Roles of carbohydrates and proteins in the staling of wheat flour tortilla
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Effects of enzymatic modification of starch, proteins and pentosans on dough and tortilla properties were determined to establish the role of these wheat components in tortilla staling. Starch, protein and pentosans were respectively modified with a-amylase, protease and transglutaminase (TG), and xylanase. Tortillas were stored at 22oC and evaluated for at least three weeks. Amylase improved shelf-stability of tortillas, produced a significant amount of dextrins and sugars, retarded decrease in amylose solubility, and weakened starch granules. However, control and treated tortillas had similar degrees of amylopectin crystallinity. Staling of tortillas appears to involve starch that reassociates into an amorphous structure. Micrographs of control dough had thin protein strands forming a continuous matrix. Protease-treated dough had pieces of proteins in place of the continuous matrix, while TG-treated dough had thicker protein strands that were heterogeneously distributed. Both treatments resulted in shorter shelf-stability of tortillas. The organization of protein in dough is important for dough structure and appears to impact tortilla flexibility. Protein solubility and SDS-PAGE results did not differentiate control and treated dough or tortillas. The fractions or molecular weight distribution are not significant determinants of protein functionality. Tertiary and quaternary protein structures of gluten may be more related to tortilla shelf-stability. The 75 ppm xylanase treatment resulted in weaker tortilla structure and significantly higher amounts of low molecular weight saccharides and sugars. Control and the 25 ppm treatment sample had a similar shelf-stability and texture profile. Pentosans may affect staling indirectly through the effect on gluten development. Fresh tortillas have amylopectin in an amorphous state, while amylose is mostly retrograded. The gluten matrix provides additional structure and flexibility to the tortilla. Pentosans may or may not be attached to the gluten network. Upon storage, amylopectin retrogrades and recrystallizes, firming the starch granules, resulting in firmer tortillas. Starch hydrolysis decreased the rigid structure and plasticized polymers during storage. It also reduced the restriction imposed by retrograded starch on gluten and allowed it more flexibility. Thus, the flexibility of tortillas results from the combined functionalities of amylose gel, amylopectin solidifying the starch granules during storage, and the changed functionality of gluten after baking.
Alviola, Juma Novie Ayap (2007). Roles of carbohydrates and proteins in the staling of wheat flour tortilla. Doctoral dissertation, Texas A&M University. Available electronically from