**4. Industrial application: a case study**

In the first two sections, a review of the main concepts and research works regarding the variables affecting rice drying were presented. In the third section, the impact of the operation variables on thin-layer drying of a South American rice variety was studied. Based on these results, in the present section two drying programs are proposed and tested to dry a South American variety in a commercial crossflow dryer, with the aim of increasing the HRY and the drying rate. The objective of this section is to provide a practical application to the results obtained on a laboratory scale.

#### **4.1 Materials and methods**

#### *4.1.1 Commercial dryer*

Runs were performed in a cross-flow commercial dryer. The dryer has two sections: a drying chamber and a tempering zone. Rice enters the dryer and recirculates, passing the drying chamber and tempering zone in each cycle, until it reaches the final MC (approximately 13%). Contrarily to what occurs in the lab-scale dryer (see Section 3), in the commercial dryer, rice temperature never reaches the drying air temperature (due to the absence of a thin layer of rice and the passage through the tempering zone between the drying cycles). Therefore, the drying programs were based on controlling the rice grain temperature (not the drying air temperature).

The drying air was taken from the environment and brought to the desired drying temperature in an industrial oven. Therefore, its RH depends on the environmental conditions. Since the drying air temperature was much higher than the ambient, the RH of the drying air was low.

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

#### *4.1.2 Experimental design*

From the experiments in the lab-scale dryer (see Section 3), it could be concluded that at low RH (which is the case in the commercial dryer) and a grain MC of 15% or higher, the air temperature (which equals the grain temperature) should be below 55° C and above 47°C. This allows a high drying rate with low HRYR (below 5 pp). Higher drying air temperatures not only increase the drying rate but also increase the HRYR.

At grain MC below 15% and low RH, the air temperature should be below 47°C, being the HRYR very low at an air temperature of 35°C (no data is available between 35 and 47°C).

During the experiments, it was also observed that the Tg plays an important role, being critical when a pronounced MC gradient is present within the kernel. Therefore, drying at temperatures above Tg should be avoided at low grain MC.

Based on these considerations, two programs were proposed and tested by triplicate:

Program A:


Program B:


#### *4.1.3 Experimental procedure*

The rice used was a long-grain Uruguayan variety, with similar dimensions and composition than Uy2 (see Section 3). All runs (at the ordinary operating conditions and the programs) were performed with the same rice variety.

A representative sample of the rice entering the dryer in each run was collected using an automatic sampler. It collected a sample of approximately half kilogram every 2 minutes during the loading of the equipment. Once the loading was completed, the samples were mixed and homogenized properly. The same procedure was followed during the unloading, to obtain a representative sample at the exit of the dryer (once the drying run was finished). The rice MC was determined by gravimetry [23], and the HRYR was determined as described in Section 3.1.5, being HRYfinal the HRY of the sample collected at the exit of the dryer and HRYinitial the HRY of the sample collected at the entrance of the dryer and gently dried in the chamber until a final MC of 13 � 0.5%.

The average drying rate of each run was defined as the average MC (on a dry basis) removed per hour and was calculated as:

$$\text{Drying rate} = \frac{\text{MCf}[\text{final}, d.b.} - \text{MCinitial}, d.b.}{\text{time}} \tag{4}$$

where MCfinal,d.b. is the MC of rice at the end of the run on a dry basis (%MC), MCinitial,d.b. is the MC of rice at the beginning of the run on a dry basis (%MC), and time is the drying duration of the run (hours).

Thermal properties of all the rice samples were measured with a differential scanning calorimeter (TA instruments, DSC Q2000) as described in [30]. The data obtained comprise the onset temperature, peak temperature, conclusion temperature, and crystal melting enthalpy (ΔH).

Pasting properties were measured with a rapid visco analyzer (Perten Instruments, RVA 4500) following AACCI Approved Method 61–02.01. The peak, trough, final, breakdown, and setback viscosities were measured.

The drying rate, HRYR, and thermal and pasting properties of programs A and B were compared with those from the ordinary drying runs (runs under the ordinary operating conditions of the industry), which maintained a constant grain temperature.

#### *4.1.4 Statistical analysis*

The standard deviation was calculated for the HRYR and the drying rates.

Analysis of variance (ANOVA) was used to compare variables. In case of significative difference among variables (p < 0.05), Tukey test was applied to determine which were the variables that differ.

### **4.2 Results and discussion**

**Table 4** shows the average HRYR and drying rate of the runs dried with program A, program B, and ordinary operating conditions of the industry. Program A and B had significantly higher drying rates than the ordinary runs. In addition, program B had a significantly lower HRYR than the ordinary runs. Although the difference between program A and program B was not significant, it was observed that program B runs tend to have lower HRYR than program A.

Based on these results, program B seems to be the most promising to improve the drying efficiency, increasing the drying rate and reducing the HRYR compared to the ordinary operating conditions. Implementing this program would reduce the drying duration of each run, increasing the reception capacity of the drying plant and, consequently, improving its productivity. At the same time, reducing the HRYR would increase the profitability of the rice obtained.

Drying conditions can affect the quality of rice, especially when high grain temperatures are reached during the drying process [31–33]. For this reason, thermal and pasting properties of the samples from program B and those from the runs under


*Different characters in the same column indicate significative difference among samples (p < 0.05). Errors correspond to two standard deviations (2σ). HRYR Head rice yield reduction and OOC = Ordinary operating conditions.*

#### **Table 4.**

*HRYR and drying rate of drying programs and ordinary commercial drying runs.*

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


*Results are the average of two runs in each condition (program B or ordinary operating conditions). PROG, Program; OOC, Ordinary operating conditions; IN, Representative sample of the loading; OUT, Representative sample of the unloading; T,Temperature; ΔH, Enthalpy.*

#### **Table 5.**

*Thermal properties of rice samples collected during drying.*

ordinary operating conditions were measured and compared. **Table 5** and **Figure 4** show these results.

Comparing the values and profiles of the samples collected during the loading (before drying) with those collected during the unloading (after drying), no significant differences were observed either for samples from Program B or for those from the ordinary operating conditions. The same behavior was observed with samples from program A (data not shown). Therefore, it could be concluded that the drying process had no effect on the cooking properties in any of the conditions tested, confirming that Program B is a suitable drying program to implement in the industry to improve the drying efficiency.

In Ref. (14), the authors found that hardness and stickiness of an aromatic longgrain rice dried using different drying methods were not significantly different to the control sample (sample dried under very mild conditions) when the drying temperature was kept below 60°C. In agreement with this, Dillahunty et al. [32] found that only drying above 55°C with exposure durations higher than 12 hours lowered the peak viscosity of a long-grain (Cypress) and a medium-grain (Bengal) rice. At temperatures below that, there were no significant differences with the control samples. In our experiments, temperature never exceeded 50°C, being below the temperature reported as critical by these researchers. Therefore, our results for a South American variety were in agreement with these findings reported for other long-grain varieties.
