Journal of Food Engineering 9 (1989) 71-80

Swelling and Pore Structure in Starchy Materials

Roberto]. Aguerre, Constantino Suárez & 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 luly 1988; accepted 29 November 1988)

ABSTRAeT

The specific sUlface areas of different starchy materials were determined by means of the t-method, their values being practically constant in the range ofwater activities ()·j()-()·8o. The straightness of the v ,-t plots observed in al! cases indicated the absence of an intrinsic microporous structure. The lack of a pore structure and the high values of the .\pecific swface areas are explained on the basis of the interlamel!ar expansion of the starch granule structures on uptake ofwater.

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 el 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 - © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain

72 R. J. Aguerre. C. Suárez. P. E. Vial/az

microporous structure. Such structures necessarily involve type 1 sorption isotherms, a fact that is not observed in practice; most of the starchy materials present type 11 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 1 and (b) type 11.

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

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

The t-method

Temperature te;

25 37·8 25 25 60 20

THEORY

Saurce

van den Berg (1981) Dustan el al. (1973) Bizot (1983) Day & Nelson (1965) Aguerre etal. (1983) Mooney etal. (1952)

The t-method is based on the concept of the standard isotherm or t­curve, which is a plot of the statistical thickness (t) of the adsorbed film against P / Ps (P = vapor pressure of the adsorbate, Ps = saturation vapor pressure) for nonporous adsorbants. The t-value is obtained by dividing the volume of adsorbate (VI)' assumed to be liquid, at a given ratio P/Ps

by the surface are a of sorption, S:

t=1O- 3 v l /S (1)

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