Effect of Alkali Concentration on Agar Yield and Quality Using a Simple Extraction Method from Gracilaria tenuistipitata

Abstract

Seaweeds are marine algae found along seashores, come in various colors such as red, green, and brown. Among these, the red variety holds the potential scope for agar production. The current study investigated the effect of using widely practiced alkaline treatments with NaOH (0%, 2.5%, 5%, 7.5%, 10% and 12.5%, w/v) on agar production using a simple and efficient protocol from red seaweed, Gracilaria tenuistipitata. Agar yield, proximate composition and physico-chemical properties (gel strength, melting and gelling temperature, syneresis, pH, color) of the produced agar were determined to investigate the suitability of the produced agar as lab-grade agar. The highest yield of agar was obtained using 12.5% NaOH treatment (24.62%), followed by 10% NaOH treatment (22.78%), 7.5% NaOH treatment (15.77%), 2.5% NaOH treatment (13.24%), 5% NaOH treatment 10.63%, 0% control (5.22%). Agar made from 12.5% concentration shown higher yield, but due to higher alkaline treatment this agar is less pure due to some seaweed residues. The highest gelling and melting temperature were found for 7.5% concentration, about 49.07 °C and 89.90 °C, respectively. Gelling and melting temperature is also satisfactory for those obtaining using 10% and 5% NaOH treatment. pH was slightly acidic for all the concentration. The reason of acidity is the acidic treatment and more neutralization facility can make it a food grade quality. Highest gel strength found for 5% NaOH treated agar followed by 7.5%, 2.5%, 12.5%, 10%, 0%. According to physico-chemical analysis, agar treated with 5% to 10% NaOH can be acceptable for better agar production. Fiber content was found to be dominated among all other proximate compositions. Fiber and carbohydrate contents were found 43.79% and 28.01% for 10% NaOH and 5% NaOH, respectively. This is comparable with the fiber (45.72%) and carbohydrate (19.78%) content of pure agar available commercially. This value is acceptable for bacterial culture in the laboratory. Agar extracted seaweed using NaOH was mixed with nutrients and cultured bacteria was cultured successfully. The 12.5% treated agar had the highest (1.69×104 CFU/g) bacterial load followed by 5%, 10%, 7.5%, 2.5% and lastly 0%. Therefore, overall result expresses, using 10% to 5% NaOH pretreatment of agar from G. tenuistipitata have acceptable physico-chemical properties and ca