ONWUNYI Chuks Atunegwu MATRIC NO 040093 Department Of ...

EFFECT OF PROCESS PARAMETERS ON SOME PROPERTIES OF
EXTRUDED SWEET POTATO (ORANGE FLESHED) STARCH
ONWUNYI Chuks Atunegwu
MATRIC NO 040093
Department Of Food Science And Technology,
College of Food Science and Human Ecology,
University Of Agriculture, Abeokuta.
A Project Report Submitted In Partial Fulfillment of the Award
of
Degree of Bachelor of Science (B. Sc) in Food Science and
Technology
OCTOBER 2010

CERTIFICATION
This is to certify that this project was carried out by Onwunyi Chuks Atunegwu with
Matriculation number 2004/0093 and is approved in partial fulfillment of the requirement for
the award of BSc. Degree of the Department of Food Science and Technology, College of
Food Science and Human Ecology, University of Agriculture, Abeokuta, under our
supervision.
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DEDICATION
This project work is dedicated to Almighty God for His greatness. The One who has kept me
all through my years on earth. I cannot thank Him enough.

ACKNOWLEDGEMENT
My sincere and unending gratitude goes to my wonderful, ever loving, caring parents, Mr.
and Mrs. Wilfred Onwunyi for their parental, financial, moral, spiritual support from birth till
now. I can’t pay you back for your efforts in moulding me into my great self but God will
continually reward you and you will live to enjoy the fruits of your labour.
I also acknowledge the suggestions, inputs and advice from my supervisor, Dr. (Mrs.) J.M
Babajide as well as my co-supervisor Dr. O. P Sobukola and all lecturers in the Department
of Food Science and Technology who sacrificed all it took to see this project through. May
God reward you bountifully. I also appreciate Professor Sanni for His input towards
execution of the project analysis, may God bless you.
I cannot end without thanking Uju (Paddy G), Mrs. Ebere Onyema, Chinasa, Uche. You all,
worked tirelessly in helping me acquire and process the Sweet Potato sample: May you never
lack manpower.
To my great friends Osagie and Tayo, I thank God I met you. You have been a great source
of inspiration to me on my path through UNAAB. I love you guys. To my Pastor, Femi
Bankole, First class for me would have been a dream but for your fervent prayers. Good
things will never elude you. This work will not be complete without mentioning the input of
two great men Gbenga Soladoye (Prof Gee) and Oladimeji Jeyeola (Sir D) towards making
sure I finished with a first class honours. May God bless you both.
Finally to my classmates you have made the race worthwhile. What will I have done without
you all. See you all at the top.

FIGURES
LIST OF FIGURES
1. Response surface plot of WAI as a function of temperature and moisture content 37
2. Response surface plot of WSI as a function of temperature and moisture content 38
3. Response surface plot of bulk density as a function of temperature and 39
Moisture content
4. Response surface plot of solid loss as a function of temperature and moisture content 40
5. Response surface plot of lateral expansion as a function of temperature 41
and moisture content

TABLE
LIST OF TABLES
1: Proximate Composition of Isolated Sweet Potato Starch 6
2. Response surface analysis of functional properties of extruded 34
sweet potato starch for all experimental.
3. Coefficient of regression equation for the physicochemical 35
properties of extruded sweet potato starch.

LIST OF PLATES
1. Aerial view of an locally fabricated single screw extruder 23
2. Extruder barrel 24
3. Extruder screw 25

ABSTRACT
The study investigated the effect of selected process parameters: barrel temperature (100-
140 o C), feed moisture (50-60%), Screw speed (100-140rpm), on some properties of extruded
sweet potato starch. Response surface methodology based on Box-Behnken design was used
to optimize process variables. Seventeen combinations including three replicates of the
central points were performed in random order for the independent variables. The results
showed a range for Water Absorption Index (WAI) (2.42-2.81), Water Solubility Index
(WSI) (0.65-0.98), Bulk Density BD (0.70-0.80), Lateral Expansion (LE) (0.8-1.02) and
Solid loss (SL) (2.5-10.0). Increasing the barrel temperature results in higher Water
Absorption Index (WAI), lower Water Solubility Index (WSI), Solid loss (SL), Lateral
Expansion (LE) and bulk density (BD). Increased screw speed decreased WAI, BD but
increased WSI. Feed moisture content had no significant effect on observed properties of
extrudates. The optimum processing conditions necessary to obtain the desired quality pasta
are combination of temperature of 100 o C, feed moisture of 60% and screw speed of 103.34
rpm.

TABLE OF CONTENTS
CONTENTS PAGES
CERTIFICATION ii
DEDICATION iii
ACKNOWLEDGEMENT iv
TABLE OF CONTENTS v
LIST OF TABLES vi
LIST OF FIGURES vii
ABSTRACT viii
CHAPTER ONE
1.0 INTRODUCTION 1
1.1 JUSTIFICATION 3
1.2 OBJECTIVES 3
CHAPTER TWO
2.0 LITERATURE REVIEW 4
2.1 SWEET POTATO 4
2.2 EXTRUSION 5
2.2.1 GENERAL DESCRIPTION OF AN EXTRUSION EQUIPMENT 7
2.2.2 OPERATING CHARACTERISTICS OF AN EXTRUSION EQUIPMENT 10
2.2.3 CHANGES IN FOOD AFTER EXTRUSION 11
2.3 FOOD STARCH 13
2.3.1 PROPERTIES AND STRUCTURE OF FOOD STARCH 15
2.3.2 HYDROLYSIS OF FOOD STARCH 16
2.3.3 DIGESTIBILITY AND DEXTRINIZATION OF FOOD STARCH 16
2.3.4 STARCH AS FOOD 16
2.3.5 USE OF STARCH AS FOOD ADDITIVE 17
2.3.6 STARCH SUGARS 17

2.3.7 INDUSTRIAL APPLICATION OF FOOD STARCH 18
CHAPTER THREE
3.0 MATERIALS AND METHOD 21
3.1 MATERIALS 21
3.2 EQUIPMENT 21
3.3 METHOD 21
3.3.1 STARCH ISOLATION 21
3.3.2 EXTRUSION COOKING 22
3.4 PROXIMATE ANALYSIS OF ISOLATED SWEET POTATO STARCH 26
3.4.1 DETERMINATION OF MOISTURE CONTENTOF ISOLATED 26
SWEET POTATO STARCH
3.4.2 DETERMINATION OF PROTEIN CONTENTOF ISOLATED 26
SWEET POTATO STARCH
3.4.3 DETERMINATION OF FAT CONTENTOF ISOLATED 26
SWEET POTATO STARCH
3.4.4 DETERMINATION OF ASH CONTENTOF ISOLATED 27
SWEET POTATO STARCH
3.4.5 DETERMINATION OF CABOHYDRATE CONTENTOF ISOLATED 27
SWEET POTATO STARCH
3.5 ANALYSIS ON EXTRUDATES 27
3.5.1 WATER ABSORPTION INDEX 28
3.5.2 WATER SOLUBILITY INDEX 29
3.5.3 BULK DENSITY 29
3.5.4 SOLID LOSS 29
3.5.5 LATERAL EXPANSION 29
3.6 STATISTICAL ANALYSIS ON FUNCTIONAL PROPERTIES 30
OF EXTRUDATES

CHAPTER FOUR
4.0 RESULTS AND DISCUSSION 31
4.1 PROXIMATE COMPOSITION OF ISOLATED SWEET POTATO STARCH 31
4.2 RESPONSE SURFACE ANALYSIS OF FUNCTIONAL PROPERTIES OF 31
EXTRUDED SWEET POTATO STARCH
CHAPTER FIVE
5.0 CONCLUSION AND RECOMMENDATION 43
5.1 CONCLUSION 43
5.2 RECOMMENDATION 43
REFERENCES 44

CHAPTER ONE
1.0 INTRODUCTION
Extrusion processing is a form of manufacture that involves the forcing of a material out of
chamber or cylinder through a small opening by taking advantage of pressure differentials
that exist between the internal and exterior of the chamber. Extrusion cooking involves the
cooking, texturizing and shaping of a wide range of plant and animal food raw materials in a
continuous operation (Nielsen, 1976). Extrusion cooking of starchy materials has become a
widely used technique to obtain a wide range of products such as snacks, breakfast, cereals,
special flours (for instant soup mixes, porridge etc) (Harper, 1989, Guy and Bouvier, 1996,
Bouzaza et al., 2000).
Consequent upon the pressure build up during fluid flow of material through the die, high
shear stresses are developed which causes structural transformation in the material. These

transformations include: Loss of starch crystalline structure, destruction of granular structure,
rupture of glycosidic bonds and new molecular interactions (Davidson, 1992; Mitchell and
Areas, 1992 Gonzalez et al., 2002). As the product leaves the die, it is in a glassy state. It
expands rapidly and as it cools, the temperature falls below the glass transition state and
strands as well as matrices form that set the structure and determines the product texture
(Blanshard, 1995). The rapid release of pressure as the food emerges from the die, causes
instantaneous expansion of steam and gas in the material to form a low density product.
Products can vary considerably depending on extruder type, operation parameters (moisture
content, screw length and speed, die diameter, temperature), type of feed material and feed
rate.
The principal food materials that could undergo extrusion are: all cereal grains, oilseeds and
legumes. An increasingly important role of food extruders are assured in the food processing
industry because of an increasing world population and shrinking conventional energy
supplies (Harper, 1989). These trends focus on the need to transform raw agriculture products
into foods of high acceptability. The versatility of extruders in combining a series of other
unit operations some of which are kneading, mixing, shearing, shaping and texturizing makes
them ideally suited for this task especially in less developed countries as well as developing
countries: Nigeria inclusive.
Sweet potato (Ipeoma batatas) is an important crop of many developing countries. Although
sweet potato originated from Central America, its ability to adapt to a wide variety of climatic
conditions, allows them to grow both in tropical and moderate temperate regions of Africa,
Asia and America (Woolfe, 1992). Villareal (1977) cited five reasons which favour sweet
potato utilization in today’s diet. These include the tremendous yield potential, high nutrient
yield per hectare, dependability, due primarily to its drought tolerance and ability to
withstand typhoon conditions and acceptability to consumers because of its palatability and

low cost. Sweet potatoes are rich in starch (6.9- 30.7%) on wet basis (Tian et al., 1991) and
starch production is a main industrial utilization of sweet potato.
Varieties of sweet potato in Nigeria include the orange fleshed variety-TIS- 1499 and the
TIB- 2. Sweet potato starch granules are reported as round, oval and polygonal shapes with
sizes ranging between 2- 42µm (Tian et al., 1991; Hover 2001). Swelling and solubility of
sweet potato starch are less than those of cassava and potato starch.
Love et al., (1983), Akinyele (1987) and Dashiell et al., (1990), reported on potential
application of extrusion cooking in Nigeria, there is however, a dearth of information on the
application of extrusion cooking to sweet potato (Iwe, 1998), despite the fact that sweet
potato is one of the most versatile energy and vitamin producers in the world (Woolfe, 1992).
The use of cooker extruders has been expanding rapidly in the food and feed industries over
the past few years there is however a need to study its effects on sweet potato starch due to its
increased consumption.
1.1 JUSTIFICATION
Dearth of information on the application of extrusion cooking on processing of sweet potato
hence a need to explore the possibility of applying extrusion cooking to creating a diversified
means of processing sweet potato.
1.2 OBJECTIVE
To determine the effect of process parameters (moisture content, temperature and
screw speed) on some properties of extruded sweet potato starch.
To determine the combination of process parameters that produce high quality
extrudates

2.0 LITERATURE REVIEW
2.1 SWEET POTATO
CHAPTER TWO
The sweet potato (Ipomoea batatas) is a dicotyledonous plant that belongs to the family
Convolvulaceae. Its large, starchy, sweet tasting tuberous roots are an important root
vegetable (Purseglove, 1991; Woolfe, 1992). The young leaves and shoots are sometimes
eaten as greens. Of the approximately 50 genera and more than 1,000 species of
Convolvulaceae, I. batatas is the only crop plant of major importance. This plant is a
herbaceous perennial vine, bearing alternate heart-shaped or palmately lobed leaves and
medium-sized sympetalous flowers. The edible tuberous root is long and tapered, with a
smooth skin whose colour ranges between red, purple, brown and white. Its flesh ranges from
white through yellow, orange, and purple.

The plant does not tolerate frost. It grows best at an average temperature of 24 °C (75 °F),
abundant sunshine and warm nights. Annual rainfalls of 750–1000 mm are considered most
suitable, with a minimum of 500 mm in the growing season. The crop is sensitive to drought
at the tuber initiation stage 50–60 days after planting and is not tolerant to water-logging, as it
may cause tuber rots and reduce growth of storage roots if aeration is poor (Ahn, 1993).
Depending on the cultivar and conditions, tuberous roots mature in two to nine months. They
are mostly propagated by stem or root cuttings or by adventitious roots called "slips" that
grow out from the tuberous roots during storage. True seeds are used for breeding only.
Under optimal conditions of 85 to 90% relative humidity at 13 to 16 °C (55 to 61 °F), sweet
potatoes can keep for six months. Colder temperatures injure the roots. They grow well in
many farming conditions and have few natural enemies; pesticides are rarely needed. Sweet
potatoes are grown on a variety of soils, but well-drained light and medium textured soils
with a pH range of 4.5-7.0 are more favourable for the plant (Woolfe, 1992; Ahn, 1993).
They can be grown in poor soils with little fertilizer. However, sweet potatoes are very
sensitive to aluminium toxicity and will die about 6 weeks after planting if lime is not applied
at planting in this type of soil (Woolfe, 1992). Because they are sown by vine cuttings rather
than seeds, sweet potatoes are relatively easy to plant. Because the rapidly growing vines
shade out weeds, little weeding is needed, and farmers can devote time to other crops. In the
tropics, the crop can be maintained in the ground and harvested as needed for market or home
consumption. In temperate regions, sweet potatoes are most often grown on larger farms and
are harvested before first frosts.
Besides simple starches, sweet potatoes are rich in complex carbohydrates, dietary fiber, beta
carotene (a vitamin A equivalent nutrient), vitamin C, and vitamin B6. Pink and yellow
varieties are high in carotene, the precursor of vitamin A.

Sweet potato has various applications and uses. It can be processed into various products for
both industrial and small scale use for consumption. One of such processing methods is
extrusion.
2.2 EXTRUSION
Extrusion processing is a form of manufacture that involves the forcing of a material out of
chamber or cylinder through a small opening by taking advantage of pressure differentials
that exist between the internal and exterior of the chamber. Extrusion cooking, hence,
involves the cooking, texturizing and shaping of a wide range of plant and animal food raw
materials in a continuous operation (Nielsen, 1976). Extrusion is a highly versatile unit
operation that can be applied to a variety of foods as shown in Table 1.
TABLE 1: FOOD APPLICATION OF EXTRUSION COOKING
Types of product Examples
Cereal-based products Expanded snackfoods, RTE and puffed breakfast
cereals, Soup and beverage bases, instant drinks,
Weaning foods, Pre-gelatinised and modified
starches, dextrins, Crispbread and croutons,
Pasta products, Pre-cooked composite flours
Sugar-based products Chewing gum, Liquorice Toffee, caramel,
peanut brittle, Fruit gums

Protein-based products Texturised vegetable protein (TVP), Semi-moist
and expanded petfoods and animal feeds and
protein supplements,Sausage products,
frankfurters, hot dogs, Surimi, Caseinates,
Processed cheese.
Adapted from Harper (1979), Harper (1987), Heldman and Hartel (1997), Jones (1990) and
Best (1994).
Extrusion cooking is performed by a cooker extruder. There major parts include the die,
barrel, feed hopper, screw etc. the type of extruder used certainly affects chemical reactions.
Larger extruders have longer barrels and relatively longer residence times than smaller table
top or laboratory scale extruders. Small extruders require considerably less feed due to the
lower through put, offering a definite advantage for preliminary studies using ingredients
available only in small quantities. Various process parameters on both products and the
extruder can affect the final product characteristics. These include the product feed rate,
moisture content, die diameter, screw speed, temperature and flow rate of thermal fluid to
barrel jackets. Various chemical and nutritional changes occur in food during extrusion. Few
general chemical or physicochemical changes can occur during extrusion cooking: binding,
cleavage, loss of native conformation, recombination of fragments and thermal degradation.

A major difference between extrusion and other forms of food processing is that
gelatinization occurs at much lower moisture levels (12- 22%). Waxy or high- amylopectin
corn starch extrusion is dominated by melting, which flows from zero order kinetics, instead
of gelatinization, which is a first order reaction (Qu and Wang, 1994)
2.2.1 GENERAL DESCRIPTION OF EXTRUSION EQUIPMENT
Extrusion equipment can be classified thermodynamically or by pressure development in the
extruder. According to the former classification, we have
1. Autogenous extruder which converts mechanical energy to heat energy during the flow
process.
2. Isothermal extruder in which a constant temperature is maintained throughout the extruder
3. Polytropic extruder that operates between extreme conditions of the autogenous and
isothermal extruders.
If classified by the manner in which pressure is developed, there are
1. Direct or positive displacement type which include the Ram or piston type extruder and the
inter-meshing counter-rotating twin screw extruder.
2. Indirect or viscous drag type which include roller type extruders, single screw extruders,
intermeshing co-rotating twin screw extruder and non intermeshing multiple screw extruder.
The main components of the extrusion equipment include
1. Drive: The drive has an AC or DC motor which is used to rotate the extruder screw in the
barrel. The drive motor also comes with a transmission and a speed variation device for
controlling the screw rotation speed. Depending on the capacity, the size of the motor can be
as small as a few horse power for a laboratory-type extruder or several hundred horse-power
for commercial extruder for full scale production.
2. Feed assembly: The feed assembly consists of dry ingredient and liquid ingredient feeders.
Pre-blended dry ingredients are held in hoppers or bins above feeders, which can be

vibratory, variable speed auger, or loss-in-weight type. Liquid ingredients are metered
through positive displacement pumps, variable orifice, variable head, or water wheels. If a
pre-conditioner is used, dry ingredients are mixed with water, steam, or other ingredients in a
closed vessel, which can be operated under pressure if needed. The uniform feeding of dry
and liquid ingredients is imperative for the consistent operation of an extruder.
3. Screw: The screw can be one piece or consist of many individual screw elements
assembled in a screw shaft. The screw is usually divided into feed, transition, and metering
sections for single-screw extruders. The feed section has deeper flights to accept the food
ingredients from the feeder. In the transition section, the food ingredients are mixed, heated,
and worked into a continuous mass. Thus, the transition section is also called the compression
section because the materials are changed from a loose, powdery state into a plasticized
dough. This is accomplished by
gradually decreasing flight depth or pitch in the direction of discharge,
heating from the barrel, and
Working of the feed material to generate frictional heat.
The metering section has shallow flights that increase the shear rate; therefore, the
temperature of material increases rapidly in this section. Twin-screw extruders are classified
by the direction of screw rotation, co-rotating or counter-rotating, and by the depth of screw
engagement—folly intermeshing, partially intermeshing, and non-intermeshing. For fully
intermeshing twin-screw extruders, the flight depth is always constant and equal to the
distance of screw overlap. The screw design consists of single-, twin-, and triple-lead types,
which refers to the number of helical-shaped flights parallel along the length of the screw. To
improve mixing and increase the conversion of mechanical energy into heat, another type of
screw design, kneading disks is usually used.

4. The barrel fits tightly around the rotating screw and is assembled by bolting or clamping
several segments together. An important parameter for the specification of the extruder is its
L/D ratio, which is the extruder barrel length divided by the extruder barrel bore diameter.
The barrels and their liners, in particular, are usually constructed of special hardened alloy to
become wear resistant. The interior surface of barrel is either smooth or grooved. The
presence of grooves increases the pumping capability of the extruder against high back
pressure (Hsieh et al., 1990).
5. The die head assembly is located at the end of the extruder barrel and holds the extruder
die plate and sometimes serves as support for the cutter. The die plate can hold many dies,
which shape the food product before it emerges from the extruder. The dies are small
openings that can be round, annular, slit, or of specially designed shape, such as alphabets or
animals and so on.
2.2.2 OPERATING CHARACTERISTICS OF AN EXTRUSION EQUIPMENT
The most important operating parameters in an extruder are:
temperature
pressure
diameter of the die apertures
shear rate.
The aforementioned parameters can be varied to provide different product characteristics.
The shear rate is influenced by the internal design of the barrel, its length and the speed and
geometry of the screw(s). The process parameters for extrusion generally include screw
speed, feed moisture, feed rate, and barrel temperature. Changes in these processing
parameters will change the motor torque, die pressure, and product temperature. Visual
product characteristics such as expansion ratio, specific length (length of extrudate per unit

mass), and bulk density also will change. Increases in screw speed at constant feed rate will
cause a quick initial increase in die pressure and motor torque, followed by a sharp decrease
to the new equilibrium value (Lu et al., 1993). The initial quick increase in die pressure and
motor torque is due to the effort of the motor drive to overcome the inertia of the screw shaft
and the momentary increase in the output. Because of a rapid decrease in the degree of fill
due to screw speed increase, both torque and die pressure fall sharply after the initial increase
(Hsieh et al., 1990, Hsieh et al., 1993). The specific energy input increases with increasing
screw speed at constant feed rate; therefore, the product temperature increases. Extrudates are
usually lower in expansion ratio,
higher in specific length, and lower in bulk density with increasing screw speed (Hsieh et al.,
1990, Hsieh et al., 1993). Increasing feed moisture lowers the torque, specific mechanical
energy input, and product temperature. Die pressure drops initially due to a lower dough
viscosity caused by moisture increase. However, as the product temperature gradually
becomes lower, the dough viscosity rises, causing die pressure to recover. Final equilibrium
die pressure will depend on the relative effect of feed moisture and product temperature on
the dough's viscosity. Extrudates are generally lower in expansion ratio and specific length,
but higher in bulk density with higher feed moisture.
An increase in feed rate causes rapid increases in motor torque and die pressure and a slower
decrease in product temperature. Increases in feed rate while maintaining constant screw
speed increase the degree of fill in the extruder barrel and, thus, raise motor torque and die
pressure. Since the specific energy input is decreased with increasing feed rate, the product
temperature decreases. Increasing feed rate usually increases both expansion ratio and
specific length, resulting in a lower bulk density. Decreasing the barrel temperature causes
the product temperature to decrease. A lower product temperature corresponds to a lower
dough temperature or a higher dough viscosity and, therefore, a higher motor torque and die

pressure. The physical characteristics of extrudates from lower barrel temperatures are
usually similar to those from higher feed moistures.
Most research to model extruders has been done with single-screw machines because twin-
screw extruders are substantially more complex. In operation, the single-screw extruder acts
as a type of pump, dragging the food through the barrel and increasing the pressure and
temperature before the food is forced through the die. For optimum pumping, the food should
stick to the barrel and slip freely from the screw surface (Heldman and Hartel, 1997).
However, if food slips on the barrel it does not move through the extruder and is simply
mixed. For this reason, the barrel wall is often grooved to minimize slipping.
2.2.3 CHANGES IN FOOD AFTER EXTRUSION
Since extrusion cooking is an HTST process, it changes the physical and chemical properties
of food. Many methods have been developed and applied to characterize extruded products.
Expansion of extrudates in both longitudinal and lateral directions can be evaluated by direct
physical measurements. Apparent bulk density is determined by weighing the extrudates held
in a container with a fixed volume. Bulk density or specific volume can be obtained using
sand or rapeseed displacement methods. Scanning electron micrographs have been used to
examine the extrudates internal porous structure. Crispness, crunchiness, hardness, and other
texture attributes can be determined objectively with a texture analyzer such as Instron or
subjectively with human sensory evaluation methods. The color of extrudates can be assessed
with a colorimeter. Flavor compounds formed can be determined with a GC-MS. Many
analytical tests have been developed to evaluate extruded products. For tests on cereal or
starchy-type products, there are the water absorption index (WAI), water solubility index
(WSI), amylograph, and gelatinization tests. The WAI is the weight of gel obtained per gram
of dry product after dispersing the dry product into water, centrifuging, and decanting the
supernatant. The WSI is the amount of dried solid recovered by evaporating the supernatant

from the WAI test. The amylograph test determines the viscosities of slurry from the finely
ground, extruded product through a preprogrammed heating and cooling cycle.
Gelatinization of starches is one of the most important changes that occur in cereal types of
foods during extrusion. Starch, in the raw or native state, exists in the form of granules with
many different shapes, ranging from round, oval to irregular, with sizes between 1 and 100
µm depending on the sources of starch. As water and heat are added during extrusion, the
starch granules swell and the amylose in the granules begins to diffuse out. Eventually, the
starch granules, consisting mostly of amylopectin, collapse and become a colloidal gel held in
a matrix of amylose. For different types of starch, the temperature range at which
gelatinization occurs differs according to the sources of starch.
Many methods have been developed to determine the degree of starch gelatinization after
extrusion. Some commonly used methods are (1) loss of birefringence (crystallinity) in
polarized light, (2) X-ray diffraction patterns, (3) differential scanning calorimetry (DSC),
and (4) enzyme susceptibility.
For proteinaceous-type products, two indices are of interest: the protein dispersibility index
(PDI) and the nitrogen solubility index (NSI). While the latter measures the percentage of
total nitrogen in extrudate that is soluble in water, the former determines the amount of
protein in the extrudate that is dispersible in water; both are conducted under controlled
conditions of extraction (Harper et al., 1981). As the amount of heat treatment during
extrusion increases, more protein in the extrudate was denatured; this will result in decrease
in both the PDI and NSI. Similar to a cereal-type product, texture is also an important quality
attribute for an extruded proteinaceous product. Both sensory and instrumental methods have
been developed. The extrudate can also be examined by utilizing scanning electron
microscopy to reveal the structural integrity of the protein matrix. Changes in proteins during
extrusion can be briefly summarized as follows: (1) physically, extrusion converts protein

odies into a homogeneous matrix; and (2) chemically, the process recombines storage
proteins in some way into structured fibers (Stanley et al., 1989).
Besides starch gelatinization and protein denaturation, many other changes may occur during
an extrusion process, such as nonenzymatic browning, enzyme inactivation, destruction of
antinutritional factors, and microorganisms.
2.3 FOOD STARCH
Starch or amylum is a polysaccharide carbohydrate consisting of a large number of glucose
units joined together by glycosidic bonds. Starch is produced by all green plants as an energy
store. It is the most important carbohydrate in the human die and is contained in such staple
foods as rice, wheat, maize, potatoes and cassava. Pure starch is white, tasteless and odorless
powder that is insoluble in cold water or alcohol. It consists of two types of molecules: the
linear and helical amylase and the branched amylopectin.
Amylose is an essentially linear polysaccharide composed of ά l-4linked to a-D-
glucopyranosyl unit. Its degree of polymerization (DP) is 1,000 to 16,000 (Molecular Weight
160,000-2,650,000), depending on the source and method of preparation. Amylose can have
several conformations. In the solid state, it probably exists most often as a left-handed, six-
fold helix. In solution, it seems to be a loosely wound and extended helix that behaves as a
random coil. Because of its helical structure, amylose is able to complex with hydrophobic
molecules. In foods, amylase often complexes with mono- and di-glycerides and/or free fatty
acids or their salts. Complexed amylose molecules retrograde less effectively. Hence,
molecules complexed with hydrocarbon chains provide greater stability to foods, so
emulsifiers and other surfactants are added to baked goods to retard staling, which at least in
part results from retrogradation (Frederick, 2008).
Amylopectin has a branch-on-branch structure. Amylopectin molecules are composed of
chains of ά l-4 linked to a-D-glucopyranosyl units; branches are formed by joining these

chains with ά l-6 linkages. The average chain length is 20 to 30 units, although branch points
are not equally spaced. The currently accepted architecture of an amylopectin molecule is that
of a long main chain to which, are attached clusters of branches, some of which are
themselves branched. This model has been termed the cluster model. The molecular weight
of amylopectin has been measured as 5 X 10 7 - 4 X 10 8 (DP 3 X 10 5 - 2.5 X 10 6 ), depending
on the source and method of preparation (Frederick, 2008).
Potato starch amylopectin occurs as a natural phosphate ester. Corn and rice cultivars with
altered polysaccharide composition have been developed. Normal corn starch is composed of
approximately 28% of the linear polysaccharide amylose and approximately 72% of the
branched polysaccharide amylopectin. Those starches that result from a mutation that makes
them composed of essentially 100% amylopectin are designated waxy starches. Waxy maize
(waxy corn) starch is an important food starch. Other amylopectin starches have been
produced but are not commercially available. High-amylose corn starches containing
approximately 55% and 70% amylose are in commercial production and find some food use.
Starches from other special corn cultivars are both available and under development. The
physical properties of amylose and amylopectin are reflected in starches. For example, high
amylose starches are difficult to gelatinize (because of the extra energy needed to dissociate
and hydrate the aggregates of amylose); form firm, opaque gels; and can be used.
to make strong, tough films. Their solutions and gels undergo retrogradation rapidly. Waxy
maize starches, even when unmodified, gelatinize easily and yield viscous, almost transparent
solutions that do not gel. In general, the properties of a starch paste or gel are a function of
the amounts of amylose and amylopectin dispersed or solubilized, the amounts of insoluble
swollen granules and granule fragments, and interactions between components. Amylose
increases gel strength; amylopectin decreases gel strength and viscosity. (Frederick, 2008)
2.3.1 PROPERTIES AND STRUCTURE OF FOOD STARCH

Starch molecules arrange themselves in the plant in semi- crystalline granules. Each plant
species has a unique starch granular size: rice starch is relatively small (2µm) while potato
starches have larger granules (up to 100µm). Although in absolute mass only about one
quarter of the plant granules in plants consists of amylase, there are about 150 times more
amylase molecules than amylopectin molecules. Amylase is much smaller than amylopectin.
Starch becomes soluble in water when heated. The granules swell and burst, the semi-
crystalline structure is lost and the smaller amylase molecules start leaching out of the
granule, forming a network that holds water thus increasing the water’s viscosity. This
process is called starch gelatinization. During cooking the starch becomes a paste and
increases further in viscosity. During cooking or prolonged storage of the paste, the semi-
crystalline structure partially recovers and the starch paste thickens, expelling water. This is
mainly caused by the retrogradation of the amylase. This process is responsible for the
hardening of bread or staling, and for the water layer on top of a starch gel (syneresis). Some
cultivated plant species have pure amylopectin starch without amylase, known as waxy
starch. The most used is waxy maize, others are glutinous rice, waxy potato starch. Waxy
starches have less retrogradation, resulting in more stable paste. High amylase starch,
amylomaize, is cultivated for the use of its gel strength (Frederick, 2008).
2.3.2 HYDROLYSIS OF FOOD STARCH
The enzyme that breaks down or hydrolyze starch into the constituent sugars are known as
amylases. Alpha- amylases are found in plants and in animals. Human saliva is rich in
amylase and the pancreas also secretes the enzyme. Individuals from a population with high
starch diet tend to have more amylase genes than those with low starch diets; chimpanzees
have very few amylase genes. Beta- amylase cuts starch into maltose units. This process is
important in digestion of starch and in brewing where the amylase from the skin of the seed
grains is responsible for converting starch to maltose (Malting, Mashing) (Frederick, 2008).

2.3.3 DIGESTIBILITY AND DEXTRINIZATION OF FOOD STARCH
Digestive enzymes have problems digesting crystalline structure. Raw starch will digest
poorly in the duodenum and small intestine, while bacteria degradation will take place mainly
in the colon. Resistant starch is starch that escapes digestion in the small intestine of healthy
individuals. In order to increase the digestibility, starch is cooked. Hence, before humans
started using fire, eating grains was not a very useful way to get energy.
If starch is subjected to dry heat, it breaks down to form pyrodextrin in a process known as
dextrinization. Pyrodextrins are brown in colour. His process is partially responsible for the
browning of toasted bread (Frederick, 2008).
2.3.4 USE OF STARCH AS FOOD
Starch is the most important carbohydrate in the human diet and is contained in many staple
foods. The major sources of starch intake worldwide are rice, wheat, maize, potatoes and
cassava. Widely used prepared food containing starch are bread , pancakes, cereals, noodles,
pasta, porridge and tortilla (Anne- charlotte, 2004). Depending on the local climate, other
starch sources are used for food such as arrow roots, arracacha, buckwheat, barley, oat,
millet, sweet potato, taro and yams. Chestnuts and edible beans such as fava, lentils, mung
beans and peas, are also rich in starch. The starch industry extracts and refines starch from
seeds by wet grinding, sieving and drying. Today, the main commercial refined starches are
cornstarch, tapioca, wheat and potato starch. To a lesser extent sources include rice, sweet
potato, sago and mung beans.
2.3.5 USE OF STARCH AS FOOD ADDITIVE
As an additive for food processing, food starches are typically used as thickners and
stabilizers in food such as puddings, custards, soups, sauces, gravies and pie fillings and salad

dressings and to make noodles and pasta. But by far the most common starch based food
ingredients are starch sugars used as sweetner in many drinks and foods.
Starch is used as an excipient, a binder in medications to aid the formation of tablets.
Resistant starch is starch that escapes digestion in the small intestine of healthy individuals.
((Frederick, 2008)
2.3.6 STARCH SUGARS
Starch can be hydrolyzed into simpler carbohydrates by acids, various enzymes or a
combination of the two. The extent of conversion is simply quantified by the Dextrose
Equivalence (DE) which is roughly the fraction of the glycosidic bonds in starch that have
been broken. Food products made in this way include:
Maltodextrin, a light hydrolyzed starch product (DE 10-20) used as a bland tasting filler and
thickener, various glucose syrups (DE 30-70), viscous solutions used as sweeteners and
thickeners in many kinds of processed foods; Dextrose (DE 100), commercial glucose
prepared by the complete hydrolysis of starch and high fructose syrup, made by treating
dextrose solutions with the enzyme glucose isomerase, until a substantial part of the glucose
has been converted to fructose.
In the United States, high fructose corn syrup is the principal sweetener used in sweetened
beverages because fructose has better handling characteristics, such as microbiological
stability and more consistent flavor. Sugar alcohol such as sorbitol, mannitol, maltitol are
sweeteners made by reducing sugars. (Frederick, 2008)
2.3.7 INDUSTRIAL APPLICATION OF FOOD STARCH
Paper making is the largest non food application for starches, globally consuming millions of
metric tonnes annually. In a typical sheet of copy paper for instance, the starch content may

e as high as 8%. Both chemically modified and unmodified starches are used in paper
making. In the wet part of the paper making process, generally called the wet end, the
starches used are cationic and have positive charge bound to the starch polymer. These starch
derivatives associate with the anionic or negatively charged paper fibres cellulose and
inorganic fillers. Cationic starches together with other retention and internal sizing agents
help to give the necessary strength properties to the paper web to be formed in the paper
making process (wet strength), and to provide strength to the final paper sheet (dry strength).
In the dry end of the papermaking process the paper is re- wetted with a starch based solution.
The process is called surface sizing. Starches used have been chemically or enzymatically
depolymerized at the paper mill or at the starch industry (oxidized starch). The size- starch
solutions are applied to the paper web by means of various mechanical presses. Together with
surface sizing agents, the surface starch imparts additional strength to the paper web and
additionally provide water hold out or size for superior printing properties. Starch is also used
in paper coating as one of the binders for the coating formulation of a mixture of pigments,
binders and thickeners. Coated paper has improved smoothness, hardness, whiteness and
gloss and thus improves printing characteristics (Frederick, 2008).
Corrugated board adhesives are the next largest application of food non starch globally.
Starch glues are mostly based on unmodified native starches, plus some additives such as
borax and caustic soda. Part of the starch is gelatinized to carry the slurry of uncooked
starches and prevent sedimentation. This opaque glue is called Stein Hall adhesive. The glue
is applied on the tip of the fluting. The fluted paper is pressed to paper called liner. This is
then dried under high heat which cause the rest of the uncooked starch to gelatinize. The
gelatining process makes the glue a fast and strong adhesive for corrugated board production.
Another large non food starch application is in the construction industry where starch is used
in the gypsum wall board manufacturing process. Chemically modified or unmodified

starches are added to the stucco containing primarily gypsum. Top and heavy weight sheets
of paper are applied to the formulation and the process is allowed to heat and cure to form the
eventual rigid wall board. The starches act as the glue for the cured gypsum rock with the
paper covering and also to provide rigidity to the board. Starch is used in the manufacture of
various glues for book binding, wall paper adhesives, sack production and bottle labeling.
Starch derivatives such as yellow dextrins can be modified by the addition of some chemicals
to form hard glue for paper work: some of those forms use borax or soda ash, which are
mixed with the starch solution at 50- 70 o C to create a very good adhesive. Sodium silicate
can be added to reinforce these formulae. Clothing or laundry starch is a liquid that is
prepared by mixing a vegetable starch in water and is used in the laundering of clothes.
Starch was widely used in Europe in the 16 th and 17 th centuries to stiffen the wide collars and
ruffs of linen which surrounded the necks of the well to do. Aside from the smooth, crisp
edges it gave to clothings, it served practical purposes as well. Dirt and sweat from a person’s
neck and wrists would stick to the starch rather than to the fibers of the clothings and would
easily wash away along with the starch. After each laundering, the starch would be re-
applied. Today, the starch is sold in aerosol cans for home use (Frederick, 2008).
Starch is also used to make some packing peanuts and some drop ceiling tiles. Textile
chemicals from starch are used to reduce breaking of yarns during weaving: the warp yarns
are sized especially for cotton. Starch is also used as textile printing thickener. In the printing
industry, food grade starch is used in the manufacture of anti- set- off spray powders used to
separate printed sheets of paper to avoid wet ink being set off (www.russellwebb.com).
Starch is used to produce bioplastics, synthetic polymers that are biodegradable. An example
is polyactic acid. For body powder, powdered corn starch is used a substitute for talcum
powder and similarly in other health and beauty products.

In oil exploration, starch is used to adjust the viscosity of drilling fluid, which is used to
lubricate the drill head and suspend the grinding residue in petroleum extraction. Glucose
from starch can be further fermented to biofuel ethanol. Hydrogen production can use starch
as the raw material using enzymes (Zhang et al., 2007)
3.0 MATERIALS AND METHOD
3.1 MATERIALS
CHAPTER THREE
Orange fleshed varieties of sweet potato (Ipeoma batatas) were purchased from Mile 12
market, Ikorodu road, Lagos state, Nigeria.
3.2 EQUIPMENT
Weighing balance (Mettler, PM400: Switzerland), Locally fabricated single screw extruder
installed at the Food Processing Laboratory of the Department of Food Science and
Technology, University of Agriculture, Abeokuta, Nigeria, Centrifuge machine (Surgifield,
Model SM 800D, England), Vernier caliper, Hot air oven (Gallenkamp Plus II oven: UK),
Water bath (Clifton, 15147: England).

3.3 METHOD
3.3.1 Starch Isolation: Starch was isolated and purified using modified procedures of
Willinger (1964) and Rosenthal and Spindola (1969). Tubers were rinsed, peeled, diced and
then, disintegrated in a domestic blender in a sodium metabisulfite water solution (750ml/l).
The mixture was sieved (0.25mm screen) and the retained solids were exhaustively rinsed on
the sieve with the sodium metabisulfite. The filtrate was allowed to stand at room
temperature and the decant was discarded. The starch sediment was treated with 0.15- 0.10%
sodium hydroxide solution and then exhaustively rinsed with distilled water until a pH 7 on a
sieve (0.105mm). The decant was then discarded. The sediment was rinsed with ethanol
(70%) and the alcohol evaporated. It was then dried in an oven at 40 o C until 12% moisture,
the starch was ground with a mortar and pestle to pass a 0.105mm sieve and then stored in
polyethylene bags.
3.3.2 Extrusion Cooking: Extrusion cooking was out with a locally fabricated single screw
extruder (Department of Agric. Engineering, University of Agriculture, Abeokuta, Ogun
State Nigeria) with L/D ratio of 304m: 118.5mm, screw diameter of 18mm, power of 0.25hp
and barrel length of 304mm respectively. The extruder operated at full speed in all runs under
the following conditions: barrel and die temperature (100, 120, 140 o C), moisture levels (50,
55 and 60%) and screw speeds (100, 120 and 140rpm). Plate 1 represents an aerial view of
the locally fabricated single screw extruder described above. Plate 2 represents the screw with
18mm diameter. Plate 3 represents the extruder barrel of length 304mm.
3.4 Proximate Analysis of Isolated Sweet Potato Starch
The starch extracted from the orange fleshed sweet potato was analyzed for the following:
3.4.1 Determination of Moisture Content of Isolated sweet potato starch

Test sample (5g) was weighed into a pre-weighed clean petri dish and placed in a well
ventilated oven maintained at 103 o C. After 3 hours, the dish was removed and transferred
into a dessicator to cool. Continuous drying was done until a constant weight was achieved
(AOAC, 2000).
Calculation
% moisture content = (M1 – M2) 100
M1 – M0
i.e. % M. C = Weight before drying – Weight after drying 100
Original weight of sample
M0 is the weight in g of dish before drying
M1 is the weight in g of dish and sample before drying
M2 is the weight in g of dish and sample after dryin

Plate 1. Aerial view of an Extruder

Plate 2. Extruder screw

Plate 3. Extruder barrel

3.4.2 Determination of Protein Content of Isolated sweet potato starch
1 g of the test sample was weighed into the digestion flask; kjeldahl catalyst tablet and the
20ml of concentrated H2SO4 was added, the flask was fixed into the digester at 410 0 C for 6
hours, until a clear solution was obtained. This was then cooled and the digest was transferred
into 100ml volumetric flask and was made up to mark with distilled water. The distillation
apparatus was set up and it was rinsed for 10 minutes, after boiling 20ml of boric acid was
pipetted into a conical flask, 5 drops of indicator and 75ml of distilled water was added; then
10ml of the digest was pipetted into the Kjeldhal distillation flask. The conical and the
distillation flask was fixed in place, then 20ml of 20% NaOH was added though the glass
funnel into the digent. The steam unit was closed and timing started as the solution of the
boric acid and indicator turns green. Afterward, the distillate was titrated with HCl (0.05N)
(AOAC, 2000).
Calculation
% Total Nitrogen = 14.01 (sample titre – blank titre) N
N = Normality of acid
10 sample weight
Therefore, % crude protein = % nitrogen conversion factor conversion factors of 6.25.
3.4.3 Determination of Fat Content of Isolated sweet potato starch
3g of the test sample was weighed into a filter paper wrapped properly, placed in a thimble
and then transferred to the extraction barrel. Petroleum ether was added through the barrel
into the extraction flask directly below it. Up to three-quarter of the flask. The water inlet was
opened and the flask was heated on the regulated heating mantle for 6 hour. After completion
of the extraction time, the thimble was removed from the extractor barrel and the flask was

dried in the oven. The flask was removed from the oven. Cooled on a dessicator and the
weight of flask plus content was taken (AOAC, 2000).
Calculation
% crude fat = W3 – W2 100
W1
W1 = Weight of sample
W2 = Weight of flask before extraction
W3 = Weight of flask after drying
3.4.4 Determination of Ash Content of Isolated sweet potato starch
3g of an aliquot of the test sample was weighed into an empty porcelain crucible, which had
been previously ignited and weighed (AOAC, 2000).
% Ash = (weight of crucible +ash) – (weight of empty crucible) 100
Sample weight
3.4.5 Determination of Carbohydrate Content of Isolated sweet potato starch
This was evaluated using difference method (100% - fat%- protein%- ash%-
moisture%)(AOAC, 2000).
3.5 ANALYSIS OF EXTRUDATES
The extrudates were analyzed for the following: Water absorption index, Water Solubility
index, Bulk density, Solid loss, Lateral Expansion. Determinations were made in triplicates
for all analysis.

3.5.1 Water Absorption Index (WAI)
The ground extrudates was suspended in water at room temperature for 30 minutes, gently
stirred during this period after which it was centrifuged at 3000rpm for 15 minutes. The
supernatant was decanted into an evaporating dish of known weight. WAI is the weight of gel
after removal of the supernatant per unit weight (Anderson et al., 1969).
WAI (g/g) = Weight of gel
Weight of original sample
3.5.2 Water Solubility Index (WSI)
This was evaluated using the method adopted for Water Absorption Index (WAI) but with
respect to the dried solid. The WSI is the weight of dry solids in the supernatant expressed as
a percentage of the original weight of sample (Anderson et al., 1969).
WSI (%) = Weight of dried solid × 100
3.5.3 Bulk Density
Weight of original sample
Samples used in measuring lateral expansion were used for bulk density measurement. Each
of the samples was weighed using laboratory balance and the length and diameter of the
sample measured using a vernier caliper (Ali et al., 1996; Alvarez- Martinez et al., 1988).
Therefore, bulk density g/cm 3 = 4m
d 2 l

Where m= mass (g)
L= length (cm)
D= diameter of extrudates (cm)
3.5.4 Solid Loss
Ten grammes of the sample were weighed in a beaker and 200ml of boiling water was added
to it. The beaker was placed in boiling water for a minimum cooking time of six minutes. The
cooking water was drained into a 250ml volumetric flask and made up to 250ml. Ten
millimeters of the cooking water was evaporated in a hot air oven at 100 o C to determine the
weight (A) of the residue (Singh et al., 2003).
Solid loss % = AB × 100
Where
CD
A is the weight of the residue
B is the volume of the cooking water (250ml)
C is the initial sample weight (10ml)
D is the volume of sample taken residue estimation (10ml)
3.5.5 Lateral Expansion
The ratio of diameter of extrudates to that of the die was used to express the expansion of the
extrudates. Six various lengths of the extrudates was selected at random during collection of
each of the extruded samples and was allowed to cool to room temperature. Also, the
diameters of the extrudates was measured at different positions along the length of each

sample taken using a vernier caliper. A sewing rope was used to measure the length of the
extrudates when not straight.
The lateral expansion (LE) was calculated using the mean of the measured diameter
(Alvarez- martinez et al., 1988; Fan et al., 1996).
LE= Diameter of extrudates
Diameters of die hole
3.6 Statistical Analysis of Functional Properties of Extrudates
A statistical package, Design Expert 7 was used and Response Surface Methodology was
used to optimize the independent variables (barrel speed, temperature and moisture levels) so
as to obtain high quality extruded sweet potato starch products.

4.0 RESULTS AND DISCUSSION
CHAPTER FOUR
4.1 PROXIMATE COMPOSITION OF ISOLATED SWEET POTATO STARCH
Table 2 shows the proximate composition of isolated sweet potato starch. Results obtained
showed that carbohydrate composition is highest. Low moisture content suggests that sweet
potato has a high microbial resistance (Oladebeye et al., 2006) Sweet potato also has a high
content of dietary fibre from the table below. Sweet potato has been identified as a significant
source of dietary fibre (Collins and Walter, 1982) and evidences suggest that increased fibre
consumption may contribute to reduction in incidence of certain diseases such as diabetes,
coronary heart diseases, colon cancer and various digestive disorders that have been reported
(Augustin et al., 1978). The high carbohydrate content makes it suitable for extrusion
process.
4.2 Response surface analysis of functional properties of extruded sweet potato starch
Table 3 shows the Response Surface Analysis for some functional properties of extruded
sweet potato starch for all experimental values from which surface plot graphs were drawn.
Water Absorption Index (WAI) had maximum and minimum values of 2.42 and 2.81
respectively. The maximum value of WAI was obtained at high temperature (140 o C) due to
the fact that exposure of starch granules to increased temperature and increased shear from
increased screw speed causes gelatinization which denatures it rendering it porous, hence
more water molecules flow into the granules absorbing more water molecules (Harper, 1987).
WAI which is the weight of sediment formed per unit mass of sample after the sample is
centrifuged and supernatant is removed, was significantly affected by screw speed and
temperature (Anderson et al., 1969). WAI is a function of gelatinization of starch which

occurs during extrusion and depends on the amylose-amylopectin interaction with water
(Noguchi, 1990). WAI increased with increasing screw speed and temperature.
Screw speed did not significantly affect functional properties of extruded samples, except for
WAI where there was a significant decrease at increased screw speed. Reports of Anderson et
al., (1969) and Conway (1971) agree that increase in screw speed reduced WAI but increased
WSI (Water Solubility Index).WAI is attributed to the dispersion of starch in excess water
and the dispersion is increased by the degree of starch damage due to the gelatinization and
extrusion of amylose and amylopectin molecules (Rayas- Duarte et al., 1998).
WSI decreased as WAI increased, having minimum and maximum values of 0.65 and 0.98 at
140 o C and 100 o C respectively. This is due to the inverse relationship existing between WSI
and temperature. Reduced temperature causes mild gelatinization which retains the WSI of
native starch to a great extent. WSI is often used as an indicator of degradation of molecular
component (Kirlby et al., 1989). It measures the degree of starch conversion during extrusion
which is the amount of soluble polysaccharides released from the starch component after
extrusion. It is also the percentage of the initial sample present in the supernatant obtained
from WAI. Temperature and screw speed had significant effects on WSI. Increased WSI in
extrudates and reduced viscosity favour high amylase availability and rapid absorption of
starch in vivo which could also increase gastric emptying rate (Asp and Bjorck, 1989).
BD was significantly affected by screw speed. Bulk density decreases with increase in screw
speed; however extrudates are higher in bulk density with a higher feed moisture (Garber et
al., 1997). Minimum and Maximum values of bulk density obtained are 0.7g/cm 3 and
0.8g/cm 3 . Maximum value of bulk density obtained is due to increased screw speed which
causes increased shear which reduces the viscosity, hence reducing the weight per unit mass.

Increased shearing from increased screw speed break macro molecules down to smaller units
resulting in decreased viscosity.
Lateral expansion refers to an increase in the area of extrudates relative to the area of the die.
Two factors governing expansion; the paste viscosity and elastic force in the extrudate. The
elastic force will dominate. An increase in temperature results in an increase in lateral
expansion with minimum and maximum values of 0.8 and 1.02 respectively; this signifies an
adequate cooking temperature for extrudates. This is as a result of expansion caused by
increased temperature and rapid cool off as it exits from the barrel. Release of pressure as the
food emerges from the die causes instantaneous expansion of steam and gas in the material.
Solid loss signifies the extent of conversion of starch from its native state. Solid loss
decreased with increase in temperature and screw speed having minimum and maximum
values of 2.50 and 10.0 respectively. This is as a result of increased gelatinization caused by
increased temperature and screw speed which converts more starch from the native state.

TABLE 2: Proximate Composition of Isolated Sweet Potato Starch
Moisture content 8.13±0.02
Crude protein 1.60±0.01
Ash content 1.59±0.03
Fat content 0.41±0.01
Crude Fibre 0.81±0.02
Carbohydrates 87.46±0.01

TABLE 3: Response Surface Analysis Of Functional Properties Of Extruded Sweet Potato
Starch For All Experimental Values.
X1( O C) X2(rpm) X3(%) WSI WAI BD SL LE
120.00
120.00
120.00
120.00
120.00
100.00
140.00
100.00
120.00
120.00
120.00
140.00
120.00
140.00
100.00
140.00
100.00
120.00
100.00
120.00
100.00
120.00
140.00
120.00
120.00
120.00
120.00
140.00
100.00
140.00
120.00
120.00
140.00
100.00
55.00
50.00
55.00
60.00
55.00
55.00
50.00
50.00
55.00
55.00
50.00
55.00
60.00
60.00
60.00
55.00
55.00
0.83
0.74
0.83
0.78
0.78
0.98
0.69
0.83
0.78
0.78
0.74
0.65
0.96
0.71
0.75
0.88
0.76
2.51
2.65
2.51
2.60
2.51
2.43
2.74
2.55
2.51
2.51
2.47
2.81
2.42
2.69
2.63
2.50
2.62
X1-Temperature ( o C), X2-Screw speed (rpm), X3-Feed moisture content (%), WSI- Water
Solubility Index, WAC- Water Absorption Capacity, BD- Bulk Density (g/cm 3 ), SL- Solid
Loss (%), LE-Lateral Expansion
0.76
0.77
0.76
0.78
0.76
0.72
0.74
0.78
0.76
0.76
0.71
0.78
0.74
0.75
0.72
0.70
0.80
2.50
10.0
3.75
8.75
2.50
5.00
5.00
8.75
5.00
3.75
3.75
2.50
10.0
2.50
2.50
2.50
3.75
0.94
0.9
0.91
0.88
0.94
0.84
0.98
0.85
0.94
0.94
0.99
0.93
0.96
0.97
0.89
1.02
0.80

Figure 1 shows that as temperature increases, WAI increases. This is because increased
temperature causes gelatinization which denatures the extrudates rendering it porous, hence
more water molecules flow into the granules absorbing more water molecules (Anderson et
al., 1969). Figure 2 shows that as temperature increases, WSI decreases. This is because
reduced temperature causes mild gelatinization which retains the WSI of native starch to a
great extent (Anderson et al., 1969). Figure 3 shows that as temperature increases, bulk
density decreases. This could be due to reduction in viscosity of native starch granules due to
increased temperature exposure (Davidson, 1992). Figure 4 shows that as temperature
increases, solid loss decreases. This is as a result of increased gelatinization with increased
temperature (Anderson et al., 1969). Figure 5 shows that as temperature increases the lateral
expansion of the extrudate increases. This could be as a result of expansion caused by
increased temperature and rapid cool off as it exits from the barrel (Best, 1994). Higher
temperature causes more expansion of the extrudates.
Table 4 shows the Coefficient of regression equation for the functional properties of extruded
sweet potato starch. Temperature, screw speed, combined effect of temperature and moisture,
quadratic function of temperature and screw speed (X1, X3, X1X2, X1 2 , X2 2 ) significantly
affect WAI with F value of 31.46 and R 2 of 0.976 . Screw speed (X3) had significant effect
on WSI and bulk density. Temperature and Screw speed (X1 and X3) had significant effect on
Lateral expansion with F value of 2.15 and R 2 value of 0.73.

WAI
X1 = A: Temperature
X2 = B: Moisture content
Actual Factor
C: Screw speed = 120.00 rpm
2.76
2.69
2.63
2.50
WAI
2.56
B: Moisture content %
60
57.5
55
52.5
50
100
110
120
A: Temperature o c
FIG 1: Response surface plot of water absorption index as a function of
barrel temperature and feed moisture content
130
140

WSI
X1 = A: Temperature
X2 = B: Moisture content
0.83
Actual Factor
C: Screw speed = 120.00
0.78
0.74
WSI
0.69
0.65
60
57.5
55
B: Moisture content %
52.5
50
100
140
130
120
110
A: Temperature o C
FIG 2: Response surface plot of Water solubility index as a function of barrel
temperature and feed moisture content

BD
X1 = A: Temperature
X2 = B: Moisture content
Actual Factor
C: Screw speed = 120.00 rpm
BD
0.78
0.77
0.75
0.74
0.72
B: Moisture content %
60
57.5
55
52.5
50
100
110
120
A: Temperature o c
FIG 3: Response surface plot of bulk density as a function of barrel
temperature and feed moisture content
130
140

SOLID LOSS
X1 = A: Temperature
X2 = B: Moisture content
Actual Factor
C: Screw speed = 120.00 rpm
SOLID LOSS
6.76
4.77
2.78
8.75
0.80
60
57.5
B: Moisture content %
55
52.5
50
100
110
120
130
A: Temperature o c
FIG 4: Response surface plot of solid loss as a function of barrel
temperature and feed moisture content
140

LATERAL EXPANSION
X1 = A: Temperature
X2 = B: Moisture content
Actual Factor
C: Screw speed = 120.00 rpm
LATERAL EX
0.96
0.92
0.88
1.00
0.84
B: Moisture content %
60
57.5
55
52.5
50
100
110
120
A: Temperature o c
FIG 5: Response surface plot of Lateral expansion a as a function of
barrel temperature and feed moisture content
130
140

Table 4: Coefficient Of Regression Equation For The Functional Properties Of Extruded
Sweet Potato Starch.
Variables WSI WAI BD SL LE
-0.049 0.033* -0.006 -0.94 12.07*
X1
X2
X3
X1X2
X1X3
X2X3
X1 2
X2 2
X3 2
F-VALUE
R 2
P - VALUE
0.025
0.079*
0.025
0.003
0.045
-0.021
-0.034
0.039
3.000
0.794
0.805
0.0006
0.092*
0.004*
0.004
0.000
0.041*
0.008*
0.002
31.46
0.976
0.0001
*Significant at p< 0.05 (5% level)
-0.001
-0.033*
0.017
0.000
0.005
-0.006
-0.006
-0.004
5.200
0.870
0.020
-0.47
-0.47
0.94
-0.31
1.88
-1.75
2.94*
1.69
2.10
0.73
0.17
12.00
0.037*
24.00
0.013
0.003
11.98
12.02
-12.02
2.150
0.730
0.162
X1 is Temperature ( o C), X2 is Moisture content (%) and X3 is Screw speed (rpm)
WSI- Water Solubility Index, WAC- Water Absorption Capacity, BD- Bulk Density (g/cm 3 ),
SL- Solid Loss (%), LE-Lateral Expansion

CHAPTER FIVE
5.0 CONCLUSION AND RECOMMENDATION
5.1 CONCLUSIONS
The recurring problem of food insecurity which stem not just from inadequte food production
but also from loss of food through spoilage has received a ray of hope by means of
diversification of the conventional methods of processing of sweet potato uncovered by this
study. This study has shown the possibility of using sweet potato for the production of pasta.
Extrudates are generally characterised by High WAI, low bulk density and lateral expansion,
therefore altering the process parameters (temperature, screw speed and feed moisture
content) result to changes in the desired qualities. The optimum processing conditions
necessary to obtain the desired quality pasta are combination of temperature of 100 o C, feed
moisture of 60% and screw speed of 103.34 rpm.
5.2 RECOMMENDATION
Improvement on the operation of the present level of
technology on the available fabricated extruders.
Microbial, storage stability and Nutritional assessment should
be carrieed out on extruded pasta.

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