Effective Diffusivity and Kinetics of Urease Inactivation and Color Change During Processing of Soybeans with Superheated-Steam Fluidized Bed

Abstract

Influences of temperature and moisture content on diffusional transport property and kinetics of both urease inactivation and color change during soybean treatment with superheated steam in the temperatures of 120–150°C were investigated. The experimental results have shown that the effective diffusivity and apparent rates of such reactions are related to temperature in a way that the rates of water transport, urease inactivation, and brown pigment formation are higher with higher temperature. In addition, these rates are also enhanced by increasing moisture content, except for the browning reaction that is inhibited by this factor due to the dilution effect. The reduction in urease activity is reasonably described by modified first-order reaction and the color change of soybeans is characterized by three Hunter parameters in which the changes of L-, a- and b-values are followed according to zero-order, modified Monod and first-order reactions, respectively. The kinetics of inactivation and color change, along with the experimental data of protein solubility and lysine content, have been suggested that the soybean should be treated with superheated steam at the temperature ranging from 120 to 135°C. The superheated-steam fluidized-bed technique can simultaneously be applied for both drying and inactivating such components in soybeans by a single operation when the initial moisture content falls within a suitable range, approximately lower than 20%d.b. At elevated initial moisture contents, two-stage drying technique is recommended in order to avoid excessive cooked soybeans.