47
REVIEW OF LITERATURE
REVIEW OF LITERATURE
CHAPTER II
REVIEW OF LITERATURE
This chapter deals with the review of research works reported for improving the
engineering aspects of copra processing under the following headings.
° Copra Processing Practices Adopted and Constraints Experienced by Farmers
° Physical, Thermal and Mechanical Properties of Coconut
° Sorption Isotherms
o Engineering Gadgets for Splitting and De-shelling of Coconut
o Thin-Layer Drying
° Deep-Bed Dying
° Copra Drying and
o Quality Characteristics of Copra and Coconut Oil
2.1. Copra Processing Practices Adopted and Constraints Experienced by Fanners
Conversion of coconut into copra for extracting the oil has been the most prominent
aspect of the coconut industry throughout the coconut producing countries of the world
(Verghese and Thomas, 1952).
Singh (1981) stated that in all the households in Kerala fresh coconuts are consumed
as an essential dietary ingredient. The daily need varies from 1 to 3 nuts in an average sized
family of 5-6 members.
Markose (1995) reported that processing of coconut products and by products other
than for coconut oil and coir has not developed much in Kerala state and as a consequence,
the coconut based economy of the farmers in the state is inextricably linked with the price
behaviour of coconut oil which exhibits unpredictable and violent fluctuations.
Aravindakshan and Venkatachalam, (1997) of Coconut Development Board,
indicated that in most of the coconut households copra is made regularly for getting oil of
good quality for edible and toiletry uses.
Thampan (1999) reported that the household consumption of coconut oil in Kerala is
in the range oi 2 to 3 kg per month. Farmers sell coconut to the local merchants who in turn
make copra and sell the product in the terminal markets. Copra yield of nuts from rainfed
gardens varies from 12 to 15 kg per 100 nuts whereas in irrigated gardens the range is 16 to
18 kg per 100 nuts.
Anithakumari and Kalavathi (2001) indicated cent percent non-adoption and poor
knowledge among small and marginal coconut farmers about post harvest technologies. The
need for developing small-scale household oil extracting units along with other tools and
gadgets for coconut cultivators had been emphasized.
Thamban and Venugopal (2002) reported that sun drying and smoking are methods
being traditionally practiced by most of the coconut farmers. Only very few (less than 1 %)
farmers who produce copra adopt the mechanical dryers like small holder’s copra dryer.
Low market price due to inferior quality of the copra produced by traditional methods of
drying is a major constraint experienced by coconut farmers.
Muralidharan et al. (2002) reported that at farm- household level, less than 4 % of
the farmers were engaged in copra production for sale.
2.1.1. Traditional method of nut splitting
Thampan (1981) stated that in India, coconut is broken transversely into two cups
using a traditional knife having a long handle and a sharp edge and is then generally sun-
dried.
Sankat (1990) reported that whole de-husked nuts were usually split into halves
using a wooden mallet and dried with kernel in the Caribbean island.
Friend (1991) reported that splitting was performed with an axe and the copra
scooped out with knife.
Hammonds (1991) reported that each coconut was broken into half cups by a sharp
blow with a file along the less sharply curved portion of the equatorial circumference in the
Solomon Islands.
Thampan (1999) reported that splitting of coconuts by the farmers is commonly done
using knives in Kerala state.
Punchihewa et al. (2001) reported that in many countries, coconut was de-husked
and the hard but brittle shell were split open in to halves using a machete. The coconut water
was drained off leaving the cups ready for the drying stage. Some fanners also practiced nut
splitting using a heavy machete even without de-husking the coconut. After nut splitting, the
water was allowed to drain off. Splitting nuts is a labour intensive and semi skilled job and
required lot of time and energy
Arulraj et al. (2002) reported that in the traditional method of nut splitting using a
knife impact force was applied opposite to the larger eye (germinating eye) which helped to
split the nuts with less force.
2.1.2. Traditional method of de-shelling
During the drying process as the moisture content gets reduced, the meat shrinks and
is scooped out from the shell. After partial drying and when the moisture content of copra
reduced to about 25-30 %, the shell and copra were separated using a traditional wooden
knife. This was done by taking individual half cups in hand and scooping out the kernel
(Thampan, 1981).
Binder et al. (1997) reported that de-shelling was done manually at an average rate
of 400 nuts per man day in the Philippines. Coconut meat was extracted manually using a
locally made metal device similar to a small blunt knife commonly used by Samoan coconut
producers.
Even in large processing units in Kerala de-shelling was done manually employing
15-20 labourers. This is a labour intensive operation and takes several hours to separate the
shell and copra. During de-shelling, lot of moisture is adsorbed by the copra if the RH is
greater than 85 %, there by increasing the total drying time and loss of fuel (unpublished
data).
2.1.3. Traditional method of copra drying
Thampan (1981) reported that sun drying is the most popular and cheapest method of
copra making, which is traditionally adopted in all the coconut producing countries. In this
method, the split nut halves (cups) were laid out in an open yard with the kernel portion
facing the sun. After two days of drying, the kernel gets detached from the shell. The
partially dried kernel was then scooped out by a thin wooden lever. The detached kernel
cups were again arranged in the yard for further drying. The drying process has to be
continued for another 4 to 5 days to achieve the desired level of drying. The cups are
covered during night or when rains suddenly break in. Wire netting or cotton netting is
sometimes hung over the yard to prevent birds from damaging the copra. In Sri Lanka, the
drying time under the sun was about 9 days. In Thailand, the preliminary drying of copra
with shell was only for one day after which de-shelling and further drying was carried out.
In Indonesia, the kernels were usually removed immediately after cracking and dewatering.
Studies conducted by Coconut Research Institute, Ceylon (now Sri Lanka), indicated
(Anonymous, 1966) that under conditions of average temperature of 27.4 °C and 7.44 h of
sunshine, the time required for drying was 9 days to reduce the moisture from 46.9 to
6 % (w.b). It was also stated that water was more near the centre (wet surface) and least in
the testa. It was also reported that copra obtained from washed nut was superior to
unwashed nut (wet meat).
Decastro and Coranera (1978) reported that pretreatment of fresh meat with the shellash prior to air drying proved to be effective in arresting mould growth.
Patil and Nambiar (1982) found that the time required for open sun drying was 9
days on cement floor, to reduce the moisture content from 46 to 6 % w.b under the ambient
conditions of 31.9 to 33.0 °C temperature and 71-79 % relative humidity (RH) with daily
6.5 to 9.7 sunshine hours. It was also observed that substitution of drying surface of mud or
cement floor with black painted Palmyra mat reduced the drying time to 7 days.
Madhavan et al. (2003) reported that the copra obtained from open sun drying had
larger population of fungi, bacteria and lipolytic microorganisms as compared to the copra
obtained from solar dryer. The acid value, free fatty acid, peroxide value and oil content
were 1.31, 0.17, 1.57 and 67.94 %., respectively in case of WCT coconut.
2.1.4. Large scale processing of copra
Thampan (1981) reported that farmers and consumers in Kerala state are benefited
by the presence of a large number of copra processing units being operated by
entrepreneurs.
Coconut Development Board (Anonymous, 1997) conducted a baseline survey and
reported the number of copra processing units and milling copra processors in Kerala state
as given in Table 2.1 and 2.2.
The above review indicated that the available farm level copra processing
technologies, have not reached the adoption stage. This may be primarily due to the
drawbacks present in these technologies / tools. Though Coconut Development Board is
extending a large number of subsidies for promoting these technologies, the impact
observed at the farmer’s level was very low. This calls for the urgent need tor fabrication of
appropriate tools / process technologies which would be acceptable to the farming
community.
Table 2.1. Distribution of Copra Processing Units in Kerala state
Sl.No. District No. of Units1. Kollam 6342. Thiruvananthapuram 897
3. Ernakulam 566
4. Thrissur 10885. Alappuzha 1041
6. Kottayam 444
7. Malappuram 5828. Palakkad 42
9. Kannur 550
10. Kasaragod 134
11. Kozhikode NA12. Pathanamthitta NA
13. klukki NA
14. Wayanad NA
Total 5928Source: CDB, Base Hue survey report 1997. NA-not available
Table 2.2. Distribution for Milling Copra Processors in Kerala
Sl.No Category (kg/ yr.) No. of Processors Percentage
1 < 20000 186 49.34
2 20001 to 50000 54 14.32
3 50001 to 100000 35 9.28
4 100001 to 300000 26 6.90
5 300001 to 500000 27 7.16
6 > 500000 49 13.00
Total 377 100.00
Source: CDB, Base line survey report, 1997
2.2. Physical, Mechanical and Thermal Properties of Coconut
The physical, mechanical and thermal properties as reported by other research
workers are reviewed and reported in the following sections.
2.2.1. Physical properties
Coconut consists of three major components, namely the husk, shell and kernel
(endosperm). The kernel is economically, the most important part of the fruit. The cavity of
the endosperm in the unripe (immature) and matured coconut is filled with a watery fluid,
called coconut water. The physical properties such as size, shape, mass, bulk density, true
density, of coconut and other crops as reported by different authors are reported here.
2.2.1.1. Constituents of coconut
Srinivasan (1967) stated that the relative proportions of husk, kernel and shell were
27.8, 52.2, 17.0, % respectively in case of fully matured coconut.
Grimwood (1975) reported that depending on variety, a matured coconut weighs 3 to
4 kg and is composed of about 35 % husk, 12 % shell, 22 % meat and 25 % water. It was
also reported that in southern India, nuts were picked immature (at about 10 months) for
preparation of best quality coir yarn fibre. Otherwise, coconuts are harvested after they are
fully ripe (twelve to 14 months from the opening of the spathe), the actual time depending
on the variety.
Reddy et al. (1980) reported that the relative proportions of kernel, shell, husk and
water vary with the maturity of the coconut. Marked variations were observed among the
varieties and among the nuts of the same variety. It was also reported that the relative
proportions of husk, shell, kernel and water were 46.5, 22.5, 27.0, and 4 %, respectively in
case of mature and dry coconut and the relative proportions of husk, shell and kernel were
47.8, 22.8, and 29.44 %, respectively in case of East Coast Tall variety of coconut.
Thampan (1981) indicated that the maximum quantity of oil was available when the
nuts were 12 months old, the reduction in the percentage of oil in 11, 10 and 9 months old
nut being 5, 15 and 33 %, respectively.
Banzon and Velasco (1982) reported that on an average fresh coconut meat contains
50 % water, 34 % oil, 7.3 % carbohydrates, 3.5 % protein, 3.0 % fibre and 2.2 % ash.
Jayasuria and Perira (1985) stated that the relative proportions of shell and kernel
were 22.5 and 35.6 %, respectively in case of dry coconut of dwarf palm.
Bosco (1997) reported that the relative proportions of water, shell and kernel were 6,
16, and 23 %, respectively in case of fully matured East Coast Tall variety of coconut.
2.2.1.2. Size and shape
The shape of the coconut can be defined either by a number on the standard chart or
by descriptive terms prepared for fruits and vegetables. Mohsenin (1970) listed out several
methods to determine sphericity and roundness and pointed out that the method adopted by
Curray (1951) was the least objectionable. Henderson and Pabis (1962) used the geometric
average of three principal dimensions for the diameter of an equivalent sphere. Later, Pabis
(1967) defined effective diameter in terms of 1000 grain weight and kernel density.
Based on the studies of Griffith and Smith (1964) for evaluating the volume of a set
of irregularly shaped pebbles, several researchers (Mohsenin, 1970; Nelson, 1980; Fortes
and Qkos, 1980; Shepherd and Bhardwaj, 1986a; Dutta et al., 1988; Kaleeniullah, 1992 and
Aviara et al, 1999) described the size of the cereal grains and oil seeds by measuring their
three principal dimensions.
Mohsenin (1970) pointed out that size and shape are inseparable in any physical
object, and both are generally necessary if the object is to be satisfactorily described. Many
seeds, grains, fruits and vegetables are irregular in shape and a complete specification of
their form theoretically requires an infinite number of measurements in perpendicular axes.
Shepherd and Bhardwaj (1986a) defined the shape of pigeon pea as prolate spheroid
and calculated sphericity and roundness as 82.2 and 81.8 % respectively.
Dutta et al. (1988) reported the shape of gram as a prolate spheroid and the mean
values of sphericity and roundness were 73.5 and 69.7 % respectively.
Gupta & Kachru (1992) reported that the average value of sphericity for turmeric
rhizomes was 0.43, which showed that the shape of the rhizome is not symmetrical to the
shape of sphere.
Kaleemullah (1992) considered the shape of groundnut kernels as oval and the
values of roundness and sphericity were calculated as 64.75 and 63.66 %, respectively.
Deshpande et al. (1993) reported that the length of soybean grain increased from
6.32 to 6.75 mm, the width from 5.23 to 5.55 mm, the thickness from 3.99 to 4.45 mm, the
geometric mean diameter from 5.09 to 5.51 mm, the sphericity from 0.806 to 0.816 in the
moisture range 8.7 to 25 % d.b., respectively.
Rai and Kumar (1995) calculated the sphericity of Kabuli chana, with and without
root and were found to be in the range of 72-80 and 78-90 %, respectively. The respective
roundness values of chana were in the range of 67-83 and 79-80 %.
Ratnambal et al. (1995) reported the shape of coconut as oval, oblong and round
depending on the variety. The shape of West Coast Tall variety of coconut was indicated as
oval.
Binder and Fletcher (1998) reported that coconut has a unique physical appearance,
but generally is ovate in shape.
The three axial dimensions of guna seeds increased with moisture content and the
seeds were described as being oval in shape (Aviara et al, 1999).
Chandrasekar and Viswanathan (1999) regarded the shape of the coffee berry as
ellipsoid and reported that the length, width and thickness of the coffee parchment did not
vary with the moisture content.
2.2.1.3. Mass
Senthil (1986) reported the mass of 2500 East Coast Tall variety of de-husked
coconut as 750 kg.
Bosco (1997) reported the mass of 260 large East Coast Tall variety of whole nut as
401.18 kg.
2.2.1.4. Bulk density
Senthil (1986) reported that the bulk density of was 500 kg / m3 in case of de-husked
and split East Coast Tall variety of coconut.
Bosco (1997) reported that the bulk density of small, medium and large coconut as
396.34, 405.79, 401.18 kg / mJ, respectively in case of East Coast Tall variety of coconut
where as in the case of partially de-husked coconut of small, medium and large size the bulk
density were 360.36, 354.49, 336.61 kg / m3, respectively.
Obe^ta (2000) reported that the bulk density of palm kernel was in the range of 450
to 325 kg / m.3 The bulk density initially decreased sharply between 5.2 and 15.30 % d.b.
from 450 to 375 kg / m3, respectively and gradually from 1 5.3 and 25.4 % d.b. from 375
to 325 kg / m3. The major reason for these results is that the volume of the palm kernel
material increases with an increase in moisture content, behaving appropriately like most
hygroscopic biological material.
2.2.1.5. True density
Dutta et al. (1988) reported that the true density decreased from 1311 to
1257 kg / m3 for gram as the moisture content increased from 9.64 to 31 % d.b.
Shepherd and Bhardwaj (1986a) indicated that the true density decreased from 1305
to 1251 kg / m3 for pigeon pea as the moisture content increased from 6.3 to 28.2 % d.b.
Kaleemullah (1992) reported that the true density decreased from 1021.95 to
1013.32 kg / m3 for groundnut kernels as the moisture content increased from
7 to 30.22 % d.b.
17
Deshpande et al. (1993) indicated that the true density decreased from 1216 to
1124 kg / m3 for soybean as the moisture content increased from 8.7 to 25 % d.b.
Rai and Kumar (1995) indicated that the true density decreased from 1319 to
1253 kg / m3 tor kabuli chana as the moisture content increased from 9.05 to 30 % d.b.
Suthar and Das (1996) indicated that the true density decreased from 1148 to
1004 kg / m3 for karingda seed as the moisture content increased from 5 to 40 % d.b.
Aviara et al. (1999) indicated that the true density decreased from 870 to 680 kg / m3
for guna seeds as the moisture content increased from 4.7 to 39.3 % d.b.
2.2.1.6. Porosity
Shepherd and Bhardwaj (1986a) reported that the porosity decreased from 40.4 to
38.2 % for pigeon pea as the moisture content increased from 6.3 to 28.2 % d.b.
Deshpande el al. (1993) indicated that the porosity decreased from 40 to 37 % for
soybean as the moisture content increased from 8.7 to 25 % d.b.
Suthar and Das (1996) reported that the porosity decreased from 57.6 to 40.7 % for
karingda seed as the moisture content increased from 5 to 40 % d.b.
The porosity increased from 38 to 41 % for guna seeds (Aviara et al., 1999) as the
moisture content increased from 4.7 to 39.3 % d.b.
2.2.1.7. Angle of repose
Dutta et al. (1988) indicated that the angle of repose increased from 25.5 to 30.4 for
gram as the moisture content increased from 8.62 to 17.6 % d.b.
Joshi el al. (1993) reported that the angle of repose increased from 30 to 52 and
from 34 to 42 degree for pumpkin seed and kernel as the moisture content increased from
4 to 27and 7 to 30.22 % d.b., respectively.
Kaleemullah (1992) reported that the angle of repose increased from 19.79 to 25.13
for groundnut kernels as the moisture content increased from 9.05 to 30 % d.b.
Rai & Kumar (1995) stated that the angle of repose increased from 19.54 to 29.10
for kabuli chana as the moisture content increased from 7.6 to 21 % d.b.
Visvanathan (1996a) indicated that the angle of repose increased from 30.8 to 37.7
for neem nut as the moisture content increased from 5 to 22 % d.b.
Aviara et al. (1999) reported that the angle of repose increased from 28.07 to 43.58
lor guna seeds as the moisture content increased from 7 to 30.6 % d.b.
2.2.1.8. Coefficient of friction
Joshi et al. (1993) reported that the static coefficient of friction varied from 0.41 to
0.76 for pumpkin seed and from 0.34 to 0.65 for pumpkin kernel over mild steel and
galvanized iron surfaces with the increase of moisture from 4 to 27 % d.b.
Carman (1996) stated that the static and dynamic coefficients of friction of lentil
seeds against galvanized sheet metal, plywood and rubber surfaces increased with moisture
content in the range from 6.5 to 32.6 % d.b.
Visvanathan et al. (1996a) stated that the coefficient of friction of neem nut
increased linearly with moisture content for surfaces such as plywood, mild steel,
galvanized iron and glass. The maximum friction was offered by plywood, followed by mild
steel, galvanized iron and glass surface. It was reported that the increased friction coefficient
at higher moisture content might be owing to the water present in the nut offering a cohesive
force on the surface of contact.
Suthar and Das (1996) reported that the static coefficient of friction of karingda
seeds varied from 0.34 to 0.91 with plywood, 0.29 to 0.80 with mild steel and 0.23 to 0.67
with galvanized iron in the moisture range of 5 to 40 % d.b. The corresponding values for
karingda kernels were 0.33 to 0.79, 0.27 to 0.69 and 0.22 to 0.67.
Aviara et al. (1999) reported that the coefficient of friction of guna seeds increased
linearly with increase in moisture content and varied from 0.41 to 0.98 according to the
Chandrasekhar and Viswanathan (1999) observed that the friction coefficient of
coffee parchment increased linearly with increase in moisture content on various test
surfaces, namely, aluminum, galvanized iron, mild steel and stainless steel. The maximum
friction was offered by the mild steel surface, followed by galvanized iron, aluminum and
stainless steel surfaces.
2.2.2. Thermal properties
The thermal properties such as thermal conductivity, thermal diffusivity and specific
heat of many materials as reported by other scientists are reviewed.
2.2.2.1. Thermal conductivity
Sreenarayanan and Chattopadhyay (1986b) observed that the thermal conductivity of
rice bran increased from 0.086 to 0.158 W/m °C when the moisture content, temperature and
bulk density of bran increased from 7 % w.b., 42 °C and 410 kg / m3 to 15 % w.b.,
68 C and 490 kg / m,3 respectively.
The thermal conductivity of soybean increased with the increase of moisture content
(Sreenarayanan et al, 1988).
The thermal conductivity of the raisins increased from 0.126 to 0.392 W/m. K. as the
moisture content increased from 14 to 80 % w.b. (Vagenas and Marinos-Kouris, 1990).
Alagusundaram et al (1991) reported that the thermal conductivities ranged from
0.169 to 0.232 W/m. K for barley, from 0.187 to 0.249 W/m. K for lentils and from 0.187 to
0.257 W/m. K for peas in the moisture range of 9 to 23 % w.b. and temperature range of
28 to 29 °C. The thermal conductivity of all the three types of seeds increased as the
moisture content or temperature increased.
Obetta (2000) reported that the thermal conductivity of palm kernel increased from
0.078 to 0.152 W/m. K as the moisture content increased from 5.2 to 25.4 % d.b.
Verma and Suresh Prasad (2000) reported that the thermal conductivity of maize
varied from 0.125 to 0.193 W/m. °C in the temperature range of 30 to 60 °C and moisture
range of 10 to 50 % d.b. The thermal conductivity of maize increased with the increase in
moisture content and temperature. They have reported that the increase in thermal
conductivity of maize with the increase in moisture content may be attributed to the fact that
the thermal conductivity of water (0.628 W/m. °C at 38 °C) was higher than that of air
(0.027 W/in. °C) in pores resulting in increase in thermal conductivity of maize.
2.2.2.2. Thermal diffusivity
Srecnarayanan and Chattopadhyay (1986b) used a transient heat flow method to
determine the thermal diffusivity of rice bran. It was observed that the thermal conductivity
of rice bran increased from 9.34 x 10'8 to 12.1 x 10‘8 m2 / s when the moisture content,
temperature and bulk density of rice bran increased from 7 % w.b., 42 °C and 410 kg / m3 to
15 % w.b., 68 C and 490 kg / in,3 respectively.
Kazarian and Hall (1965) and Shepherd and Bhardwaj (1986b) calculated the
thermal diffusivity from the measured values of specific heat, bulk thermal conductivity and
bulk density. The calculated values of diffusivity obtained were 6.1 to 11.6 % greater than
the measured values of wheat at various moisture levels, using transient heat flow method.
Obetta (2000) reported that the thermal diffusivity of palm kernel was determined by
using the experimental setup of Dickerson (1965) and reported that the values increased
from 13.92 to 18.33 m2/s as the moisture content increased from 5 .2 to 25 .4 % d.b.
Verma and Suresh Prasad (2000) reported that the thermal diffusivity of maize
varied from 8.33 x 10 '8 to 10.17 x 10 _s m2 /s in the temperature range of 30 to 60 °C and
moisture range of 10 to 50 % d.b. The thermal diffusivity increased with increase in
temperature from 30 to 60 °C, while it increased with moisture content up to 40 % d.b. and
thereafter decreased with further increase in moisture content. This phenomenon was
explained by the fact that that the bulk density of maize decreased to a minimum value at 40
% d.b. moisture content and subsequently increased at high moisture level.
2.2.23. Specific heat
Sreenarayanan and Chattopadhyay (1986a) determined the specific heat of rice bran
by the classical method of mixtures. The specific heat of rice bran had a linear positive
correlation with moisture content and temperature. The specific heat values increased from
0.4048 to 0.4976 cal /g °C when the moisture content was increased from 6.388 to 18.036 %
w.b. for the lowest mean temperature of 38.2 C while the values increased from 0.4140 to
0.5290 cal /g C when the moisture content increased from 4.265 to 18.036 % w.b. for the
highest mean temperature of 63.1 °C.
Obetta (2000) reported that the specific heat of palm kernel was determined by the
method of mixtures and reported that the value increased from 2.508 x 10'3 to
3.177 x 10-3 J / Kg °C as the moisture content increased from 5.2 to 25.4 % d.b.
Verma and Suresh Prasad (2000) determined the specific heat of maize by the
method of mixtures and reported that the value varied from 2.002 to 2.996 Id/ kg °C in the
temperature range of 30 to 60 C and moisture range of 10 to 50 % d.b.
Deshpande and Bal (1999) determined the specific heat of soybean by the method of
mixtures and reported that it exhibited a linear positive correlation with moisture content in
the moisture range from 8.1 to 25 % d.b. for the mean temperature of 315 K. The specific
heat values increased from 1.926 to 2.912 kJ/ kg K in the given moisture range.
2.2.3. Mechanical properties
The mechanical properties such as surface area and calorific value of shell and husk
as reported for coconut and other crops were reviewed and are given in the following sub
sections.
2.2.3.1. Surface area
Baten and Marshall (1943) reported that the surface and volume measurements are
needed in calculating respiration, water loss and absorption. The surface area predicted was
also required for maturity and heat treatment studies. For the calculation of surface area for
the unpicked fruits the transverse diameter gave the best surface area predictions as in the
case of apples and for the picked fruits the surface area -weight relationship gave the best
results.
The method of measuring the surface area by peeling the apples, tracing the peeling
and planimetering the traced area is the most commonly followed method for apples (Baten
and Marshall, 1943; Frechette, et al. 1966).
The linear regression equations proposed by many authors to relate the weight
surface area were similar (Mohenin, 1980).
Narayanan, et al. (1992) reported that the surface area of East Coast Tall variety of
coconut was determined by tracing the cut slices of copra on a graph sheet.
Bosco (1997) determined the surface area of East Coast Tall variety of coconut by
tracing the cut slices of kernel on a tracing sheet and the area was calculated using leaf area
measurement meter.
2.2.3.2. Calorific value
Senthil (1985) reported the calorific value of shell as 5246 kcal / kg in case of East
Coast Tall variety of coconut.
Annamali et al. (1989) reported that the calorific value of coconut shell can be
assumed to be about 3500 kcal / kg.
2.3. Engineering Gadgets for Splitting and De-shelling of Coconut
The engineering gadgets for splitting and de-shelling coconut as reported by other
research workers are reviewed.
2.3.1. Nut splitting
Prats (1954) developed a machine to crack the shell and dry the meat to produce high
grade copra which was almost equal to desiccated coconut in quality. The machine consisted
of a drum with ribs of angle iron, which were chained with iron balls. As the drum rotated
the coconuts tumbled against each other and against the iron ball, getting cracked in the
process.
Liang (1977) and Sarig (1979) reported that mechanical crackers built on the
principle of compression were better than those relying on impact for shell cracking. It was
reported that careful observation lead to the conclusion that the lack of consistent control of
nut deformation as well as the lack of controlling the moisture of nuts at cracking was
probably responsible for the shortcomings. The magnitude of deformation was controlled by
the difference between the size of the nut and the clearance between two compression or
cracking edges.
Liang (1980) reported that compressing nuts in a shell to a constant deformation was
one of the most widely used means to crack macadamia nuts for kernel extraction. The
deformation was induced by forcing nuts through a openings which were smaller than the
nuts. The size difference between the nut and the opening was set to be equal to the desired
deformation. It was also reported that a tapered clearance was found to be capable of
compressing nuts of all sizes to the same deformation provided the orientation of the nuts
were controlled.
2.3.1.1. Effect of knife angle on splitting forces
Fellar (1959) investigated the effect of shear angle of standing stalks without counter
edge and reported that energy was minimum at 60° shear angle for alfalfa. The shear
strength of alfalfa stem internodes varied from 40 to 1800 N / cm2 (Halyk, et al, 1968). A
pendulum impact tester was also used to apply impact loading to potato tubers and corn
kernels (Parke, 1963; Jindal, etal, 1978).
Pickett el al. (1969) observed that both stalk stiffness and resistance to penetration
were affected by the diameter of the stalk.
Prasad and Gupta (1975) studied the mechanical properties of maize stalk as related
to harvesting in a pendulum impact shear test apparatus and reported optimal valve of 23
ofor the bevel angle, 55 for shear angle and 2.65 m / s cutting velocity.
McRandall and McNulty (1980) concluded that the grass stem resistance to
penetration was dependent on dry matter content and knife edge angle.
Pearson (1987) reported that an increase in oblique angle usually reduces the cutting
forces especially for wide cutting devices.
Dogherty and Gale (1991) indicated that there was no significant effect of blade rake
angle and thickness of blunt blades when cutting above the critical speed of 30m/s.
Surayanto (1993) reported that low speed cutting tests on spikelets and bunch stalk
of oil palm were conducted to investigate the effects of design factor blade, mode of
operation and material properties on specific force and energy requirement per unit cut area.
It was reported that knife edge angle, oblique angle and shear angle, significantly affected
the cutting force and energy requirement for both the stalk and spikelet. The cutting force
was optimum at 15 to 25 knife edge angle. Increasing oblique angle from 0 to 40 reduced
the cutting force to 47 % for spikelet and 45 % for stalk.
Viswanathan el al. (1996) conducted experiments to determine the optimum values
of cutting velocity, knife bevel angle and shear angle on the energy required to cut cassava
tubers. The results suggested that the specific cutting energy of the tuber were minimum for
cutting velocities in the range of 2- 2.5 m / s, shear angles of 60-75 and bevel angles of 30
to 45 .
2.3.2. Mechanical de-shelling
Rey (1955) reported a knife shaped shallow spoon which moved back and forth upon
the rotation of a cam and in the process the coconut meat was scooped in fragments.
Mix (1957) designed a shelling machine for removing the shell from the fresh
coconut meat. It was reported that a knife was inserted in an eye of the coconut and the
coconut was then engaged by an endless toothed chain, the teeth of which engaged the shell
of the coconut to move the coconut on the knife and to break the shell from the meat which
was guided by an operator.
Blandis and Glaser (1973) used water under pressure to separate the coconut meat
from the shell. It was indicated that the half nut was filled with a rubber plug and a nozzle
which passed vertically through the rubber plug which was attached to a power line. The
used water was removed through a water outlet with its half pipe end piece which was
radically introduced between the rubber plug over lap and the rim of the half nut and moved
along the rim concentrically to the nozzle. In this process the shell and meat were separated.
Stickiness of the solid particles is the most important factor among those physical
properties, which affect movement in the drum. The friction between the particles, which is
a function of stickiness, determines the ease of rolling of particles over each other. Kelly and
O’ Donnell (1977) and Hodgson and Keast (1984) have used a parameter called static
coefficient of friction, which represents the effect of friction between the particles. This
correlation predicts the pattern of solids movement and computes the angle at which forces
acting on the particles at the edge of the flight cancel out each other. This angle is called
kinetic angle of repose which is given below.
Ohler (1979) reported that shelling was the greatest problem in case of cashew nuts.
This was because the nut was irregular in shape; the kernel was thus susceptible to breakage
which fetches low price in the market.
Kelly (1992) and Wang el al.(1995) reported that equation (2.1) has been validated
to show the point of cascade, at which the forces acting on the particles were in equilibrium
up to a value of v= 0.4, which is equivalent to a 2 in diameter drum rotating at about 20
RPM.
2.4. Sorption Isotherms
The concept of EMC-ERH, the temperature effect on EMC-ERH, sorption hysteresis
for various food products as reported by various researchers are reviewed and presented in
the following sections.
2.4.1. Concept of EMC-ERH
All food materials display characteristic vapour pressure at constant moisture content
and temperature. The moisture in food always tends to approach the equilibrium with the
temperature and vapour pressure of its surrounding gaseous atmosphere. If the conditions of
the surrounding atmosphere are not changed for a sufficiently long time, then the
equilibrium is established, at which the vapour pressure and temperature of the food
material and its surrounding are the same. At equilibrium, no further change in the moisture
content of the food occurs (Shatadal and Jayas, 1992).
The interactions between moisture in the surroundings and the foods are usually
studied in the systems in equilibrium. A sorption isotherm that represents these interactions
macroscopically (Multon, 1988) is a graph between EMC (as ordinate) and ERH or aw(as
abscissa) for any given temperature. A typical sorption isotherm for food materials is
sigmoidal in shape and is divided into three successive parts. The first part of the isotherm
which falls approximately between axis and represents the “monolayer’ where the water
molecules are strongly bound to the polar groups (primary adsorption sites) by high energy
hydrogen bonds. These water molecules possess a specific and rigid orientation; therefore,
their mobility and chances of taking part in any biochemical reactions are practically zero
(Multon, 1988). The water molecules in the monolayer may be considered to form an
integral part of the solid phase, which displays none of these functional properties of pure
water (Trollerand Christain, 1978).
The second part of the isotherm which fells approximately between aw = 0.2 and
aw = 0.65, corresponds to the binding of several layers of water molecules super-imposed on
the previous layers, to which they are attached by hydrogen bonds of decreasing energies.
The water molecules in these layers have decreasing binding energies and their mobility
remains limited. Their chances of taking part in biochemical reactions can only be very
restricted (Multon, 1988). The third part of the isotherm, for aw greater than 0.65 and almost
asymptotic to aw of 1.0, represents the water retained by capillarity, solution formation and
osmosis. The binding energy and degree of mobility of water in the third part is almost equal
to that of the pure water and, therefore, the water in this zone can participate in biochemical
reactions (Van den Berg and Bruin, 1981).
2.4.2. Effect of temperature on EMC and ERH
The EMC values decrease with increase in temperature at a constant water activity
for most food materials. Isotherm curves therefore are lowered with increasing temperature.
Increased motion of water molecules with increasing temperature may be the reason for
such influence (Kapsalis, 1987). Exception to this rule is that all the food materials which
contain large amount of low molecular weight constituent in a mixture of high molecular
weight biopolymers. At low humidity, water sorption is due mainly to biopolymer and
increase of temperature has the usual effect of lowering the EMC values. In the intermediate
relative humidity range, sugar and other low molecular weight constituents begin to absorb
the moisture. Higher temperature in this range favors endothermic dissolution, which uses
relatively large amount of water and results in crossing over the higher temperature isotherm
above the lower temperature isotherm. A good example of this phenomenon is shown for
sultana raisins (Saravacos et al., 1986).
The values of equilibrium moisture content decrease with the increase of temperature
in the case of dehydrated mushroom (Pandey& Aich, 1989).
Ayranci et al. (1990) reported that the equilibrium moisture content of dried apricot,
fig and raisins decreases with increasing temperature in the water activity region below 0.6
and the opposite result was observed above 0.6 water activity. The unusual temperature
effect on isotherms above water activity 0.6 was a result of dissolution of sugar in this water
activity range and thus the food products at higher temperatures were holding more water.
Nathenel (1962) argued that moisture content of 7 % in copra could be permissible
in view of the possibility of moisture absorption under humid condition and that 8 %
moisture in copra regarded as critical moulding moisture percentage.
Patil et al. (1982) found that the average value of equilibrium moisture content as 3.8
%. This value increases beyond 4 % mainly due to the effect of relative humidity of ambient
air, i.e. when the relative humidity was above 80 % between June and October in Kerala.
Head et al. (1999) indicated that copra with moisture contents ranging from 5.5
to 32 % were placed in a commercial store and examined. The storage period was 2, 4
and 8 weeks. It was found that all the samples reached a moisture level of 5-7 %
within 2 weeks of storage indicating the range of equilibrium moisture content of copra
as 5-7 %.
Guarte et al. (19,96) reported that the equilibrium moisture content of copra was 4 -5
% in case of coconut dried in thin layer under Philippine conditions.
The equilibrium moisture content of copra as reported by Bustrillos and Banzon
(1949) and Rajashekaran, et al. (1961) is presented in Table 2.3.
Table 2.3. Equilibrium moisture content values of copra at 30 °C
Relative Humidity (RH) %
Equilibrium moisture content, % w.b.
Bustrillos and Banzon (1949) Rajashekaran et al. (1961)
30 °C 40 °C At room temperature
Adsorption Desorption
10 0.32 0.11
20 0.80 0.23 1.0 1.2
25 0.80 0.32 - -
30 0.96 0.45 1.9 2.2
35 1.10 0.60 - -
40 1.30 0.78 2.5 3.0
45 1.55 0.99 - -
50 1.80 1.21 3.0 3.5
55 2.05 1.48 - -
60 2.31 1.80 4.0 4.4
65 2.61 2.09 - -
70 2.95 2.41 4.8 5.5
75 -3.32 2.74 - -
80 3.73 3.13 bo 7.0
85 4.30 3.60 - -
90 - 4.76 9.0 9.5
95 - 6.0 - -
100 - - 11.0 11.4
2.4.3. Sorption hysteresis
The term used to denote the difference between desorption and adsorption EMC
values for the same ERH is called “hysteresis effect”. Sorption isotherms for a food over the
full range of water activity from a hysteresis loop in which desorption curve lies above the
adsorption curve (Shatadal and Jayas, 1992). Rajashekaran et al.( 1961) found that hysteresis
exists between the isotherms for coconut and was greatest at low temperature and low
relative humidity. The hysteresis reduces with the increase of temperature and relative
humidity.
2.5. Thin-Layer Drying
Considerable number of crops is artificially dried in heated air or non-ambient
drying systems (Karathanons & Belessiotis, 1977; Pathak et al, 1991; Hansen et al, 1993).
Simulation models for the drying process have been used to design new or improve existing
systems for the drying process.
In thin layer drying experiment Curate et al. (1996) reported the effect of drying air
temperature up to 100 C. Position of halved de-husked coconuts with respect to the
direction of the air stream were investigated at constant air velocity of 0.5 m / s and tropical
dew point temperature of 25 C, so that good quality copra and coconut oil were produced at
the shortest drying time. Drying characteristics of copra was shown by the moisture
reduction against the drying time which is significantly affected by the drying temperature,
being fastest at highest temperature, but not by the position at which the halves were placed
on the dryer. Drying time to reach the desired moisture content of 7 % wet basis is
significantly reduced when the temperature of the air stream was increased from 40 to 70 °C
o obut not between 90 and 100 C. At the drying temperature to 90 C both copra and coconut
oil quality were unaffected. Drying air temperature and position of the halves in relation to
the sensory properties of copra showed that the highest temperature of 100 C caused
excessive browning, but the taste and smell were not affected. Neither drying temperature
nor nut position affected significantly the various quality characteristics of coconut oil and
this was also in the case of fatty acid composition. The results suggested that temperature of
o 490 C is optimum for the production of high quality copra and coconut oil in the shortest
drying time.
2.5.1. Models
The drying kinetics of the materials may be described completely using material and
drying medium properties viz., thermal conductivity, thermal diffusivity, moisture
diffusivity, interface heat and mass transfer coefficients (Sokhansaj, 1984; Vagenas &
Karathanos, 1993). However, sometimes the drying constant ‘K’ is used instead of transport
properties of foods. A commonly used simple model, assuming that the resistance for water
transport is all over the surface of the particle, is represented by an equation analogous to
Newton’s law of cooling (Parry, 1985) which is given below
The thin-layer drying equation (2.2) describes the drying phenomena in a unified
way, regardless of the controlling mechanism. The thin layer equation has been used for
estimation and prediction of drying time for several products and for generalization of
drying curves. The drying constant is a suitable quantity for the purpose of process design,
optimization and in cases where a large number of iterative model calculations are needed.
This is due to the fact that the drying constant embodies all the transport properties into a
simple exponential function. There are four prevailing transport phenomena during drying
(internal heat transfer, internal mass transfer, external heat transfer and external mass
transfer) which may describe the drying process analytically (Jayas et al, 1991). These four
classical partial differential equations demand considerable computing time for their
a\ T) O Li
2.5.2. Comparison of drying models
Wang and Singh (1978) applied four models viz., single exponential equation, and
diffusion equation for a sphere, Page equation and Thompson’s quadratic equation for single
layer drying of rough rice. It was observed that the diffusion model gave the worst fit and
Thompson’s model was found to be the best. Sharma et al. (1982) developed a two-term
exponential model for rough rice and explained rough rice drying more logically by
considering the drying equation to be a two-compartment model.
Bala (1983) fitted three equations viz., single exponential equation, Page equation
and two-term exponential equation to the experimental data of malt. The two-term
exponential equation always gave the best fit. The Page equation was better than the single
exponential equation. Diamante et al. (1991) fitted several mathematical models to the
drying rates of sweet potato slices and found that the modified Page equation gave the best
fit. Pathak et al. (1991) conducted experiments on drying of rapeseed and concluded that the
thin-layer drying model for rapeseed for temperatures 50-200 °C can be described by Page’s
equation.
Karthanos and Belessiotis (1999) applied a thin-layer model, such as the Page
equation for air drying data of high sugar-containing agricultural plant products, such as
currants, sultanas, fig and plums and found a good relationship. The Page equation was
successful for the modeling of drying fresh fruits; it failed to predict the drying behavior
when the drying was continued for moisture contents below 15 % d.b., which is the water
content usually required to attain shelf stability of dried fruits. This was attributed to the
Page equation being accurate only in cases where weight reduction is manly due to water
evaporation. Deviation from the Page equation occurs where there is a further reduction in
weight due to the decomposition of sugars at relatively high temperatures and moisture
contents lower than that typical of dried fruits. Thus the pages equation may be applied
exclusively for relatively wet products or in those cases in which the only weight reduction
mechanism is that of water evaporation.
Lopez et al (2000) observed that the simple exponential model and Page model have
good prediction capability for the thin-layer drying behavior of vegetable wastes (mixture of
lettuce and cauliflower leaves). The Page equation fitted adequately the drying behavior of
green chilies (Ahmed & Shivhare, 2001).
Sogi et al. (2003) reported that Page equation adequately described the drying
behavior from 50 to 90 0 C at loading density of 4.12 kg / m2 in case of tomato seeds.
The drying models being used to describe the convective drying of grains have not
been applied to copra drying.
2.6. Deep-Bed Drying
McEwen and O’Callaghan (1954) proposed that the deep bed drying of grain can be
represented by a number of thin layers in series and they developed a semi-graphical method
of solution. Van Andersal (1955) developed a partial differential equation model and solved
the partial differential equation by predictor-corrector method. Boyce (1965 and 1966)
developed a digital computer model for deep bed drying of barley and validated the model
with experimental results.
Henderson and Henderson (1968) presented and compared the experimental results
by using an analytical procedure for computing moisture contents of small grains as it was
dried in deep-bed, by the use of a digital computer and thin layer drying theory. The effects
of several parameters on the drying process were also investigated.
Verma et al. (1974) conducted experiments on paddy drying in batch dryer for
predicting the performance of farm batch dryers working with beds of 30 cm depth. They
also observed the heat and moisture variation during drying with air flow rates between 4.5
to 6.5 leg / h / kg of bone dry grain at drying air temperature of 40 and 45°C.
2.7. Copra Drying
The essential requirement of copra drying is to bring down the moisture content of
the wet meat from 50-55 % to 5-6 % w.b. (Purseglov, 1977; Thampan, 1981; Kochhar,
1986; Sankat, et al, 1991; Persely, 1992; Lozada, et al, 1995; Guarte, et al, 1996;
Singh, et al, 1999; Madhavan, et al, 2003).
2.7.1. Methods of drying
The different drying methods as reported by other research workers are reviewed.
2.7.1.1. Solar drying
A simple solar dryer constructed from bamboo sticks, plastic sheet and small nails
was found suitable for drying copra, salted fish, cassava and sliced potatoes. Advantages
were utilization of solar energy under hygienic circumstances and low cost. However, this
has limited applicability during cloudy weather, low drying temperature and limited life of
plastic sheet (Anonymous, 1978).
Boulder (1980) reported a small copra dryer of 200 nuts capacity. The dryer was
built with wood and bamboo racks. Plastic was used for covering the roof. Copra could be
dried in 15 h.
Lambert (1980) used the tip-up solar dryer for drying green copra and half nuts. In
one experiment kernels from 600 nuts were dried to obtain 112 kg of copra in 3 days. In
another experiment 75 kg of copra was obtained with a mean moisture content of 7.5 % after
27.7 h of sunshine. Kernels pre-dried in the half nut gave copra of the best quality.
Patil (1984a) developed two types of solar dryers for drying copra. The low cost
polyethylene solar dryer was made of 10 mm MS bar frame and 200 gauge double layered
transparent plastic cover with perforations for air circulation to reduce the drying time from
nine to six days with increased capacity of 50 % compared to open sun drying. The second
dryer was of cabinet type made from jack wood frame. The drying surface was 22 gauge
corrugated GI sheet painted in black, with coir fibre insulation below it. The drying chamber
was made of 3 mm window glass on sides and 3 mm acrylic plastic on the top. The
reflectors of 24 gauge aluminum foil were provided from 3 sides and castor wheels
facilitated the movement of the dryer for constant sun tracking and transportation to short
distances. The drying time was reduced to four days in the dryer compared to nine days on
concrete floor even at double the spreading density compared to open sun drying.
Sepratomo (1990) studied the performance of a partially solar heated drying system
for copra made from a corrugated iron sheet which was simulated on a computer. An
expression for the drying rate of shallow half coconuts was determined. It was reported that
the techno economic performance of the dryer was good under Indonesian conditions.
Supratomo (1990) used a solar hot air generator for three coconut drying trials. The
solar hot air generator basically comprised of a large cylindrical shed, oriented East-West.
The drying cabinet consisted of two drying chambers and had two heat sources. The first
chamber was 1 m long and 0.8 m wide which would dry approximately 166 kg of fresh meat
with moisture content of 43 % laid out on 10 racks of 16.6 kg each which produced 100 kg
of copra.
Maiava (1992) reported that drying of copra in a short time was a great problem in
the Islands of South Pacific. With a view to determine the appropriate types of dryer to dry
copra experiments were conducted in Western Samoa in 1989, for the simulation of solar
drying of copra using simple passive direct solar dryer. In the first experiment the operating
characteristics namely, air flow rate, temperature, humidity, driving pressure and heat loss
of the dryer with lm2 drying area to dry 16 kg of extracted coconut meat was investigated.
Good quality copra was obtained as characterized by it’s off white colour appearance and
the final moisture content.
Frederic (1950) reported a simple Ceylon copra kiln having a fire pit, copra grill and
a corrugated iron roof with a covered verandah. The total capacity was 10,000 nuts / year.
The drying process required 5 days with 8 to 10 firings of double row of shells.
Varghese and Thomas (1952) modified and adopted the Malaysian Kiln to suit
Indian conditions. The drying chamber was made of red earth with bamboo grills, which
was loaded with coconut halves. These were placed one over the other on the drying
platform. A perforated iron sheet hung 0.5 m below the grill platform served as heat
spreader. The drying time was 34 h with a temperature of 53-60 C.
The early smoke dryers were known as tapahan dryers in Philippines. They were
further improved to obtain quality copra (Anonymous, 1962). The improved versions were
known as De vapor and taphan.
Rajashekaran et al. (1961) reported that the use of indirect dryers has been promoted
in India for making good quality copra. Different models of low cost indirect dryers have
been designed and developed by the Central Plantation Crops Research Institute (CPCRI),
Kasaragod to suit small and medium sized coconut holdings. These dryers have indirect
heating and natural air convection arrangements.
Menon (1968) described the Pearson patented copra kiln having iron structure for
drying chamber of 6 m x 6 m x 30 m with 24 m x 2.1 m verandah. There were four
compartments with a central chimney with heating from two furnaces located at either side
of the chamber. The total capacity per year was 20,000 nuts. A series of control doors were
fitted on the sides and at the bottom of the chamber to control the temperature of the hot gas,
Saiz (1971) compared kiln dryer and oil fired forced hot air dryer and reported that
the time required in the latter type dryer was 24 h compared to 3 - 4 days in the former, with
ease of operation and less labour requirement. Experience in Papua New Guinea showed
that these factors resulted in production of better quality copra at lower cost than was
possible with the kiln dryer.
Mansoor (1973) reported a hot air dryer constructed out of cheap materials. I he
drying time required was 64 h, and total capacity was about 7 tonnes of copra.
The dryer designed by Ibarra Guz of Philippines was reported to be suitable for big
plantations (Anonymous, 1973) and the fuel used was coconut shell, charcoal or diesel oil.
The copra produced with charcoal was reported to be cleaner and whiter, whereas diesel oil
fuel deteriorated the quality of copra. The copra dryer has three sections viz., drying
chamber, blower and valve. The capacity of drying chamber was 1000 nuts and the nuts
were dried at 82 °C for 12 to 16 h.
Cooke (See Grimwood, 1975) in Malaysia designed copra kiln suitable for 10 acre
plantation having capacity of 100 nuts / batch and fuel required was 100 shells. The net
firing time was reported to be 15 h and total time required for drying was 24 h. The kiln for
30 acre plantation had a capacity of 200 nuts / batch with fuel requirement of 500 shells.
The net firing time was 23 h and overall time required was 30 h.
Thampan (1981) reported that in Kerala, kiln drying is combined with sun drying, as
it is believed that smoke drying cures the copra and makes it less susceptible to mold
infection. In the more common kilns in Kerala, brick or mud walls are provided with tiled or
thatched roof. In the Philippines, smoke dryer are called ‘Tapahan’ with a capacity of 400-
4000 nuts per batch. Some of them are very large requiring more area. Split coconuts to be
dried were kept on the grill, with the kernel portion of the bottom layer facing upwards. The
upper layers are arranged downwards up to a thickness of 30 cm. Coconut shells of even
size are interlocked and laid loosely in single or double rows in the fire pit. In order to avoid
excessive emission of smoke, some shells are ignited outside the kiln and brought in when
they are burning. The fire moves slowly burning the arranged shells without smoke. Six to
eight firings are required with a drying time of three to four days. The drying temperature
should not exceed 70 C in the initial hours and subsequent drying should be completed at
60 °C.
Timminis (1994) reported that the kiln used to dry coconut kernels vary from
country to country in the South East Asian region.
Lozada, et al. (1995) developed a copra dryer popularly known as Los Banos multi
crop dryer. It mainly consisted of a unique burner which produced heat for 4-5 h without
tending. Ventilation doors were provided for proper utilization of heat produced.
Rodrigo (1996) found that the Standard Ceylon copra kiln designed in the 1960 was
still considered to be the most economical for drying coconut in Sri Lanka.
Sudaria and Piedraverde (1996) surveyed the fanners direct type ‘tapahan’ copra
dryer to monitor the configuration of copra dryer, depth of heat source, fuel consumption
and time of drying. The copra dryers with square, parabolic and rectangular pit
configuration were constructed and evaluated at varying depths of heat source of 2.3 m, 1.8
m, and 1.3 m. Square dryer at 1.8 in depth performed significantly better compared to other
types of dryers.
Rachmat et al. (1999) developed a pit dryer for copra production in Indonesia. The
main components were made of bamboo, coconut leaves and concrete blocks. It was
recommended that a door or regulator was needed at the entrance of the chimneys exhaust of
the combustion chamber to reduce heat loss. The average drying rate was 1.2 % w.b. / h and
cumulative energy consumption was 20.3 kcal / h from the combustion of coconut husks and
shells. The cumulative drying efficiency was 10.72 % and the dried copra had oil
content of 62.1 % and free fatty acid content of 1.4 %.
Singh et al. (1999) developed a simple smoke free collapsible kiln type copra dryer
at CPCRI Kasaragod to suit medium size plantation crop holdings & processing units. The
dryer mainly consisted of a burner, a galvanized iron heat exchanger and mild steel angles
as supporting frame with ventilation holes. The dryer capacity was 1000 to 1500 coconuts
per batch and the total drying time was 24 h. The dryer costs about Rsl2, 000 and the area
required to house was 8 nr. The cost of drying was Rs 2.18 / kg and the thermal efficiency
of the dryer was 30.7 %.
2.7.1.3. Drying by indirect heat
The chula (means “firing place” in Malayalam) copra dryer developed by M/s
Tyneside Foundry & Engineering Co. Ltd England (Anonymous, 1959) mainly consisted of
an air heater with furnace and power driven fan and a drying chamber. The damper and ash
trap were provided in the chimney. The maximum capacity of drying chamber was 1000
nuts and after shell removal the capacity could be doubled. Drying time required was 17 h.
On an average 28633 half shell, 2425 kg of firewood or 2830 number petioles were required
as fuel to produce 1 tonne of copra. The temperature control was found to be difficult to
maintain when husks were used as fuel.
Millan and Barrau (1959) described the tunnel type (Atahiti Kiln) French hot air
dryer, which accommodated end to end three trolleys on iron rails. Each trolley could hold
seven rows of trays. The main hot air flue was a slopping pipe under the concrete slab
supporting the trolleys. At the end of the kiln remote from the furnace the flue rose to floor
level, where it was connected to two parallel return pipes running on either side of rails.
These pipes opened into a chimney on either side of the kiln at the furnace end. The kiln
capacity was 1200 nuts / day. Each firing required about 300-400 husks. The dryer was
reported to produce 60 - 70 tonnes of copra during 250 days of operation. Total tray area
was 104 m2 with each tray carrying 64.6 kg kernel.
Le Gall and Dercle (1960) reported Kumkum type Tagabe hot air dryer with a
capacity of 300-400 kg of fresh kernel and drying time as 48 h.
The tongo hot air dryer (Anonymous, 1968) used old oil drums for furnace. The
drying chamber had three compartments with nine racks each. The drying capacity was
1600 nuts / batch. The pre-drying with shells was done in open sun for two days and kernel
without shell was dried in the dryer for another 2 days. Thereafter open drying was adopted
for 2-3 days.
Tumaneng et al. (1970) described the construction and testing of ordinary hot air
copra dryer. In this dryer, fuel was burnt in the drum heat exchanger. The hot air produced
rises by convection into an indirectly heated drying chamber above the furnace. The drying
efficiency of the indirectly heated drying chamber was evaluated using three different piling
schemes of the half nuts. Coconut husk or shell was utilized as fuel.
Gritnwood (1975) reported that in indirect heat dryers any type of fuel can be used,
even the dry agricultural waste materials available in the plantations. Generally coconut
husk, shells, fronds, dry firewood etc., were used as fuel. Copra was smoke free with
minimum deterioration during storage. But the higher consumption of fuel and higher cost
of installation were its limitations. The product of combustion does not come into contact
with the coconut kernel and as such high quality, smoke-free white copra could be
produced.
De Castro (1976) reported the performance of 1000 nuts capacity Kukum hot air
dryer developed in the Philippines. He suggested the drying process in two or three stages
for moisture to travel to the surface. This increased the efficiency of drying and minimised
the scorching. The use of knotted jute sacks covering on copra was found suitable for
efficient drying and heat distribution on drying platform. This dryer was further improved to
accommodate a maximum of 2000 nuts.
Masuhit and Domingo (1978) developed the integrated charcoal kiln and indirect
dryer which could be operated simultaneously utilizing the heat generated from the kiln. The
dryer had drying tray which facilitated even drying of coconut meat or any other material
and the kiln was having a metal lid which facilitated the loading of material to be charcoal
and also serves as heat exchanger of the dryer.
Cruz (1978) also developed an integrated system of copra drying and charcoal
manufacture. In this process, product gases from the charcoaling process supplied part of
heat required in drying process. Recirculation of an average 60 % of hot exhaust and from
copra dryer resulted in considerable fuel saving. Fifty per cent of the shell from a lot to be
dried was required to supply the heat for drying.
Fanguin (1979) reported a copra oven designed by the IRHO and installed at Grand
Devin Plantation in Ivory Coast. It was natural draught hot air dryer fueled by coconut
husks. It was operated by two persons and the output was 1100 kg copra /day of operation.
The total drying capacity was reported to be 1.5 million nuts / year in 300 working days.
Thampan (1981) indicated that indirect heat dryers essentially consist of a kiln and a
heating unit. The Kiln may be of a single compartment or several compartments, either
vertical or laterally placed adjacent to a closed combustion system designed to obtain the
maximum heat. The combustion unit consisted of a furnace or fire box usually made of fire
bricks from which flue pipes of suitable dimensions passed beneath the kiln along its full
length and were connected with a dampened chimney at the end opposite to the furnace. The
flue pipes were inclined vertically up to 5° for easy flow of flue gases. In some dryers,
thermal efficiency is improved by providing additional furnace with damper controls.
Sufficient inlets for the entry of cold air were provided on the walls and below the flue
pipes. The gases of burning fuel in the furnace passed through the flue pipes and heated the
air around them. The hot air so produced moved into the kiln where the split nuts or copra
were arranged. As the hot air passed through the layers of split nuts or copra, it removed the
moisture and the moisture laden air was exhausted by dampened ventilators at the top of the
kiln. For efficient drying, the drying bed thickness was not allowed to exceed 30-40 cm.
Here the copra does not come into contact with smoke, but only with the uncontaminated
hot air because the flue pipes were connected to a chimney for the smoke to escape to
outside.
Patil and Singh (1984 b) reported a tray type (mixed flow) mechanical dryer for
obtaining good quality copra even drying rainy season. Electrical energy was used for
heating. The dryer was fabricated from wood, GI sheet, asbestos rope etc. It has drying
chamber, air distribution unit, plenum chamber, heating unit and blower. Drying chamber
accommodated air distribution unit in the centre with copra trays on its both sides. The air
was blown by 1.5 hp motor operated blower on 8 kW heaters. The hot air was then
circulated through the material on the trays. The capacity of the dryer was 1000 nuts / batch
and drying time required at 60 °C drying air temperature was 30 h.
The small holder’s dryer has a capacity to dry 400 coconuts (Patil, 1983). The fuel
requirement was about 128 kg of coconut husk and shell mixed. The fuel is fed every 15
minutes to keep the fire burning. The drying air temperature is maintained at 60 °C by
adjusting the valves in the chimney. After 10-15 h of drying, the partially dried copra could
be scooped out from the shell. The copra cups are to be raked every two hours for uniform
drying. The total drying period is over four days with overnight breaks till the moisture
content of copra is reduced to 6 %. The actual drying time is about 36-40 h.
Lozada (1987) developed a utility model of UPLB copra dryer comprising of a
drying chamber and plenum chamber, which was having vertical walls and inclined side
walls provided with ventilation parts which had doors and a heat diffuser metal plate for
equal heat distribution in the drying chamber and the drying chamber was provided with
slotted flooring and side walls.
Sreenarayanan el al. (1989) conducted experiments, in a laboratory model thin layer
dryer set up for the determination of drying characteristics of copra at air flow rates of
19.57, 38.37 and 82.88 m3 / h and drying air temperatures of 50, 65, 80 and 95 C. For
reducing the moisture content of copra from 50 % (w.b) to 7 % (w.b), it took only 20 h of
drying in the mechanical dryer at an optimum temperature of 65o C as compared to 7-10
days for sun drying.
Annamalai et al. (1989) developed a simple and cheaper dryer working on indirect
heating and natural convection principles using dry agricultural waste as fuel to suit medium
size plantation crop holdings and processing units. It consisted of a drying chamber, burning
cum heat exchanging unit, chimney and can hold 1000-1200 coconuts or 375 kg of ripe
arecanut per batch. The drying time required was about 33-37 h for copra and about 87 h for
arecanut. The thermal efficiency of the dryer was 1.8.7 to 28.4 %.
2.7.2. Heat tolerance of the copra
Samson (1971) studied the heat tolerance of coconut meat. The properties evaluated
were browning of colour, lysine destruction and protein solubility at pH of 2.8 and 10.5 and
nitrogen solubility index. It was reported that meat could tolerate up to 100 °C air
temperature and up to 105 C air temperature for 60 minutes without significant loss of
protein solubility. However, marked loss of lysine availability occurs if heated at 120 °C.
2.7.3. Effect of temperature on drying
The studies on mechanical drying of cup copra by Rajashekaran et al. (1961) in
cabinet through draught dryer indicated that at constant parameter of 1.37 m / s air velocity,
26 kg / m2 loading density and 15 % RH (i) the drying rate was markedly affected by
temperature of drying air; (ii) the charring of coconuts at higher temperature occur only at
the end of drying and (iii) the drying rate tends to minimize below 20 % moisture content
(d.b).
Cunade (1977) discussed the procedures for copra pre-processing, processing and
post processing in Philippines and found that temperature up to 105 °C could be used for
drying without charring during early stage of drying when moisture content was high. At
120 cubic ft/min. air flow rate, the drying times at 37.8, 60, 75 and 90 °C were 38, 20, 9 and
8 h, respectively.
Mantilla (1982) reported that the drying platform elevation for drying coconut
should be 1.24 m, which can dry copra in 16 h. Meat size had no significant effect on drying
rate.
Patil (1983) reported that in the small holder’s dryer, the drying temperature was
maintained at 70 to 80 °C and the drying time required to dry copra is 40 h.
The I.R.H.O hot air copra dryer maintains a temperature of 65-75 C around the grid
and its whole length was hotter than the above temperature and copra becomes brown and
looses value (Anonymous, 1984).
Supratomo (1990) reported that higher the temperature, shorter the drying time in
case of coconut. This phenomenon was due to the fact that the temperature of the drying air
had great influence on the drying rate especially for high moisture content kernels. Too high
an air temperature can damage copra. During the trials it was reported that the copra slightly
dulled at 60 C, remain more or less white with slight yellowing at 70 C and distinctly
reddish at 80 C.
Suresh and Shah (1992) reported that the type of dryer determines two
characteristics of coconut, viz., browning due to thermal damage and growth of bacteria.
Oozing of oil takes place if drying is not controlled and it makes the product unacceptable.
Drew et al. (1993) reported that the waste heat recovery system was capable of
delivering 3.38 m3/s of hot air for the commercial scale deep-bed dryer at 75 °C, the
temperature found to be most suitable for drying copra.
Sudaria and Piedraverde (1996) reported drying time of 8.67 h and drying rate of
31.08 kg / h in case of coconut dried in improved farmer’s direct type ‘taphan’ dryer in the
Philippines.
Rachmat el al (1999) reported the average drying rate of 1.2 % w.b / h and
cumulative energy consumption of 20.3 kcal / h from the combustion of coconut husks and
shells. The cumulative drying efficiency was 10.72 %.
2.7.4. Effect of air velocity on drying of copra
Rajashekaran et al. (1961) reported that in coconut moisture diffuses only slowly in
the surface. It was felt that variations in velocity of drying air may not influence much the
drying rate. A series of experiments were conducted at constant air temperature of 65 °C but
with varying air velocities (0.92 to 1.37 m / s). The drying curves for lowest and highest
velocities tried showed no significant difference in the drying rate.
2.7.5. Effect of relative humidity on drying
Sill (1965) studied the relationship between rate of drying and relative humidity of
drying air. When the copra was dried at 71 °C the case-hardening occurred. It was reported
that water was removed faster from surface layer than the diffusion of water from centre to
surface. Two methods were suggested to prevent it viz., i) low temperature slow drying, and
ii) drying at higher temperature with high humidity of drying air. Since the enthalpy of the
air at high relative humidity is more it increases the rate of drying. At 65.5 °C temperature of
drying air, the optimum relative humidity was found to be 35 % for first 12 h and 20 % in
the final stage.
Del Rosrio (1966) studied the optimum conditions for drying the small pieces of
coconut kernel of 3.5 cm, 0.2 cm x 1 cm. The upper temperature limit for drying was found
as 70 °C, air velocity 16.1 km / h and relative humidity of 60 %.
Ramaswamy et al. (1970) studied the drying characteristics of cup copra and
suggested phased drying at two different temperatures for quicker drying and to get good
quality copra.
Supratoino (1990) reported that the recycling rate has a considerable effect on the
relative humidity of the drying air, especially for high copra moisture contents. During first
hour of drying, higher the re-cycling rate, higher would be the difference between R.H of
the air in the first and the last rack. Warm, dry air can considerably damage copra by
causing its surface to harden. In order to obtain copra of excellent quality Sills (1956)
recommended using air of a relative humidity of around 35 % for the first 12 h, and then
reducing it to 20 % to complete the drying.
2.7.6. Effect of loading density on drying
Rajshekharan et al. (1961) reported that the minimum loading density in a through
draught dryer was 26 kg/m2 and at lower capacities the drying was not efficient due to
channeling (non mixing) of hot air whereas there was no significant change when the
capacity was doubled.
In Saraoutou when drying was done with fresh shelled meat from un-husked split
nuts (green copra) with grid load of about 110 kg / m 2 the time for drying depends on the
thickness of the layer of copra, density, free circulation of hot air and the fuel used
(Anonymous, 1984).
2.8. Quality Characteristics of Copra and Coconut Oil
The quality characteristics of copra and coconut oil as reported by other research
workers were given in the following sections
2.8.1. Copra quality
The quality of milling copra ultimately determines the quality of the oil and the
residual cake. Good quality copra would yield oil with free fatty acid content (FFA) of less
than one percent even without refining. The quality of copra is influenced by (i) moisture
content (ii) colour and cleanliness (iii) microbial load (iv) rubberiness (v) case hardening
and (vi) charring.
Moisture is the most important factor influencing the quality of copra. Copra with a
moisture content of less than six percent is considered good quality as it is not easily
damaged by insects, moulds or microorganisms. At CPCRl, Kasaragod, an electronic
moisture meter to determine the moisture content of copra was developed
(Madhavan, 1985), based on the electrical conductivity of the kernel. It is very handy and
has an accuracy of ± 0.5 % w.b.
Henderson (1952) found infected copra even at 4 % moisture content under humid
conditions (above 85 % relative humidity).
Southern (1957) reported that rubberiness of copra was determined by the variety
from which the product comes and the stage of maturity of the nut. Nuts from dwarf variety
and also unripe nuts yield rubbery copra. This copra undergoes rapid deterioration and good
copra also becomes susceptible to microbial infestation when mixed with rubbery copra.
Nuts from sulphur deficient palms also yield rubbery copra.
Grimwood (1975) indicated that if the initial drying temperature is too high, case
hardening can occur, preventing the moisture from the interior of the meat from diffusing
rapidly to the outside layers. Case hardened copra develops a hard smooth surface covering
a wet core. While drying copra, the initial temperature should not exceed 70o C in the early
stages and 60o C subsequently, to avoid charring and turbid oil with a burnt odour.
Patil (1984 a) reported case hardening of copra when nuts were dried in kilns and hot
air dryer with no control of temperature.
2.8.2. Bio-deterioration of copra
Defective methods of processing and high moisture content of the copra are the
major factors responsible for copra deterioration. Bacterial action starts first during the
initial stages of processing followed by subsequent mold infection and insect attack. A gap
of more than four hours between the splitting and commencement of drying facilitates the
activities of bacteria, on the wet surface of the kernel followed by mould attack (Ward and
Cooke, 1932).
Fungi also cause deterioration of copra followed by bacteria. P.Frequentam was
found to cause spoilage even at a moisture content of four percent (Nair and
Sreemulanathan, 1970).
Rao el al. (1971) observed the presence of Botrvo diplodia theobromae during the
blackening of coconut kernel.
Paul (1969) and Susamma et al. (1981) isolated a number of fungi and bacteria from
deteriorated copra. The fungi isolated were R. Stolonifer, R.Orysae, Mucor hiemlis, P.
Citrimim, Cvlvularia Senegalensis, cochliobolus hiatus, Paecilomyces Lilanicus,
Aspergillus orysae and Aspergillus fumigatus, Bacteria causing spoilage were identified as
B. Subtilis, E, aerogenes, Pseudomonas fluorescence and Sarlina lutea.
2.8.3. Storage of copra
Use of proper packing materials is also an important factor. Plastic lined gunny bags
can be used for safe storage of copra even during the rainy season (Krishna Marar &
Padmanabhan, 1960).
Mathen et al. (1968) found that multi walled paper bags could keep copra free from
insects up to three months and those treated with pyrethrum could keep up to 9 months.
Lever (1969) reported that sulphuring copra is effective for storing for longer periods
during rainy season. Fumigation of copra with methyl bromide at the rate of 3 kg / 100 m3
for 48 hours with a gas proof sheet is recommended when copra is stored in godowns.
Fumigation with sulphur dioxide as a fog is also resorted to for better shelf life. Fumigation
with a mixture of carbon dioxide and ethylene oxide (99:1) was effective against insects and
pests of copra. .
Patil (1982) found copra in storage was affected by excessive mould growth when
the relative humidity was greater than 85 % at room temperature or greater than 95 % at
40o C. In many milling establishments in India, it is a usual practice to dry under sun for one
of two days before bagging and storing. Painting of upper surface of the roof of the storage
structure with white reflective paint has been reported to reduce temperature fluctuations
within 10o C, thus preventing serious condensation effect. The walls also should be shaded
from direct sunlight and should be provided with sufficient number of adjustable ventilators.
Studies at CPCRI, Kasaragod (Anonymous, 1988) have indicated that copra stored in
tin containers and polythene bags and fumigated with biogas, neem leaf gas, carbon dioxide
and sulphur dioxide are effective in controlling microbial infestation during storage.
Lozada, et al. (1995) reported that the study of Madamba indicated that at 8 % and
below levels of moisture content moulds will not grow on copra and reported EMC of copra
as 5 % under Philippine conditions.
2.8.4. Coconut Oil
Coconut oil is classified under the lauric acid group of plant oils, and over 90 % of
its fatty acid is saturated. It has the lowest percentage of unsaturated fatty acids (oleic,
linoleic ) with reported values from 3.7 % (Banzon and Velasco, 1982) to 8.3 %
(Levitt, 1967).
Coconut oil, because of its high content of saturated fatty acids is highly resistant to
oxidative rancidity and retains pleasing flavour when refined. It is also easily digestible and
stable in flavour (Mittal and Pendharkar, 1992).
Prasad and Azeemoddin (1995) considered coconut oil as one of the richest sources
of vegetable oil in the world and also important oil in the group of lauric oils.
Guarte et al. (1996) reported the mean values of oil content in the range of
60 to 65 % on dry basis. Drying air temperature up to 100 C has no significant influence on
quality of coconut oil. Coconut oil extracted from copra dried up to 100 C remains stable in
terms of oil content, acid number, saponification number, ansidine value and free fatty acid
content. The fatty acids are the most important constituents of oil and the balance between
various acid components determines the properties and uses of coconut oil. Coconut oil is
valued by soap manufacturers due to its high lauric acid content.
Rachmat (1999) found oil content of 62.1 % and free fatty acid content of 1.4 % in
case of copra dried in a pit dryer in Indonesia.
Naresh (2000) reported that the free fatty acid, acid value, peroxide value and
saponification value as 0.104, 0.381, 0.625, and 252 respectively in case of West Coast Tall
variety of coconut.
Thampan (2003) observed that compared with other fats and oils coconut oil has the
highest content of glycerol content. Coconut oil is more or less constant in composition
irrespective of the country of origin. It has the highest saponification value (251-263) and
the lowest iodine value (8 to 9.6). On account of its low iodine value it is classed as a non
