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.