Kinetic and spectroscopic investigation of oxidative and protein structural stability in protein bars fortified with elderberry extract and Diospyros lotus concentrate

Maintaining oxidative, textural, and structural stability during storage remains a major challenge in the development of high-protein functional foods. This study investigated protein bars fortified with elderberry extract and Diospyros lotus concentrate during 28 days at 25 °C. The fortified formulation was prepared by adding 0.98 g elderberry extract and 19.18 g D. lotus concentrate per 100 g of base formulation. Peroxide value, moisture, hardness, antioxidant capacity, browning index, and FTIR-derived protein secondary structures were monitored and kinetically modeled. After 28 days, fortified bars showed lower peroxide value (5.35 vs. 5.97 meq O₂/kg), less hardening (47% vs. 124%), and higher DPPH radical-scavenging activity (35.0% vs. 11.5%) than the control. Browning and secondary-structure profiles also differed between formulations. The empirical models adequately described these temporal changes. Overall, fortification favorably modified short-term quality evolution, while kinetic modeling provided a quantitative framework for characterizing storage-related changes in functional protein bars.