2.0 LITERATURE
REVIEW
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
form 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. Te concentration is done
by boiling of some of the water and it is often done under vacuum to reduce the
temperature required.
2.3 NUTRITIONAL
VALUE AND HEALTH BENEFITS
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)
|
|
|
|
|
|
ZEnergy
|
257kg
(6 kcal)
|
|
Carbohydrate
|
4.7g
|
|
Fat
|
3.3g
|
|
Protein
|
3.5
|
|
Vitamin
A equivi
|
27ug
(3%)
|
|
Riboflavin
(vit. Biz)
|
0.14
mg (9%)
|
|
Calcium
|
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 e 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).
2.5 PRODUCTION
OF YOGURT
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 product, 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.
THE FLOWCHART FOR YOGURT PRODUCTION
Milk Mix
|
Heat treatment (850c for 15 mins)
|
Homogenize
|
Cool to incubation
temperature (430c)
|
Inoculate with starter
culture
|
Cooling (50c)
|
Storage
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 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
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
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
|
100g
|
|
Energy
|
824kj
(197kcal)
|
|
Carbohydrates
|
2.8lg
|
|
Fat
|
21.33g
|
|
-Saturated
|
18.915g
|
|
Protein
|
2.02g
|
|
Vitamin
c
|
1mg(1%)
|
|
Calcium
|
18mg(2%)
|
|
Iron
|
3.30mg(25%)
|
|
Magnesium
|
46mg(13%)
|
|
Phosphorus
|
196mg(14%)
|
|
Potassium
|
220mg(5)
|
|
Sodium
|
13mg(1%)
|
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
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
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
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,
• Design of process equipments including heat exchangers,
pipelines, mixers, extruders and pumps;
• Determining the functions of ingredients during product
• Determining the functions of ingredients during product
development;
• Intermediate or final product quality control;
• Shelf life testing;
• Evaluation of food texture and sensory assessment (Pelegrine
• 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, is the Newtonian viscosity and 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).
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,
The apparent viscosity, ia, for power-law fluids (Steffe, 1996) is,
Μ = f (g) = K.gn = K.gn-1
(4a)
g
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.
2.10.2.1. 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).
2.10.2.2 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).
2.10.2.3. 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 will be 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)