Comparative Study of Conventional Ohmic and Hybrid Ohmic-Vacuum Heating for White Mulberry Syrup Concentration: Bioactive Compound Retention, Energy Consumption, and Multi-objective Optimization

This study systematically compared conventional ohmic heating and hybrid ohmic-vacuum heating technologies for white mulberry syrup concentration, evaluating the interactive effects of voltage gradient (10–30 V/cm) and pressure (50–100 kPa) on bioactive compound retention, energy efficiency, and process optimization. Multi-objective optimization was performed using Non-dominated Sorting Genetic Algorithm II (NSGA-II) to simultaneously maximize bioactive compounds while minimizing processing time and energy consumption. Results demonstrated that increasing voltage gradient from 10 to 30 V/cm reduced processing time by 75.3% at atmospheric pressure, though ohmic-vacuum conditions exhibited 81–295% longer processing times due to reduced electrical conductivity at lower temperatures. Total phenolic content decreased by 28–48% across all treatments, with best retention (33.1 mg GAE/100 mL) achieved at 50 kPa and 10 V/cm. Antioxidant capacity showed excellent preservation (62.13–86.12%), peaking at 86.12% under intermediate conditions (75 kPa, 20 V/cm). Total flavonoid content exhibited dramatic variability (60.22–171.20 mg CE/100 g), with maximum retention at 50 kPa and 10 V/cm, while 20 V/cm proved universally detrimental regardless of pressure. Counterintuitively, conventional ohmic heating demonstrated superior energy efficiency, with specific energy consumption 2.5-3.7-fold lower than hybrid ohmic-vacuum method. NSGA-II optimization identified the best conditions at 83.47 kPa and 26.82 V/cm, which yielded the following experimental responses: 30.85 mg GAE/100 mL for total phenol content, 84.73% for antioxidant capacity, 78.14 mg CE/100 g for total flavonoid content, 12.24 min for processing time, and 2.87 MJ/kg water for energy consumption.