other but they are significantly different from other samples at (p<0.05). 40% tigernut milk + 40% coconut milk + 20% whole milk produced a clean natural colour with a smooth appearance.
4.2.2   Flavour
      The flavour produced from the fermentation of the yogurt is essentially due to the production of acetaldehyde and other volatile aromatic compounds resulting from the breakdown of carbohydrates by the relevant microbes (Ihekoronye and Ngoody; 1985). It  is seen from table 2 that  100% whole milk was most preferred interms flavour  and was significantly different from all other samples. Flavour or yogurt depends on lactic acid, acetaldehyde, acetic acid and diacetyle (Douglas, 1985).
4.2.3   Taste/Sourness
      Sources is a desirable characteristic  of yogurt which expresses the level of tang produced as a result of the production lactic acid by the action of lactobascilus bulgaricus on lactose (sugar in milk) in the substrates (Moore, 2004). From table 2 this on sources for the yogurt samples indicate that whole milk yogurt was most preferred in terms of taste flowered by 40% tigernut  milk + 40% coconut milk + 20% whole milk and they are significantly different form other samples.
4.2.4               TEXTURE
      Texture or consistency is an attribute of the yogurt to flow without forming hagging insoluble particles on the inner side of the containers (Moore, 2004). It refers to the property of the yogurt to exhibit smoothness and good flow behaviour or properties. From table 2, the result revealed that whole milk yogurt was most desirable followered by 40% tigernut milk + 40% coconut milk + 20% whole milk and was significantly different from all other samples. 30% tigernut milk + 790% coconut milk has the least desired consistency rated as dislike when compared with the other yogurt sample blends. All other samples containing 100% tigernut milk, 100% coconut milk or blend were not significantly different from each other at (p>0.05).
4.2.5               GENERAL ACCEPTANCE
      Generally, it could be deduced from the results that the control (100% whole milk yogurt) was most desirable followered by 40% tigernut milk + 40% coconut milk + 20% whole milk and was significantly different from all other sample at (p>0.05). All samples containing 100% tigernut milk, 100% coconut milk and the blend of the two were not significantly different from each other. Base on the sensory evaluation. It appeared that sample 50% tigernut milk 50% coconut milk was the third most referred. According to sinful (2009) on the use of tigernut (Cyperus esculantus), cow moil and their composite as substrate for yogurt production, consumers are more inclined to accept yogurt made form two composite substrate than one substrate.

Fig. 1. Apparent viscosity of yogurt samples at shear rate of 0.5 s-1

Fig. 2. Apparent viscosity Vs shear rate of yogurt samples

Fig. 3. Shear stress Vs shear rate of yogurt samples

Rheological parameter

       K (Pa.s)








5. 1      Conclusion
      The results obtained from the physical properties indicate the pontenelitity blend of coconut milk and tigernut milk in the production of yogurt.
      The results obtained from the reheological study, shown that the apparent viscosity decreased with increase in shear rate indicating shear thinning behaviour characteristics. The coconut Tigernut yogurt blends studied exhibited pseudoplastic behaviour because their “n” values were generally below I (n<1) it was evident that shear stress also increased with increased in shear rate.
      The sensory evaluation of yogurt produced from coconut – tigernut milk studied did not show a different trend in variation statistical (p>0.0.5) between samples produced in terms of appearance, testes, flavour and texture. However, there was no significant different (p> 0.05) between sample 100% tigernut yogurt, 100% coconut yogurt, 30% tigernut yogurt + 70% coconut yogurt 50% tigernut yogurt + 50% cocnut yogurt and 70% tigerunt + 30% coconut yogurt in their appearance, flavour, taste, texture and general acceptance. The increase in protein demand in developing countries led to effort in finding alternative sources of protein in plant seeds. However, data obtained from this study shows that coconut – tigernut yogurt could be helpful in meeting a significant portion of the daily needs of these nutrients.
5.2       Recommendation                 
      The high cost of imported milk and milk production in Nigeria and Africa seen to have made consumers more ready to accept milk produced from plant source.
      This research work revealed that products from tigernut and coconut should be encourage so as to solve the problem of protein calorie malnutrition in Africa and improve the protein intake of the Nigeria population.
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