Accelerated aging of rice by controlled microwave treatment.

To obtain the desired technological properties (pasting, texture, and rheology) of naturally aged rice (AR), the aging process of freshly harvested rice was accelerated by controlled microwave treatment at 540 W for 1-3 min. Similar to AR, the rice microwave treated for 2 min showed increased pasting viscosities (peak, trough, breakdown, final, and setback) and pasting temperature, enhanced gel hardness and strength, and reduced gel adhesiveness. The mechanism by which microwaves accelerated rice aging was illustrated. Microwave treatment promoted the formation of protein disulfide bonds and the release of free phenolic acids, which enhanced protein gel network and cell wall strength. This phenomenon inhibited the swelling of starch granules and consequently modified the technological properties of rice. The crystalline structure and fatty acid content of rice flour was uninvolved in the mechanism, but the microwave-induced micromechanical change (intercellular cleavage to intracellular cleavage) of rice endosperm may be involved.

Microwave pretreatment promotes the annealing modification of rice starch.

Early indica rice starch can only be modified to a small extent by annealing treatment (ANN), which limits its application. Microwave pretreatment (MW) with different intensity was used to enhance the effectiveness of ANN, and single ANN or MW treatments were used as controls. MW pretreatments per se exhibited insignificant or minor effect on the starch. However, MW pretreatment with appropriate intensity could significantly promote the structural and physicochemical modification of the starch in subsequent ANN, including the enhancement of long- and short-range crystalline structures, the increases in gelatinization enthalpy, particle size, peak viscosity, breakdown and G' value, and the decreases in tan δ value. The MW pretreatment could untangle the entanglements between starch chains by inducing violent movement of the chains, which facilitated the molecular rearrangement and interaction during subsequent ANN, thereby promoting the structural and physicochemical changes. This study provided new insights into the annealing mechanism of starch.