the effect of variety and drying on the engineering … varieties of cassava demands that the effect...

International Journal of Scientific Knowledge (Computing and Information Technology)

Volume 1 Issue 5, Jan 2013 @2012 IJSK & K.J.A. All rights reserved

www.ijsk.org 1493 -ISSN 2305

13

The Effect of Variety and Drying On the Engineering

Properties of Fermented Ground Cassava

Chukwuneke J.L1, Achebe C.H

1, Okolie P.C

1 and Okafor E.A

2

1Department of Mechanical Engineering, Nnamdi Azikiwe University, Nigeria

2School of Engineering, Energy Centre, Robert Gordon University, Aberdeen.

E-mail: [email protected]

Abstract: - Many food processing industries use cassava as their basic raw material. The existence of improved varieties of cassava demands that the effect of variety on the engineering properties of fermented ground cassava is studied in order to generate data for design and optimum performance of various driers used in cassava processing. This paper attempts to provide data on the engineering properties such as moisture content, specific heat capacity, thermal conductivity, thermal diffusivity, bulk density and mass transfer coefficient of some cultivars as well as determine the effect of variety on the drying and engineering properties of fermented ground cassava using two high yield improved cultivars (Tms 30572 and NR 8082) and one native cultivar (Akpu Bonny). General models were also obtained, which could be used to predict the specific heat and thermal conductivity at different moisture content of the cultivars. The specific heat capacity obtained ranged from 1.53kJ/kgK to 3.49kJ/kgK while the thermal conductivity ranged from 0.24W/m

oC to 0.51W/m

oC for dried fermented ground cassava of these

cultivars. The highest moisture content of 48.66% was obtained after fermentation with the NR 8082 variety. At same moisture content, the specific heat capacity and thermal conductivity were found to be different for cultivars with different proximate compositions but similar for cultivars with close proximate compositions. The drying rate, thermal diffusivity and mass transfer coefficient of each cultivar varied with proximate composition, hygroscopy, product surface area and change in density after drying. Therefore, the same drying conditions cannot be used for drying the different cultivars except if they have close engineering properties.

Keywords: Fermented ground cassava, Specific heat capacity, Thermal conductivity, Thermal diffusivity, Bulk density, Effect of variety, Cultivars, Drying rate, Moisture Content, proximate compositions

.

1. INTRODUCTION

1.1. Background of the Study

The design, simulation, optimization, operation and control of food processing operations require basic engineering properties of foods. Food technologists require data on engineering properties for various purposes such as process design, quality assessment and evaluation.

Cassava is widely grown in Africa and some other parts of the world. It plays a central role in

the diets of most Nigerians. This is why [1] attributed garri (a staple food from drying of fermented cassava flour) as the commonest food amongst the teeming poor West African people. Fermented cassava flour is, therefore, one of the most important cassava products and the methods of processing depend on local customs. However, the industrial utilization of cassava

mailto:[email protected]




14

roots into various food and non-food uses is expanding by the day.

Consequently, food industries need to understand the various engineering or physical properties of ground cassava in other to use it as a raw material. These properties include;

1. Specific heat capacity which is the amount of heat (kJ) needed to raise the temperature of a unit mass (kg) by a unit change in temperature (k).

2. The thermal conductivity which is the amount of energy, in form of heat, "conducted through a body of unit area and unit thickness in unit time when the difference in temperature between the faces causing heat flow is unit temperature difference [2].

3. The thermal diffusivity determines how fast heat propagates or diffuses through a material.

Knowledge of these engineering properties is necessary not only because they are important on their own but they are the commonest indicators of other properties and qualities [3].

These engineering properties are known to be affected by density, moisture content and temperature, and will help the engineer to generate data for the design and operation of driers for cassava processing. Drying is an inevitable unit operation in the food processing industries since dehydrated foods normally last longer due to the absence of microbial activities.

1.2. Aims and Objectives

A review of pertinent literature revealed that such data on engineering properties of foods were lacking [3]; Hence the objectives of this study are to determine; at various stages of drying fermented ground cassava into garri:

1. The moisture content. 2. The specific heat capacity. 3. The thermal conductivity. 4. The thermal diffusivity. 5. The bulk density. 6. The mass transfer coefficient. 7. To determine the effect of variety on the drying and engineering properties of fermented ground cassava.

1.3. Significance of Study

The significance of this work cannot be overlooked just as cassava itself. Knowledge of the engineering properties is necessary because they are the commonest indicators of other food qualities. Engineering properties are important data needed for quality assessment and evaluation, design, operation and control of driers since drying is an inevitable stage in the production of both food and agricultural products.

Also, knowledge of these properties and how they are affected by drying will enhance better understanding of the heat and mass transfer phenomena for drying of agricultural products.

Moreover, it provides an opportunity to apply the principles of chemical and Mechanical engineering processes which will help as a guide to any interested industrialist for future applications taking into account the engineering properties of ground cassava.

1.4. Scope / Limitations

The study attempts to determine the various engineering properties such as moisture content, specific heat, thermal conductivity and thermal

diffusivity of fermented ground cassava only. The study is also restricted to three varieties of




15

cassava; two improved ones (Tms 30572 and NR 8082) and a native variety.

2. METHODOLOGY

2.1. Sample Preparation

This involved the preparation of cassava cultivars, one native cultivar (Akpu Bonny) and two high yield improved cultivars (Tms 30572 and NR 8082) all obtained from Root-crops Development Centre, Agricultural Research Institute, Igbariam, Anambra State, Nigeia. The cultivars were harvested in the month of April,

peeled, washed, grated and packed in three different sacks for pressing. They were then allowed to ferment for 72 hours. The mashed cassava was sieved with a mesh of 2.4mm. A hot circulating air oven was used to determine the drying rates.

2.2. Determination of Engineering Properties

I. Moisture Content

The moisture content was determined before and

after fermentation. Three different crucibles

were washed and dried in an air oven for about

40mins; 5.0g of each sample were carefully

weighed into each of the identified crucibles, it

is then dried in a hot circulating air oven at

105°C for 24hours. Further drying was

continued until the final weight of dry sample

became stable. The initial moisture content of

the samples was calculated as the total moisture

loss divided by total sample weight and

presented in percent wet basis.

100

1

21

W

WWX w

(1)

Where: Xw = moisture content on wet basis, W1 = Initial weight of sample, W2 = Final weight of sample

after drying.

The moisture content on dry basis was calculated using; 1

100

1

21

W

WWX d

(2)

The moisture content on dry basis at any time‘t’ was also calculated using the equation;

Xt (dry basis) =

(3)

Where; W2 is the weight of dried sample and W1 is the weight of sample at any time t.

II. Proximate Compositions

The Association of Analytical chemists method was used to determine the carbohydrate, moisture, protein, fat, fibre and ash (dry ashing

method in a furnace) content of the samples [4]. The proximate composition was determined after drying in an air oven at 70

oC for 24hours.

III. Ash Content Determination




16

A silica dish was heated at 600oC, cooled and

weighed; 2.0g of the sample was carefully

transferred into the dish. The dish was then

placed into a muffle furnace and ashed at 600oC

for 3hours. It was then allowed to cool in

desiccators before taking the final weight. This

procedure was carried out for all the sample

And the ash content was calculated thus; % Ash =

(4)

IV. Fat Content Determination

Soxhlet extractor was used. An extraction flask was thoroughly washed and dried in hot oven for 30mins. It was placed in desiccators to cool. 2.0g· of sample was accurately weighed and transferred into a rolled ashless filter paper and placed inside the extractor thimble. The thimble was placed into soxhlet extractor. Some reasonable quantities of petroleum ether, about three-quarter of the volume of the flask were

placed inside the extraction flask. The heater was switched on and the set up was heated for about 3hours at low temperature so that the temperature of the petroleum ether is not exceeded. Finally, the extracted oil was dried at 100

0C and weighed. The difference in the

weight of empty flask and the flask with oil gives the oil content of the sample.

The percentage fat is thus calculated as; % fat =

(5)

Where; A = weight of empty flask, B = weight of sample, C = weight of flask + oil after drying.

V. Moisture Content Determination

The sample for proximate analysis was heated at 60

0C in a hot air circulating oven for 24hours.

The moisture content was determined using 2.0g of sample on dry weight basis.

VI. Fibre Content Determination

2.0g of the sample was collected (W1) 150ml of

preheated H2S04 was added and heated to

boiling for 30mins. It was then filtered and the

residue washed three times with hot water.

150ml preheated KOH was added and heated to

boiling for another 30mins, filtered, washed

three time with hot water and three times with

acetone before drying at 1300C for 1 hour. The

sample was then weighed (W2), ashed at 500oC

and the ash also weighed (W3).

The fibre content is thus calculated; % Fibre 1001

32

W

WW (6)

Where; W1 = weight of sample, W2 = weight after pretreatment & drying, W3 = weight after ashing.

VII. Protein Content Determination

2.0g of the dried sample was weighed into 30ml kjeldahl flask and 15m1 cone H2S04 and 1.0g of the catalyst mixture (kjeldahl catalyst mixture, a mixture of 20g potassium sulphate, 1.0gram copper sulphate and a pinch of selenium

powder) were added. The mixture was heated until a greenish clear solution appeared (about 30mins), it was heated for more 30mins before allowing to cool. 10ml of distilled water was added to avoid caking. The digested solution




17

was then transferred to the kjeldahl distillation apparatus. A 50ml receiver flask containing 5ml boric acid indicator solution was placed under the condenser of the distillation apparatus so that the tip is about 2cm inside the solution. The digested sample in the apparatus had10ml of 40% NaOH solution added through the funnel stop-cork.

The distillation commenced immediately the

steam by-pass was closed and the inlet stop-cork

on the steam jet arm of the distillation apparatus

opened. When distillation reached about 35ml

mark on the receiver flask, the process was

stopped by closing the inlet stop-cork first, then

opening steam by-pass. The resulting solution

was then titrated to the first pink colour with

0.1M HCl.

Calculation follows thus; 1001000

1005025.61.001.14.

sampleofweight

HClVol (7)

Where; 14.01 = Nitrogen standard, 50 = dilution factor, 6.25 = crude protein factor, 0.1 = concentration of HCI, 1000 - ppm, 100ml distilled.

VIII. Carbohydrate Content Determination

The carbohydrate content was determined by difference (i.e. by subtracting other constituents in each sample from 100%).

IX. Determination of Specific Heat Capacity

Specific heat capacities were determined using the various proximate compositions of the samples. These were obtained by applying [5].

Cp = 4.180xw + 1.711xp + 1.929xf + 1.547xc + 0.908xa (8)

Where; Cp is the specific heat capacity in kJ/kgK and x are respective mass fractions of water, protein, fat, carbohydrate and ash, present in each cultivar.

X. Determination of Thermal Conductivity

The thermal conductivities of the samples were obtained by substituting the various proximate composition of the sample in the expression developed by [6].

K = 0.24Xc + 0.155Xp + 0.16Xf + 0.135Xa + 0.54Xw (9)

Where; K is the thermal conductivity W/m0C of sample.

XI. Determination of Thermal Diffusivity

Thermal diffusivity ( ) determines how fast heat propagates or diffuses through a material. This was determined from the thermal conductivity K, density ρ, and specific heat Capacity Cp using the dimensionally correct equation developed by [7].

= k/ ρCp (m2/s) (10)

XII. Determination of Bulk Density




18

The bulk density was determined using a centrifuge tube, at intervals of drying until the density becomes constant. The weight of the centrifuge tube and its content (sample) was noted, it was later tapped until there was no longer change in volume, and the centrifuge tube and its content were reweighed.

The weight was later obtained. The bulk density was calculated using the expression.

sampleofVolume

sampleofWeight (kg/m3) (11)

XIII. Determination of Mass Transfer Coefficient

This is a function of drying rate. The drying rate per unit area is proportional to the bulk density difference or concentration difference [8].

pfAdt

dm

(12)

pfKcAdt

dm

Adt

dmKc

pf

1 (13)

Where; f and p are the bulk density of feed and products respectively, dm = change in mass after each interval of drying, dt = time difference, A = surface area of the particles given as

2

ppp DMNA (14)

Where; Np = Number of particles per unit mass, Mp = Mass of products, pD = Mean particle diameter = Dpd, d = Mass fraction retained on the sieve. A small quantity of about 0.5g of the dried product was counted as accurate as possible to obtain the number of particles per unit mass. The samples were then sieved in sieves of 0.8mm, 1.6mm and 2.0mm respectively to obtain the mean particle size diameter.

3. PRESENTATION OF DATA

3.1. Moisture content at Harvest: Weight of sample used =5.0g in each case

Table 1: Weight of sample at harvest

Cultivar Weight of crucible (g) Weight of crucible + wet sample (g) Crucible + sample (dried)(g)

NR8082 29.04 34.04 30.64

Tms30575 29.17 34.17 31.045

Native 28.54 33.54 30.368

Table 2: Weight of sample at intervals of drying and Weight of dried sample after 48hours




19

2.1. Weight of sample at intervals of drying

Cultivar Weight of empty petridish(g)

Weight of petridish + sample (g)

Weight after 10 mins (g)







NR8082 12.137 26.137 25.367 24.347 23.949 23.782 23.711 23.704 23.704

Tms30575 17.577 22.577 22.464 20.737 20.395 20.321 20.281 20.261 20.261

Native 19.845 24.845 23.648 22.905 22.633 22.590 22.586 22.586 -

2.2. Weight of dried sample after 48hours

Cultiver NR 8082 Tms 30572 Native

Weight (g) 1.5385 2.3578 2.9597

3.2. Proximate Analysis: Weight of sample used = 2.0g in each case

Table 3: Table for Fat, Protein, Ash and Moisture content calculations

3.1. Table for Fat content calculations

Cultivar Weight of empty flask (g) Empty flask + extracted oil (g) Weight of oil

NR 8082 32.664 32.664 Nil

Tms 30572 33.930 33.930 Nil

Native 33.724 33.724 Nil

3.2. Table for Protein content calculations

Cultivar Initial burette reading (cm3) Final burette reading (cm

3) Volume of HCl used (cm

3)

NR 8082 20.60 20.70 0.1

Tms 30572 20.40 20.60 0.20

Native 20.70 20.90 0.20

3.3. Table for Ash content calculations

Cultivar Weight of empty silica dish (g)

Silica dish + sample after treatment (g)

Weight of sample after pretreatment (g)

Sample + silica dish after ash(g)

Weight of ash (g)

NR8082 26.210 26.284 0.074 26.22 0.01

Tms30572 29.062 29.121 0.059 29.07 0.008

Native 28.380 28.483 0.103 28.39 0.01

3.4. Table for Moisture content calculations




20

Cultivar Weight of crucible + sample (g) Weight of crucible + sample after drying (g)

Weight of moisture (g)

NR 8082 22.492 22.456 0.036

Tms 30572 16.749 16.699 0.043

Native 22.098 22.059 0.039

Table 4: Table for calculation of bulk density during drying

NR 8082 Tms 30572 Native Cultivar

Time (min)

Mass of Sample (g)

Volume of Centrifuge Tube (cm

3)

Time (min)

Mass of Sample (g)


3)

Time (min)

Mass of Sample (g)


3)

10 3.930 0.006 10 4.887 0.009 10 3.803 0.0060

20 3.210 0.006 20 3.160 0.007 20 3.060 0.0065

30 2.812 0.006 30 2.818 0.0075 30 2.788 0.009

40 2.645 0.007 40 2.744 0.012 40 2.745 0.0095

50 2.574 0.012 50 2.704 0.017 50 2.741 0.017

60 2.567 0.016 60 2.684 0.045 60 2.738 0.025

3.3 Tabulated Results and Simple Calculations: (Mass of sample used = 5.0g in each case)

Table 5: Moisture content of cultivars before fermentation

Cultivars Mass of dried crucible + wet sample (g)

Mass of crucible + sample after drying for 24hours

Weight of moisture (g)

% moisture content (wet basis)

NR 8082 34.040 30.640 3.40 68

Tms 30572 34.170 31.045 3.125 62.5

Native 33.640 30.368 3.172 63.44

Table 6: Moisture content of cultivars after dewatering and fermentation for 72 hours (wet basis)

Cultivar Mass of petridish + wet sample (g)

Mass of petridish + sample after drying to stable weight (g)

Weight of moisture (g) expelled

% moisture content (wet basis)

NR8082 26.137 23.704 2.433 48

Tms30572 22.577 20.261 2.316 46.32

Native 24.845 22.586 2.559 45.18




21

Table 7: Moisture content at intervals of drying (wet basis)

Cultivar 10mins 20mins 30mins 40mins 50mins 60mins

NR 8082 53.09 25.05 9.54 3.04 0.27 -

Tms 30572 82.08 17.73 4.99 2.24 0.75 -

Native 33.74 11.64 1.71 0.15 - -

Table 8: Hygroscopic properties of cultivars

Cultivar MC (g) MD (g) % water reabsorbed

% Ash

% Fat

% Protein

% Fibre

% Moisture

% Carbohydrate

NR8082 2.567 2.892 12.66 1.05 Nil 0.22 3.20 1.80 93.73

Tms30572 2.684 3.039 13.23 1.50 Nil 0.44 2.55 2.15 93.36

Native 2.741 3.081 12.40 0.15 Nil 0.44 4.60 1.95 92.86

MC = Mass of cassava after drying to final moisture (g), MD = Mass of dried sample 48 hours later after drying (g)

Table 9: Effect of Variety on the surface Area and Mass Transfer Coefficient of the Cultivars

Cultivar Surface Area (m2) Density before

drying (kg/m3)

Density after drying (kg/m

3)

Mass transfer Coefficient Kc

m/s(x10-6

)

NR 8082 0.897 655 160.44 1.632 x 10-6

Tms 30572 0.415 543 59.64 3.25 x 10-6

Native 0.850 551.2 171.3 2.332 x 10-6

Table 10: Effect of variety on the drying rate of the cultivars

Cultivar Total drying time

(mins)

Drying rate after

20 mins (kg/hr)

Drying rate after

40 mins (kg/hr)

Drying rate of stable

weight (kg/hr)

NR 8082 50 5.37 3.53 2.61

Tms 30572 60 5.52 3.38 2.32

Native 50 5.82 3.38 3.07




22

Fig. 1: Drying curve for the cultivars

Fig. 2: Specific Heat of Cultivars at Various Moisture Contents

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60

% M

ois

ture

Co

nte

nt

(dry

bas

is)

Time (mins)

NR 8082

Tms 30572

Native

0

0.5

1

1.5

2

2.5

3

3.5

4

0 10 20 30 40 50 60Spe

cifi

c H

eat

cap

acit

y (

kJ/k

gK)

% Moisture Content (wet basis)

NR 8082Tms 30572Native




23

Fig. 3: Thermal Conductivity of Cultivars at Various Moisture Contents

Fig. 4: Bulk density variations with Moisture Contents

0

0.1

0.2

0.3

0.4

0.5

0.6

0 10 20 30 40 50 60The

rmal

Co

nd

uct

ivit

y (W

/m0C)

% Moisture Content (wet basis)

NR 8082

Tms 30572

Native

0

100

200

300

400

500

600

700

0 20 40 60 80 100

Bu

lk d

en

sity

(kg

/m2)

% Moisture Content (dry basis)

NR 8082Tms 30572Native




24

Fig. 5: Thermal Diffusivity Variations with Moisture Contents

4. DISCUSSION

4.1. Moisture Content

The moisture contents obtained before and after fermentation by drying the cultivars with a circulating air oven is shown in Tables 6 and 7 respectively. The initial moisture content of cassava at harvest was not stable and may be dependent on season. The cultivars used here were harvested in the month of April. NR 8082

had the highest moisture content of 68% at harvest, but Native cassava had the highest moisture expelled after drying. This suggests that the native cassava was more porous than the improved cassava in agreement with the studies in [9].

4.2. Drying Rate

The moisture content (dry basis) decreases as drying time increases for all the cultivars during the drying process as shown in fig.1. The total drying time differed at approximately the same moisture content for all the cultivars, Tms 30572 with almost the same moisture content (wet basis) with the Native cassava after fermentation, had the longest total drying time of 60mins while native cassava had the shortest total drying time of 50mins. Fig.1 suggests that two cultivars with same initial moisture content did not have same drying characteristics. Other properties like the differences in chemical

composition of the cultivars may have contributed to this.

The drying rate decreased as the drying time increased for all the cultivars as show in table 2.1. This implies that the drying rate decreased with increase in moisture content (wet basis) as against that reported by [9]. The drying rate of NR8082 and Tms 30572 at stable weight was low compared with native cassava. The native cassava had the highest drying rate of 3.07kg/hr at stable weight with shortest drying time which also confirms its highly porous property, as show in table 2.1.

0.00E+00

2.00E-04

4.00E-04

6.00E-04

8.00E-04

1.00E-03

0 20 40 60 80 100

The

rmal

dif

fusi

vity

𝛼(m

2 /s)

% Moisture Content (dry basis)

NR 8082

Tms 30572

Native




25

4.3. Proximate Composition

Table 8 shows the proximate composition of the cultivars. The improved cultivars had a slightly higher percentage of carbohydrate (93.36 - 93.73%) than the native cultivar (92.86%). All the cultivars do not contain fat since there was no oil extract from them. The geographical location of these cultivars as well as time of planting and harvest may have been responsible for this. Tms 30572 and the native cultivar have the same protein content higher than that of

NR8082. The native cultivar ranked highest in fibre content but contains the least percentage of ash. These might have contributed to its shortest drying time. The type of drier used might have equally affected the total drying time as well as the drying rate. Also the differences in the drying rate can be attributed to the difference in their chemical compositions. The same drying condition cannot therefore be used for drying different cultivars.

4.4. Specific Heat Capacity and Thermal Conductivity

In order to determine the effect of moisture content on specific heat capacity and thermal conductivity, the data in table 2.2 and the model equations of (8) were used to calculate the specific heat capacity and (9) was used to calculate the thermal conductivity at various range of moisture contents. The specific heat capacity increased with moisture content (wet basis) for all the cultivars. The native cultivar (fig. 2) had a different specific heat capacity than the other cultivars. The native cultivar seems to have a lower ash and carbohydrate content from other cultivars (Table 8) which might have contributed to this difference. The two improved cultivars exhibited close specific heat capacities probably due to the fact that they had approximately the same carbohydrate content. Tms 30572 and NR 8082 had very close specific heat at different moisture contents showing that cultivars with similar chemical composition (ash and carbohydrate) might have

the same specific heat capacity. The specific heat capacities of Tms 30572 and NR 8082 were high compared with the native cultivars, probably because of their high carbohydrate and ash contents. More heat energy will therefore be required in other to dry cultivars with high ash and carbohydrate content. The specific heat obtained from this work ranged from 1.53kJ/kgK-3.49kJ/kgK which compare favorably with that of [9]; who obtained up to 3.5kJ/kgk.

The thermal conductivity of each of the cultivars also increased with moisture content (wet basis). The native cultivar from (fig. 3) still exhibited a different thermal conductivity relative to other cultivars. The two improved cultivars had almost the same thermal conductivity at different moisture contents. Tms 30572 and NR 8082 still exhibited higher thermal conductivity than the native cultivar.

4.5. Bulk Density

The bulk density of each cultivar decreased as the moisture content (dry basis) decreased during drying as shown in fig.4, but it was almost stable with moisture content below 13%

dry basis. This goes to suggest the hygroscopic property of cassava as shown by [8] during the drying of a native cassava cultivar. At particular moisture content under the same drying




26

conditions the bulk density differed for each cultivar which meant that the mass transfer rate would also differ for all the cultivars, since mass

transfer rate is a function of the density of the sample dried.

4.6. Thermal Diffusivity

The thermal diffusivity increased as moisture decreased, but was almost constant below the hygroscopic moisture of about 13%. The thermal diffusivity also differed slightly for each cultivar when determined at the same moisture as shown in fig.5. Thermal diffusivity determines how fast heat propagates or diffuses through a material. From the results, the native cultivar and NR8082

with higher density had low thermal diffusivity while the diffusion of heat is fastest in Tms 30572. NR8082 was the least porous (least thermal diffusivity from fig.5) and also has high carbohydrate and least protein content; this should have contributed to the low thermal diffusivity.

4.7. Hygroscopic Property and Porosity

The dried ground cassava from each cultivar was reweighed after 48 hours of drying and the results show that 12.4% - 13.23% moisture was reabsorbed after the drying by the various cultivars. All the cultivars exhibited hygroscopic property. Tms 30572 reabsorbed the highest moisture, indicating that it is more porous and hygroscopic than the other cultivars. Its highest thermal diffusivity also goes to confirm this. This must have also led to the increase in the

rate of mass transfer of moisture from Tms 30572. This is in contradiction with the results of the native cultivar’s porosity, which expelled the highest moisture with the shortest drying time. It can therefore be said that the diffusion of heat and mass through the pores of a cultivar does not depend on porosity alone but can also be affected by the chemical compositions of the cultivar, as reported by [9].

4.8. Surface Area and Mass Transfer Coefficient

The surface area was obtained when the fermented ground cassava cultivars were dried to stable weight. Tms 30572, with the least surface area of 0.415m

2 has the longest drying

time. The Native Cultivar with surface are of 0.85m

2 dried faster than Tms 30572 suggesting

that the drying time does not entirely depend on the surface are. The proximate composition of

the cultivars has a great effect on the drying rate even when the cultivars surface area is small. Tms 30572 has the highest mass transfer coefficient which was due to its low product surface area and relatively lower density difference.

5. CONCLUSION AND RECOMMENDATION

The proximate composition of the cultivars had a very great impact on their engineering properties. The difference in the drying rate could therefore be attributed to the difference in their chemical composition. It can be said that the moisture content, specific heat capacity, and

porosity also affected the rate of diffusion of heat and mass through the cultivars. The specific heat capacity and thermal conductivity of each cultivar varied with the proximate composition, water content and density, indicating that cultivars with similar chemical composition had




27

same specific heat capacities as could be seen in the improved cultivars. More heat energy would be required to dry cultivars with high carbohydrate and ash content. There was no trace of fat in all the cultivars probably as a result of the geographical location of the samples.

Thermal diffusivity of each cultivar varied with the proximate composition, porosity, density as

well as the moisture content. Fermented ground cassava of any of these cultivars should not be dried below their equilibrium moisture content about 13%), else moisture will be reabsorbed from the atmosphere. From all indications, the improved variety is more porous, has high hygroscopic property and exhibited high rate of mass transfer of moisture.

The mass transfer coefficient differed for each cultivar and was highest for cultivars with high protein content and high drying rate.

Therefore, the same drying condition cannot be used for drying the fermented ground cassava cultivars except if they have close engineering properties.

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the effect of variety and drying on the engineering … varieties of cassava demands that the effect...

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