## 4 Comprehensive Review
### A Comprehensive Overview of Bioactive Compounds in Different Food Drying Systems
#### 4.1.5 Cold Plasma  
Cold plasma (CP) is an environmentally friendly emerging technique that has gained prominence in studies focused on food preservation and drying methods. This technique is based on the generation of non-equilibrium plasma, where a high electrical current is used to excite molecules in the surrounding atmosphere. This process leads to the formation of several reactive components, mainly ionized molecules, with gas temperatures typically ranging from 30 to 60 \({}^{\circ}\)C. Owing to these characteristics, cold plasma enables enzyme inactivation, cell wall disruptions, and the oxidation or alteration of compounds without the need for high temperatures, elevated pressures, or prolonged exposure times [96; 97]. Some of the main methods used to generate cold plasma include dielectric barrier discharge (DBD), corona discharge, glow discharge, radiofrequency discharge, gliding arc discharge, atmospheric pressure plasma jets, and multi-pin discharge. In addition to reactor types, parameters such as the frequency, voltage, and gas composition can directly influence the effects of cold plasma on food matrices [96]. The use of cold plasma as a pretreatment for food drying is currently the focus of several studies due to its ability to induce microstructural changes on food surfaces, thereby increasing diffusion rates and reducing the drying time and energy consumption. These advantages are particularly relevant in the preservation of heat-sensitive compounds, such as phenolic compounds [2; 98]. Table 7 summarizes the main studies on cold plasma applications. These studies primarily focused on using cold plasma as a pretreatment to optimize drying processes, reducing processing times and energy costs, minimizing the degradation of heat-sensitive compounds, and enhancing desired sensory and physicochemical properties. [PERSON] et al. [99] observed that frequency had a direct impact on the effects caused by cold plasma. Nearly all tested frequencies (200, 500, and 800 Hz) resulted in higher levels of \(\beta\)-carotene, total phenolic compounds, and antioxidant capacity compared to untreated samples. At 500 Hz, the DPPH activity was similar to that of the fresh sample. The cold plasma pretreatment also promoted greater pore formation on the surface of the samples, which facilitated moisture diffusion and significantly reduced the drying time--from 270 min in untreated samples to 105 min with the most effective pretreatment (200 Hz). Similarly, [PERSON] et al. [96] studied _Crocus sativus_ (saffron) subjected to various exposure times to cold plasma before convective drying. The plasma treatment significantly reduced the sample moisture content and shortened the drying time by 16% compared to the control. Additionally, a 30 s pretreatment resulted in higher concentrations of crocin and picrocroccin--heat-sensitive bioactive compounds--with increases of 12.28% and 19.29%, respectively, relative to untreated samples. These findings confirm that cold plasma acts not only as a physical modifier but also as a barrier against the thermal degradation of bioactive compounds, better preserving the functional properties of foods. These positive effects of cold plasma are recurrent in the literature. For instance, [PERSON] et al. [2] reported that combining a cold plasma pretreatment and ultrasound-assisted convective drying in goldenberries led to the highest retention of vitamin C (80.87%), the total phenolics (85.57 mg GAE/100 g), and the antioxidant capacity (27.06%) compared to treatments without plasma. [PERSON] et al. [101; 102] also found significant increases in phenolic and flavonoid contents in _Centella asatica_ L. treated with plasma activated water, with superior values regardless of the drying method applied (vacuum, convective, or microwave). Although all studies have reported benefits in preserving bioactive compounds, few have quantified the energy consumption associated with drying operations. Tabibian