Effects of Trehalose Content and Water Activity on the Fracture Properties of Deep-fried Wheat Flour Particles and Freeze-dried Porous Waxy Corn Starch Solids

Wheat flour-based batter containing 0 to 20 % trehalose was deep-fried, dried and held in various water activity (a(w)) conditions. The effects of trehalose content and a(w) on oil content, water sorption, isothermal mechanical relaxation, and fracture properties were investigated. For comparison, the fracture properties of freeze-dried porous waxy corn starch solids were also investigated. The 10 % trehalose sample had the lowest oil content, water content, and a(w). A force-reduction value (∆F) of the samples was evaluated as a typical mechanical relaxation parameter. ∆F gradually increased with increasing a(w) and sharply increased above a specific a(w) presumed to be associated with the glass to rubber transition. Compared to ∆F values among the glassy samples, 10 and 20 % trehalose samples had higher ∆F values (were more rigid) than 0 and 5 % trehalose samples. From the fracture measurements of the glassy samples, the first fracture force increased linearly and the number of fracture peaks decreased linearly with increasing a(w). At each a(w), 10 % trehalose had the lowest first fracture force and the highest the number of fracture peaks. Freeze-dried porous waxy corn starch solids showed similar fracture properties to deep-fried samples. These findings suggest that around 10 % trehalose content is optimal for producing deep-fried foods with a brittle texture.