Optimization of hybrid infrared – hot air drying of red beet pretreated with cold plasma: Drying kinetics, physical properties and microstructure

This research evaluated the impact of cold plasma pretreatment and an alternating infrared-hot air hybrid dryer on the drying characteristics and quality of red beet. The study was designed to improve drying efficiency while preserving structural integrity and limiting quality losses. Red beet samples were subjected to three plasma pretreatment times (0, 15 and 30 s), three infrared power levels (300, 600 and 900 W), and two pulse ratios (1 and 2). The findings indicated that cold plasma pretreatment numerically reduced drying time by approximately 20% and resulted in lower shrinkage and improved rehydration ratio. However, these effects were not statistically significant (p > 0.05) for most parameters, likely due to the thin sample geometry and the dominant effect of infrared power. Statistically significant effects (p < 0.05) were observed only for infrared power and pulse ratio on drying rate and shrinkage. Among the drying conditions, increasing infrared power to 900 W markedly improved moisture removal and reduced total drying time by about 25%. Pulse ratio also had a noticeable effect, with pulse ratio 1 generally producing a faster drying rate and better physical quality than pulse ratio 2. SEM observations of the optimization-based optimum (0 s plasma, 600 W, PR1) revealed a porous and uniform structure. Additionally, a plasma-treated sample (15 s plasma, 600 W, PR1) was imaged to compare microstructural effects; it showed surface modifications without statistically significant improvements in drying kinetics. Overall, the Page model provided the most accurate fit for moisture ratio prediction.