Improving The Texture of Low-Sodium Fish Cakes Using Agar Extract from Gracilaria fisheri by Hot Water and Alkaline Methods

Abstract

Background and Objectives:  Conventional fish cakes typically contain approximately 2–3% salt to promote myofibrillar protein solubilization and gel formation; however, salt reduction often compromises texture, water holding capacity, and overall product quality. Thailand’s coastal regions are rich in edible red seaweeds, particularly Gracilaria fisheri (G. fisheri) in southern Thailand, presenting strong potential for health oriented food applications. The functional properties of agar are highly dependent on extraction methods, which in turn affect its physicochemical characteristics and performance in food systems, particularly under sodium reduced conditions. Despite extensive documentation of individual extraction techniques, comparative evidence linking extraction induced physicochemical differences of agar to textural performance in low sodium food systems remains limited. In addition, G. fisheri derived agar offers sustainability benefits by supporting coastal livelihoods and environmental health. However, its application as a texture modifying agent in low sodium seafood products where salt reduction compromises protein gelation has not been systematically investigated. In particular, the comparative performance of agar obtained via the hot water extraction method (HWEM) and the pre-alkaline extraction method (PAEM) in restructured fish cakes under low sodium conditions, as well as their relationships with key textural attributes  hardness, springiness, cohesiveness, and chewiness remain unclear. Therefore, this study evaluates G. fisheri  agar extracted by HWEM and PAEM as functional texturizers in low sodium fish cakes, focusing on physicochemical properties, textural performance, and potential for sustainable sodium reduced product development.

Methodology: Fresh G. fisheri seaweed was collected from southern Thailand, washed, and dried before agar extraction. Two methods the hot water extraction method (HWEM) and the pre-alkaline extraction method (PAEM)  were applied. For HWEM, dried seaweed was extracted in deionized water, while PAEM involved NaOH pretreatment followed by hot water extraction. Extracted agar solutions were filtered, cooled, and stored for analysis. Agar yield, color (L*, a*, b*), and gel strength were determined using standard analytical methods. Low sodium fish cakes were prepared from tilapia fillets using five formulations (1% salt), varying by agar type (HWEM or PAEM) and concentration (5% and 10%), and compared with a control (no agar) and a commercial product (2% salt). Fish cakes were steamed, cooled, and analyzed for pH, color, cooking loss, and texture profile parameters, including hardness, springiness, and gumminess.

Main Results: This study evaluated the effects of two agar extraction methods, HWEM and PAEM, on the physicochemical properties of agar derived from G. fisheri and its functionality as a texturizing agent in low sodium fish cakes. Agar yield, gel characteristics, and color were first examined. PAEM (29.12%) produced a significantly higher agar yield than HWEM (19.94%), confirming the role of alkaline pretreatment in enhancing agar recovery. However, agar obtained by HWEM exhibited superior gel strength at 0.46 ± 0.02 gforce/mm^2, indicating better gel network integrity, which are desirable attribute for food applications requiring visual clarity. In contrast, PAEM agar showed lower gel strength at 0.32 ± 0.02 gforce/mm², likely due to polysaccharide degradation and structural modification during alkaline treatment. Food application part, Low sodium fish cakes (1% salt) were formulated with agar extracted by HWEM or PAEM at 5% and 10% concentrations and compared with a control and a commercial product (2% salt). Agar addition significantly influenced textural and physicochemical properties (p<0.05). Fish cakes containing HWEM agar exhibited increased hardness and gumminess with increasing agar concentration (from 5% to 10% agar), while springiness remained relatively unchanged. In contrast, PAEM agar resulted in slightly reduced hardness (207.49 ± 21.58 N at 5% agar and 202.66 ± 29.22 N at 10% agar) and gumminess at higher concentrations, possibly due to over aggregation and weakened gel cohesiveness. These results demonstrate that agar can compensate for reduced salt-induced protein gelation and maintain desirable texture in low sodium formulations. Cooking loss decreased with increasing agar concentration for both extraction methods, reflecting improved water holding capacity. Agar supplemented samples showed cooking loss values comparable to the commercial product, was not significant differences (p>0.05), despite containing half the salt level, highlighting agar’s effectiveness in yield retention. Fish cake pH values were slightly higher in PAEM treatments due to residual alkali, but remained within acceptable limits. No significant differences in lightness (L*) were observed among fish cake samples containing 5% and 10% agar from either extraction method (p > 0.05); however, the commercial product showed the highest L* value.  From a commercial and sustainability perspective, HWEM offers a clean label, environmentally friendly approach suitable for health-oriented and premium food products, while PAEM provides higher yields and firmer gels for industrial applications. Together, these findings demonstrate the potential of G. fisheri agar to support sodium reduction, enhance product quality, and contribute to sustainable food and bioeconomy development in Thailand.

Conclusions: Both HWEM and PAEM produced functional agars from G. fisheri suitable for low sodium meat products, though with distinct advantages. PAEM yielded a higher extraction efficiency and enhanced water retention, supporting its applicability in industrial scale processing despite the requirement for alkaline neutralization. In contrast, HWEM offered an environmentally friendly alternative with simpler processing and lighter color, supporting health oriented and sustainable product development. Adopting a dual path strategy enables Thailand to reduce dependence on imported agar, enhance local value chains, and support coastal community livelihoods. The utilization of locally sourced seaweed aligns with BCG economy principles, food security objectives, and sustainable marine resource management.