Quality retention and energy optimization in ohmic vacuum concentration of white mulberry syrup

This study systematically compared hybrid ohmic-vacuum heating technology with conventional ohmic heating at atmospheric pressure (100 kPa, used as the control treatment) for white mulberry syrup concentration. The experimental design consisted of two operational modes: (a) conventional ohmic heating at atmospheric pressure (100 kPa, baseline control), and (b) hybrid ohmic-vacuum heating at reduced pressures (50 and 75 kPa). Each mode was evaluated at three voltage gradients (10, 20, and 30 V/cm). Multi-objective optimization was performed using 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% in the control (atmospheric pressure), whereas under ohmic-vacuum conditions (50 and 75 kPa), processing times were 81-295% longer than the control treatment 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 mL), with maximum retention at 50 kPa and 10 V/cm. Counterintuitively, the control treatment demonstrated superior energy efficiency, with specific energy consumption 2.5-3.7-fold lower than the hybrid ohmic-vacuum method. NSGA-II optimization identified the best compromise conditions at 83.47 kPa and 26.82 V/cm, yielding: TPC = 30.85 mg GAE/100 mL, DPPH = 84.73%, TFC = 78.14 mg CE/100 mL, processing time = 12.24 min, and SEC = 2.87 MJ/kg water evp.