**3. Industrial drying techniques**

Generally, drying involves the coexistence of complex processes, which may be operated simultaneously. Initially, the energy is transferred from hot drying agent to fresh product. Then, the unbounded moisture evaporation (free water) occurs, and eventually, water particles bound inside the cellular structure. These particles exposed to diffusion and thus migration, and transferred to the product surface, where the water is lastly evaporated [22]. The attributes of dried products can be effectively influenced by the parameters of process, such as pressure, temperature, gas feed rate, and relative humidity. In addition to protein formulations characteristics, such as composition and excipients type, and solutes concentration, viscosity as well as solvent type [23]. A large assortment of dryers has been developed to dehydrate and preserve these products to meet different quality and cost requirements. As indicated by [3], over 400 dryer kinds have been described in the reviews, although only about 100 types are commonly use. Differences in dryer design are due to the product physical characteristics, modes of heat input, temperatures and pressure operating, dried product qualifications, etc. Conventionally, food stuffs are dried by open sun drying system. Though this system is still common in many places for non-commercial use, there have been several efforts to improve advanced drying systems for products on a commercial scale. It is obligatory to enhance the drying systems to reduce waste [4]. Regarding this concern, this section will shed light more on the most common industrial drying techniques applied in the food processing process.

### **3.1 Rotary drying**

Rotary drying is also known as tumbling dryer. It is considered the oldest continuous and the most common high-volume dryer used in industry. It has developed more adaptations of the technology than any other dryer classification. Rotary dryers stand out for their flexibility and applicability to a large number of materials, as well as their high processing capacity [24]. Large amounts of granular substantial with particles of 10 mm or larger that are not too fragile or heat-sensitive or cause any other solids handling problems are dried in rotary dryers [25]. Conventional rotary dryers have flights, which lift solids and make them cascade across the dryer section (active phase) when there is effective fluid-particle contact [26]. It is one of the many drying methods existing in unit operations of chemical engineering. The drying takes place in rotary dryers, which consist of a cylindrical casing usually made of steel plates rotating on bearings, lightly inclined horizontally. Usually, its length is 5–90 m, while its diameter is 0.3–5 m, and rotates at 1–5 [25]. It is regularly worked at a negative interior pressure to stop dust get-away. Wet feedstuff is presented into the system upper edge and the feed proceeds through it by force of rotation, head influences and shell inclines. Then the dried stuff is withdrawn at lower edge. The direct heat rotary dryer diagram is illustrated in **Figure 1**. Gas flow direction throughout the cylinder in the case of the solids is dictated by the processed substance characteristics. Current flow is utilized for heat-susceptible substances even with high entrance gas temperature because of the gas's fast cooling through initial surface moisture evaporation. While for other substances, the countercurrent flow is desired for higher thermal efficiency. In the first status, the solid flow rate is boosted by gas flow, whereas it retarded in the second status [27–30].

*Drying Technology Evolution and Global Concerns Related to Food Security… DOI: http://dx.doi.org/10.5772/intechopen.109196*

#### **Figure 1.** *Diagram of direct-heat rotary dryer.*
