Swelling and pore structure in starchy materials

Journal of Food Engineering 9 ( 1989) 7 l-80

Swelling and Pore Structure in Starchy Materials

Roberto J. Aguerre, Constantino Sk-ez & Pascual E. Viollaz

Departamento de Industrias, Facultad de Ciencias Exactas y Naturales, Ciudad Universitaria, (1428) Buenos Aires, Argentina

(Received 28 October 1987; revised version received 12 July I 988; accepted 29 November 1988)

ABSTRACT

The specific suface areas of diflerent starchy materials were determined by means of the t-method, their values being practically constant in the range of water activities O*lO-0.80. The straightness of the u,--t plots observed in all cases indicated the absence of an intrinsic microporous structure. The lack of a pore structure and the high values of the specific su$ace areas are explained on the basis of the interlamellar expansion of the starch granule structures on uptake of water.

INTRODUCTION

It is an experimental fact that high molecular weight polymers usually undergo swelling on water uptake. Considerable literature on this exists (Hermans, 1946; Smith, 1947; Benson et al., 1950; Hellman & Melvin, 1950). Stamm and Millett ( 1941) postulated that such swelling occurs as a result of the creation of a transient capillary structure along with an increase in the number of active sites exposed within the structure. The concept of a transient capillary structure is in agreement with the experimental fact found by Hermans ( 1946) that molecules such as glycerol do not penetrate into dry cellulose unless a pathway has been created by a certain amount of moisture. Furthermore, the large differences in the values for surface area in macromolecules such as proteins and starches measured using nitrogen and water (Labuza, 1968) is in accord with Stamm and Millett’s transient capillaries.

The large differences in surface areas previously mentioned, together with low porosity usually found in starches (Hellman & Melvin, 1950; Andrieu & Stamatopoulos, 1986), suggest the existence of an intrinsic

71 Journal of Food Engineering 0260-8774/89/$03.50 - 0 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain

72 R. J. Aguerre, C. Sua’rez, P. E. Viollaz

microporous structure. Such structures necessarily involve type I sorption isotherms, a fact that is not observed in practice; most of the starchy materials present type II isotherms for water sorption which are characteristic of nonporous materials (Gregg & Sing, 1982); both types of isotherms are shown in Fig. 1, where (a) represents type I and (b) type II.

The aim of the present work was to elucidate whether or not starch contains any appreciable microporous structure. For this purpose the ‘t’

modified procedure developed by Mikhail et al. (1968) was used to analyze the moisture sorption isotherms for several starchy materials. The results obtained are compared with those corresponding to a mineral system whose structure and sorption properties are completely characterized.

MATERIALS

Water vapor sorption isotherms for the following products were investigated:

Material Temperature (“C) Source

Maniac (desorption) Sorghum (desorption) Potato starch (adsorption) Wheat (desorption) Rough rice (desorption) Natural montmorillonite (desorption)

25 vandenBerg(1981) 37.8 Dustan et al. (1973) 25 Bizot (1983) 25 Day & Nelson (1965) 60 Aguerre et al. (1983) 20 Mooney et al. ( 1952)

THEORY

The t-method

The t-method is based on the concept of the standard isotherm or t-

curve, which is a plot of the statistical thickness (t) of the adsorbed film against P/P, (P = vapor pressure of the adsorbate, P, = saturation vapor pressure) for nonporous adsorbants. The t-value is obtained by dividing the volume of adsorbate (v,), assumed to be liquid, at a given ratio P/Ps by the surface area of sorption, S:

t= 1o-3 z&s (I)

where, when zlr has units of cm3/g and S is expressed in m2/g, t is obtained in run. According to eqn (1) any sorption isotherm for a non-

Swelling and pore structure in starchy materials 73

water actlvlty I

Fig. 1. Characteristic isotherms of (a) microporous and (b) nonporous materials.

porous solid can be converted into a straight-line plot. For a given point on the isotherm, the volume or weight adsorbed is converted to vi ; the value for the corresponding P/Ps is read from the t-curve for nonporous adsorbants, and v, is plotted against t. At low pressures the amount adsorbed in a porous solid will be the same as that corresponding to a free surface, except when the width of the pore is approximately the diameter of the sorbed molecule. The q-t plot in this region will then be a straight line, the slope of the line being the surface of the adsorbant.

In this method the t-values determine the entire volume of a pore group (eqn (2), from M&hail et al., 1968). It is accordingly very important that the correct choice is made of the t-curve to be used for the analysis. The appropriate t-curve is based on nonporous adsorbants which have approximately the same free energies of sorption as the adsorbant to be analyzed. It was found that the BET-C constant is adequate for selecting the appropriate t-curve (Mikhail et al,, 1968). If the C constant for a given adsorbant does not coincide with that of any of the standard t-curves, an interpolated t-curve can be drawn, without introducing any significant error.

When the amount adsorbed is plotted against t, three different types of curves are observed (Broekhoff & de Boer, 1969); a schematic representation is given in Fig. 2. A straight line from the origin (Fig. 2(a)) indicates the absence of micropores or capillary condensation, the surface area of the adsorbant being the slope of the line, designated as S,. When capillary condensation occurs, the u,-t plot deviates from the

74 R. J. Aguerre, C. Sua’rez, P. E. Viollaz

(0) “1 I S

c’ , /’

t

“1 Ibf

l/L- c/ - '-5

<*

t -

v, @) _/45/ l___- _--,

__e- ,I

xs I’

t

Fig. 2. Three characteristic types of z-plots: (a) external surface sorption; (b) capillary condensation; (c) micropore and external surface sorption.

straight line (Fig. 2(b)), with a sharp increase in the amount of adsorbed water. When adsorption occurs in micropores, at a certain value of PIPS the q-t plot assumes a less steep slope (Fig. 2(c)) and the new slope gives the surface area which is available for adsorption, that is, the surface area of the wider capillaries and the outer surface of the adsorbant.

An application of the t-method for determining the specific surface areas of a hypothetical microporous adsorbant is given later. For simplicity, the pore walls are visualized as parallel plates. A schematic representation of the pore filhng is illustrated in Fig. 3. During the process it is assumed that the surfaces S, , S,, S, , . . . , Si, are filled succes- sively with one, two, three and i statistical layers. The total volume adsorbed is

n n 21*= C z/i= C Si(ti-tti_1) (2)

i=l i=l

If

ti-ti_,=At for Mien


Fig. 3. Schematic visualization of pore filling.

then

Vl = ~ Vi= At i: Si (3) i=l i=l

From eqn (3), the remaining specific surface area unfilled can be calculated, making the difference between two successive accumulated volumes,

Au, -=

VI I,,+, - VI I,, =s,

At t;+, -t, 1+1

In the limit, when t tends to zero, eqn (4) reduces to

dv, -= S dt

(4)

(5)

From eqn (5) it can be seen that from a v, - t plot can be obtained the available surface at different t values.

RESULTS AND DISCUSSION

The equilibrium sorption data for the different materials considered were processed according to the simple BET equation and found to give

76 R. J. Aguerre, C. Suhez, P. E. F/iohz

good straight lines in the region up to about 0.5 water activity. From the slopes and intercepts of these lines, values of ~1, (the volume of water required to cover the whole surface area) and the constant C were calculated; both magnitudes are reported in Table 1.

These C values were used to select the most appropriate standard t-

curve from the equilibrium data on water sorption in nonporous solids reported by Hagymassy et al. (1969). In those cases where the values of C for the product did not coincide with that for any of the four t-curves reported by the aforementioned authors, interpolation was used. The isotherms for the different products investigated were converted into q-t plots as follows. Pairs of ~,-a~ values were taken from the smoothed isotherms at a, intervals of O-05, starting with a, = 0.05, and the t-values were read from the standard t-curves. The q-t values so obtained are plotted in Figs 4 and 5 (the range of water activities for each material is

TABLE 1 Surface Areas Obtained from U, -1 Plots and Comparison with BET Areas

Material BET-C (constant) BETarea

WIti

Surface area

Wlgl

Maniac 29.1 320 312 Sorghum 38.8 270 26.5 Potato starch 10.5 249 265 Wheat 22.7 260 252 Rough rice ll,O 145 149 Natural montmorillonite 5.4 341 347

Fig. 4. v,-t Plots for different products: 0, natural montmorillonite; 0, wheat; 0, rough rice.


Fig. 5. v,-t Plots for different products: 0, sorghum; 0, maniac, 0, potato starch.

indicated in the figures). It is observed from these figures that the relationship between o, and t is linear for a wide range of a, values, showing slight deviations from the straight line only at the extreme values of a,.

The specific surface area for each material was obtained from the slope of the straight line, according to eqn ( 1). For the purposes of computation the density of the liquid water was assumed to be 1 g/cm3. The specific surface areas calculated by the t-method and from the BET monolayer value are given in Table 1. The statistical thickness of the water monolayer was taken as O-3 nm, according to Hagymassy et al.

( 1969). It can be observed that there are no significant differences between the values obtained from the two methods.

The linearity of the water sorption isotherms plotted by the t-method can be interpreted as evidence of a constant starch-water interfacial area which exists from the completion of one monolayer of water until about three have been adsorbed. It is observed that despite a constant surface area, the volume of sorbed water increases with the increasing water activity.

At this point of the analysis, it is interesting to discuss some aspects related to the sorption of water on natural montmorillonite and to compare the t-curve for this material with the t-curves for the starchy materials. The equilibrium data for montmorillonite-water plotted in Fig. 4 show a linear relationship between the volume of sorbed water and the thickness t, such as was found for the starch-water systems. Only one type of capillary structure could possess these properties. This is a ‘parallel plate’ pore, effectively infinite in length and width compared

78 R. J. Aguerre, C. Sua’rez. P. E. Viollaz

with the distance between plates. This structure is clearly identified in the expanding clays such as the natural montmorillonite (Mering, 1946; van Olphen, 1969). During sorption, the water is sorbed between the parallel plates promoting the lattice expansion. The attraction between sheets is weak enough for water molecules to force them apart and enter into the spaces promoting the swelling of the clay. According to Norrish (1954) the average number of water layers between sheets is limited to between 2 and 4, depending on the water activity. It can be seen in Fig. 4 that the natural montmorillonite gives a constant sorption surface area at least up to the second layer, for a water activity of about 0%

Because of the similarity between the u,-t plots of the clay considered and the starchy materials, it appears that similar sorption structures exist in the starch to those observed in clays. Such structures have been reported by different authors (Badenhuizen, 1969; Hoseney et al., 1986). According to those authors the microlamellation of the starch granules results from the single granule growth through apposition, forming concentric layers of variable width. Based on such a configura- tion and on the straightness of the u,-t plots found in all the materials investigated, it appears that the uptake of water takes place between the concentric layers. The laminar structure within the starch granule would explain the large differences in the values of specific surface area measured with nitrogen and water (Hellman & Melvin, 1950). Similar results were reported by Mooney et al. ( 1952) for natural montmoril- lo&e. These results are consistent with the ‘parallel plate’ pore structure. The plates are held together by forces too great for the energy of adsorption of nitrogen to overcome, thus bringing about separation. When water molecules penetrate between the plates, it seems reasonable that the plates first separate only enough to admit one layer of molecules, and when this layer is filled completely, a second layer may begin forming. The nitrogen adsorption method presumably gives the external area, whereas the water adsorption method gives the total external and internal surface area. This is in agreement with the low porosities reported by Andrieu and Stamatopoulos (1986) for the system wheat pasta-water.

The small deviations from the straight lines found at low water activities in almost all the materials investigated can be attributed to the appearance of new sorption areas as a result of swelling. This is probably due to the increase of the sorption surface area from a value equal to the external surface area (the only surface available at the start of the adsorption), to the total surface area attainable at the monolayer value. On the other hand, the limitation of the swelling would be consistent with a decrease of the sorption area observed at high water activities.


According to van den Berg ( 198 1) such a limitation of the swelling could be due to the existence of a substructure of microfibrils crossing the depth of concentric layers.

CONCLUSIONS

From the application of the t-method to the water sorption isotherms of starchy materials, it was found that these materials behave similarly to montmorillonite from the sorption point of view. This result allowed the calculation of sorption surface areas, though such calculation is justified only if the existence of a transient pore structure is assumed. Such phenomenological observations result from the structural similarity between both kinds of materials (the existence of lamellar structures) and the agreement between sorption surface areas obtained from the BET equation and the t-method.

REFERENCES

Aguerre, R. J., Sutirez, C. & Viollaz, P. E. ( 1983). Moisture desorption isotherms of rough rice. J. Food Technology, l&345-5 1.

Andrieu, J. & Stamatopoulos, A. A. (1986). Moisture and heat transfer model- ling during durum wheat pasta drying. In Drying ‘86, Vol. 2, ed. Arun S. Mujumdar. Hemisphere Publishing Corp, Washington, New York, London.

Badenhuizen, N. P. (1969). The Biogenesis of Starch Granules in Higher Plants. Appleton-Century-Crofts, New York.

Benson, S. W., Ellis, D. A. & Zwanzig, R. W. ( 1950). Surface areas of proteins. III. Adsorption of water. J. Am. Chem. Sot., 72, 2102-5.

Bizot, H. ( 1983). Using the GAB model to construct sorption isotherms. In Phy- sical Properties of Foods, Vol. II, ed. R. Jowitt, F. Escher, B. Hallstriim, H. F. Th. Meffert, W. E. L. Spiess & G. Vos. Applied Science Publishers, London & New York.

Broekhoff, J. C. P. & de Boer, J. H. (1969). The surface area in intermediate pores. Proc. Int. Symp. on Surface Area Determination, International Union of Pure and Applied Chemistry, in conjunction with The Society of Chemical Industry, ed. D. H. Everett & R. H. Ottewill. Butterworths, London, pp. 97-121.

Day, D. L. & Nelson, G. L. (1965). Desorption isotherms for wheat. Trans. ASAE, 8,293-7.

Dunstan, E. R., Chung, D. S. & Hodges, T. 0. (1973). Adsorption and desorption characteristics of grain sorghum. Trans. ASAE, 26,667-70.

Gregg, S. J. & Sing, K. S. W. (1982). Adsorption, Surface Area and Porosity. Second edition, Academic Press, New York.

Hagymassy, J., Brunauer, S. & Mikhail, R. S. (1969). Pore structure analysis by water vapor adsorption. I. t-Curves for water vapor. J. Cofloid Znterjace Sci., 29,485-91.

80 R. J. Aguerre, C. Surirez, P. E. Viollaz

Hellman, N. N. & Melvin, E. H. (1950). Surface area of starch and its role in water sorption. J. Am. Chem. Sot., 725 186-8.

Hermans, P. H. ( 1946). Contributions to the physics of cellulose fibres. Elsevier Publishing Co., Amsterdam, Brussels, London & New York.

Hoseney, R. C., Zeleznak, K. J. & Yost, D. A. (1986). A note on the gelatiniza- tion of starch. Starch/Stiirke, 38,407-9.

Labuza, T. P. (1968). Sorption phenomena in foods. Food Technology, 22, 263-72.

Mering, J. (1946). On the hydration of montmorillonite. Trans. Faraday Sot., 42B, 205-19.

M&hail, R. S., Brunauer, S. & Bodor, E. E. (1968). Investigations of a complete pore structure analysis. J. Colloid Znter$zce Sci., 26,45-53.

Mooney, R. W., Keenan, A. G. & Wood, L. A. (1952). Adsorption of water by montmorillonite. I. Heat of desorption and application of BET theory. J. Am. Chem. Sot., 74,1367-71.

Norrish, K. ( 1954). The swelling of montmorillonite. Disc. Faraday Sot., 18, 120-34.

Smith, S. E. (1947). The sorption of water by high polymers. J. Am. Chem. Sot., 69,646-5 1.

Stamm, A. J. & Millet, M. A. (1941). The internal surface of cellulosic materials. J. Phys. Chem., 45,43-54.

van den Berg, C. ( 198 1). Vapour sorption equilibria and other water-starch inter- actions; a physico-chemical approach. Agricultural University Wageningen, The Netherlands.

van Olphen, H. (1969). Determination of surface areas of clays-Evaluation of methods. Proc. Int. Symp. on Surface Area Determination, International Union of Pure and Applied Chemistry, in conjunction with The Society of Chemical Industry, ed. D. H. Everett & R. H. Ottewill. Butterworths, London, pp. 255-71.

Swelling and pore structure in starchy materials

Documents

Transcript of Swelling and pore structure in starchy materials