**1. Introduction**

Rice is a staple crop that feeds almost half of the world population [1]. It is usually harvested at a moisture content (MC) of 16 to 22% and needs to be dried to a MC of 13% or lower for safe storage [2]. Considering that rice must be dried immediately after harvest to prevent spoilage, drying becomes a critical process during the harvest season.

Increasing the drying rate is relevant because it allows a larger amount of rice to be dried in a given period of time, relieving the frequently occurring bottleneck

generated when rows of producers' trucks are waiting to deliver their wet rice at the industrial plant.

During drying, moisture is removed from the surface of the rice grain. Initially, there is enough water available at the surface, making the drying rate rapid. After a short period of time, drying is limited by water diffusion from the interior to the surface of the kernel. This generates an increasing moisture gradient inside the kernel, with the center having a higher MC than the surface [1]. MC gradients are present during the entire drying process, and they depend on the drying rate, which is determined by the rice grain MC and the drying air conditions (temperature, relative humidity, and flow rate) [3]. The surface of the kernel tends to the equilibrium moisture content (EMC), which is given by the drying air temperature (T) and relative humidity (RH) [4, 5]. A higher T or lower RH decreases the EMC, increasing the drying rate and the MC gradient.

The MC gradient generates tension at the surface of the grain, where the cells tend to shrink as moisture is lost, compressing the center [6]. This is associated to the formation of fissures and cracking during drying. Fissured kernels are more prone to breaking during milling. Therefore, the head rice yield (HRY), defined as the mass percentage of rough rice that remains as head rice (kernels that are at least threefourth of its original length) after milling, tends to decrease with the presence of fissured kernels. In addition, fissured kernels affect the functional properties of milled rice, and thus the sensory quality [5].

Glass transition temperature (Tg) also plays an important role in fissures formation. Tg is the temperature range of transition of the amorphous regions of starch from a glassy into a rubbery state. Starch is the main component of rice and is formed by amylose and amylopectin chains. Glass transition occurs at the branching points of amylopectin [7]. The glassy state has a low expansion coefficient, specific volume, specific heat, and diffusivity but high viscosity and modulus of elasticity. Contrarily, the rubbery state has higher expansion coefficient, specific heat, specific volume, and diffusivity [5, 8]. The Tg increases as the grain MC decreases. During drying, the MC at the surface of the grain is lower than that at the center. This could cause the center of the grain to be in the rubbery state, while the surface is in the glassy state. The differences in the properties of the two states increase the tensions generated by the MC gradient and play an important role in terms of kernel fissuring potential [8].

To prevent this, at least in part, a process called tempering is used between drying passes. During tempering, rice is held in bins for a certain period of time. The purpose is to allow the MC gradients generated during drying to subside, reducing the tensions inside the kernels and therefore preventing kernels' fissuring [5, 9].

The drying rate is also affected by grain composition and geometry. Therefore, different varieties may respond differently to the same drying air conditions [1, 10].

As a result of this, the need arises to find suitable drying programs for each variety, reducing the drying time while minimizing fissures formation. A compromise should be made between the drying rate and the MC gradient generated during drying, which could lead to fissures formation, especially when two states (glassy and rubbery) coexist within the same kernel.

Several authors studied the impact of the drying air conditions and Tg on the drying rate and HRY [2, 5, 11]. Most of the studies were conducted using long-grain rice laid out in a thin layer, to ensure that all the rice is subjected to the same conditions.

As previously exposed, rice variety also plays an important role in relation to drying. Long-grain and medium-grain rice showed different behavior during drying *Improving the Efficiency of Rice Drying: Impact of Operational Variables on the Drying… DOI: http://dx.doi.org/10.5772/intechopen.112970*

[10]. This was attributed to differences in the kernels' dimensions. Medium-grain kernels were thicker, so moisture had to travel a longer path in its migration to the surface (compared to long-grain kernels).

Different drying methods, including experiments in commercial dryers, were also studied by some researchers. In Ref. [12], continuous drying of rough rice was compared with intermittent drying, while [13] investigated rough rice drying in fixed and fluidized bed dryers. Natural drying (shade and sun drying) was compared with heated air drying in [14] using different drying methods in a commercial and a lab scale.

The research published so far is mostly on varieties developed in the United States or Asia. There is very little literature on South American varieties, which have their own characteristics given by climatic conditions, cultivation practices, and genetics.

The present chapter introduces a review on the main variables affecting the drying efficiency, understanding the drying efficiency as the combination of drying rate and HRY. Then, the impact of the operating conditions on the efficiency of rice drying was studied for a South American variety using a thin-layer lab-scale dryer. Finally, an industrial application of the previous results is shown for a South American variety dried in a commercial cross-flow dryer.
