Abstract:
Background: The global population is projected to grow more than one-third by 2050, and food demand estimated to exceed another 70%. With escalating needs for protein, animal protein production would eventually outbalance future resources. The search for a plant protein requires immediate attention to sustain natural resources and enhance food security. In this scenario, the sustainable cultivation, and attractive nutritional and functional values of red seaweeds, such as Porphyra and Pyropia, spotlight such seaweeds as reliable alternative protein sources.
Objectives: The present study aimed to systematically optimise the protein extraction protocols for red seaweeds (Porphyra umbilicalis, Pyropia virididentata and Pyropia cinnamomea), and investigate the physicochemical properties and functionalities of the derived seaweed protein extracts.
Methods: Four protein extraction methods were investigated, namely water, alkaline, ultrasound-assisted and enzyme-assisted extractions. Physicochemical properties of the obtained seaweed protein extracts (SPEs), including amino acid composition, molecular weight distribution, protein functional groups and thermal stability were analysed. The functional properties of the SPEs, such as solubility, emulsifying and foaming properties, were examined and compared to commercial whey and soy protein isolates. The potential antioxidant capacity of SPEs was evaluated using the DPPH and FRAP assays.
Results: The optimised conditions for each extraction protocol were as below: water extraction (biomass-water ratio of 1:40, extraction at 23 °C for 4 hr), alkaline extraction (biomass-water ratio of 1:30 at pH 12, 23 °C for 4 hr); ultrasound-assisted extraction (biomass-water ratio of 1:40 at 200 W and 20 kHz for 5 mins); enzyme-assisted extraction (biomass-water ratio of 1:40 at pH 9, 23 °C for 16 hr with Alcalase). Across all extraction protocols, SPEs derived from Alcalase-treatment exhibited the greatest protein yield of 193.20 mg protein/g seaweed and a purity of 43.01% (p < 0.05). Alcalase extraction efficiency doubled that of polysaccharidases, reaching up to 68% extraction yield. SPE following enzyme-assisted extraction comprised comparable proportions of essential amino acids to the commercial isolates. All SPEs had a molecular weight distribution ranging between 20 to below 10 kDa, typically lower than those from commercial isolates. From the FTIR analysis, the SPEs showed slight variances in the
absorbances of amide I, II and III bonds, referring to different protein structures. Inspecting thermal properties through Differential Scanning Calorimetry revealed higher thermal stability across all SPEs than commercial isolates. The functional properties of SPEs were consistent with their physicochemical properties. For most of the functional properties, some SPEs produced more impressive performances than commercial isolates. All SPEs demonstrated good solubility with minimal fluctuations across different pHs; in particular, SPEs obtained from enzyme-assisted extraction had over 95% solubility. All SPEs also consisted of good emulsifying properties; in particular, those obtained from enzyme-assisted extraction achieved an emulsifying capacity of 62 mg2/g and emulsifying stability of 77.03 mins. Moreover, the SPEs illustrated good foaming properties; in particular, SPE following alkaline extraction showcased up to 280% foaming capacity, whereas the SPE after enzyme-assisted extraction formed a stable foam for 82.99 mins.
Conclusions: Based on the results of this study, red seaweed proteins, especially ones extracted enzymatically, could be a promising protein source for developing functional foods and have significant potential to be incorporated in the formulation of various food products.