Application of microwave-vibro-fluidized bed drying in production of instant rice | |
| Author | Jiraporn Sripinyowanich |
| Call Number | AIT Diss. no.FB-11-01 |
| Subject(s) | Drying apparatus Rice--Drying Microwave cooking Food--Effect of heat on |
| Note | A dissertation submitted in partial fulfillment of the requirement for the degree of Doctor of Philosophy in Food Engineering and Bioprocess Technology, School of Environment, Resources and Development |
| Publisher | Asian Institute of Technology |
| Series Statement | Dissertation ; no. FB-11-01 |
| Abstract | This study aimed to assess the potential of a single-mode applicator-microwaveassisted vibro-fluidized bed (SMA-MVFB) dryer to produce instant rice. The SMA-MVFB dryer was fabricated, and its performance was evaluated. Vibro-fluidized bed (VFB) drying was also evaluated for its performance to compare with the microwave-assisted drying. To improve rehydration capability of instant rice, freezing was conducted as a predrying treatment of cooked rice. Effects of pre-drying, microwave-assisted drying, and drying temperatures on drying characteristics of cooked rice, energy consumption during drying, and instant rice quality were investigated. An empirical drying model was developed to describe the drying kinetics of cooked rice. Different rehydration techniques were applied to evaluate their effects on quality of reconstituted rice. Microwave efficiency of the SMA-MVFB dryer was compared with that of a multimode applicator (MMA)-MVFB dryer. Their microwave generation and absorbance efficiency at different levels of microwave power density (0.82, 1.04, and 1.70 kW/kg) was investigated in water system. Results showed that the types of applicator affected the absorbance efficiency. Almost 100% efficiency of microwave absorbance was achieved when heating water in the SMA. To evaluate drying performance of the developed SMAMVFB dryer, moist paddy, which was the raw material for cooked and instant rice production and enabled easier operation of the dryer, was used as a drying material. Four drying techniques, including 850-W MVFB drying at ambient temperature (AT) and moderate temperature (MT) of 60°C, and VFB drying at MT and high temperature (HT) of 150°C, was used to dry paddy from 25 to 17% (wet basis, wb). The combined technique of MT-MVFB drying was the best in terms of decreased drying time and increased drying rate. It also yielded milled rice with high percentages of head rice and whiteness. The hybrid system of MT-MVFB drying was proven to be superior to the other drying techniques. This research confirmed applicability of the developed SMA-MVFB dryer. Its ability for cooked rice drying was next examined. Three drying factors, including pre-drying treatment (with and without freezing), microwave-assisted drying (MVFB and VFB), and drying temperatures (110-185°C), were varied to investigate their relative effects on drying characteristics of cooked rice, energy consumption during drying, and instant rice quality. An air velocity and vibration were fixed at 5 mis and 1200 rpm, respectively. During the process of drying cooked rice from 60 to 10% (wb), drying rate increased, whereas drying time decreased with the three drying factors. The highest drying rate of 0.554 g water/g dry matter/min and the shortest drying time of 2 min were hence provided by MVFB drying of frozen cooked rice at l 85°C. The drying profile of cooked rice was also dependent on the three factors. MVFB drying of unfrozen cooked rice at 110-160°C respectively took place in heating-up, constant rate, and falling rate periods, whereas the other drying treatments showed no constant rate period. A new model (MR = exp(-ktn) + bt + c) was proposed in this study as an alternate to 11 commonly used drying models. The proposed model described the MVFB and VFB drying data most satisfactorily. The effective diffusivity values substantially increased with the three factors and were between l.42x10·7 and 5.86x10·7 m2/s. Their activation energy values were in the range of 21.19 and 23.90 kJ/mol. The lowest specific energy of 0.664 kWh/kg water was also provided by MVFB drying of frozen cooked rice at the highest temperature of 185°C. These results presented best practice of a freezing pretreatment and MVFB drying to improve drying characteristic of cooked rice and save energy required during drying. In terms of instant rice quality, the freezing pretreatment was good in increasing whiteness but poor in increasing grain porosity resulting in higher bulk density, iii longer rehydration time, and a slower rehydration rate. MVFB-dried samples were less whitish and denser than the VFB ones. The quality was improved when drying temperature was up to 160°C. Based on these findings, unfrozen cooked rice subjected to VFB drying at l 60°C was superior to the other samples since it was ranged in the whitest class, had an elaborated porous structure, had the lowest bulk density value, and presented the best rehydration characteristic. After rehydration, it was whiter, bigger, and softer than freshly cooked rice. Conclusively, quality of instant rice was not improved by freezing pretreatment and MVFB drying. Quality of the best instant rice sample (unfrozen cooked rice subjected to VFB drying at l 60°C; named FEBT (Food Engineering and Bioprocess Technology) instant rice) after rehydration was investigated both in excess and adequate water systems. The rehydration techniques included addition of vigorously boiling water and microwave cooking at 800 W. These techniques took 7 and 6 min and their suitable water-to-rice ratios were 1. 74 and 2.50, respectively. To describe the rehydration characteristics of the FEBT instant rice, addition of water at constant temperatures of 30, 70, and 95°C into the rice was also applied as representatives of the ambient, warm, and hot water temperatures, respectively. It was found that an increase in rehydration temperature resulted in increased rehydration rate and amount of absorbed water. Microwave cooking yielded the reconstituted rice with the closet dimensional and textural properties to freshly cooked rice. When compared with the commercial instant rice products, the FEBT instant rice had similar bulk density but had a more elaborated void network and greater whiteness. The microwave-reconstituted FEBT instant rice also had the closest dimensional and textural properties to freshly cooked rice. The FEBT instant rice was preferable to the commercial products when the same rehydration method was applied. |
| Year | 2011 |
| Corresponding Series Added Entry | Asian Institute of Technology. Dissertation ; no. FB-11-01 |
| Type | Dissertation |
| School | School of Environment, Resources, and Development |
| Department | Department of Food, Agriculture and Natural Resources (Former title: Department of Food Agriculture, and BioResources (DFAB)) |
| Academic Program/FoS | Food Engineering and Bioprocess Technology (FB) |
| Chairperson(s) | Athapol Noomhorm |
| Examination Committee(s) | Rakshit, Sudip Kumar;Anal, Anil Kumar;Soni, Peeyush |
| Scholarship Donor(s) | RTG and AIT Fellowships |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 2011 |