Tomowo Mihori

A new non-Fickian diffusion model for water migration in starchy food during cooking

When a starchy food such as a rice grain or a strand of noodle is boiled, a high moisture region is generated at the surface spreading inward, producing a low moisture core. The characteristic features of the change of moisture profile were recently observed by nuclear magnetic resonance imaging (MRI). However, the profile cannot be described by any existing mathematical model based on the principle that water molecules are driven by the gradient of moisture content. In this paper, a new mathematical model is proposed using a new concept, water demand (WD). In this model, migration of water is driven by the gradient of WD, which is defined as the difference between the ceiling moisture content and the existing moisture content. This model is demonstrated to have a potential to describe the characteristic features of the change of moisture profile in starchy food during boiling.

The rate of starch gelatinization as observed by PFG-NMR measurement of water diffusivity in rice starch/water mixtures

Abstract The gelatinization rate of rice starch was studied by observing the water diffusivity measured by a PFG (Pulsed-Field-Gradient)-NMR method. A rice starch/water mixture in glass capillary tubes was heated and cooled in a stepwise manner. The water diffusivity in these samples decreased as heating time increased. This was recognized as the increase in the amount of carbohydrate polymer dissolved in the aqueous phase during heating. Consequently, the measured water diffusivity was converted into the moisture content in the dissolved polymer solution. The time course of changes in this moisture content was approximated as a first-order process, and a rate constant in the order of 10 −2 s −1 was obtained for 66–80 °C. This value is comparable to that in the literature measured by a DSC isothermal method, while those measured by other methods such as flow consistency and gravimetric methods were 100-fold smaller.

Fracture stress of fish meat and the glass transition

Abstract Freezing a high moisture food may involve the crystallization of pure water and therefore concentrate the solute, which sometimes forms a concentrated amorphous solution (CAS). The CAS may be formed via hot air drying as well, although most hot air dried foods available on the market are porous and/or in powder form and are inconvenient for use in fracture tests. In this paper, the traditional Japanese dried food ‘katsuo-bushi’, a highly smoked, dried fillet of bonito fish meat (which can be regarded as a CAS in itself) was used as samples to measure the compressive fracture stress. The compressive fracture stress of 0% moisture Katsuo-bushi was found to be constant at temperatures between 25 °C and −196 °C, and this value was comparable to that of 0% moisture CAS of completely frozen bonito meat estimated using a simple model for a frozen food. On the other hand, fracture stress of katsuo-bushi with 15–20% moisture changed greatly at temperatures between 0 °C and −90 °C, showing the feature of yielding. The temperature of brittle-ductile transition was different from the glass transition temperature as defined by DSC.

Fracture stress of frozen food analyzed by a two-component model consisting of pure water ice and concentrated amorphous solution

Abstract Fracture stress of soybean curd was measured at temperatures between − 20 °C and − 196 °C by compression tests. Fracture stress increased as the temperature decreased until it reached a characteristic temperature; below this temperature the fracture stress was constant. Fracture stress varied with the moisture content of the sample measured before freezing; the lower the moisture content, the higher the fracture stress. We attempted to analyze the fracture stress of the frozen soybean curd using a mathematical model in which frozen soybean curd was regarded as a two-component system consisting of pure water ice and concentrated amorphous solution (CAS). Since the volumetric fraction of pure water ice in the system is required for the analysis, the degree of freezing of soybean curd with varied moisture content was estimated as a function of temperature using a hypothetical phase diagram for soybean curd. Based on these data, fracture stress of CAS was calculated using a series model and a parallel model. The calculated fracture stress of CAS was found to be a unique function of temperature and independent of moisture content before freezing.