Moisture sorption isotherms of fufu and tapioca at different temperatures

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Journal of Food Engineering 34 (1997) 203-212 0 1998 Elsevier Science Limited. All rights resewed Printed in Great Britain 0260-8774198 $19.00+0.00 SO260-8774(97)00072-l

ELSEVIER

Moisture Sorption Isotherms of Fufu and Tapioca at Different Temperatures Lateef Oladimeji

Sanni,

Charles Ateren & Ayoade Kuye’”

“Department of Food Science and Technology, University of Agriculture, Abeokuta, Nigeria ‘Department of Chemical Engineering, University of Port Harcourt, PMB 5323, Port Harcourt, Nigeria (Received

6 October

1995; accepted

13 August 1997)

ABSTRACT Moisture sorption characteristics for two products of cassava, namely fufu and tapioca, at 25”C, 32°C and 45°C were studied for water activity ranging from 0.1 to 0.96. At a given water activity, the results show that the moisture content decreases with increasing temperature for jkfu and tapioca. Eight sorption models were used to analyze the data. The GAB model showed the best fit, whereas the BET model was the poorest over the whole range of water activity. Also, estimates of the net isostetic heats of sorption and their dependence on moisture content were presented for each product. 0 1998 Elsevier Science Limited. All rights reserved

INTRODUCTION Cassava (Manihot esculenta) is an important source of carbohydrates to many people in the tropics. Once harvested, the roots must be consumed or processed within a few days. Consequently, various processing techniques have been reported in the literature. In general these involve dehydration, in which the moisture content is reduced to about 12 to 14% (Onayemi & Oluwamukomi, 1987). A fundamental property of biological material, which influences dehydration, shell life prediction and storage stability, is the water sorption characteristics (Labuza, 1975). The sorption characteristics are often represented by moisture sorption isotherms (MSIs), which are plots of moisture content versus water activity a, at certain temperatures. MSIs over a wide range of temperatures are required for designing dehydration processes, predicting drying rates and evaluating rehydration characteristics of dehydrated foods (Rim & Bhowmik, 1994). *To whom all correspondence

should be addressed. 203

204

L. 0. Sanni et al.

For Nigerian foods, such information is relatively scarce. Ajibola (1986a,b) presented the desorption isotherms for plantain and gari, while Onayemi and Oluwamukomi (1987) presented such information for lafun, gari, yam flour and instant pounded yam flour. Ajisegiri and Sopade (1990) presented the sorption data for millet. We are not aware of any published work on the MSIs for fufu and tapioca. The primary objective of this paper is to present the MSIs for fufu and tapioca at three different temperatures. The data will be interpreted using well established models with a view to obtaining the model that best represents the data. Finally, the isosteric heat of sorption for these products will be evaluated.

MATERIAL

AND METHODS

Material

Cassava roots of variety designated as TMS 30572 were obtained from the University farm, University of Agriculture, Abeokuta. The tubers are processed into the required products within 2-3 h of harvesting. The products are fufu and tapioca. Preparation of fu&

Processing of cassava roots to fufu followed the traditional method described by Akingbala et al. (1991). Cassava roots were peeled, washed and diced into smaller pieces, before steeping in water to ferment for 4 days. The soft cassava pieces were washed and later mashed in water. Fibers were then removed from the mashed cassava pulp by pressing it over a fine sieve. The filtrate was allowed to settle for 4 h. The resultant fufu was dewatered by pressing, and later sun-dried for 2 days. It was then milled, sieved and stored in polyethylene bags for future use. Preparation of tapioca

Processing of cassava roots to tapioca followed the traditional method described by Oyewole and Obieze (1995). Fresh cassava roots were peeled, washed in water and then grated. The resulting cassava pulp was diluted with water and sifted to extract starch milk solution from the pulp. The filtrate starch milk was allowed to settle for 7 h before decanting off the water. The thick starch cake was further dewatered and pulverized. The pulverized starch cake was then roasted in a flat hot pan under direct flame heating with constant stirring. The dried irregular flakes and grains that resulted is the tapioca. Determination

of equilibrium

moisture

content

The equilibrium moisture content was determined using the gravimetric method, as described by Pawar et al. (1992), at temperatures of 25, 32 and 45°C. The temperatures were chosen to simulate the variation in temperature across Nigeria. Triplicate samples, 20 g each, were placed on the top of a wire mesh inside the desiccator containing saturated salt solutions in the water activity range 0.11-0.96. The different salt solutions used are the same as those reported by Ezeike (1988). The desiccators were kept in an incubator at the desired temperature maintained to an

Moisture sorption isotherms of fufu and tapioca

205

accuracy of + 1°C. The samples were weighed daily, and equilibrium conditions were considered attained when three consecutive readings gave identical values. The equilibrium moisture content at each water activity was calculated on a dry basis. For most of the products, equilibrium was attained within 13-15 days, and samples which showed mould growth, especially at high a,, were discarded and the experiment repeated. PREDICTION

MODELS

Over the years, many models have been developed for predicting experimental sorption data. Some of these models are purely empirical, whereas others are based on certain fundamental principles such as thermodynamic considerations (Craspiste & Rotstein, 1982). In general, the equations are of the forms: M = M(a,,T)

(1)

M = M(a,)

(2)

where M is the equilibrium moisture content (kilograms water/kilograms solid), a, is the water activity and T is the temperature. Note that eqn (2) is restricted to the isothermal case. For this work we have used eight different models for predicting the MS1 for fufu and tapioca. It has been observed that most models can only predict the sorption for a particular range of activities (Labuza, 1968; Craspiste & Rotstein, 1982). Hence, our use of different models was to ascertain whether any of them would fit our data for the whole range of interest. The different models are shown in Table 1. In the models the constants are calculated using a developed multiple regression fortran program. To facilitate our comparison of the different models we have chosen to calculate the average deviation, defined as percentage

average deviation = -

100

lmca, - mactl

NE

mact

(3)

where N is the number of observations, and meal and mact are respectively the calculated and experimentally measured values of the equilibrium moisture content. As can be seen from Table 1, some of the models include the effect of temperature. Also, the constants are in some cases related to certain properties. For example, in the GAB model, m, is the monolayer moisture content (kilograms water/kilograms solid), and cl and c2 are respectively related to monolayer and multilayer properties. In addition, the influence of temperature on GAB constants may be calculated with an Arrhenius form of equation (Kim & Bhowmik, 1994). That is m,(T) = mkexp(AHIRT) c,(T)=c;

exp[(H, -H,)IRT]

c*(T) = c; exp[(H,-

H,)IRT]

where mb, c;, c; are pre-exponential factors, AH is an Arrhenius-type H&J mall ’ is the heat of sorption of the first layer, H,/kJ mol-’ sorption of a multilayer, and H JkJ mol-’ is the heat of condensation vapor.

(4) (5) (6)

energy factor, is the heat of of pure water

L. 0. Sanni et al.

$ --

d u’s

E

0

0” G-

F-i

.E

d

207

Moisture sorption isotherms of fufu and tapioca

Finally it has been observed that the sorption Clapeyron relationship (Labuza, 1968):

phenomena

obey the Clausius-

Qs

d(ln a,> d(W)

= -

(7)

R

where Qs is the net isosteric heat of sorption. For this work eqn (7) is used to compute Qs for different equilibrium moisture contents. RESULTS

AND DISCUSSIONS

Sorption isotherms Experimental moisture content values obtained for fufu and tapioca at three different temperatures are shown in Figs 1 and 2. In all cases at least three

12

10

6

4

2

0 0.0

I

0.1

I

0.2

I

0.3

1

1

1

0.4

0.5

0.6

I

0.7

t

0.8

1

0.9

1 .o

Water activity Fig. 1. Effect of temperature on sorption isotherms of fufu (solid lines calculated GAB model; symbols are experimental values).

using the

208

L. 0. Sand et al.

experimental moisture content values were used. The curves show that an increase in temperature decreases the moisture content for a given water activity. Similar results have been reported by other investigators (Onayemi & Oluwamukomi, 1987; Ajisegiri & Sopade, 1990; Kim & Bhowmik, 1994). A close look at Figs 1 and 2 will, however, reveal that the sorption characteristics for fufu are slightly different from that for tapioca. For water activities less than 0.35 the rate of change of moisture content with water activity for tapioca is higher than that for fufu at a given temperature. Thus, it would appear that the processing technique may have a significant effect on the sorption characteristics, since fufu and tapioca are derived from the same raw material (cassava). Tables 2 and 3 show the ability of the different models to predict the sorption isotherms for fufu and tapioca respectively over a water activity range of 0.11 to 0.96. In these tables, Nd is the number of points for which the deviation is greater than 10%. Deviation is defined as deviation (%) = 100

lmcar- mactl mact

(8)

16

GAB modul

v

25°C

32% w 46°C 0

Water activity Fig. 2. Effect of temperature on sorption isotherms of tapioca (solid lines calculated the GAB model; symbols are experimental values).

using

‘,O’)

Moisture sorption isotherms of fufu and tapioca

With the exception of the data for tapioca at ZYC, the GAB model gave the best prediction (lowest average and maximum deviations). Also, for the GAB model it can be seen that the deviation is less than 10% for about 97% of the data points. For fufu, the Bradley model appears to be the second best, whereas for tapioca it is the Henderson model. For the range of interest, the BET model gave the worst prediction. Thus, our data validate the earlier assertion that the BET model is good for a narrow range of water activities (Labuza, 1968; Craspiste & Rotstein, 1982). Figures 1 and 2 show the temperature dependencies of the isotherms calculated using the GAB constants shown in Tables 2 and 3 respectively for fufu and tapioca. Figure 1 shows that the GAB model fits the data well, thus confirming the high correlation coefficient shown in Table 2. For tapioca, it can be seen that the GAB model is not as good, especially for low water activities (less than 0.35). This explains the relatively low correlation coefficients (see Table 3). The temperature dependencies of the GAB constants calculated using linear regression are shown in Table 4.

Comparison Model

At 25°C GAB Bradley Chung Pfort Oswin Smith Halsey Henderson BET

TABLE 2 Models for Fufu at Various Temperatures

of Different

cl

c2

w,

Ave. dev.

Ma-X. dev.

I.208 3.166 3.198 3.666 5.557 5.911 5.919 68.442

- 3.796 - 7.730 ~ 8.217 - 9.630 14.164 14.448 - 10.646 - 170.220

0.995 52 - 0.99003 0.990 03 0.990 17 0.978 38 0.977 37 0.979 63 0.735 34

11.11 22.22 11.11 88.89

con: coef

Nd

126 12.76 33 780 7518 5.113 206 800 ;.;W& lo-‘*

0.692 4 O-680 6 0.392 6 0.224 3 3.148 3.491 2.903 2.49

4.891

At 32°C GAB Bradley Chung Pfort Oswin Henderson Smith Halsey BET

41.11 - 8.399 22 920 6.63 2.33 x lo- “’ 4.229 55 320 0.994 9

0.7115 0.6915 0.378 3 0.262 3 2.454 3.258 3.013 2.563

4.461

2.507 2.853 2.877 3.941 5.102 7.090 8.540 70.108

- 6450 - 9.943 - 10.579 12.203 - 10.933 26.612 19.429 - 187.542

0.982 16 -0.98744 0.987 44 0.988 6 0.989 23 0.973 17 0.966 75 0.736 22

0.00 0.00 11.1 I 22.22 11.11 22.22 33.33 88.89

At 45°C GAB Henderson Bradley Chung Pfort Oswin Smith Halsey BET

18.17 8.85 x lo- ” -8.611 24 300 5.498 3.587 27 970 0,823 6

0.6612 2.309 0.641 0.454 8 0.274 3 2.681 2.917 2.54

4-273

I-473 2.187 4.704 5.276 6.948 11.214 11.823 71.341

- 3.284 - 4.076 9.320 12.018 17.376 39.265 26.325 - 188.503

0.995 69 0.998 17 -0.98893 0.988 93 0.98 148 0.964 08 0.948 05 0.73181

0.00 0.00 0.00 11.11 22.22 22.22 55.56 88.89

0.00 o-00 0.00 0.00

L. 0. Sanni et al.

210

Comparison Model

of Different Cl

At 25°C Henderson GAB Chung Pfort Bradley Oswin Smith Halsey BET

TABLE 3 Models for Tapioca at Various Temperatures c2

9.05 x lo27.2 31970 _ 12.02 7.471 5.151 127300 1.061

I1

m,

2.557 0.620 1 0.382 1 0.688 1 0.247 3.197 3.257 - 2.489

5.806

Ave. dev.

MaX. dev.

4.573 5.093 5.959 6.007 7.489 10.083 11.612 69.058

- 10.323 15.219 28.367 25.938 23.823 53.432 39.895 - 166.336

0.988 63 0.963 39 0.989 24 - 0.989 24 0.969 3 1 0.966 12 0.93102 0.743 06

11.11 11.11 11.11 11.11 22.22 11.11 22.22 88.89

con: coet

Nd

At 32°C GAB Henderson Bradley Chung Pfort Oswin Smith Halsey BET

9.203 3.83 x lo-O9 - 8.239 23 680 5.937 4.014 24 490 0.905 9

0.528 2.059 0.672 7 0.413 9 0.299 6 2.914 2,737 - 2.538

6.4

5.029 7.477 10.240 11.058 13.208 18.474 18.909 72.975

9.815 19.566 48.314 55.573 41.672 94.356 65.968 - 187.580

0.91844 0.98172 -0.97873 0.978 73 0.940 28 0.943 84 0.88 631 0.735 29

0.00 22.22 22.22 11.11 55.56 33.33 66.67 88.89

At 45°C GAB Henderson Bradley Chung Pfort Oswin Smith Halsey BET

7,762 1.46 x 1O-08 - 6.98 20 460 4.499 2.932 9086 0.7172

0.550 9 1.927 0.616 8 0.5018 0.3205 2.407 2.554 - 2.587

4.935

4.137 7.004 10.152 10.967 13.743 19.182 20.089 74.406

7.532 17.631 46.171 53.371 41.226 97.086 67.203 - 202.828

0.913 11 0.986 09 - 0.98135 0.98135 0.945 29 0.947 82 0.892 28 0.732 17

0.00 33.33 22.22 11.11 55.56 44.44 77.78 88.89

Temperature

Dependencies

TABLE 4 of GAB Constants

c,(T)

m,(T) AH

con:

0.6322 0.3015

5.0253 7.4885

0.9355 0.7325

cz(T)

c;

HL -H,,,

Con: coej

c;

HL -H,,,

Con: coef.

1.43 x lo-” 2.65 x 10W7

73.4225 45.0617

0.9701 0.8510

0.2995 0.1250

2.1214 3.8467

0.7353 0.5931

coef

Fufu Tapioca

for Fufu and Tapioca

Moisture sorption isotherms of fi&

and tapioca

311

Heat of sorption The net heats of sorption, calculated by applying eqn (7) are shown in Fig. 3 for fufu and tapioca. Fufu shows higher sorption energy levels than those for tapioca at moisture contents lower than O-052 kg water/kg solid. At moisture contents higher than 0,052 kg water/kg solid, tapioca has a higher sorption energy than fufu.

CONCLUSIONS MSIs at three temperatures were presented for fufu and tapioca. Eight different models were used to assess the goodness of fit for the experimental data. The GAB model gave the best fit for the sorption isotherms, whereas the BET model gave the poorest fit. Hence, the temperature shift of the isotherms could be expressed using the GAB constants. Fufu showed a higher sorption energy level than tapioca at moisture contents lower than O-052 kg water/kg solid.

I

I

1

I

I

T

I

l Fufu

v

0.03

Tapioca

I

I

t

I

I

1

0.04

0.05

0.06

0.07

0.08

O*OQ

Moisture Fig. 3. Net heat of sorption

content

as a function

(kg water/kg

of moisture

content

Cl.10

solid)

for fufu and tapioca.

212

L. 0. Sunni et al.

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Smith, S. E. (1947). The sorption 646

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