Seasonal variation in nitrogenous components and bioactivity of protein hydrolysates from Porphyra dioica

The red macroalga Porphyra dioica has been harvested and consumed for centuries. Based on its nutritional composition, availability and consumer familiarity, significant potential exists to develop this species as a source of high-value functional food ingredients. Therefore, a detailed assessment of the natural variation in P. dioica nitrogenous components was performed to identify the optimal season for biomass harvesting with high bioactive peptide potential. Kjeldahl nitrogen analysis revealed that total nitrogen (TN) and protein nitrogen (PN) contents in P. dioica (expressed as (w/w) dry weight) from western Ireland ranged from 2.48 to 4.94% and 1.90 to 4.30%, respectively. Significant differences in protein contents were observed between samples collected in summer and winter months. Electrophoretic analysis also showed differences in the protein profiles of P. dioica collected at different times of the year. P. dioica protein extracts were hydrolysed with the food-grade proteolytic preparations, Alcalase 2.4 L and Flavourzyme 500 L, and significant seasonal differences were observed in in vitro bioactivity assays. The oxygen radical absorbance capacity (ORAC) and ferric reducing antioxidant power (FRAP) values of the hydrolysates ranged from 229.5 to 1015.3 and 4.1 to 28.7 μmol Trolox equivalent per gram of freeze-dried powder, respectively. The P. dioica hydrolysates also inhibited angiotensin-converting enzyme (ACE; half maximal inhibitory concentration, IC₅₀, 0.34 to 1.78 mg mL⁻¹) and dipeptidyl peptidase-IV (DPP-IV; IC₅₀, 1.14 to 5.06 mg mL⁻¹). The results demonstrate the potential of P. dioica hydrolysates as health-enhancing food components or natural food preservatives due to their enzyme inhibitory and antioxidant activities.