Drying of liquid foods is a very complicated simultaneous heat and mass transfer process, in which the water diffusion plays a crucial role. The water diffusion coefficient that decreases sharply with decreasing water content must be known in order to predict the drying process. Methods for determining such water diffusivities have been developed on the basis of the regular regime concept. We have developed a very simple method for determining the water diffusivity from the regular regime isothermal drying curves, and applied to various sugar or carbohydrate solutions. The drying behavior of single droplets or slab-shaped samples of sugar solutions were numerically calculated by a drying model with the concentration-dependent water diffusion coefficient determined. Good agreements between experimental and calculated results were obtained. Enzyme inactivation rate constants were measured experimentally over a wide range of sugar concentrations. By using these data the inactivation behavior of enzymes in sugar solutions were simulated. The enzyme was stabilized during drying because the inactivation rate decreases very sharply with decreasing water contents. The numerical calculations simulated the experimental data well.
Enhancing effects of heat transfer rate and thermal efficiency of superheated steam treatment (SHS) combined with far infrared heating (FIH) were compared with those of superheated steam treatment only by using a semi-practical compact apparatus fabricated in this study. Heat transfer rate was calculated from the constant drying rate of samples wetted enough with water. The samples were treated at three superheated steam temperatures (120, 150, and 200°C) or at those temperatures combined with four far infrared heater temperatures (250, 300, 350 and 400°C) . When only 1/5 of SHS heat flow was supplied in addition to the FIH heater, the heat transfer rate was increased 2 times or more compared with that for SHS only. Thermal efficiencies for the SHS heating process + 400°C FIH were about 2-4 times larger than those for SHS only, depending on the superheated steam temperature. When a low temperature (about 120°C) with a small amount of steam was combined with high temperature FIH (about 400°C), a highly enhanced effect on both heat transfer rate and thermal efficiency was obtained. Furthermore, the SHS + FIH process greatly reduced the heating up time of the apparatus and the heat flow to maintain a constant temperature in the heating chamber.
Rice is a major economic crop in Iran, where it is usually dried using a batch-type dryer until the final moisture content (FMC) reaches below 9%. The influence of low FMC (about 9%) produced by four drying temperatures (30, 40, 50, and 60°C) on the physical properties, milling, and cooking qualities of short grain aromatic rice were investigated. The obtained data were also compared with standard FMC (about 12%) treatments which were dried in a batch type dryer. The experimental result showed that for low FMC samples, head rice yield (HRY) was increased by 4% at drying temperatures of 30°C and 40°C but was reduced by about 20% at 60°C in comparison with control samples (25°C, 60% RH) . The water uptake ratio (WUR) and volume expansion ratio (VER) were decreased significantly as the drying temperatures were increased. We can conclude that low FMC (9%) could produce higher HRY, if the drying temperature is 40°C or less. In addition, high drying temperatures (50°C and 60°C) caused increases in the number of fissured kernels, energy consumption and decreases in HRY, WUR, and VER. Maintaining the HRY and keeping a cooking quality at maximum level are the main task for the optimization of drying process. It was occurred at drying temperature less than 40°C with a low FMC for aromatic rice. Therefore, a drying temperature of more than 40°C would result in deterioration of quality of japonica type aromatic rice, followed by low FMC.
A theoretical analysis was given for the kinetics of an observable in general first-order reaction process (R→P) . It was assumed that reaction degree x≡ [P] / ( [R] + [P] ) obeyed a first-order reaction rate equation and that the observable o of sample during reaction was related with reaction degree through the mixing rule of a power-law type oν= (1-x) oνR+xoνP where oR and oP represent the values of observable, being proper to the reactant and the product respectively (oR<oP) . It was demonstrated for ν<1 that an observable-time curve could possess an inflection point at o*= (1-ν) 1/νoP with the maximum growth rate do/dt= (1-ν) 1/ν-1oPK1. These results coincide in the ν→0 limit with those obtained for a logarithmic (ln o) mixing rule.
Knowledge of survival behaviors of microorganisms on solid surfaces is important to assess and control the risk of cross contamination for food being processed on the surface. Here we report the survival behavior of Escherichia coli left on polypropylene coupons with or without nutritious soil subjected to drying at room temperature. When E. coli cells were left on the coupons with 0.85% saline solution, decrease in the viable cell number was observed in two stages, each of which followed the first order kinetics. The specific death rate was 0.21 h-1 initially and reduced to 0.11 h-1 after the water content reached 3.9 g-water/g-solid. When E. coli cells were left with Luria-Bertani broth, as a model soil, of different water contents, the viable cell number did not show a distinct decrease until the water content reached 3-4 g-water/g-solid. After the water content went down below the threshold, the specific death rate was roughly 0.1 h-1 though deviation was large especially at the final stage of drying. E. coli survived on the surface even after 2 days, and coexistence of nutrients enhanced the final level of viable cells.
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