Moisture sorption isotherms of some Nigerian food grains

J. stored Prod. Res. Vol. 21, No. 2. pp. 53-58, Prmted m Great Britain. All rights reserved

1985 Copyright

0

0022-474X185 $3.00 + 0.00 1985 Pergamon Press Ltd

MOISTURE SORPTION ISOTHERMS OF SOME NIGERIAN FOOD GRAINS A. 0. DENLOVE and A. 0. ADE-JOHN Department of Chemical Engineering, University of Lagos, Lagos, Nigeria (Received

9 December

1983)

Abstract-A fluidized bed apparatus that can be used to determine moisture sorption isotherms of granular food products is described. This apparatus was used to obtain moisture sorption isotherms for maize, cowpeas, groundnuts and soyabeans at temperatures of 30,40 and 50°C. Hysteresis was observed in the isotherms for the four products and the equilibrium moisture content was lower for the oily grains (soyabeans and groundnuts) at the same value of relative humidity. The Hailwood and Horrobin equation was used to correlate the experimental data and the values of the correlating constants are tabulated. The Brunauer, Emmett and Teller equation was also used to obtain estimates of the monolayer water content for the four grains.

INTRODUCTION

The concept of equilibrium moisture content is important in the study of drying and preservation of foodstuffs because the equilibrium moisture content determines the minimum moisture content to which a food product can be dried under a given set of conditions. A fundamental purpose of drying foodstuffs is to lower the moisture content of the food to a level at which there is no danger to growth of undesirable microorganisms. When a food product is exposed to an atmosphere of uniform relative humidity and constant temperature, the inter-granular relative humidity reaches a constant value known as the equilibrium relative humidity after a period of time. A plot of the moisture content at equilibrium on a dry basis against the equilibrium relative humidity or water activity (a) is known as the moisture sorption isotherm. The moisture sorption isotherm of a product is specific at a given temperature and the shape of the curve for most biological products is sigmoid (S-shaped). The moisture sorption isotherm depends on whether the equilibrium was approached by wetting (adsorption) or by drying (desorption). A closed loop hysteresis has been observed by many workers, Babbitt (1945), Houston (1952), Benson and Richardson (1955), Bushuk and Winkler (1957), Chung and Pfost (1967) and Pixton and Warburton (1971, 1975), for most biological products, with the desorption isotherm showing the larger equilibrium moisture content at a given equilibrium relative humidity. Many models and more than 75 equations have been proposed to describe moisture sorption isotherms. Boquet et al. (1979) concluded that the Hailwood and Horrobin (1946) equation was the best and most versatile. This equation is based on a model of a two phase system consisting of water vapour and a solid solution of water and the polymers that make up the bio-materials. In this study, an apparatus that can be used to rapidly determine moisture sorption isotherms for granular food products is described. Moisture sorption isotherms were determined for four granular food products, maize, cowpea, groundnuts and soya beans of Nigerian origin and at three temperatures (30, 40, 50°C). The Hailwood and Horrobin (1946) equation was used to correlate the experimental data, and the Brunauer, Emmett and Teller (1938) equation was used to estimate values of the monolayer water content for the four products.

MATERIALS

AND

METHODS

Appuratus

A flow diagram of the apparatus is given in Fig. I. The equilibrium cell consisted of a rectangular perspex fluidized bed (25 x 25 mm) and the fluidizing air was provided by a high speed, high pressure fan. The fluidizing air passed in a closed cycle through the heat exchanger, electric heater, acrylic

A. 0.

54

DENLOYE and A. 0. ADE-JOHN

FI.w( kz----l

I

r-l

Fluidizad bed Equilibrium Cell t

valve

I ’

Heat t Exchanger

Electric Heater

r

I Sulphurlc Soln

Fan

Acid

bath

Fig. 1. Flow diagram of an apparatus for determining moisture sorption isotherms for granular food products.

sulphuric acid bath, equilibrium cell and a low pressure drop flowmeter. The heat exchanger was installed to provide cooling of the circulating air and the thermostatically controlled electric heater provided the necessary heating of the air. Sulphuric acid solutions of varying concentration in the bath provided atmospheres of different relative humidity. A stainless steel filter gauze at the base of the column ensured that entrained droplets of sulphuric acid solution did not enter the equilibrium cell. Uniform conditions existed in the cell as a result of fluidization, and the inherent high rate of heat and mass transfer between the fluid and particles led to a rapid attainment of equilibrium. Sample preparation

Bulk samples of about 10 kg of each of the food product were obtained from a local market in Ketu, Lagos and each sample was then split into three. One batch was kept as a reference and was stored in a deep freezer maintained at - 15°C. The second batch was dried in a vacuum oven maintained at a temperature of 30°C and 5 mm Hg pressure for 18 h. This dried batch was used for the determination of the adsorption or wetting isotherms. The third batch was soaked in distilled water, maintained at a temperature of 30°C for 24 h, and then allowed to equilibrate in a closed container maintained at 10°C for about 10 days. This batch was used to obtain the desorption of drying isotherms.

/

P

Desorption

o

AdsorptIon

A DosorptIon o

0

0.2 Water

Adsorption

0.4 activity

0.6 (a

0.6

1.0

1

Fig. 2. Moisture sorption isotherm for cowpeas (30, 50°C).

0

02

0,4 Water

06

activity

06

10

(17 )

Fig. 3. Moisture sorption isotherm for maize (3o’C).

Moisture sorption isotherms

55

Relative humidity determination

A sample of about 5 g was taken from the appropriate batch and placed in the equilibrium cell. The fan was switched on and the air flow was adjusted so that the fluidized bed bubbled gently. The air flowrates during the experiment runs ranged from 50 to 80 l/min, but was kept at a constant value for each run. For operation at 30°C the electric heater was switched off, and it was found necessary to cool the air leaving the blower by turning on the water flow to the heat exchanger. For operation at 40°C and above, the water flow to the heat exchanger was turned off, and the circulating air was heated by the electric heater. Equilibrium was monitored by withdrawing the equilibrium cell from the circuit, rapidly weighing it and installing it back to the circuit. Equilibrium which was attained after about 30min was indicated by a constant weight (change in weight of cell less than 1 mg) of the equilibrium cell. The air flow was continued for an additional 15 min after equilibrium had been indicated. The relative humidity and temperature in the equilibrium cell which were monitored continuously using a digital psychrometer were also constant at equilibrium. The constant values of both the temperature and relative humidity were recorded. Equilibrium moisture determination

After equilibrium was attained, the food sample was quickly transferred to a lidded container of known weight. The container without its lid was then placed in a vacuum oven which was maintained at 80°C and at a pressure of 5 mm Hg for an initial drying period of about 10 h. The drying conditions were chosen by experiment to ensure a rapid evaporation of water vapour from the sample without deterioration of the sample. The sample was then removed from the oven and the container was covered, cooled in a desiccator and weighed. The sample was then dried for two further periods of 4 h and then reweighed till a constant weight (change in weight of sample less than 0.1 mg) was obtained. Moisture content on a dry basis was then calculated. The experiments were carried out twice, and the results (Figs 2-5) represent the average of two experimental measurements. The results of the two measurements were always close, the maximum discrepancy being less than 5%. RESULTS

AND DISCUSSION

The moisture sorption isotherms for the four food grains are presented in Figs 2-8. Figures 2, 3, 4 and 5 present the adsorption and desorption moisture sorption isotherms at 3O.O”C for cowpeas, maize, soyabeans and groundnuts respectively. The isotherms have the usual sigmoid shape and show hysteresis to a varying degree, with the desorption isotherm having a greater moisture content than the adsorption isotherm at the same value of relative humidity. The hysteresis effect appears more noticeable in the intermediate relative humidity ranges. The maximum hysteresis effect in the moisture content for maize, cowpeas, groundnuts and soyabeans

b

Oesorption

0

Adsorption

L

0

0.2

0.4 Water

activity

Fig. 4. Moisture sorption isothem

0.6

0.6

10

((11

for soyabeans (30°C).

0

0.2

0.4 Water

0.6 activity

0.6

10

(~7)

Fig. 5. Moisture sorption isotherm for groundnuts (30°C).

A. 0. DENLOVE and A. 0. ADE-IOHN

56

I

I 0

TEMPERATURE 40%

I

I

I

I

0.2

0.4

Q6

0.8

Water Fig. 6. Adsorption

?

activity

I

10

(0)

moisture isotherms for four food grains (40°C). The full lines represent

Horrobin

correlation

Hailwood

and

for the four grains.

are 0.009, 0.006, 0.002 and 0.003 respectively. The oily grains (soyabeans and groundnuts) seem to exhibit a lower hysteresis effect than the non-oily grains (maize and cowpeas). Pixton and Warburton (1971) in fact observed no hysteresis effect in their moisture sorption isotherm for groundnuts. The presence of oil in the grains therefore appears to reduce the hysteresis effect observed in food grains. The hysteresis effect also appears to wane with increase in temperature (Fig. 2). Figure 6 compares the adsorption moisture isotherms for the four grains. It is apparent that the isotherms for the non-oily grains are higher than those for the oily grains. It appears that the presence of oil in the oily grains suppresses the equilibrium moisture content. A similar effect was observed by Pixton and Warburton (1971). The effect of temperature on the moisture sorption isotherms is shown in Figs 7 and 8 for maize and groundnuts respectively. Equilibrium moisture content at constant relative humidity decreases as temperature is increased from 30 to 50°C. The effect of temperature is however slight both for the oily and non-oily grains. The temperature coefficient (k?x/aT) is of the order of O.OOYC- for the four products. The temperature coefficient appears to decrease slightly with increasing moisture content, and is also slightly lower for the oily grains. The experimental data obtained for maize and groundnuts at 3O.O”C were compared with the data of Gough (1975) and Pixton and Warburton (1971) and the results were in agreement within about 10%.

I

I

I

I

0.2

0.4

0.6

0.8

Water

Fig. 7. Desorption

moisture

activity

isotherms

temperatures.

(0

I 1.o

0

1

for maize at different

Fig. 8. Desorption

0

3O’C

A

40-c

I

I

I

I

I

0.2

0.4

06

0.8

1.o

Water

activity

moisture

(0)

isotherms

different temperatures.

for groundnuts

at

Moisture sorption isotherms Table 1. B.E.T. Monolayer Values (x,) and Horrobin’s constants (A, B and C) for some food grains. (Adsorption isotherm) 30°C A B c %I

1.097 10.72 8.616 0.0606

30°C A B C 5,

0.0683 14.84 10.16 0.0548

30°C A B C I*

2.484 28.161 26.21 0.0293

30°C A B C %I

0.9074 22.39 20.04 0.0360

Cowpeas 40°C 1.174 11.90 9.865 0.0571 Maize 40°C 0.1340 15.12 10.39 0.0529 Groundnuts 40°C 3.932 28.85 28.56 0.0230 Soyabeans 40°C 1.983 20.99 19.61 0.0350

57

Table 2. B.E.T. Monolayer Values (x,) and Horrobin’s constants (A, B and C) for some food grains. @sorption isotherm)

50°C

30°C

Cowpeas 40°C

1.431 12.480 10.64 0.0548

1.029

1.118

10.35 8.172 0.0637

11.60 9.480 0.0572

50°C

30°C

0.1841 16.04 11.184 0.0577

A B C %I

30°C

50°C 5.611 29.190 30.56 0.02 1

A B C %I

50°C 2.086 22.29 21.02 0.0332

0.1274 12.52 7.940 0.0580

2.702 26.160 26.160 0.3020

30°C A B C -%I

0.942 1 21.23 19.80 0.0370

50°C 1.428 12.094 10.21 0.0548

Maize 40°C 0.2334 14.95 10.140 0.0540

0.1190 14.30 9.584 0.0556 Groundnuts 40°C

5o’c

4.336 26.23 26.11 0.0238 Soyabeans 40°C 1.932 20.44 18.90 0.340

5.848 27.84 29.34 0.02 I

5o’c -

1.987 21.90 20.48 0.0331

An estimate of the monolayer moisture content, X, was obtained using a least square analysis to fit the Brunauer et al. (1938) equation to the various isotherms using values of relative humidity less than 45%. The results for the various isotherms (Tables 1 and 2) show that there is little difference between the values obtained from the adsorption and desorption isotherms, the adsorption values being slightly greater. The monolayer moisture content decreases as temperature is increased, and is higher for the non-oily grains. Salwin (1959) has suggested that the monolayer moisture content is the optimum point for the stability of dehydrated foods. He observed that above and below this moisture level certain chemical and biochemical reactions proceeded at a faster rate. The utility of using the Brunauer et al. (1938) equation to predict optimum moisture content values for food products has however been questioned by Hayakawa and Matas (1978). Hailwood and Horrobin (1946) proposed the following expression for correlating experimental moisture sorption isotherms.

where a is the relative humidity and x is the equilibrium moisture content. A, B and C are empirical constants. The above expression was used to correlate the experimental results and the constants A, B and C were obtained using a least square analysis. Values of the constants are listed in Tables 1 and 2 for the adsorption and desorption isotherms of the four grains at temperatures of 30, 40 and 50°C. The equation described the experimental data adequately and the smooth lines in Fig. 6 are drawn based on the expression. The constants A, B and C generally increase with increase in temperature and the constants for desorption are also generally lower than those for adsorption. REFERENCES Babbitt J. D. (1945) Hysteresis in the adsorption of water vapour by wheat. Nature, Land. 156, 265-266. Benson S. W. and Richardson R. L. (1955) A study of hysteresis in the sorption of Polar gases by native and denatured proteins. J. Am. them. Sot. 77, 2585-2590. Boquet R., Chirife J. and fglesias H. A. (1979) Equations for fitting water sorption isotherms of foods, III. Evaluation of various three-parameter models. J. fd Tech. 14, 527-534. Brunauer S., Emmett P. H. and Teller E. (1938) Adsorption of gases in multimolecular layers. J. Am. them. Sot. 60, 309-3 19. Bushuk W. and Winkler C. A. (1957) Sorption of water vapour on wheat flour, starch and gluten. Cereal Chem. 34, 73-86.

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A. 0. DENLOYEand A. 0. ADE-JOHN

Chung D. S. and Pfost H. B. (1967) Adsorption and desorption of water vapour by cereal grains and their products, Part III. A hypothesis for explaining the hysteresis effect. Trans. Am. Sot. agric. Engrs 10,556-557. Cough M. C. (I 975) A simple technique for the determination of humidity equilibria in particulate foods. J. stored Prod. Res. 11, 161-166. Hailwood A. J. and Horrobin 0. S. (1946) Adsorption of water by polymers. Analysis in terms of a simple model. Trans. Furadu), Sot. 42B, 84-92. Hayakawa K. I. and Matas J. (1978) Moisture sorption isotherms of coffee products. J. fd Sci. 43, 1026-1027. Houston D. F. (1952) Hygroscopic equilibrium of brown rice. Cereal Chem. 29, 71-76. Pixton S. W. and Warburton S. (1971) Moisture content-relative humidity equilibrium at different temperatures of some oil-seeds of economic importance. J. stored Prod. Res. 7, 261-269. Pixton S. W. and Warburton S. (1975) The moisture content-relative humidity relationship of rice bran at different temperatures. J. srored Prod. Res. 11, 1-8. Salwin H. (1959) Defining minimum moisture content for dehydrated foods. Fd Tech.. Champaign 13, 594-595.