Carrageenan Production and Gel Quality: Interspecific Responses of Eucheumatoid Algae to Extraction Method and Low-Salinity Stress

Low-salt stress caused by rainfall and freshwater runoff severely restricts the large-scale cultivation of the tropical algae Eucheumatoid. In this study, five commercial Eucheumatoid species, including Kappaphycus alvarezii (Hainan green and red strains, Malaysia red strain), Malaysia Kappaphycus striatum and Hainan Eucheuma denticulatum, were selected as the experimental materials. We compared the differences in carrageenan yield, physicochemical properties, and five textural parameters of carrageenan extracted by alkaline extraction and ultrasound-assisted enzymatic extraction methods, as well as those under a low-salinity gradient. Among the five algal species assessed with two extraction methods, Hn-E showed the best comprehensive carrageenan performance (gel yield = 45%; gel strength = 210 g·cm−2, sulfate content = 32%, viscosity > 250 mPa·s; TPA: hardness > 150 g, cohesiveness = 0.73, chewiness = 122.5 g, springiness > 0.88, resilience = 0.72). Enzyme-assisted extraction produced the poorest overall indices for Hn-G and Hn-R. Under low-salinity gradients, 18‰-cultured Ma-R achieved the highest yield (44.5%) and sulfate content (36%). Viscosity increased with salinity without significant inter-species divergence at the same salinity. Gel strength and textural parameters peaked at 24‰, moderately dropped at 32‰ and minimized at 18‰; at each salinity level, these parameters followed the descending order: Hn-E > Ma-R > Ma-Ks > Hn-R > Hn-G. The result of Fourier-transform infrared spectra revealed κ-type carrageenan signatures in all samples and absence of λ-type signals; the dominant ι-type peak was found in alkali-extracted carrageenan of Hn-R and enzyme-assisted extracted carrageenan of Hn-E. This study demonstrates that salinity fluctuation and extraction strategy jointly determine the quality and type of carrageenan, providing references for stabilizing carrageenan quality under low-salinity disturbance and a foundation for germplasm screening and extraction protocol optimization of Eucheumatoid aquaculture in rainfall-prone coastal regions.