| Abstract | Okara is an insoluble pulp residue waste byproduct of tofu production and usually presents in a homogeneous wet mass with light yellow color. Since okara still consists of useful nutrients for human being, it can be possibly used as an ingredient in a variety of processed foods to reduce calorie intake and increase dietary fiber. However, utilization of okara is not easy because it has a tendency to deteriorate very fast due to its high water activity and organic content. Accordingly, okara must be dehydrated quickly to avoid such spoilage, often requiring high temperature and thus resulting in a hard brownish dry product of poor quality. An attempt has been made in the present study by investigating an improved okara drying method in a spouted bed of inert particles to obtain an economically viable better quality final product. Drying experiments with raw okara were carried out in a continuously moving bed of inert particles subjected to vortex-like motion in an experimental setup. The experimental variables in their respective ranges included the mass of Teflon pellets used as inert particles (0.4-1.2 kg), feed rate (0.5-1.4 kg/h), inlet air temperature (100-145°C), and airflow rate (195-271 m3/h). The effect of the experimental variables on the dryer performance was evaluated in terms of final moisture content of dry okara, product recovery, relative humidity and temperature of exit air, specific water evaporation rate, specific heat consumption and drying efficiency. Quality evaluation of dry okara powders included the color properties, particle density, water absorption, soy lipid absorption, and equilibrium moisture content isotherm. Effect of drying methods on the quality of dry okara was also investigated for spouted bed and hot air drying at similar drying air temperatures. Utilization of dry okara powder to replace up to about 30% wheat flour in selected food recipes was evaluated by a preference test based on a nine-point hedonic scale. The experimental dryer showed good performance in general and produced dry okara with final moisture content ranging from 5 to 33% on wet basis (wb) depending upon the operating conditions such as mass of Teflon pellets, feed rate and inlet air temperature. Thorough movement of Teflon pellets used as inert particles in the drying chamber was important to ensure steady-state operation and uniform drying, and to prevent agglomeration of okara under extreme operating conditions. The product recovery was between 80 and 90% on dry basis in most experiments and depended significantly upon the airflow rate and mass of the Teflon pellets. The relative humidity of exit air mainly depended upon the inlet air temperature along with the feed rate and airflow rate indicating minor influence. The exit air temperature was highly dependent on the inlet air temperature and increased proportionally with increasing inlet air temperature. The specific water evaporation rate in okara drying increased with an increase in the feed rate and mass of Teflon pellets. However, the specific heat consumption decreased with an increase in the okara feed rate. Results showed that specific heat consumption for okara drying in a bed of inert particles was about 3 to 4 times higher in comparison with that of free water. Drying efficiency varied in general between 23 and 38% and was about 1.2 times of the thermal efficiency. The main factors influencing the drying efficiency were feed rate and airflow rate, respectively. Drying of okara in hot air oven showed an increase in the moisture removal rate with increasing of the air temperature. It appeared that okara dried during the falling rate period. A quadratic equation described the drying of okara in hot air oven satisfactorily. Equilibrium moisture content isotherms of spouted-bed and hot-air dried okara samples exhibited similar trends manifested by a sigmoid-shaped curve. The experimental equilibrium moisture content isotherms of okara were best represented by the GAB, Oswin and Smith equations with a successively decreasing goodness of fit in that order. The method of drying as well as temperature did not appear to influence the equilibrium moisture contents of okara. Comparative evaluation of quality indicated that the okara samples dried in spoutedbed were of significantly lighter color in appearance than hot-air dried okara powders for similar drying temperatures as depicted by the values of whiteness, hue angle and chroma index. In addition, drying air temperature had little effect on the whiteness, hue and chroma of okara in spouted bed drying in comparison with the hot-air drying in oven. Chroma index of hot-air dried okara was slightly higher than the spouted-bed okara, but hue angle showed the reverse trend. However, both chroma index and hue of spouted-bed dried okara were relatively close to that of control sample dried at 55°C indicating that drying of okara in a bed of inert particles produced a product of superior color in comparison with oven drying method. In general, particle density of okara dried in spouted-bed was lower than the hot-air dried samples and appeared to be influenced by the dryer operating conditions. Both spouted-bed and hot air dried okara powders showed a significant reduction in their water and soy lipid uptake in comparison with that of control sample dried at 55°C. However, water absorption values of spouted-bed dried okara were relatively higher than in case of hot-air dried okara. Results showed that dry okara powder could be used to replace up to 20% wheat flour in the preparation of batter used for coating tempe prior to deep-frying. The sensory evaluation indicated that the modified preparation was well accepted in terms of visual appearance and taste. |