water-absorption characteristics of various pearl millet cultivars and sorghum grown in northern...

Journal of Food Process Engineering

28

(2005) 282–298.

All Rights Reserved.

282 ©

Copyright 2005, Blackwell Publishing

WATER-ABSORPTION CHARACTERISTICS OF VARIOUS PEARL MILLET CULTIVARS AND SORGHUM GROWN IN

NORTHERN NIGERIA

M.H. BADAU

1,3

, I. NKAMA

1

and I.A. JIDEANI

2

1

Department of Food Science and TechnologyUniversity of Maiduguri P.M.B. 1069, Maiduguri

Nigeria

2

Biological Sciences ProgrammeSchool of Science and Science Education

Abubakar Tafawa Balewa University P.M.B. 0248, Bauchi

Nigeria

Accepted for Publication April 25, 2005

ABSTRACT

Ten pearl millet cultivars and one sorghum cultivar were evaluated forphysical properties and chemical composition. Water-absorption characteris-tics of these grains were also determined using Peleg’s equation. Resultsindicated wide variations in thousand-kernel weight and volume, grain hard-ness, percent floaters, in addition to considerable variations in protein, fatand crude fiber. Peleg’s equation gave a reasonable fit to experimental data.Peleg constants

K

1

were obtained for the pearl millet cultivars. Temperaturedependence of the reciprocal of the Peleg constant

K

1

was determined usingan Arrhenius equation. Activation energies were in the range of 1.405–6.572 kJ/mol. A linear relationship was proposed to describe the relationshipbetween the rate of absorbed water per unit change in temperature andactivation energy.

INTRODUCTION

Pearl millet grain is an important component in the diet of the poor anda major source of calories and a vital component of food security in thesemiarid areas of the developing world (FAO and ICRISAT 1996). It couldbe used to produce varieties of dishes that require preliminary preparationssuch as soaking in water, dehulling, fermentation, germination, drying and

Blackwell Science, LtdOxford, UKJFPEJournal of Food Process Engineering0145-8876Copyright 2005 by Food & Nutrition Press, Inc., Trumbull, Connecticut.283282298Original Article

WATER-ABSORPTION CHARACTERISTICSM.H.

BADAU, I. NKAMA and I.A. JIDEANI

3

Corresponding author. TEL: 00234-76977382; EMAIL: [email protected]

WATER-ABSORPTION CHARACTERISTICS 283

milling to produce fine flour and grits. The treatments used depend on theproduct to be produced.

Soaking is the most common preliminary process applied to millet grainduring the production of various millet-based food products (ndaleyi, kunu,masa, etc.). Soaking is also an important operation in rice parboiling (Engels

et al.

1986), Ogi preparation from sorghum (Adeyemi 1983) or millet (Nkama

et al.

2000) and in vegetable-milk processing (Nelson

et al.

1976).Peleg (1988) proposed an empirical equation to model water-absorption

characteristics of food materials. The equation is simple compared with otherequations based on Fick’s Law of diffusion (Crank 1975). It has been appliedto various food materials by other researchers (Sopade and Obekpa 1990;Sopade

et al.

1992, 1994; Sopade and Okonmah 1993) and was found suitable.Although, Peleg’s equation has been applied to various food materials, thereseems to be inadequate information on its application on pearl millet cultivarsin this environment. Soaking is an important aspect of the malting process.To understand the malting properties of some pearl millet cultivars, theirsoaking characteristics need to be investigated. Therefore, this study wasundertaken to investigate suitability of Peleg’s equation in understanding thesoaking characteristics of some pearl millet cultivars.

PELEG’S EQUATION

Peleg (1988) proposed that:

(1)

where

M

t

=

moisture content at a known time (% dry basis);

M

0

=

initialmoisture content (% dry basis);

t

=

soaking time (h);

K

1

=

Peleg’s constant 1(h/% dry basis);

K

2

=

Peleg’s constant 2 (1/% dry basis).As

t

Æ•

where

M

t

=

Equilibrium moisture content (% dry basis).The equation is applicable to the nonlinear segment of the sorption curve

because beyond this region (

M

t

-

M

0

) is approximately constant; rearrangingEq. (1) gives:

(2)

M Mt

K K tt = ++0

1 2

M MKt = +0

2

1

t

M MK K t

t -= +

01 2

284 M.H. BADAU, I. NKAMA and I.A. JIDEANI

This implies that plotting against ‘

t

’ gives a straight line with

K

1

as

the ordinate intercept and

K

2

, the gradient of the line. This plot allows thePeleg’s constants to be determined.

Sopade and Obekpa (1990) observed that

K

1

is a function of temperatureand

K

2

is a constant for a food material and hence could be used as acharacteristic sorption parameter. Sopade

et al.

(1992) indicated that

K

1

couldbe compared to a diffusion coefficient and the Arrhenius equation could beused to describe the temperature dependence of the reciprocal of the Peleg’sconstant

K

1

in the following manner:

(3)

where

K

0

=

Arrhenius constant at a reference temperature;

E

a

=

activationenergy (kJ/mol);

R

=

universal gas constant (

=

8.318 kJ/mol/K);

T

=

absolutetemperature (K).

On linearization, Eq. (3) becomes:

(4)

When is plotted against , a straight line of slope is

obtained from which the activation energy can be calculated and sensitivityof the constant to temperature can be assessed.

MATERIALS AND METHODS

Ten pearl millet cultivars, namely GB 8735, SOSAT C-88, GWAGWA,EX-BORNO, LCRI – IC 9701, G.I – 14.9, ICMV – IS 94208, ICMV – IS94206, G.I – 297-1 and ZANGO, and one sorghum cultivar (ICSV 111) wereused for the soaking studies. They were obtained from ICRISAT experimentalstation at Bagauda, Kano and Lake Chad Research Institute, Maiduguri.

Sample Preparation

The grains were cleaned by removing foreign matter, broken, crackedand damaged grains. The quartering procedures of Lees (1975) were used forsampling.

t

M Mt - 0

1

10K

K E RTa= -( )exp

ln ln1 1

10K

KE

R TaÊ

Ëˆ¯ = - Ê

Ëˆ¯ÊË

ˆ¯

ln1

1KÊË

ˆ¯

1

TÊË

ˆ¯

E

RaÊ

Ëˆ¯


Chemical Composition

AOAC (1990) methods as reported by Badau

et al.

(2002) were used todetermine the moisture, fat (petroleum ether extracts), ash, crude fiber andprotein by the micro-Kjeldah method. Soluble carbohydrate was determinedby difference. All the determinations were done in triplicate and reagents wereof analytical grade.

Water Absorption

About 50 g of each of the grains was soaked in 250 mL of distilled waterin a 400-mL conical flask. Weight gain during 24 h of soaking at 10C, 32Cand 50C was determined. The grains were kept in a refrigerator (Thermocool400, YB 080313, Port-Harcourt, Nigeria) at 10C, at room temperature(32C), and in a water bath (Clifton, 14634 Nickel Weston-S-Mare, ElectroLtd., Avon, England) at 50C. Weights of soaked grains were taken at 30-minintervals within a 0- to 12-h period, and at 2-h intervals within the 12- to24-h period. The soak water was drained off the grains by the use of a sieve.The free water was allowed to drain from the grain. Water absorbed by thegrains with respect to soaking time was determined by subtracting the originalweight of grains from the weight of the water-absorbed grains (Ituen

et al.

1985). Soaking of grains in water continued until they stopped absorbingwater (i.e., the moisture absorption capacity was reached). Total solids of eachsoak water for each beaker containing the 50 g for each pearl millet cultivarand the sorghum cultivar was determined at each interval of time, by evapo-rating off the water with a water bath initially and laboratory oven at 105Covernight. Total solids of the soak water were determined and added to theweights gained during soaking, to obtain corrected weight (without solid loss)(Sopade

et al.

1992). Determinations were done in triplicate.

Statistical Analysis

Linear regression analysis and a c2 test were carried out as described byMead et al. (1993). The goodness of fit between the experimental and pre-dicted amounts of water absorbed was determined using:

(1) c2 test statistics (Mead et al. 1993) with the experimental values asthe observed and the predicted values as the expected;

(2) the root-mean-square deviation (RSMD) as defined (Engels et al.1987; Sopade et al. 1992) below:

Sn

M Pei rii

n

= -( )=Â1 2

1


where S = root mean square of deviation; n = number of experimental points;Mei = experimental moisture content (% dry basis); Pri = predicted moisturecontent (% dry basis).

RESULTS AND DISCUSSION

Physicochemical Characteristics

The physicochemical characteristics of the 10 pearl millet cultivars usedin this study have been reported by Badau et al. (2002) and that of sorghumby Sopade et al. (1992). Badau et al. (2002) reported that there were widevariations between the physical and chemical properties of the pearl milletcultivars.

Water Absorption

Figures 1 and 2 show water-absorption characteristics of the pearl milletcultivars and the sorghum cultivar used as control. The water-absorptionbehavior of the grains for the three temperatures increased very rapidly withinthe first 5 h and then became almost steady from then up to the 24th hour ofsoaking.

The water absorption at 50C was more rapid than at 32C and 10C.Generally more water was absorbed at 50C than 32C and 10C. Temperatureappears to accelerate the rate of water absorption. Other workers have reportedsimilar increases in water absorption as temperature increases (Sopade et al.1992).

Peleg’s equation was used to fit the experimental data within the non-linear segment of the curves.

Equation (2) was used for all the millet grains studied.Figures 3 and 4 show the application of Peleg’s equation to the

experimental data on the 10 pearl millet cultivars and one sorghum cultivarstudied. The parameters from the linear regression analysis are shown inTable 1. The coefficient of determination R2 varied from 0.9745 to 0.9989.The constant K1 decreased with decrease in temperature for all the grainsstudied (Table 1).

The relationship between the reciprocal of K1 and temperature wasstudied using Eq. (4).

When was plotted against with as the gradient of

the line and ln K0, the intercept and activation energy (Ea) of the temperaturedependence of the reciprocal of the Peleg’s constant K1 was calculated fromall the curves (not shown), which is presented in Table 2. Table 2 also showsthe mean water absorbed by the grains per unit change in temperature. Theactivation energy ranged from 1.405 to 6.572 kJ/mol and mean water absorbed

ln1

1KÊË

ˆ¯

1

TÊË

ˆ¯

E

RaÊ

Ëˆ¯


FIG. 1. WATER-ABSORPTION CHARACTERISTICS OF PEARL MILLET CULTIVARS OF: (a) SOSAT-C88, (b) ZANGO, (c) EX-BORNO, (d) LCRI – IC 9701, (e) ICMV – IS 94206,

(f) ICMV – IS 94208

0

10

20

30

40

50

60

70

01.

5 34.

5 67.

5 910

.5 12 18 24

283K305K323K

0

10

20

30

40

50

60

70

01.

5 34.

5 67.

5 910

.5 12 18 24

283K305K323K

0

10

20

30

40

50

60

70

01.

5 34.

5 67.

5 910

.5 12 18 24

283K305K323K

0

10

20

30

40

50

60

70

01.

5 34.

5 67.

5 910

.5 12 18 24

283K305K323K

0

10

20

30

40

50

60

01.

5 34.

5 67.

5 910

.5 12 18 24

283K305K323K

0

10

20

30

40

50

60

70

01.

5 34.

5 67.

5 910

.5 12 18 24

283K305K323K

Time (h)

Wat

er a

bsor

ptio

n (g

H2O

/100

g d

ry s

olid

)

a b

c d

e f


FIG. 2. WATER-ABSORPTION CHARACTERISTICS OF PEARL MILLET CULTIVARS OF: (g) GWAGWA, (h) G.I – 14.9, (i) GB 8735, (j) G.I – 297-1, (k) SORGHUM (ICSV 111)

0

10

20

30

40

50

60

70

01.

5 34.

5 67.

5 910

.5 12 18 24

283K305K323K

0

10

20

30

40

50

60

70

01.

5 34.

5 67.

5 910

.5 12 18 24

283K305K323K

0

10

20

30

40

50

60

70

01.

5 34.

5 67.

5 910

.5 12 18 24

283K305K323K

0

10

20

30

40

50

60

70

80

01.

5 34.

5 67.

5 910

.5 12 18 24

283K305K323K

0

10

20

30

40

50

60

70

80

01.

5 34.

5 67.

5 910

.5 12 18 24

283K305K323K

Time (h)

Wat

er a

bsor

ptio

n (g

H2O

/100

g d

ry s

olid

)

g h

i

k

j


FIG. 3. APPLICATION OF PELEG’S EQUATION TO THE EXPERIMENTAL DATA ON PEARL MILLET CULTIVARS OF: (a) SOSAT-C88, (b) ZANGO, (c) EX-BORNO, (d) LCRI – IC 9701, (e)

ICMV IS 94206 AND (f) ICMV IS 94208

Time (h)

(d

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 10 20 30

283K

305K

323K

a

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 5 10 15 20 25 30

283K

305K

323K

b

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 10 20 30

283K

305K

323K

c

0

0.1

0.2

0.3

0.4

0.5

0.6

0 5 10 15 20 25 30

283K

305K

323K

d

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 10 20 30

283K305K323K

e

0

0.1

0.2

0.3

0.4

0.5

0.6

0 5 10 15 20 25 30

283K305K323K

f

t/ (M

t –M

0)


FIG. 4. APPLICATION OF PELEG’S EQUATION TO THE EXPERIMENTAL DATA ON PEARL MILLET CULTIVARS OF: (g) GWAGWA, (h) G.I – 14.9, (i) GB 8735, (j) G.I – 297-1 AND (k)

SORGHUM ICSV 111

Time (h)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 10 20 30

283K

305K

323K

g

0

0.1

0.2

0.3

0.4

0.5

0.6

0 10 20 30

283K305K323K

h

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 10 20 30

283K

305K

323K

i

0

0.1

0.2

0.3

0.4

0.5

0.6

0 10 20 30

283K

305K

323K

j

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0 10 20 30

283K305K323K

t/ (M

t – M

0)

k


per unit change in temperature within 12 h of soaking ranged from 0.234 to0.518 g H2O/K.

The mean of K2 and K1 values predicted from regression analysis wereneeded to propose equations (Table 3) based on Peleg’s equation, to modelwater absorption of these grains at each of the temperature of soaking. Theagreement between the proposed equation and experimental data is presentedin Table 4.

TABLE 1.

VALUES OF CONSTANTS IN PELEG’S EQUATION

Material Temperature (K) K1 K2 Mean K2* R2

Pearl millet cultivarsSOSAT C-88 283 3.22 ¥ 10-2 2.20 ¥ 10-2 1.95 ¥ 10-2 0.9842

305 3.04 ¥ 10-2 1.85 ¥ 10-2 (2.180 ¥ 10-3) 0.9788323 1.22 ¥ 10-2 1.80 ¥ 10-2 0.9771

ZANGO 283 5.71 ¥ 10-2 1.93 ¥ 10-2 2.19 ¥ 10-2 0.9804305 5.08 ¥ 10-2 1.82 ¥ 10-2 (5.48 ¥ 10-3) 0.9894323 1.44 ¥ 10-2 1.02 ¥ 10-2 0.9967

EX-BORNO 283 4.86 ¥ 10-2 1.79 ¥ 10-2 1.76 ¥ 10-2 0.9915305 2.98 ¥ 10-2 1.76 ¥ 10-2 (3.00 ¥ 10-3) 0.9915323 2.11 ¥ 10-2 1.73 ¥ 10-2 0.9914

LCRI – IC 9701 283 3.63 ¥ 10-2 1.22 ¥ 10-2 1.09 ¥ 10-2 0.9948305 3.58 ¥ 10-2 1.06 ¥ 10-2 (1.14 ¥ 10-3) 0.9801323 1.42 ¥ 10-2 1.00 ¥ 10-2 0.9788

ICMV – IS 94206 283 3.56 ¥ 10-2 2.45 ¥ 10-2 1.90 ¥ 10-2 0.9801305 2.96 ¥ 10-2 1.92 ¥ 10-2 (5.55 ¥ 10-3) 0.9874323 1.48 ¥ 10-2 1.34 ¥ 10-2 0.9779

ICMV – IS 94208 283 3.53 ¥ 10-2 2.33 ¥ 10-2 2.00 ¥ 10-2 0.9974305 2.77 ¥ 10-2 2.03 ¥ 10-2 (3.46 ¥ 10-3) 0.9899323 1.40 ¥ 10-2 1.64 ¥ 10-2 0.9844

GWAGWA 283 5.98 ¥ 10-2 2.33 ¥ 10-2 1.75 ¥ 10-2 0.9901305 2.89 ¥ 10-2 1.85 ¥ 10-2 (6.31 ¥ 10-3) 0.9897323 1.23 ¥ 10-2 1.08 ¥ 10-2 0.9787

G.I – 14.9 283 2.66 ¥ 10-2 1.99 ¥ 10-2 1.60 ¥ 10-2 0.9964305 2.48 ¥ 10-2 1.78 ¥ 10-2 (5.05 ¥ 10-3) 0.9942323 1.36 ¥ 10-2 1.03 ¥ 10-2 0.9983

GB. 8735 283 6.05 ¥ 10-2 2.44 ¥ 10-2 2.17 ¥ 10-2 0.9793305 6.00 ¥ 10-2 2.07 ¥ 10-2 (2.36 ¥ 10-3) 0.9745323 2.45 ¥ 10-2 2.00 ¥ 10-2 0.9941

G.I – 297-1 283 3.28 ¥ 10-2 1.91 ¥ 10-2 1.38 ¥ 10-2 0.9844305 3.05 ¥ 10-2 1.14 ¥ 10-2 (4.63 ¥ 10-3) 0.9899323 2.34 ¥ 10-2 1.08 ¥ 10-2 0.9879

Sorghum cultivarICSV 111 283 2.29 ¥ 10-2 1.47 ¥ 10-2 1.59 ¥ 10-2 0.9989

305 1.90 ¥ 10-2 1.05 ¥ 10-2 (4.97 ¥ 10-3) 0.9958323 0.90 ¥ 10-2 1.00 ¥ 10-2 0.9974

* Values in brackets are the standard deviations of the means.


The water-absorption characteristics of the pearl millet cultivars studiedindicated an initial high rate of water absorption followed by slower absorp-tion in the later stages. Many researchers have similar curves for some cerealgrains (rice, sorghum) and grain legumes (soyabeans, cowpea) (Engels et al.1986, 1987; Sopade and Obekpa 1990; Sopade et al. 1992, 1994; Sopade andOkonmah 1993). Also the effect of temperature on the water absorption of thepearl millet cultivars was in agreement with the report of Quast and de Silva(1977), Kon (1979), Hsu et al. (1983), Sopade and Obekpa (1990), Sopadeet al. (1992), Sopade and Okonmah (1993) and Sopade et al. (1994). Gener-ally, it was observed that the higher the temperature, the greater the waterabsorbed.

Observing the rate of water absorption at the 12th hour of soaking andfor the three temperatures, it indicated that sorghum absorbed the most water.This agrees with the reports of Sopade et al. (1992). G.I – 297-1 had thehighest water absorption among the pearl millet cultivars, while GB 8735recorded the least. These could be as a result of varietal differences.

There were no definite relationships observed between the quantity ofwater absorbed per unit time and the physicochemical properties (Badau et al.2002) of the grains. However, it has been reported that there are many param-eters that affect water absorption, which include moisture content, proteincontent, carbohydrate and fat contents (Sefa-Dedeh and Stanley 1979; Hsuet al. 1983; Hsu 1983; Ituen et al. 1985).

TABLE 2.ACTIVATION ENERGY OF THE TEMPERATURE DEPENDENCE OF THE RECIPROCAL OF THE PELEG’S CONSTANT K1 AND WATER ABSORBED BY THE GRAINS PER UNIT

CHANGE IN TEMPERATURE

Material Activation energykJ/mol

Mean water absorbed(g H2O/K)

Pearl millet cultivarsSOSAT C-88 4.032 0.347ZANGO 5.728 0.518EX-BORNO 3.467 0.351LCRI – IC 9701 3.908 0.284ICMV – IS 94206 3.650 0.327ICMV – IS 94208 3.845 0.332GWAGWA 6.572 0.449G.I – 14.9 2.789 0.337GB. 8735 3.758 0.264G.I – 297-1 1.405 0.275

Sorghum cultivarICSV 111 3.883 0.234


TABLE 3.

PREDICTED EQUATIONS FOR WATER-ABSORPTION CHARACTERISTIC OF VARIOUS

PEARL MILLET CULTIVARS AND SORGHUM

Materials Temperature (K) Predicted equation

Pearl millet cultivars

SOSAT C-88 283 Mt = M0 +

305 Mt = M0 +

323 Mt = M0 +

ZANGO 283 Mt = M0 +

305 Mt = M0 +

323 Mt = M0 +

EX-BORNO 283 Mt = M0 +

305 Mt = M0 +

323 Mt = M0 +

LCRI – IC 9701 283 Mt = M0 +

305 Mt = M0 +

323 Mt = M0 +

ICMV – IS 94206 283 Mt = M0 +

305 Mt = M0 +

323 Mt = M0 +

ICMV – IS 94208 283 Mt = M0 +

305 Mt = M0 +

323 Mt = M0 +

GWAGWA 283 Mt = M0 +

305 Mt = M0 +

323 Mt = M0 +

100

32 22 2 20

t

t. .+100

3 04 1 85

t

t. .+100

1 22 1 80

t

t. .+100

5 71 1 93

t

t. .+100

5 08 1 82

t

t. .+100

1 44 1 02

t

t. .+100

4 86 1 79

t

t. .+100

2 98 1 76

t

t. .+100

2 11 1 73

t

t. .+100

3 63 1 22

t

t. .+100

3 58 1 06

t

t. .+100

1 42 1 00

t

t. .+100

3 56 2 45

t

t. .+100

2 96 1 92

t

t. .+100

1 48 1 34

t

t. .+100

3 53 2 33

t

t. .+100

2 77 2 03

t

t. .+100

1 42 1 64

t

t. .+100

5 98 2 33

t

t. .+100

2 89 1 85

t

t. .+100

1 23 1 08

t

t. .+


Hsu et al. (1983) and Ituen et al. (1985) obtained a negative correlationbetween absorption capacity and kernel size. It appears that the effect ofchemical composition of food materials on water-absorption capacity is stilluncertain (Sopade et al. 1992). Sefa-Dedeh and Stanley (1979) made refer-ence to protein and obtained positive correlation using cowpea; while Hsuet al. (1983) used soyabean and did not obtain any correlation.

The range of the coefficient of determination (R2) was reasonably highand this indicates a very good fit to the experimental data and it can besuggested that the equation could be used to describe water-absorptioncharacteristics of the pearl millet cultivars at the three temperatures. Theresults obtained are in agreement with earlier reports (Sopade and Obekpa1990; Sopade et al. 1992, 1994; Sopade and Okonmah 1993). The K2 valuesat 32 and 50C were also close enough to assert the fact that K2 is relativelyconstant over the temperature range, and hence could be used as a sorption

G.I – 14.9 283 Mt = M0 +

305 Mt = M0 +

323 Mt = M0 +

GB. 8735 283 Mt = M0 +

305 Mt = M0 +

323 Mt = M0 +

G.I – 297-1 283 Mt = M0 +

305 Mt = M0 +

323 Mt = M0 +

Sorghum cultivar

ICSV 111 283 Mt = M0 +

305 Mt = M0 +

323 Mt = M0 +

Materials Temperature (K) Predicted equation

100

2 66 1 99

t

t. .+100

2 48 1 78

t

t. .+100

1 36 1 03

t

t. .+100

6 05 2 44

t

t. .+100

6 00 2 07

t

t. .+100

2 45 2 00

t

t. .+100

3 28 1 91

t

t. .+100

3 05 1 14

t

t. .+100

2 34 1 08

t

t. .+

100

2 29 1 47

t

t. .+100

1 90 1 05

t

t. .+100

0 90 1 00

t

t. .+

TABLE 3. CONTINUED


constant (Sopade et al. 1992). Hsu (1983), Engels et al. (1986) and Hendrickxet al. (1987) observed that the diffusion coefficient during water sorptionvaries directly with temperature. They used the Arrhenius equation to definethe relationship. The constant K1 in the Arrhenius equation has been shownto be inversely related to temperature (Sopade and Obekpa 1990). GWAGWAhad the highest activation energy, while G.I – 297-1 had the lowest. Therefore,the reciprocal of K1, for GWAGWA is more sensitive to temperature, whileG.I – 297-1 is less sensitive.

Peleg (1988) reported that the unit of K1 is hour per percent weight; theunit of the reciprocal will therefore be percent weight per hour. Hence, thesensitivity of the reciprocal of K1 to temperature will give an indication ofthe temperature effect on rate of water absorbed (percent weight per hour) andhow critical the control of temperature needs to be when food materials areundergoing water sorption (Sopade et al. 1992). The mean water absorbed perunit change in temperature within the nonlinear section of the curves is afunction of the activation energy (Table 2). A linear regression analysisshowed that the following equation: W = 0.044 Ea; R2 = 0.712 could be

TABLE 4.APPLICATION OF THE PREDICTED EQUATIONS TO THE EXPERIMENTAL DATA – c2 (CHI

SQUARE) – TEST AND ROOT-MEAN-SQUARE DEVIATION (RMSD)

Material Soaking temperature (K)

283 305 323

c2-value RMSD c2-value RMSD c2-value RMSD

Pearl millet cultivarsSOSAT C-88 25.48 (30)* 0.98 29.66 (30) 1.07 34.78 (26) 1.09ZANGO 20.10 (30) 0.79 29.08 (30) 0.01 34.60 (26) 1.08EX-BORNO 39.37 (30) 1.05 37.48 (30) 1.04 35.33 (26) 1.05LCRI – IC 9701 26.30 (30) 0.98 38.65 (29) 1.03 35.53 (26) 1.47ICMV – IS 94206 34.92 (30) 1.10 28.44 (28) 1.09 33.31 (25) 1.06ICMV – IS 94208 39.53 (30) 0.01 29.78 (28) 1.17 34.36 (26) 1.08GWAGWA 31.77 (30) 0.99 39.76 (30) 1.30 34.93 (26) 1.04G.I – 14.9 29.97 (30) 1.11 39.18 (30) 1.08 33.15 (26) 1.33GB. 8735 21.91 (30) 0.81 30.72 (30) 1.026 37.23 (26) 1.03G.I – 297-1 31.32 (30) 1.13 31.86 (29) 1.11 28.07 (24) 1.29

Sorghum cultivarICSV 111 35.15 (30) 1.58 35.60 (30) 1.50 30.91 (24) 1.38Critical values† df = 24 df = 25 df = 26 df = 28 df = 29 df = 30

5% 36.4 37.6 38.9 41.30 42.6 43.810% 33.2 34.4 35.6 37.9 37.9 40.3

* Values in brackets are the corresponding degrees of freedom.† Source: Mead et al. (1993).


used to describe the relationship, where W = mean water absorbed andEa = activation energy. This could imply that the lower the activation energy,the lower the rate of water absorbed per unit change in temperature forthe pearl millet cultivars. Hence, temperature control during soaking ofGWAGWA and ZANGO needs to be more precise.

It has been observed that both the experimental and predicted absorbedwater were significantly (c2, 5, 10% level of significant) close, so as to confirmthe suitability of the proposed equations. The maximum RSMD was about1.30 g water/100 g dry grains and 1.08 g water/100 g dry grains forGWAGWA and ZANGO, respectively. Generally, the proposed equationsapproximated to experimental data better at 30C for ICMV – IS 94206 andICMV – IS 94208, better at 10C for SOSAT C-88, ZANGO, LCRI – IC 9701,GWAGWA, G.I – 14.9 and GB 87.35 and better at 50C for EX-BORNO, G.I– 297 and ICSV 111. Similarly, Sopade et al. (1992) reported that theirproposed equations approximated better at 30C. This difference could be asa result of varietal differences and environmental conditions (Sopade andObekpa 1990). Sopade and Obekpa (1990) reported that temperature gradientbetween the grains and soak water at nonambient temperature affect absorp-tion during early part of soaking. Irrespective of the cause of the deviation,the suitability of Peleg’s equation at the soak temperature is evident.

CONCLUSION

The suitability of Peleg’s equation in modeling water absorption duringsoaking of various pearl millet cultivars used in this study has been demon-strated. The proposed equations could be used by interested processors toreasonably predict water absorption at any given time and specified tempera-ture for a known period of soaking or exposure to water during rainfall priorto harvesting. The Arrhenius equation was used to describe the relationshipbetween the reciprocal of K1 and temperature. The resulting activation energygave a good indication of the rate of change of water absorbed with temper-ature and could serve as an index for the level of control of sorption operations.

ACKNOWLEDGMENT

The assistance rendered to Mr. M.H. Badau (in the form of publishedarticles on water absorption) by Dr. P.A. Sopade and Professor Micat Pelegis highly acknowledged. Also financial support in part by a grant fromINTSORMIL and ROCAFREMI is gratefully acknowledged. We thankICRISAT and LCRI for supplying the samples.


REFERENCES

ADEYEMI, I.A. 1983. Dry-milling of sorghum for Ogi manufacture. J. CerealSci. 1, 221–227.

AOAC. 1990. Official Methods of Analysis, 15th Ed., Vols 1 & 2, Associationof Official Analytical Chemists, Washington, DC.

BADAU, M., NKAMA, I. and AJALLA, C.O. 2002. Physicochemical char-acteristics of pearl millet cultivars grown in Northern Nigeria. Int. J. FoodProp. 5, 37–47.

CRANK, J. 1975. The Mathematics of Diffusion, 2nd Ed., Oxford UniversityPress, Oxford, UK.

ENGELS, C., HENDRICKX, M., DE SAEMBLANX, S., DE GRYZE, I. andTOBBACK, P. 1986. Modelling water diffusion during long grain ricesoaking. J. Food Eng. 5, 55–73.

ENGELS, C., HENDRICKX, M. and TOBBACK, P. 1987. Limited multi-layer absorption of brown, parboiled rice. Int. J. Food Sci. Tech. 22, 219–223.

FAO AND ICRISAT. 1996. The World Sorghum and Millet Economics. Facts,Trends and Outlook. Food and Agriculture of the United Nation, viabledelle Terme di Caracalla, 00100 Rome, Italy and International CropsResearch Institute for the Semi-Arid Tropics, Panancheru 502324, AndiraPradesh, India.

HENDRICKX, M., ENGELS, C. and TOBBACK, P. 1987. Three-dimensional TLM models for water diffusion in white rice. J. Food Eng.65, 187–197.

HSU, K.H. 1983. Effect of temperature on water diffusion on soyabean. J.Food Sci. 48, 1364–1365.

HSU, K.H., KIM, C.S. and WILSON, L.A. 1983. Factors affecting wateruptake of soyabeans during soaking. Cereal Chem. 60, 208–221.

ITUEN, E.U.U., MITTAL, J.P. and ADEOTI, J.S. 1985. Water absorption incereal grains and its effects on their rapture stress. J. Food Eng. 8, 147–158.

KON, S. 1979. Effect of soaking temperature on cooking and nutritionalquality of beans. J. Food Sci. 44, 1328–1334.

LEES, R. 1975. Food Analysis: Analytical and Quality Methods for theFood Manufacturer and Buyer, 3rd Ed., Leonard Hill Books, London,UK.

MEAD, R., CURNOW, R.N. and HASTED, A.M. 1993. Statistical Methodsin Agriculture and Experimental Biology, 2nd Ed., Chapman & Hall,London, UK.

NELSON, A.I., STEINBERG, M.P. and WEI, L.S. 1976. Illinois processpreparation of soymilk. J. Food Sci. 41, 57–72.


NKAMA, I., TEGOMOH, M.T. and ADDY, E.O. 2000. Studies on themalting characteristics of some local cereals. J. Arid Agric. 10, 139–143.

PELEG, M. 1988. An empirical model for description of moisture sorptioncurves. J. Food Sci. 53, 1216–1219.

QUAST, D.G. and DE SILVA, S.D. 1977. Temperature dependence of hydra-tion rate and effect of hydration on the cooking rate of dry legumes. J.Food Sci. 42, 1299–1303.

SEFA-DEDEH, S. and STANLEY, D.W. 1979. The relationship of micro-structure of cowpeas to water absorption and dehulling properties. CerealChem. 56, 279–386.

SOPADE, P.A. and OBEKPA, J.A. 1990. Modeling water absorption in cow-peas, soyabean and peanut at three temperatures using Peleg’s equation.J Food Sci. 55, 1084–1807.

SOPADE, P.A. and OKONMAH, G.N. 1993. The influence of physicochem-ical properties of foods on their water absorption behaviour: A quantita-tive approach. ASEAN Food J. 8, 32–36.

SOPADE, P.A., AJISEGIRI, E.S. and BADAU, M.H. 1992. The use of Peleg’sequation to model water absorption in some cereal grains during soaking.J. Food Eng. 1, 269–283.

SOPADE, P.A., AJESIGIRI, E.S. and OKONMAH, G.N. 1994. Modelingwater absorption characteristics of some Nigerian varieties of cowpeaduring soaking. Trop. Sci. 34, 297–305.

water-absorption characteristics of various pearl millet cultivars and sorghum grown in northern...

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