**3. Impact of operating conditions on the efficiency of rice drying**

As seen in the previous section, there are numerous variables that affect the drying rate and the HRY, among which are the operating (drying air) conditions. The operating conditions needed to increase the drying rate are usually opposite to those needed to improve the HRY.

The aim of the present section is to study the impact of the operating conditions on the milling quality and drying duration of a South American long-grain rice variety (Uy2) at different stages of the drying process. To this purpose, a thin-layer lab-scale dryer was used to dry rice under different controlled drying air conditions. The drying process at each condition was modeled using Page's equation.

#### **3.1 Materials and methods**

#### *3.1.1 Rice sample*

Rice of the Uruguayan variety Uy2 was collected from a single producer in the south-east region of Uruguay. The MC was determined by gravimetry [23]. The harvest MC of the paddy lot was 20.5 1.0%. The sample was homogenized and stored in a refrigerating chamber at 4.3 1.8°C until use. Before each experiment, the amount needed was removed from the chamber and left in sealed bags at room temperature for at least two hours.

#### *3.1.2 Experimental procedure*

Drying runs of long-grain rice Uy2 at different MC levels and using different drying air conditions (temperature and relative humidity) were performed. The drying air velocity was set at 0.4 m/s for all runs and the air T and RH were constant during each run.

**Table 1** shows the experimental design. Runs were carried out at different temperatures, below and above the Tg, and two RH were tested at each T. The RH was chosen to have the same EMC values (7% and 10%) at all the air T levels, meaning that the grain surface MC was also the same (since the grain surface MC equals the EMC soon after the drying begins). The RHs for each combination of EMC and air T were calculated using the modified Chung-Pfost equation for long grains [24]. For some runs, the air conditions set could not be reached (due to limitations of the drying system). In those cases, the runs were set at the closest condition possible (see **Table 1**). At the air T of 47°C, a greater variation of the milling quality was observed between the two EMC conditions (compared to other air temperatures). Therefore, two extra runs at two different RHs (corresponding to EMC of 8% and 12%) were added.

*Improving the Efficiency of Rice Drying: Impact of Operational Variables on the Drying… DOI: http://dx.doi.org/10.5772/intechopen.112970*

