Characterization of Physicochemical Properties of Xanthan/Curdlan Hydrogel Complex for Applications in Frozen Food Products
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Syneresis, expulsion of moisture from food, remains a major challenge in frozen products, due to post-production temperature fluctuation during distribution, transportation, and storage. Curdlan, an FDA approved microbial polysaccharide, produces a hydrogel under certain conditions exhibiting good gel strength during freeze-thaw abuse, better than similar polysaccharides. However, curdlan alone cannot effectively suppress syneresis in aqueous solutions. The present study aims at developing an effective hydrogel complex containing curdlan and a secondary biopolymer to reduce or eliminate syneresis. Polysaccharides, kappa-carrageenan, guar, locust bean, and xanthan gums were investigated. Xanthan gum, when added to curdlan, was found capable of eliminating syneresis for up to five freeze-thaw cycles (FTCs) while exhibiting stability in several rheological experiments. The xanthan curdlan hydrogel complex (XCHC) was further investigated to elucidate its structure in relation to its high elasticity and moisture holding capacity. A gel and indications of syneresis was clearly seen in magnetic resonance images of curdlan alone, whereas XCHC was homogeneous. The three dimensional network, indicated by frequency sweeps, of curdlan was responsible for its gel structure. Though not three dimensional, elasticity and angular frequency dependency for XCHC were significantly closer to curdlan than xanthan alone. Addition of xanthan to curdlan restricted spin-spin relaxation times of XCHC to intermediate and slower exchange regimes, promoting the polymer's interaction with water while inhibiting intermolecular interactions found in curdlan. Furthermore, the effects of pH and NaCl concentration on the rheology of XCHC were examined. Though both pH and NaCl were found to reduce the hydrogel's moduli, XCHC remained stable over the pH range 4 to 8 through FTCs and were not significantly different. Statistically, the modulus measurements of samples treated with 5-200 mM NaCl were slightly different and fluctuated throughout the FTCs. Nonetheless, the slope of the storage modulus over all NaCl-treated samples averaged ~9% different from untreated XCHC, indicative of similar gel behavior despite lower elasticity. Finally, the hydrogel was incorporated into a simulated pumpkin pie filling. Treating the filling with XCHC delayed moisture migration by one FTC, yielded a brighter color, reduced variability and minimized surface cracking that could lead to decreased quality and microbial growth.