Freeze–thaw stabilization of sweet potato starch gel by polysaccharide gums

Abstract

Nine polysaccharide gums (sodium alginate, carboxymethyl cellulose, curdlan, gellan, guar gum, gum arabic, κ-carrageenan, locust bean, and xanthan) were compared for their stabilizing effects in sweet potato starch gel against repeated freeze–thawing (FT) treatments. The gums were added in starch gel at 0.3 or 0.6% (w/w, based on total gel weight), and total solid content in the gel was adjusted to 7% (w/w) with starch. The gels containing starch and gum were repeatedly freeze–thawed up to five times by storing at −18 °C for 20 h and then at 25 °C for 4 h. Water release (syneresis) was measured by vacuum-filtering the freeze–thawed gels. Among the gums tested, alginate, guar gum, and xanthan were highly effective in reducing the syneresis. For example, guar gum, at 0.6%, showed the least syneresis (33.0%, w/w based on initial water content) after five FT cycles, which was less than half that of pure starch gel. At 0.3%, however, xanthan was more effective than guar gum in reducing syneresis. Xanthan reduced paste viscosity significantly, whereas guar gum and alginate increased the viscosity, but there was little relation between pasting viscosity and syneresis. The gums remained in the gel matrix during the syneresis without a significant loss. Recrystallization of starch (retrogradation) induced by FT treatment was also retarded by the presence of gums, and sodium alginate was more effective in retarding the retrogradation than xanthan or guar gum.

Introduction

As the demand for ready-to-eat food items rises, a variety of frozen food products are continually introduced in markets. Upon freezing, however, moisture in the foods transforms into ice, often resulting in physical stress to the food matrix. When a frozen food is thawed for consumption, the moisture is readily separated from the matrix and it causes softening of the texture, drip loss, and often deterioration of overall quality (Rahman, 1999).

In many frozen food items, starch contributes improving consistency and texture, normally through pasting and gelling (Holmes & Solender, 1981). Syneresis (water loss), retrogradation, and textural change in starch gel during freezing and thawing have been reported by many researchers. Dreher, Trinsley, Scheerens, and Berry (1983) reported that a gel from corn or buffalo gourd root starch showed approximately 80% water loss by repeating the freeze–thawing (FT) cycle up to 14 times. Wu and Seib (1990) also reported that waxy barley and waxy maize starch gels gave up 55% water by seven FT cycles. In addition, a lima bean starch gel treated by three FT cycles has shown a higher syneresis (65%) than those (55–60%) of corn and potato starch gels (Hoover & Martin, 1991). Wheat and rice starch gels treated by two FT cycles showed the syneresis of 37 and 26%, respectively (Baker & Rayars, 1998).

The syneresis and related physical property changes induced by FT could be reduced by adding a minor amount of hydrocolloids, but the cryoprotecting mechanism of the hydrocolloids is not fully understood (Budiaman and Fennema, 1987a, Budiaman and Fennema, 1987b, Igoe, 1982). The effect of hydrocolloids depends on their origin and molecular structure, and on the various environmental factors, such as pH and ionic strength of the gel.

Muhr and Blanshard (1984) reported that xanthan, sodium alginate or gelatin did not affect the amount of ice formation or the uniformity of ice nucleus in sucrose solution, but they delayed the rate of ice formation. When xanthan gum was added in starch gel, however, amylose retrogradation, syneresis, and rheological change of the starch gel were reduced although ice crystal formation and amylopectin retrogradation were not prevented (Ferrero, Martino, & Zaritzky, 1994). Despite many studies on the effect of gum addition, comparative studies among the various food gums in starch gel have rarely been carried out.

In the present study, various polysaccharide gums were compared for their effects in stabilizing a sweet potato starch gel against repeated FT cycles, and the cryoprotecting behaviors of the selected gums were characterized.