## 4 Comprehensive Review
### A Comprehensive Overview of Bioactive Compounds in Different Food Drying Systems
#### 4.1.6 Infrared  
Infrared (IR) radiation is a type of electromagnetic radiation that covers the wavelength range between 0.75 and 1000 \(\upmu\)m, which lies just below the visible red light spectrum--hence the name \"infrared\". IR radiation is emitted or detected by any object with a temperature above absolute zero (0 K). It can be classified into near-infrared (0.75-3.0 \(\upmu\)m), mid-infrared (3.0-25 \(\upmu\)m), and far-infrared (25-1000 \(\upmu\)m) regions [43; 103]. Infrared drying works by delivering intense heating to the sample. Due to the penetration capability of certain IR frequencies, the molecules within the food vibrate, generating internal heating. This mechanism enables highly efficient heating with minimal heat loss to the environment. Near-infrared, due to its lower frequency and greater penetration depth, is considered more suitable for thick food products. In contrast, far-infrared has a higher frequency and shallower penetration, making it more appropriate for thin layers [43; 103]. The infrared dehydration technique offers several advantages, including its faster drying, lower energy consumption, high heat transfer efficiency, uniform heating, and reduced quality losses. It also positively influences sensory attributes such as color, flavor, and texture; preserves antioxidants and essential nutrients; allows direct heating without affecting the surroundings; ensures precise process control; and is compact and cost-effective [51; 104]. Recent findings support the efficiency of infrared drying in reducing the drying time and energy consumption while preserving the physicochemical properties of fruits. For example, in the drying of _Phoenix dactylifera_ L. (date fruit), the use of infrared radiation significantly reduced the drying time by up to 50% compared to conventional convective methods, while better retaining the color, total phenolic content, and antioxidant activity [105]. These results emphasize the potential of infrared technology not only in improving the process efficiency but also in maintaining the nutritional and functional quality of thermosensitive compounds. Table 8 summarizes the applications of this technology in various food matrices. Infrared radiation has a high penetration capacity, allowing the volumetric heating of tissues. This heating occurs from the inside out, breaking cell walls and releasing bioactive compounds, such as phenolics and flavonoids, which are bound to polysaccharides and proteins [111]. The release of these compounds occurs mainly through the conversion of bound forms into free forms, increasing their solubility and, consequently, their bioavailability. Infrared radiation also facilitates the hydrolysis of glycosidic and ester bonds, which helps release flavonoids such as catechin and epicatechin, potentially increasing the levels of total phenolics and flavonoids [106; 112]. One of the main advantages of using infrared radiation is the reduced exposure time to heat, which lowers the risk of the thermal and enzymatic oxidation of these compounds. Shorter drying times prevent the phenolic and flavonoid degradation caused by prolonged exposure to heat [111; 112] and also minimize enzymatic degradation reactions that can compromise antioxidant activity. Indeed, studies have shown that infrared drying, when carefully controlled, preserves antioxidant properties by maintaining bioactive compound levels [107]. The presence of phenolic compounds and flavonoids, both with antioxidant properties, is essential to maintaining a food's antioxidant capacity. Infrared radiation, by releasing bioactive compounds from their bound forms, increases the active fraction of these compounds in extracts, potentially improving their radical scavenging capacity [111; 112]. Furthermore, the thermal process can promote the formation of pigments, such as melanoidins, via the Maillard reaction, which also contributes to antioxidant activity [109]. However, infrared radiation should be used with caution to avoid the degradation of heat-sensitive bioactive compounds.