Comparative evaluation of the digester-screw press and a hand-operated hydraulic press for palm fruit processing


Owolarafe, O.; Faborode, M.; Ajibola, O.

Journal of food engineering 52(3): 249-255

2002


In order to demonstrate the strength and possible weaknesses of the digester-screw press (DSP) system for small-scale oil palm fruit processing, a comparison was made of its performance and that of the erstwhile hand-operated hydraulic extraction system. Indices of evaluation include oil yield and quality, and operational economics. The results indicate that the throughput of the DSP system was four folds of that of the hydraulic system, whilst also operating at higher oil extraction efficiency (89.1%). There was no significant difference between the quality of the palm oil obtained from the two systems. However, the economic analysis of the systems indicates that at throughput of 0.75 t/h and above, the DSP system was more economical than the hydraulic system in terms of equipment, labour, material and floor space requirement and revenue accruing from the processing operation.

JOURNAL
OF
FOOD
ENGINEERING
ELSEVIER
Journal
of
Food
Engineering
52
(2002)
249-255
www.elsevier.com/locateljfoodeng
Comparative
evaluation
of
the
digester—screw
press
and
a
hand-operated
hydraulic
press
for
palm
fruit
processing
Oseni
K.
Owolarafe,
Michael
0.
Faborode
*,
Obafemi
0.
Ajibola
Department
of
Agricultural
Engineering,
Obafemi
Awolowo
University,
Ile-Ife,
Nigeria
Received
25
May
2000;
accepted
27
May
2001
Abstract
In
order
to
demonstrate
the
strength
and
possible
weaknesses
of
the
digester—screw
press
(DSP)
system
for
small-scale
oil
palm
fruit
processing,
a
comparison
was
made
of
its
performance
and
that
of
the
erstwhile
hand-operated
hydraulic
extraction
system.
Indices
of
evaluation
include
oil
yield
and
quality,
and
operational
economics.
The
results
indicate
that
the
throughput
of
the
DSP
system
was
four
folds
of
that
of
the
hydraulic
system,
whilst
also
operating
at
higher
oil
extraction
efficiency
(89.1%).
There
was
no
significant
difference
between
the
quality
of
the
palm
oil
obtained
from
the
two
systems.
However,
the
economic
analysis
of
the
systems
indicates
that
at
throughput
of
0.75
t/h
and
above,
the
DSP
system
was
more
economical
than
the
hydraulic
system
in
terms
of
equipment,
labour,
material
and
floor
space
requirement
and
revenue
accruing
from
the
processing
operation.
©
2002
Elsevier
Science
Ltd.
All
rights
reserved.
Keywords.•
Palm
fruit;
Palm
oil;
Screw
press;
Oil
extraction
1.
Introduction
Palm
oil,
the
most
important
product
of
the
oil
palm
(Elaeis
guineensis),
is
the
world's
main
edible
oil
as
well
as
the
most
important
oil
for
soap
making
(Bek-Nelson,
1974).
High
quality
palm
oil
is
required
for
domestic
and
industrial
applications.
The
commercially
impor-
tant
quality
parameters
of
palm
oil
are
its
free
fatty
acid
(FFA)
value
and
oxidation
level.
Other
quality
para-
meters
include
colour,
peroxide
value,
iodine
value,
moisture
content,
specific
gravity,
refractive
index
and
viscosity
(Hartley,
1988,
Chap.
14).
The
palm
fruit
is
a
sessile
drupe,
one-seeded
fruit
enclosed
in
a
fleshy
pulp
of
variable
shape
of
about
20-50
mm
long.
The
fruit
has
a
kernel
consisting
of
an
irregularly
shaped
mass
of
white
soluble
proteins
or
albumen
enclosing
a
cylindrical
embryo,
an
endocarp
or
shell,
a
mesocarp
or
fleshy
part
(pulp)
with
cells
filled
of
oil
(in
which
individual
bodies
are
clubbed
together)
and
a
coloured
exocarp
or
outer
skin
(Tropical
Agricultur-
alist,
1998;
Rajanaidu,
1994).
Corresponding
author.
E-mail
address.•
(M.O.
Faborode).
The
unit
operations
involved
in
palm
oil
processing
include
fruit
sterilisation,
fruit
loosening/stripping,
di-
gestion,
oil
separation
and
clarification
(Fig.
1).
Fruit
sterilisation
is
a
heat
rendering
and
moisture
absorption
process
which
inactivates
the
lipolytic
enzymes
in
the
fruit
mesocarp
tissue.
The
hydrolysis
of
the
colloidal
mucilage
in
the
cell
wall
and
the
breakdown
of
car-
bohydrate
molecules
into
glucose
molecules
initiates
osmotic
pressure
in
the
cell.
The
pressure
of
the
liquid
fat
assists
in
heat
transmission
of
the
cell
walls.
In
mechanical
processing
of
palm
fruit,
sterilisation
en-
hances
fruit
recovery
during
fruit
stripping
and
facili-
tates
fruit
digestion
by
softening
the
mesocarp
tissue.
Fruit
loosening
or
stripping
refers
to
the
separation
of
fruits
from
bunches,
quarters
or
spikelets.
It
is
usually
done
to
facilitate
handling
of
fruits
in
subsequent
op-
erations.
Fruit
digestion
means
crushing
and
detach-
ment
of
the
steamed
or
heat-weakened
mesocarp
from
fruit
nuts.
The
main
purpose
of
fruit
digestion,
which
is
a
form
of
size
reduction
and
wet
comminution
opera-
tion,
is
to
break
up
the
pulp
of
the
fruit
and
liberate
oil
from
the
cells
in
which
it
is
contained.
The
extent
of
the
digestion
of
the
fruit
determines
the
degree
of
exposure
of
the
oil
cells.
Thus,
sterilisation
and
digestion
opera-
tions
are
essential
pre-treatment
operations
for
palm
fruit.
Oil
separation
entails
separating
the
crude
oil
from
0260-87741021$
-
see
front
matter
©
2002
Elsevier
Science
Ltd.
All
rights
reserved.
P11:
50260-8774(01)00112-1
Fresh
Fruit
Bunches
V
Quartering
of
Fruit
Bunches
V
Sterilisation
of
Quarters
in
V
Digestion
and
Pressing
In
DSP
Fruit
Stripping
Digestion
of
Fruits
in
a
agester
V
Hydraulic-Pressing
of
Digested
Fruit
Clarification
of
extracted
oil
250
0.K
Owolarafe
et
al.
I
Journal
of
Food
Engineering
52
(2002)
249-255
Hydraulic
DSP
Fig.
1.
Flow
chart
of
hydraulic
and
DSP
systems.
the
mash,
while
clarification
is
the
separation
of
pure
oil
from
the
sludge
in
boilers
or
clarification
tanks.
All
the
operations
are
carried
out
in
different
ways
in
large
scale,
intermediate
and
small-scale
plants.
The
methods
employed
invariably
influence
the
yield
of
oil
and
its
quality.
In
the
large
scale
plant,
all
the
processing
operations
are
fully
mechanised,
with
sophisticated
conveyance
systems.
The
use
of
screw
expellers
in
oil
separation
and
the
recapturing
of
oil
by
a
sludge
centrifuge
for
further
clarification
enables
over
90%
of
the
oil
to
be
extracted
(Hartley,
1988,
Chap.
14).
How-
ever,
there
is
a
contention
that
screw
expellers
are
gen-
erally
too
expensive,
such
that
small-scale
processors
find
it
difficult
to
embrace
the
technology
(Badmus,
1991).
The
throughput
of
a
small-scale
processing
plant
is
expected
to
be
in
the
range
1.0-1.5
t/h
(Badmus,
1991).
Most
small-scale
processing
plants
are
unable
to
meet
this
expectation
due
to
the
level
of
technology
being
employed.
Consequently,
a
large
quantity
of
fruits
are
not
processed
promptly,
leading
to
considerable
losses
especially
in
the
peak
period
of
palm
fruit
processing.
Delay
in
processing
of
the
fruits
also
results
in
low
quality
palm
oil.
While
palm
oil
from
small
scale
pro-
cessing
plant
is
expected
to,
at
least,
meet
the
quality
criteria
for
standard
palm
oil,
this
is
hardly
always
the
case.
The
FFA
value
for
standard
palm
oil
is
fixed
at
3-5%.
and
its
moisture
content
1%,
while
special
palm
oil
should
have
FFA
below
3%
and
moisture
content
of
less
than
1%.
A
variety
of
small-scale
mechanical
processing
de-
vices
have
been
developed.
The
hand-operated
hydraulic
system,
which
uses
a
separate
digester
(horizontal
or
vertical
types),
is
very
popular
and
prevalent
(Ajibola
et
al.,
1998).
The
oil
yield
of
the
hydraulic
system
is
generally
between
70%
and
90%
depending
on
the
strength
of
the
man
who
operates
the
hand
pump
and
whose
work
efficiency
diminishes
in
the
latter
part
of
the
work-day
due
to
tiredness.
A
new
concept,
the
digester—screw
press
(DSP)
sys-
tem,
was
recently
developed
by
the
Nigerian
Institute
for
Oil
palm
Research
(NIFOR),
Benin
City
(Owolarafe,
1999).
The
system
has
the
digestion
and
pressing
units
combined
into
a
single
machine.
The
complete
system
also
comprises
of
a
steriliser
and
a
clarifier
(as
in
the
hydraulic
system).
The
development
of
this
machine
was
aimed
at
stemming
the
losses
incurred
by
small-scale
processors
during
the
peak
season
of
palm
fruit
pro-
cessing
mostly
in
April
and
September.
In
order
that
the
good
attributes
of
the
new
system
be
not
achieved
at
the
expense
of
oil
yield
and
oil
quality,
this
study
was
un-
dertaken
to
obtain
a
comparative
evaluation
of
the
emerging
technology
with
the
existing
hydraulic
system,
in
order
to
ascertain
its
appropriateness
for
small-scale
palm
fruit
processing.
2.
Material
and
methods
2.1.
Process
equipment
The
equipment
constituting
the
digester
screw
press
system,
Fig.
1,
used
in
the
study
include:
(a)
A
steriliser
which
is
a
cylindrical
drum
(cp900
mm
x
1800
mm),
divided
into
three
compart-
ments,
namely,
the
furnace,
the
water
chamber
and
the
fruit
chamber.
The
water
chamber
and
the
fruit
chamber
are
demarcated
by
an
elliptical
plate,
which
is
perforated
to
facilitate
steam
intake
into
the
fruit
chamber.
(b)
A
hand-operated
400
mm
long
palm
fruit
stripper
consisting
of
a
shaft
with
stripping
arms
enclosed
in-
side
a
cylindrical
drum,
perforated
to
allow
passage
of
fruits
into
the
discharge
chute.
(c)
A
diesel
engine-powered
DSP.
The
DSP
essen-
tially
consists
of
an
upper
part
(horizontal
digester
with
beater
arms)
and
a
lower
part
(the
screw
press
with
worm
in
a
cage)
resting
on
a
supporting
frame.
The
horizontal
digester
is
1200
mm
long
and
360
mm
in
diameter.
It
is
provided
with
a
feed
hopper
at
one
end,
and
discharges
its
content
into
the
screw
press
150
mm
away
from
the
other
end
(Fig.
2).
Thus,
with
the
arrangement
of
the
digester
and
screw
press,
16
11
15
13
14
0
4
8
0
12
1
DIGESTER
FRUIT
INLET
HOPPER
9
SCREW
THREAD
2
FRAME
10
SCREW
SHAFT
3
GEAR
BOX
11
PRESS
TOP
COVER
4
GEAR
BOX
FLANGE
12
INCLINED
BOTTOM
COVER
5
PRESS
SHAFT
FLANGE
13
DIGESTED
FRUIT
OUTLET
6
FRUIT
TRANSFER
POT
14
SCREW
PRESS
FRUIT
INLET
POT
7
CONE
ADJUSTMENT
NUT
15
NUT-
FIBRE
OUTLET
GAP
8
CONE
16
PERFORATED
CAGE
0.K
Owolarafe
et
al.
I
Journal
of
Food
Engineering
52
(2002)
249-255
251
Fig.
2.
Digester
screw
press
parts.
the
content
of
the
digester
is
subjected
to
further
crushing
and
pulverisation
in
the
press
and
this
is
ex-
pected
to
enhance
oil
recovery.
(d)
A
clarifier,
consisting
of
the
oil
separating
and
drying
units
as
well
as
an
auxiliary
water
reser-
voir
and
a
furnace.
The
oil
separating
unit
consists
of
three
concentric
cylinders
with
the
inner
one
equipped
with
a
sieve
which
receives
the
crude
oil.
The
complete
suite
of
devices
making
up
the
hydraulic
system,
as
found
in
small-scale
palm
oil
processing
centres
(Ajibola
et
al.,
1998),
also
includes
a
stripper,
a
steriliser
and
a
clarifier
as
well
as
the
following:
A
separate
diesel
engine-powered
horizontal
digester
consisting
of
a
cylinder
cage
of
cp360
mm
diameter
and
1200
mm
long,
provided
with
a
hopper
at
one
end
and
a
discharge
chute
beneath
the
other
end.
The
whole
assembly
is
mounted
on
a
supporting
frame
of
750
mm
height.
A
hand-operated
hydraulic
press.
2.2.
Comparative
oil
extraction
For
the
two
systems
used,
the
fruits
were
run
through
the
machines
once
without
recycling.
The
digestion
and
pressing
times
were
determined
based
on
this.
Digestion
is
considered
adequate
when
a
consistent
uniform
paste
has
been
obtained.
The
operating
conditions
were
en-
sured
to
be
optimal
based
on
a
preliminary
experiment.
The
batch
experiment
was
replicated
twice
for
each
system.
2.2.1.
Digester—screw
press
system
Fresh
fruit
bunches
were
quartered
(cut
into
ap-
proximately
four
pieces)
and
sterilised
for
1
h
in
the
steriliser.
The
sterilised
quarters
were
passed
into
a
stripper.
About
480
kg
of
hot
stripped
fruits,
being
the
full
capacity
of
the
two
sterilizers
used
at
the
small-
scale
unit
of
NIFOR,
were
processed
using
the
DSP.
The
digestion/pressing
times
were
noted.
The
crude
oil
collected
was
then
transferred
into
the
clarifier
and
the
time
taken
for
complete
clarification
was
also
noted.
Pure
oil
collected
was
weighed
using
a
Mettler
750
scale
with
0.02
kg
accuracy.
2.2.2.
Hydraulic
pressing
system
For
the
hydraulic
system,
480
kg
of
fruit
were
also
digested
in
a
separate
horizontal
digester
and
subse-
quently
subjected
to
oil
expression
in
the
hand-operated
hydraulic
press.
Digestion
time
and
pressing
time
was
noted
and
the
crude
oil
weighed.
The
crude
oil
collected
was
clarified
and
the
pure
oil
collected.
2.2.3.
Analysis
of
results
The
throughput
of
each
of
the
two
systems
was
cal-
culated
as
the
ratio
of
the
weight
of
fruit
processed
in
252
0.K
Owolarafe
et
al.
I
Journal
of
Food
Engineering
52
(2002)
249-255
tonnes
to
the
time
taken
in
hours.
The
pure
oil
yield
was
taken
as
the
ratio
of
the
weight
of
oil
collected
over
the
weight
of
fruit
processed.
The
extraction
efficiency
was
calculated
as
the
ratio
of
oil
yield
to
the
oil
content
of
the
palm
fruit.
2.3.
Analytical
methods
Some
physical
properties
(specific
gravity
and
colour)
and
chemical
quality
parameters
of
the
purified
oil
were
determined.
The
chemical
quality
parameters
were
FFA,
saponification
value,
peroxide
value,
iodine
value
and
moisture
content.
The
specific
gravity
was
determined
using
a
10
ml
constant
volume
specific
gravity
bottle,
while
Munsell
colour
charts
were
used
for
colour
determination.
Titration
and
titrimetric
methods
as
recommended
by
AOAC
(1998)
were
used
in
the
deter-
mination
of
FFA
and
saponification
number.
The
per-
oxide
value
and
iodine
value
were
determined
by
titration
and
Wijs
methods,
respectively,
while
the
oil
content
and
moisture
content
of
the
oil
were
determined
by
Soxhlet
extraction
method
(AOAC,
1998).
2.4.
Economic
analysis
The
economic
analysis
was
based
on
a
work
study
of
the
material
and
labour
input
against
returns
for
the
small-scale
processing
unit
of
NIFOR.
The
number
of
items
of
each
equipment
required,
labour
requirement
and
space
requirement
at
different
throughput
were
es-
timated
for
each
of
the
systems.
The
total
cost
of
equipment
including
running
cost,
maintenance
cost
and
labour
cost
were
also
calculated.
The
estimated
gross
returns
over
a
period
of
time
for
each
throughput
and
the
net
profit
were
determined.
The
yearly
processing
cost
and
revenue
were
based
on
the
peak
production
periods
(April
and
September).
NIFOR,
being
a
gov-
ernment
establishment
has
20
working
days
per
month,
giving
a
total
of
40
days
for
the
yearly
processing
period
(Table
5).
Furthermore,
the
Institute
operates
between
8.00
a.m.
and
4.00
p.m.
daily
giving
a
total
of
8
h/day,
out
of
which
2
h
are
used
for
preprocessing
operations,
thus
leaving
6
h
for
the
actual
operation.
The
calculation
of
daily
throughput
was
thus
based
on
6
h
effective
processing
time.
3.
Results
and
discussion
3.1.
Oil
yield,
processing
time
and
extraction
efficiency
Table
1
shows
the
influence
of
processing
system
on
the
processing
time,
oil
yield
and
extraction
efficiency.
The
results
indicate
that
there
was
a
significant
differ-
ence
between
the
digestion/pressing
time
and
clarifying
time
of
the
hydraulic
system
and
the
DSP
system.
Table
1
Effect
of
processing
system
on
processing
time,
oil
yield,
extraction
efficiency
and
labour
requirement
in
processing
480
kg
of
fruits
System
processing
Hydraulic
system
Digester—screw
parameter
press
system
Digestion/pressing time
lh
24
min
22
min
Clarifying
time
2
h
8
min
1
h
8
min
Oil
yield
(%)
24.4
27.6
Extraction
efficiency
(%)
78.6
89.1
Labour
requirement
10
man-hour
6
man-hour
The
use
of
the
DSP
machine
enabled
the
throughput
of
processing
to
be
increased
four
fold
when
compared
with
that
of
the
hydraulic
system.
The
clarifying
time
was
also
reduced
by
half.
Obviously,
the
thoroughness
of
the
digestion
operation
(crushing
and
pulverisation
of
the
fruit
into
a
pulp)
and
the
pressing
method
determine
how
easily
oil
is
released
during
extraction.
The
use
of
the
screw
press
in
the
DSP
enhances
the
size
reduction
operation
thus
allowing
for
better
exposure
of
the
oil
cells
and
subsequently
easing
the
flow
of
oil
from
the
cells.
The
duration
of
the
whole
processing
operations
was
also
similarly
affected.
Thus,
in
a
continuous
clari-
fying
process
as
this,
the
time
of
completion
of
the
di-
gestion
and
pressing
operations
affect
the
clarifying
time
as
clarification
of
oil
does
not
end
until
oil
from
the
expression
process
is
clarified.
Hartley
(1988,
Chap.
14)
had
observed
that
screw
pressing
as
a
unit
operation
generally
has
a
high
throughput.
The
higher
oil
yield
obtained
from
DSP
system
(Table
1)
equates
to
higher
extraction
efficiency
of
89.1%
for
the
DSP
system
com-
pared
to
78.6%
for
the
hydraulic
system.
Using
the
DSP
has
a
significant
effect
on
the
final
oil
yield
and
the
ex-
traction
efficiency
of
palm
fruit
processing.
This
may
be
attributed
to
the
fact
that
there
is
further
reduction
of
the
oil
bearing
material
notably
the
mesocarp
tissues
thereby
exposing
more
of
the
oil
cells.
The
high
pressure
developed
in
the
press
enables
more
oil
to
be
expressed
from
the
exposed
cells.
3.2.
Crude
oil
and
sludge
contents
The
analysis
of
the
contents
of
the
crude
oil
from
the
two
systems
is
presented
in
Table
2.
The
result
indicates
that
the
DSP
system
produced
crude
oil
with
higher
oil
content
(79.3%)
than
the
hydraulic
system
(67.0%).
This
result
is
in
conformity
with
the
earlier
results
on
the
extraction
efficiencies
of
the
two
systems.
The
higher
oil
Table
2
Effect
of
processing
system
on
constituents
of
crude
oil
and
sludge
Parameter
Crude
oil
Sludge
Hydraulic
DSP
Hydraulic
DSP
Oil
content
(%)
67.0
79.3
11.7
11.1
Moisture
content
(%)
22.2
11.3
78.4
79.4
Solid
impurity
(%)
10.8
9.7
9.9
9.5
0.K
Owolarafe
et
al.
I
Journal
of
Food
Engineering
52
(2002)
249-255
253
content
of
the
DSP
crude
oil
may
be
attributed
to
the
fact
that
there
was
better
digestion
and
pressurization
of
the
fruit
mash
enabling
more
oil
to
ooze
out
of
the
fruit
cells.
There
was
no
significant
difference
between
the
solid
impurities
of
the
crude
oil
from
both
systems.
Similarly,
there
was
no
significant
difference
between
the
sludge
contents
of
the
two
systems
(Table
2).
The
con-
tents
of
the
sludge
actually
depend
on
the
efficiency
of
the
clarifier.
The
clarifier
oil
losses
were
estimated
to
be
1.7%
and
1.6%
for
the
hydraulic
and
DSP
systems,
re-
spectively.
Olie
and
Tjeng
(1974)
obtained
1.10%
and
2.29%
for
hydraulic
and
unit
screw
presses,
respectively.
3.3.
Palm
oil
quality
From
Table
3,
we
note
that
the
palm
oil
from
the
two
systems
met
the
required
standard
for
ordinary
palm
oil.
There
was
no
significant
difference
between
the
quality
parameters
of
palm
oil
samples
from
the
two
systems.
That
they
are
classified
as
ordinary
palm
oil
is
due
to
their
FFA
level,
which
in
actual
fact
is
not
strictly
de-
pendent
on
the
processing
system,
but
rather
on
the
delay
in
processing
the
fresh
fruit
bunch.
Special
palm
oils
are
characterised
by
low
FFA
and
moisture
content,
and
are
increasingly
in
high
demand
in
large
scale
in-
dustries
since
the
cost
of
refining
(bleaching)
is
relatively
low
compared
with
that
of
the
ordinary
palm
oil.
Any
of
the
two
systems
would
produce
this
grade
of
palm
oil
once
the
fruits
are
promptly
processed
after
harvest.
3.4.
Economic
analysis
3.4.1.
Comparison
of
capital
cost
requirement
Fig.
1
shows
the
effect
of
throughput
on
the
fixed
cost
of
the
two
systems.
Fig.
3
presents
the
fixed
cost
of
the
DSP
system
as
a
percentage
of
that
of
the
hydraulic
system.
At
lower
throughput,
the
fixed
cost
of
the
DSP
system
was
higher,
while
the
reverse
is
the
case
at
higher
throughput.
This
is
due
to
the
high
initial
capital
cost
of
the
DSP
system
and
its
flexibility
to
accommodate
higher
throughput.
The
hydraulic
press
system
needs
more
units
of
the
machines
involved,
at
higher
costs,
to
accommodate
higher
throughput.
Table
3
Effect
of
processing
system
on
palm
oil
quality
OSP
es
%
of
Hydraulic
15
2
2.5
Throughput
(Tonnoo/Hour)
Fig.
3.
Fixed
cost
of
DSP
as
percentage
of
hydraulic
at
different
throughputs.
3.4.2.
Comparison
of
labour
requirement
The
labour
requirement
of
the
hydraulic
system
and
the
DSP
system
at
the
throughput
used
in
the
experi-
ment
were
10
and
6
man-hour,
respectively
(Table
1).
Table
4
shows
the
projected
labour
requirement
of
the
two
systems
from
0.5
up
to
2.0
t.
The
result
indicates
considerable
difference
between
the
labour
require-
ment
of
the
hydraulic
system
and
the
DSP
system.
The
difference
becomes
magnified
as
the
throughput
was
increased.
The
higher
labour
requirement
for
the
hy-
draulic
system
at
higher
throughput
is
due
to
the
need
for
additional
hands
to
handle
the
extra
units
of
the
machines
at
higher
throughput.
Thus,
the
DSP
is
more
economical
in
labour
use
and
also
has
higher
overall
efficiency.
3.4.3.
Comparison
of
processing
cost
and
revenue
of
the
hydraulic
and
DSP
systems
The
estimated
processing
(running)
cost
per
annum
for
the
DSP
system
as
a
percentage
of
the
hydraulic
system
is
shown
in
Table
4.
It
can
be
observed
that
the
DSP
system
has
lower
cost
than
the
hydraulic
sys-
tem.
As
earlier
indicated,
increase
in
throughput
for
the
120
100
80
60
40
20
Quality
parameter
Value
in
processing
system
Standard
value'
Hydraulic
DSP
Special
palm
oil
Ordinary
palm
oil
Free
fatty
acid
3.7
3.9
1.0-2.5
3.5-5.0
Moisture
content
(%)
1.2
1.5
<0.1 >0.1
Saponification
value
197
202
Iodine
value
52
54
53
±
1.5
45-56
Peroxide
value
0.5
0.7
0.57-0.60
0.57-2.61
Specific
gravity
0.9007
0.9005
0.90
0.90
Colour
Red Red
Red
Red
'Standard
values:
Swern
(1945)
and
Bek-Nelson
(1974).
254
0.K
Owolarafe
et
al.
I
Journal
of
Food
Engineering
52
(2002)
249-255
Table
4
Projected
labour
(man/h)
requirement
of
hydraulic
and
DSP
systems
up
to
2
t
of
fruit/h
Throughput
0.5
0.75
1.0
1.5
2.0
H
DSP
DSP
H
DSP
H
DSP
H
DSP
Firing
1 1 1
2 2 2 2 2 2
Quartering
and
Weighing
3 3
4 4
5
5
6 6
8 8
Loading,
unloading
of
steriliser
stripping
of
fruit
and
collection
3 3
4 4 4 4
6 6 6 6
Fruit
digestion
and
pressing
of
mash
2
1
5
6
1
7
2
8
2
Removal
of
pressed
cake
residue
1
3
4
4
4
Disposal
of
cake
1 1
3
4
2
4
2
4
2
Clarification
and
collection
of
oil
2 2 2 2 2 2 2 2 2 2
Supervision
1 1 1 1 1 1 1 1 1 1
Total
labour
required
14
12
23
24
28
17
32
21
35
23
hydraulic
system
will
entail
high
labour
requirement.
At
the
same
time,
operating
and
maintenance
costs
are
in-
creased
as
a
result
of
the
additional
machines.
The
overall
effect
manifests
in
higher
processing
cost
for
the
hydraulic
system,
in
comparison
with
the
DSP
system.
This
represents
another
comparative
advantage
for
the
DSP
over
the
hydraulic
system.
Based
on
the
extraction
efficiency
of
the
two
systems
and
prevailing
market
cost
of
palm
oil,
the
expected
monthly
revenue
from
the
DSP
system
was
found
to
exceed
that
of
the
hydraulic
system
by
14%,
while
the
processing
cost
per
tonne
(of
fruit)
for
the
DSP,
at
a
throughput
of
1.3
t/h,
was
found
to
be
92.4%
of
that
of
the
hydraulic
system.
Furthermore,
the
oil
yield
and
sales
per
tonne
were
found
to
be
113.6%
of
the
hydraulic
system.
The
expectation
is
that
the
processing
cost
of
the
DSP
system
will
decrease
at
a
faster
rate
as
the
quantity
of
fruits
processed
increases,
while
oil
yield
and
sales
from
the
DSP
would
increases
more
rapidly
than
for
the
hydraulic
system.
Consequently,
the
investment
pay-
back
period
for
the
DSP
system
is
expected
to
be
shorter
than
for
the
hydraulic
system
given
the
same
initial
capital
investment.
4.
Conclusion
The
comparative
evaluation
of
the
DSP
system
which
is
being
developed
and
the
conventional
hydraulic
press
system
undertaken
in
this
study
revealed
that
the
DSP
system
has
many
comparatively
favourable
attributes
and
is
to
be
preferred
to
the
hydraulic
system.
The
throughput
capacity,
extraction
efficiency
and
conse-
quent
economic
advantage
of
the
DSP
system
over
the
hydraulic
system
further
enhances
its
suitability
for
small-scale
palm
fruit
processors
particularly
those
op-
erating
at
throughput
of
about
1.0
t/h
of
fruits.
How-
ever,
the
quality
of
palm
oil
from
the
two
systems
compares
favourably
well.
The
adoption
of
this
new
technology
by
small-scale
processors
should
go
a
long
way
in
preventing
waste
of
harvested
fruits
usually
ex-
perienced
in
the
peak
season
of
production.
Table
5
Estimated
processing
cost/year
of
the
DSP
system
as
a
percentage
of
the
hydraulic
system
(20
working
days/month
and
at
7.8
t/day)
Item
Cost
(%
of
hydraulic
system)
1.
Fruits
100.00
2.
Diesel
167.7
3.
Wood
100.00
4.
Operating
labour
64.5
5.
Maintenance
(10%
of
fixed
capital)
76.3
6.
Supervision
(20%
of
operating
labour
cost)
64.5
7.
Miscellaneous
material
(10%
of
76.3
maintenance
cost)
8.
Sales
expense
(20%
of
items
1-7)
92.1
Total
variable
cost
92.1
A
period
of
two
full
months
was
taken
as
a
year
based
on
the
peak
seasons
of
April
and
September.
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