Influence of inulin/oligofructose on the acid-induced aggregation and gelation of soy proteins and silken tofu

The market of functional (health-promoting) foods has reached an all-time high level, with a projected total sale of {dollar}35.86 billions in 2006 in the United States alone. Prebiotics, i.e., compounds that can promote the growth of beneficial host microflora in the colon, represent a special group of such functional ingredients. Understanding the underlying mechanisms by which prebiotics and other food components interact can help food scientists to develop high-quality functional foods. The overall objective of my dissertation research was to elucidate the mechanisms and effects of fructan-type prebiotics, including inulin (long-chain) and oligofructose (short-chain), on the gelation of soy proteins and textural properties of a silken tofu prepared with glucono-delta-lactone (GDL) as a coagulant.; Differential thermal scanning, turbidometry, dynamic and static rheological testing, and electron microscopy were used to determine structural changes, interactions, and network formation of soy proteins in an acidic environment. The results showed both fructants stabilized native soy proteins (delaying thermal transitions by 1.9--2.3°C). Both fructans improved the rheological properties of GDL-coagulated soy protein gels upon heating, showing 14.4--45.5% enhancements in gel rigidity. Microscopic analysis revealed a denser protein cross-linking gel structure having greater compactness and smaller pore sizes when treated with fructans. While both prebiotical compounds suppressed GDL-induced soy protein aggregation at ambient temperature, oligofructose did not significantly affect its "cold" gelation. However, the inulin treatment promoted protein network formation and increased rigidity of cold-set soy protein gels by 10.1--13.6%. The addition of 2% inulin produced a silken tofu with higher gel hardness and rupture force while maintaining good cohesiveness and deformability. These data demonstrated that inulin and oligofructose influenced the performance of soy proteins in a complex manner. Namely, in dilute aqueous protein solution, the excluded volume effect may be the major driving force by which these polyhydroxyl compounds stabilized protein molecules. When protein concentration was increased, the addition of these fructans, especially inulin, further promoted protein-protein interactions possibly by reducing water activity during thermal gelation, thereby producing gels with increased rigidity.; Overall, this study produced evidence of fructan-soy protein interactions in an acidic environment; such interactions significantly altered the protein gelation process and modified the gel texture. Thus, it is possible to improve the physical characteristics of gel-type soy products by incorporation of fructan-type prebiotics. Future studies are warranted to clarify the precise nature of interactions between soy proteins and fructans, and to evaluate the sensory characteristics and shelf-stability of silken-type tofu products containing these prebiotics.; Keywords. Inulin, Oligofructose, Prebiotic, Glucono-delta-lactone, Silken tofu.