Temperature-Driven Maillard Conjugation and Phenolic Changes in Dried Lychee Pulp: Implications for Antioxidative Enhancement

Thermal aging is an effective strategy for improving the functional properties of fruit-based ingredients via physicochemical modifications. This research investigates the effect of thermal aging on the physicochemical, antioxidant, and Maillard conjugation properties of dried lychee pulp aged at 50, 60, 70, and 80 °C for 20 days under controlled relative humidity. Comprehensive analyses were performed, including total phenolic content (TPC), total flavonoid content (TFC), individual phenolic profiles, saccharide composition, free amino acid content, degree of glycation (DG), peptide molecular weight distribution, and antioxidant activity assessed by ABTS, DPPH, and FRAP assays. The results demonstrated that aging at moderate temperatures (60-70 °C) significantly enhanced TPC, TFC, and antioxidant activity, alongside an increased degree of glycation, peaking at approximately 47% at 70 °C. Principal component analysis (PCA) confirmed strong correlations between these compositional changes and antioxidant responses. In contrast, aging at 80 °C led to the degradation of thermolabile phenolics, sugars, and amino acids, resulting in reduced antioxidant activity compared with non-aged samples. Overall, the results highlight a temperature-dependent balance between constructive Maillard conjugation and thermal degradation, identifying 60-70 °C as an optimal aging range for improving the functional quality of dried lychee pulp. These findings provide mechanistic insight into thermal modulation of fruit bioactivity and support the potential application of controlled thermal aging in the development of value-added functional food ingredients.