Encapsulation of betacyanin extract from red dragon fruit peel with hydrocolloids : molecular mechanism study and application in plant-based protein beverage

Abstract

This research aimed to investigate the stability of betacyanins from red dragon fruit peel for application in a high-protein plant-based beverage. Betacyanins were extracted using ultrasound-assisted extraction (UAE), and HPLC was used to identify the primary components. Density Functional Theory (DFT) was then employed as a screening tool to model the interaction between the principal betacyanin structure and various hydrocolloids (maltodextrin (MD), guar gum (GG), sodium alginate (SA), and low-methoxyl pectin (LMP)). HPLC analysis revealed that betanin and phyllocactin are the main betacyanins in extract. DFT calculations showed that SA–Betacyanins (-361.259 to -108.935 kJ/mol) and LMP–Betacyanins (-375.372 to -97.496 kJ/mol) complexes exhibited the lowest binding energies, indicating greater stability and stronger bonding. Therefore, SA and LMP and mixed with MD (maltodextrin-sodium alginate (MD-SA) and maltodextrin-low methoxyl pectin (MD-LMP)) were selected for encapsulation of the betacyanin extract via freeze drying. The 40% MD was used as a control, while MD + SA (1–3%) and MD + LMP (1-3%) were used as treatments. The resulting encapsulated powders were evaluated for physical and chemical characteristics, encapsulation efficiency (EE), and betacyanin content (BC). The encapsulated powders were then introduced into a high-protein plant-based beverage (pea/rice protein), subjected to pasteurization (72°C/15s and 85°C/30s), and monitored during refrigerated storage (4°C) for 14 days. Results showed that composite encapsulants (MD-SA2% and MD-LMP2%) exhibited significantly higher BC (93.81 mg/100 g and 80.00 mg/100 g, respectively) compared to MD alone (77.93 mg/100 g), which aligned with the DFT predictions. The FTIR analysis revealed that the bonds formed between betacyanins and the hydrocolloids were consistent with the DFT results. However, a contradictory finding was observed upon application. During initial thermal processing (pasteurization), MD (control) provided the superior protective effect, retaining the most betacyanins, especially at 85°C. Conversely, during the 14-day cold storage, the MD-SA2 formulation demonstrated the highest stability, with the lowest first-order degradation rate constant (k) and the longest half-life (t1/2), particularly in the 72°C/15s sample (t1/2 ≈ 3.45 days). This discrepancy is likely due to the neutral pH (~7.0) of the high-protein plant-based beverage and shear forces during homogenization, which may have compromised the protective integrity of the pH-sensitive hydrocolloids (SA and LMP). This study highlights a critical trade-off: while MD is superior for surviving initial heat shock, MD-SA2 is more effective for long-term shelf-life stability.