Effects of Freezing Pretreatment, Microwave-Assisted Vibro-Fluidized Bed Drying and Drying Temperature on Instant Rice Production and Quality

Abstract

Three drying factors including pre-drying treatment (freezing), microwave-assisted drying (single-mode applicator-microwave vibro-fluidized bed [SMA-MVFB] and vibro-fluidized bed [VFB] drying) and drying temperatures (110-185C) were varied to investigate their effects on drying characteristics, energy consumption and instant rice quality. During drying cooked rice from 60 to 10% (wet basis), 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 obtained from SMA-MVFB drying of frozen cooked rice at 185C. This treatment also required the lowest energy of 0.204 kWh. However, in terms of product quality, unfrozen cooked rice subjected to VFB drying at 160C was superior as it was ranged in the whitest class, had the lowest bulk density and shrinkage values, and presented the best rehydration characteristic. After rehydration, it was whiter, bigger and softer than freshly cooked rice. PRACTICAL APPLICATIONSInstant rice has many advantages such as short preparation time, ease to carry and long shelf life. However, the existing products are not popular due to their poorer quality and higher price as compared with freshly cooked rice. As new food technology has continuously emerged, potential assessment of new technology for instant rice production would be imperative. Cooked rice is a difficult-to-handle material to be dried because of its sticky nature. So, a specific dryer is required to prevent agglomeration of cooked rice. Therefore, the effect of a new hybrid single-mode applicator-microwave vibro-fluidized bed (SMA-MVFB) drying on instant rice production and quality was investigated. In addition, a porous structure of instant rice is vital to ensuring its good rehydration capability and claimed to be improved by freezing before drying. Consequently, this study presented potential applications of SMA-MVFB drying and freezing pretreatment as well as drying temperature for preparing instant rice with good quality.

Effect of puffing conditions on physical properties and rehydration characteristic of instant rice product

“…Softer texture of the rehydrated rice than the normal cooked rice was due to their puffed and porous structure following the volume expansion. These results agree with Sripinyowanich & Noomhorm () observations of rice with higher porosity resulted in rehydrated rice with softer texture.…”

“…( ) reported the instant rice with low bulk density to have higher porous structure enabling the rice to be penetrated by water in a relative shorter time. Similar observation was reported by Sripinyowanich & Noomhorm () for higher rehydration capacity of highly expanded instant rice. The instant rice with volume expansion ratio of 1.71–1.94 (bulk density 0.33–0.36 g cm −3 ) exhibited the rehydration ratio of 2.21–2.23.…”

“…The amylose and short‐chain amylopectin was found to be associated with the stickiness of cooked rice and longer cooking time, resulting in larger amount of leached starch (Chiang & Yeh, ). Furthermore, Sripinyowanich & Noomhorm () observed that the reconstituted instant rice production with high drying temperature resulted in lower stickiness.…”

Optimization of operating process parameters for instant brown rice production with microwave-followed by convective hot air drying

“…Apart from its high drying rate, the vibro-fluidized bed dryer could reduce the sticky problem of cooked rice during drying. Interestingly, Rewthong et al (2011) and Sripinyowanich and Noomhorm (2013) proposed the idea to apply the freezing step prior to drying in order to raise the porous structure of instant rice.…”

“…Generally, the production of instant rice includes several basic steps such as pretreatment, cooking, and drying process (Rewthong et al, 2011;Sripinyowanich and Noomhorm, 2013). The operating conditions for each step should be properly selected depending on the forms of rice and warming method prior to serving.…”