Effect of subcritical water–mediated liquefaction on compositions and bioactivities of sea lettuce (Ulva prolifera) extracts

Sea lettuce (Ulva prolifera) has become a coastal pollutant despite its benefits for human health. This study investigated the effect of subcritical water (SW) liquefaction on the changes in the composition and physiological activities of sea lettuce. Proteins and polysaccharides from sea lettuce were hydrothermally hydrolyzed with SW at 200–250 °C, and these hydrolysates underwent Maillard reaction, yielding a maximum amount of melanoidin at 250 °C. The SW at 250 °C was effective for the extraction of phenolic compounds from sea lettuce. 2,2′-Azino-bis-(3-ethylbenzothiazoline-6-sulfonic) acid (ABTS) radical inhibition activity and angiotensin-converting enzyme (ACE) inhibition activity indicated that the thermochemical changes induced by SW enabled the conversion of sea lettuce into strong antioxidative and antihypertensive biomaterials. Sea lettuce subjected to a temperature of 150 °C exhibited the best prebiotic activity, whereas lettuce liquefied at 300 °C showed strong antimicrobial activity. The taste profiles of sea lettuce indicated that the simple taste attribute of the control extract was transformed into a complex taste attribute through SW. In particular, sea lettuce subjected to 250 °C had the highest sweetness and umami. Consequently, this study demonstrated that sea lettuce liquefied by SW could be used as a biomaterial and a taste enhancer in food systems, and the SW had notable potential for the upcycling utilization of sea lettuce in the functional food industry.