Comparative Evaluation of Drying Methods for Vegetable Waste Aimed at Producing Natural Functional Food Ingredients

This study presents a comparative evaluation of four drying methods for carrot, red beet, and pumpkin pomace to produce natural functional food ingredients. The work addresses the valorization of 35-45% vegetable processing waste-a rich source of bioactive compounds-aligning with circular bioeconomy principles and Kazakhstan's goals for deep processing of agricultural raw materials. The compared methods were convective drying (CD), ultrasound pretreatment + convective drying (US + CD), vacuum-microwave drying (VMD), and ultrasound pretreatment + vacuum-microwave drying (US + VMD). Drying kinetics, water activity, physicochemical and functional properties of powders, retention of bioactive compounds, color characteristics, thermal stability, and sensory attributes were assessed. Kinetics were fitted using Midilli et al., Page, and Weibull models. US + VMD provided the highest drying acceleration (6-11 times faster than CD), reaching final moisture of 5.1-5.9%, water activity a_w 0.27-0.31 in 80-170 min, and bioactive compound retention of 90-95% (carotenoids 92-95%, betalains 90-94%). It also delivered superior flowability (Carr's index 22.5-30.4%), dispersibility (80-88% in 30 s), and thermal stability (75-85% at 200 °C). Acceleration varied by raw material: maximum for beet (up to 11×) due to soluble sugars and nitrates, minimum for pumpkin (5.5-8×) due to dietary fibers and pectins, and intermediate for carrot (6-9×) influenced by carotenoids' dielectric properties. The results highlight US + VMD's strong potential for producing functional powders to replace synthetic additives in food systems. Effective method selection and parameter optimization require consideration of raw material type and rheological characteristics.