Freeze–thaw stabilization of sweet potato starch gel by polysaccharide gums
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.
Section snippets
Materials
Sweet potato starch was supplied by Halla Industry (Cheju, Korea). Gum arabic (The Gum Arabic Company, Sudan), sodium alginate (Roland and Company, USA), carboxymethyl cellulose (CMC) (Showa Chemicals, Japan), curdlan (Takeda Chemical Industry, Japan), κ-carrageenan (Marcel Trade Corporation, Philippine), locust bean gum (Cesalpinia Food SPA, Italy), guar gum (Lotus Gum and Chemicals, India), gellan gum (Kelcogel, The Nutrasweet Kelco Company, USA), and xanthan (Keltrol, The Nutrasweet Kelco
Pasting viscosity
Pasting viscosity of sweet potato starch was changed by gum presence, and the change depended on the gum type (Table 1). Locust bean and guar gums, which have galactomannan structure, displayed similar viscosity profiles in which pasting temperature was decreased, whereas peak viscosity and breakdown were increased significantly. Viscosity increase by guar gum in starch paste has been reported (Christianson, Hodge, Osborne, & Detroy, 1981). From experiments with wheat flour, Rojas, Rosell, and
Conclusions
Guar gum, alginate, and xanthan were effective in stabilizing sweet potato starch gel against repeated FT treatments. Xanthan reduced paste viscosity, possibly through strong network formation with starch. When excess xanthan (0.6%, in this experiment) was used, however, the tendency of intermolecular interactions could cause the adverse effect of destabilizing and hardening the gel. Guar gum was useful not only as an excellent freeze–thaw stabilizer, but as a viscosity enhancer. Alginate was
Acknowledgements
This research was financially supported by a grant from the Korea Science and Engineering Foundation (Project No. 2000-2-22000-002-3).
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