Modelling moisture sorption isotherms for maize flour

Modelling moisture sorption isotherms for maize flour

ARTICLE IN PRESS Journal of Stored Products Research 44 (2008) 179–185 www.elsevier.com/locate/jspr Modelling moisture sorption isotherms for maize ...

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ARTICLE IN PRESS

Journal of Stored Products Research 44 (2008) 179–185 www.elsevier.com/locate/jspr

Modelling moisture sorption isotherms for maize flour O.J. Oyeladea,, T.Y. Tunde-Akintundeb, J.C. Igbekac, M.O. Okeb, O.Y. Rajid a

Procter Department of Food Science, University of Leeds, Leeds LS2 9JT, UK b Ladoke Akintola University of Technology, PMB 4000, Ogbomoso, Nigeria c Department of Agricultural and Environmental Engineering, University of Ibadan, Ibadan, Nigeria d Centre for Epidemiology and Biostatistics, University of Leeds, Leeds LS2 9JT, UK Accepted 3 October 2007

Abstract The sorption isotherm of food material is pertinent in the processing and storage of food products. Adsorption and desorption isotherms for maize flour were investigated using the static gravimetric method over the range of temperature (27–40 1C) and water activity (aw) (0.10–0.80) commonly experienced in the tropical environment. The experimental data were compared with five widely recommended models in the literature for food sorption isotherms (GAB, modified GAB (MGAB), modified Oswin (MOE), modified Henderson (MHDE), and modified Chung–Pfost (MCE)). The GAB, MGAB, and MOE models were found to be acceptable in predicting the moisture sorption isotherms for maize flour. Overall, the MGAB appears to be most suitable for fitting the adsorption and desorption moisture isotherms data for the maize flour. r 2007 Elsevier Ltd. All rights reserved. Keywords: Maize flour; Storage; Sorption isotherms; Acid solutions; Equilibrium moisture content

1. Introduction Maize (Zea mays L.) is an important grain and is among the millet group of cereals. At the time of harvest, maize is reported to have a moisture content ranging from 28% to 35%, wet basis (Asiedu, 1989). Drying and size reduction processes are common practices that are carried out to produce maize flour as an alternative way of presenting the grain to commerce in tropical developing countries. In Nigeria, it is the most popularly grown and consumed cereal in all ecological zones (Iken et al., 2002). It is also a principal source of food in many other developing and developed countries (Samapundo et al., 2007). Knowledge of the sorption characteristics is essential in regard to stability in storage and acceptability of food products, drying process modelling, design and optimization of drying equipment, aeration, calculation of moisture changes which may occur during storage, and for selecting appropriate packaging materials (Ngoddy and Bakker-Arkema, 1975; Corresponding author. Tel.: +44 113 2747904.

E-mail addresses: [email protected], [email protected] (O.J. Oyelade). 0022-474X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jspr.2007.10.005

Erol et al., 1990; Kumar, 2000; Aviara and Ajibola, 2002; Viswanathan et al., 2003; Chowdhury et al., 2005; Akanbi et al., 2006; Samapundo et al., 2007). Aviara et al. (2006) have reported the static gravimetric technique as the preferred method for determining the moisture sorption isotherms of food products. This method has several advantages over the manometric and hygrometric techniques, and these are: ability to determine the exact dry weight of the sample, minimization of temperature fluctuation between samples and their surroundings or the source of water vapor, registering the weight change of the sample in equilibrium with the respective water vapor pressures, and achieving hygroscopic and thermal equilibrium between samples and water vapor source. The differences in experimental techniques adopted affect the sorption properties of foods (Al-Muhtaseb et al., 2002). Saturated salt solutions are commonly used to create the necessary micro-climate for the sorption experiments. This has necessitated the production of template-like data to prepare the standard solutions referred to by several researchers (Chowdhury et al., 2005; Aviara et al., 2006). Acid solutions can also control the relative humidity (r.h.) of air at different temperature levels. Using acid solutions

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for conditioning the micro-climate for research studies in developing countries has advantages because of their ready availability, and there is also the lack of CO2 absorption by acid compared with KOH or some other salt solutions. The choice of acids as desiccants can therefore reduce the risk of uncertainty in determinations of water activity during sorption studies. The European Project Group COST 90 on Physical Properties of Foods and the American Society of Agricultural Engineers have recommended some isotherm equations as the preferred ones for food isotherms and hence are the widely adopted food isotherms models (Chowdhury et al., 2005; Aviara et al., 2006). Because crop processing has a marked influence on the physicochemical and nutritional attributes of food products (Igbeka and Olumeko, 1996; Oyewole and Afolami, 2001; Falade et al., 2003), it is desirable to investigate the use of these isotherms for maize flour. Sopade and Ajisegiri (1994) have constructed moisture sorption isotherms for a variety of maize grains. The main aim of this study was to determine the sorption isotherms of maize flour over a range of temperatures and humidities commonly experienced in the tropical environment. The specific objectives include the presentation of the influence of temperature on sorption isotherms, the hysteresis loop, and modelling of the adsorption and desorption moisture isotherms using five widely recommended isotherm models. 2. Materials and methods 2.1. Material preparation Matured and dried white maize of a local variety (Akure local) that was used in this study was purchased in a market at Ogbomoso, Nigeria. The preparation of the flour was in accordance with the traditional procedure. Grains were cleaned, seed coats removed with a locally available hand mill, and winnowed. The decorticated maize grains were then milled to obtain the maize flour. Initial moisture content (m.c.) of the flour was determined in triplicate according to the AOAC (1990) procedures. This involved placing 1 g of the flour in an oven that was set at 130 1C for 1 h and reweighing.

uniform single layer in the plastic containers used as moisture pans, were placed in desiccators at each aw point and temperature level (Ajibola, 1986). In this way the desiccators were maintained at aw values of 0.1, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, and 0.80, respectively. The desiccators were placed in a Genlab (England) Incubator (Model M75CPD) to maintain the required temperature. Samples were monitored for equilibration by weighing at intervals until constant weights were attained. The moisture content of the equilibrated samples (the e.m.c.) was then found by calculation from the original m.c. and the known change in weight (Igbeka et al., 1975; Oyelade et al., 2001). 2.4. Determination of desorption isotherms Samples used in the adsorption experiments were also used to construct desorption isotherms by transferring the samples in the range 0.15–0.80aw, in a step-wise manner to the immediate lower humidity level at the same temperature. The desiccators were placed in the incubator for desorption until the equilibrium conditions were reached when three consecutive measurements gave similar readings (Ajibola, 1986; Oyelade et al., 2001; Falade et al., 2003). 2.5. Isotherm equations and modelling Five widely recommended isotherm equations (GAB, modified GAB (MGAB), modified Oswin (MOE), modified Henderson (MHDE), and modified Chung–Pfost (MCE)) that were investigated with the experimental data are shown in Table 1 in the form M ¼ f ðaw ; TÞ. The SAS procedure for non-linear regression (Proc NLIN) was used Table 1 Sorption isotherm models used for maize flour Model

M ¼ f ðaw ; TÞ

GAB



abcaw ð1  caw Þð1  caw þ bcaw Þ

MGAB



aðc=tÞbaw ð1  baw Þ½ð1  baw þ ðc=TÞbaw Þ

MOE

2.2. Micro-climate

 M ¼ ða þ bTÞ 

aw 1  aw

c

1=c

The required micro-climate was prepared based on previous experience by using H2SO4 (Oyelade, 1997; Oyelade et al., 2001). The baseline properties of H2SO4 provided by Perry and Green (1984) were used for this purpose.

MHDE

2.3. Determination of adsorption isotherms

GAB ¼ Guggenheim, Anderson and de Boer equation, MGAB ¼ modified Guggenheim, Anderson and de Boer equation, MOE ¼ modified Oswin equation, MHDE ¼ modified Henderson equation, MCE ¼ modified Chung–Pfost equation. M ¼ equilibrium moisture content (%, dry basis); a, b, c ¼ models, constant parameters; T ¼ temperature, 1C.

The static gravimetric method was used. Three replicates each of weighed maize flour (1–1.03 g), sufficient to give a

MCE



lnð1  aw Þ aðT þ bÞ

  1 ðT þ bÞ lnðaw Þ M ¼  ln c a

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RSS ¼

n X

ðM cal  M pred Þ2 ,

(1)

i¼1

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P ðM cal  M pred Þ2 SEE ¼ , n

(2)

RES , (3) TES where Mcal is the e.m.c. by experiment, % wet basis; Mpred is the predicted e.m.c. by models, % wet basis; RES is the residual sum of squares; and TES is the total error sum of squares.

R2 ¼ 1 

3. Results and discussion 3.1. Adsorption moisture isotherms The e.m.c.’s at different aw determined using the AOAC procedure were used in plotting the adsorption isotherms shown in Fig. 1. The isotherms followed the characteristic type II (sigmoid shape) classification, an indication that the adsorption in maize flour was a multilayer adsorption typical of cereal samples with micro-capillary structure (Ertugay and Certel, 2000). Isotherms of food products high in starch content also demonstrate this attribute (Onayemi and Oluwamukomi, 1987; Kumar, 1994, 2000). 3.2. Moisture sorption hysteresis loop

Equilibrium moisture content, %d.b

The hysteresis loops obtained for maize flour were large enough to conform to the typical type of hysteresis that has been reported for starchy foods (Al-Muhtaseb et al., 2002). The loops displayed a tendency of executing a closed loop (the hysteresis loop) between the upper and lower bounds of r.h. (0pawp1), and were affected by temperature 18 16 14 12 10 8 6 4 2 0

27 °C

32 °C

37 °C

(Fig. 2). This attribute has been well documented for certain food products by several researchers (Ajisegiri, 1987; Sopade and Ajisegiri, 1994; Aviara et al., 2006).

3.3. Isotherm predictive models The five widely recommended models which were investigated for maize flour are given in Table 1. The unknown values that were estimated (constants) for each of the models, and the indices for estimating the errors associated with the models, which are the coefficient of fit (R2), RSS and SEE, are shown in Tables 2 and 3 for the adsorption and desorption isotherms, respectively. All the five tested models were generally suitable for predicting the adsorption isotherms for maize flour, with their R2 values ranging from 0.930 to 0.994. There is no clear distinction in R2 values (0.9944 and 0.9943) displayed by the GAB and MOE models, which indicated the same degree of very high reliability of prediction with the equations in terms of coefficient of fit. Closely following this in reliability for R2 values are the MGAB (R2=0.994) and MHDE (R2=0.990) models. MCE with R2 of 0.930 gave the least degree of reliability in this context. However, despite the high R2 values the residual plots displayed systematic patterns for some of the models (see Figs. 3 and 4). This shows the value of the residual plot as an index of assessing the closeness of fit because the R2 could be viewed as a measure of systematic departure from linearity.

Equilibrium moisture content, % d.b

to fit the experimental e.m.c.–aw data in modelling the sorption isotherms. The accuracy of the models was evaluated by using three different indicators, namely residual sum of squares (RSS), standard error of estimate (SEE), and coefficient of determination (R2). These indicators of errors are defined in Eqs. (1)–(3)

16 14 12 10 8 6 4 2 0

0.2

0

0.1

0.2

32 °C desorption

0.3 0.4 0.5 0.6 Water activity, aw

0.7

0.8

0.9

Table 2 Estimated values for fitting models with their evaluation indicators (adsorption)

40 °C

0.4 0.6 Water activity, aw

32 °C adsorption

Fig. 2. Hysteresis plot for maize flour.

Equation

0

181

0.8

Fig. 1. Adsorption isotherms for maize flour.

1

GAB MGAB MOE MHDE MCE

Equation constants a

b

c

4.4966 4.6320 11.3701 0.000288 225.5

71.1404 0.8754 0.0884 21.9347 15.5831

0.8868 1471.7 0.4076 1.7466 0.2175

RSS

SEE

R2

26.1364 24.3558 26.7877 47.9391 44.7751

0.6771 0.6537 0.6856 0.9171 0.8863

0.9944 0.9939 0.9943 0.9898 0.9302

a, b, c, are the model constants; RSS, residual sum of squares; SEE, the standard error of estimate; and R2, the coefficient of fit.

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182

It explores the amount of variability in the response variable explained by the predictors. Therefore, using the three modes of closeness of fit indicator, the closeness of fit parameters

were generally lower for the prediction of adsorption isotherms in comparison with the desorption isotherms. MGAB appeared the most suitable for predicting the adsorption isotherms with the least RSS, SEE and highest R2 (see Table 2) and also displayed a randomized residual plot (see Fig. 3). The MGAB model also appeared most suitable for the desorption isotherms (see Table 3 and Fig. 4) despite not having the least RSS and SEE. MOE which had the least RSS, SEE and highest R2 for predicting the desorption emc displayed a systematic residual plot (see Fig. 4). Therefore, Figs. 5 and 6 show the comparison of experimental isotherms and the prediction of the adsorption and desorption isotherms, respectively, with the MGAB model. A survey of the literature indicated that models such as GAB, Oswin, Halsey, Chung–Pfost, BET, and modified BET are suitable to fit the e.m.c.–aw data of corn flour and related corn-based products under some specified

Table 3 Estimated values for fitting models with their evaluation indicators (desorption) RSS

Equation Equation constants a GAB MGAB MOE MHDE MCE

b

SEE

R2

c

4.7223 80.4004 0.8762 4.8814 0.8631 1532.0 12.2281 0.1037 0.3958 0.000281 13.8259 1.7997 208.6 9.8714 0.2149

32.7329 30.7363 29.1108 49.7439 46.1885

0.7578 0.7343 0.7146 0.9342 0.9002

0.9944 0.9939 0.9943 0.9898 0.9302

a, b, c, are the model constants; RSS, residual sum of squares; SEE, the standard error of estimate; and R2, the coefficient of fit.

3

1.5

Standardized residuals

Standardized residuals

2 1 0.5 0 -0.5

0

5

10

15

20

-1 -1.5 -2

0 0

5

10

15

20

-1 -2 EMC, % d.b

2

2 1 0 0

5

10

15

20

-1 -2

Standardized residuals

1.5 1 0.5 0 -0.5 0

5

10

15

-1 -1.5 -2 -2.5

EMC, % d.b

EMC, % d.b

2 1.5 Standardized residuals

Standardized residuals

1

-3

EMC, % db

3

-3

2

1 0.5 0 -0.5

0

5

10

15

20

-1 -1.5 -2 -2.5

EMC, % d.b

Fig. 3. Adsorption isotherms predictive models, residual plots for (a) MHDE, (b) GAB, (c) MGAB, (d) MOE, and (e) MCE models.

20

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2

183

3

1 0.5 0 -0.5

0

5

10

15

2

Standardized residuals

Standardized residuals

1.5

20

-1

1 0 0

5

10

15

20

-1 -2

-1.5 -3

-2

Emc, % d.b

Emc, % d.b

2

2

1.5

1 0 0

5

10

15

20

-1 -2 -3

Standardized residuals

Standardized residuals

3

1 0.5 0 -0.5

0

5

10

15

20

-1 -1.5 -2

Emc, % d.b

Emc, % d.b

-2.5

Standardized residuals

2 1.5 1 0.5 0 -0.5

0

5

10

15

20

-1 -1.5 -2

Emc, % d.b

Fig. 4. Desorption isotherms predictive models, residual plots for (a) MHDE, (b) GAB, (c) MGAB, (d) MOE, and (e) MCE models.

conditions (Hsien, 1994; Ikhu-Omoregbe, 2000; Vega-Galvez et al., 2006). This study confirms the type II isotherm shape previously observed for cereal products (Ertugay and Certel, 2000), and shows that the widely recommended GAB, MGAB, MOE, MHDE, and MCE models are suitable for predicting the e.m.c.–aw of corn flour under practical storage conditions in the tropics. 4. Conclusions The AOAC procedure that was used for the determination of e.m.c. may not necessarily produce the same set of curves when other known methods for moisture determi-

nation such as ISO are applied. Therefore, it may be necessary to apply appropriate correction factors depending on the circumstances in which the isotherm curves are to be used. The following conclusions are drawn from the study into the moisture sorption isotherms of maize flours at 27, 32, 37, and 40 1C, respectively. (1) The adsorption and desorption isotherms are sigmoidal in shape and are influenced by temperature. (2) Three (GAB, MGAB, and MOE models) out of the five commonly recommended models are acceptable for describing the adsorption and desorption isotherms for maize flour at 27, 32, 37, and 40 1C.

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Equilibrium moisture content, % wet basis

18

27 °C

32 °C

37 °C

32 °C Exptal

37 °C Exptal

40 °C Exptal

40 °C

Acknowledgment

27 °C Exptal

16

O.J. Oyelade is grateful to the British Council for the provision of a Commonwealth Academic Staff Scholarship.

14 12 10

References

8 6 4 2 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Water activity

Fig. 5. Comparison of experimental and MGAB predictive adsorption isotherms.

Equilibrium moisture content, % wet basis

18

27 °C

32 °C

37 °C

40 °C

27 °C Exptal

32 °C Exptal

37 °C Exptal

40 °C Exptal

16 14 12 10 8 6 4 2 0

0

0.2

0.4

0.6

0.8

1

Water activity Fig. 6. Comparison of experimental and MGAB predictive desorption isotherms.

(3) The three acceptable models were found to predict the adsorption isotherms for maize flour better in comparison with the desorption isotherms at all the tested temperatures. (4) The MGAB model proved the best for predicting the adsorption moisture isotherms for maize flour at the investigated temperatures. (5) Although the MOE gave the least standard error of estimate, least residual sum of squares and highest coefficient of fit, the residuals plot displayed a systematic departure from the fitted curve and hence the MGAB model is most acceptable for describing the desorption isotherms for maize flour at the investigated temperatures. (6) The adsorption and desorption isotherms show the occurrence of moisture sorption hysteresis in maize flour at all the investigated temperatures. (7) The size of the total hysteresis appears to reduce as the temperature increases.

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