Improvement of gel properties of low-salt shrimp (Litopenaeus vannamei) surimi by sodium alginate: Effect of mannuronic acid/guluronic acid ratios

The impact of different mannuronic acid (M)/guluronic acid (G) ratios (2:1, 1:1, and 1:2) in sodium alginate (SA) on the gel properties of low-salt whiteleg shrimp surimi was investigated, and the corresponding mechanism was elucidated in this study. The results showed that all three types of SA (M-block dominant, M/G balanced, and G-block dominant) could improve the texture characteristics of low-salt shrimp surimi gel (SSG), strongly modulated by the M/G ratio. As the G content of the SA increased, the gel strength, hardness, and storage modulus of low-salt SSG were significantly enhanced ( P  < 0.05). After adding SA at the M/G ratio of 1:2, the highest water holding capacity (86.13 %) and the lowest cooking loss (1.16 %) were observed in the low-salt SSG. These phenomena could be attributed to the structural differences between the M unit (with an equatorial C5 carboxyl group, flexible) and the G unit (with an axial C5 carboxyl group, rigid) of SA. SA with high G content had a superior ability to construct a rigid network, which restricted protein mobility, increased the probability of protein-protein interactions, and thereby promoted protein cross-linking and disulfide bond formation. Meanwhile, the protein conformation in the low-salt SSG changed from α-helix to β-sheet after adding SA. These effects collectively contributed to forming a dense gel network, leading to better texture and water retention capacity of low-salt SSG. In conclusion, the M and G components of SA play crucial roles in establishing the protein-polysaccharide network, which is essential for developing high-quality low-salt SSG.