**2. Variables affecting the efficiency of rice drying**

This section presents a review of the main variables affecting the drying efficiency, understanding the drying efficiency as the combination of drying rate and HRY.

#### **2.1 Drying air conditions**

The drying air conditions have an impact on the drying rate and the HRY. Severe drying conditions (high drying air temperatures, flow rate, and/or low RH) increase the MC gradient, increasing the drying rate but decreasing the HRY.

In Ref. [10], the authors study the HRY of rice dried at different drying conditions. At the milder condition (higher EMC), the HRY was not affected during the entire drying process. However, as the drying conditions became more severe (lower EMC), the HRY decreased during drying. The more severe the drying condition, the sooner the HRY begins to decrease once drying starts. This could probably be attributed to the formation of more pronounced MC gradients.

An increase in the drying air temperature increases the drying rate, even for the same EMC [2]. The HRY is also susceptible to air temperature, even with equal EMC conditions. This is probably because higher drying rates cause more pronounced MC gradients inside the kernel, increasing tensions and favoring fissures formation. The HRY reduction is even more pronounced when drying above Tg, probably due to the coexistence of glassy and rubbery zones (surface and core, respectively) in the same rice kernel [15].

Regarding the drying air RH, when reduced (at a given temperature), the drying duration is shortened [16]. This could be related to an increase in the drying potential of the air.

#### **2.2 Glass transition temperature**

Tg is the temperature range corresponding to the transition of the amorphous regions of starch from a glassy to a rubbery state. It is a useful material descriptor due to its good correlation with structural and thermodynamic properties [17]. Tg depends on the composition of the rice grains, particularly starch. The amylose/amylopectin ratio plays an important role in the Tg. The higher the amylose content, the higher the Tg, which is associated to chain-chain interactions of linear chains of amylose that induce partial crystallinity [18]. On the contrary, high amylopectin starches show lower Tg. This was attributed to the formation of gel balls by the short-branched chains in amylopectin molecules. The gel balls require less energy to move than long linear chains, reducing the Tg. Water also reduces the Tg, acting as a plasticizer [7, 18]. Therefore, the Tg of a rice kernel depends on its MC. This means that at a certain temperature, a kernel could be in the glassy or in the rubbery state, depending on its MC.

The abrupt change in several properties of the material in the glass transition range can be used for its determination [17]. There are changes in two groups of properties: rheological and thermodynamic properties. Differential scanning calorimetry (DSC) is used to determine Tg based on changes in thermodynamical properties, such as heat capacity. Dynamic mechanical thermal analysis (DMTA) is another methodology used and is based on the change in rheological properties, such as the storage and loss moduli. Both methodologies showed good results and proved to be suitable for Tg determination [8, 19].

**Figure 1** shows the state diagram for three Uruguayan rice varieties (Uy1, Uy2, and Uy3), built using DSC [20]. A significative difference between the Tg of Uy1 and Uy3 in the MC range of 12 to 16% was found, proving that different varieties could have differences in the Tg. This was attributed to differences in the starch composition and in the kernels' dimensions of both varieties.

Tg plays an important role in the rice drying process. Drying at temperatures above the Tg increases the drying rate since thermal conductivity and mass diffusivity are higher in the rubbery state. However, the possible presence of glassy and rubbery regions within the same kernel (due to differences in the MC given by the MC

#### **Figure 1.**

*State diagram for three Uruguayan varieties (reprinted, with permission, from [16]). Tg, glass transition temperature; MC, moisture content.*

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

gradient formed) could increase the tensions inside the grain, favoring fissures formation and reduction of the HRY [4]. **Figure 2** represents this situation in a state diagram [15]. A rice kernel with initial MC of 19% is heated during drying to a temperature of 55°C, going from the glassy to the rubbery zone. As drying continues, the center (more humid) remains in the rubbery zone, while the surface tends to the EMC in the glassy zone. This increases the tensions that already exist inside the kernel due to the MC gradient. Increasing the air relative humidity (RH) increases the EMC, and consequently reduces the MC gradient. This enables a greater part of the kernel to remain in the rubbery state (when drying above the Tg), and therefore increases the HRY [4].

In Ref. [21], a glass transition mapping inside a rice kernel during drying was performed by modeling. It was found that when the drying air temperature is higher than the Tg, the outer layers of the kernel go from the glassy to the rubbery state due to a rapid temperature rise at the beginning of drying. Then, as MC decreases, a transition from the rubbery back to the glassy state could occur (as shown in **Figure 2**). Simulation suggests that fissures initiate more easily from the tensile zones, where the transitions from a rubbery to a glassy state occur, probably because of the stress concentration in the interface between the regions of expansion and contraction caused by the change of state.

#### **2.3 Kernels dimensions and composition**

The dimensions and composition of the grain kernels affect their behavior during drying. As the kernel thickness increases, the fissured kernel percentage also increases [22]. It was hypothesized that the thicker kernels experience a greater MC gradient than thinner kernels when exposed to the same drying condition. This leaves thicker kernels more susceptible to fissuring. Confirming these results, a study with Bengal (medium-grain), Cypress, and Kaybonnet (both long-grain) varieties dried under the

#### **Figure 2.**

*Representation of a rice kernel (whole grain, center, and surface) in a state diagram. MC, moisture content, EMC = equilibrium MC,Tg = glass transition temperature (reprinted, with permission, from reference [11]).*

#### *Drying Science and Technology*

same drying conditions shows that medium-grain Bengal has a faster and more pronounced HRY reduction compared to the other two varieties [10].

The chemical composition (amylose and protein content) and physicochemical properties of rice also impact the HRY [3]. Drying of both low and high amylose rice, at the same level of grain temperature, shows quality differences [14].

The drying rate is also affected by the rice grain variety, influenced by the kernel dimensions and/or composition [2].
