Sargassum latifolium samples were collected during late spring from the intertidal zone of Hurghada, Egypt (27° 12′ N, 33° 50′ E). The macroalgal biomass was air-dried and milled in a home blender.
1.1. Alginate extraction
In the extraction process, milled seaweed biomass (1.5 % w/v) was acidified with 2 % citric acid solution with shaking (200 rpm) for 2 h at room temperature. The residual biomass was separated by filtration and washed with distilled water. For alginate extraction, the acidified algal biomass was suspended in 3% Na2CO3 solution at different alkali: alga ratio (20, 40, and 60 mL/g). The different extraction temperatures ranged from 25 to 45º C, and lasted for 1 to 3 h. For each experimental run, sodium alginate was collected by filtration and precipitated with absolute ethanol (1:2 v/v). The mixture was maintained at 4º C overnight. The precipitate was collected by vacuum filtration and allowed to dry at room temperature. The crude alginate yield was calculated from the following equation:
Alginate yield (%) = [dry wt. of obtained alginate / dry wt. of sample] × 100
1.2. Block distribution and M/G ratio
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Alginate sample (30 mg) was hydrolyzed in 10 mL HCl (0.3 M) at 100 ºC for 2 h. After cooling, the mixture was centrifuged (6000 rpm, 45 min), and the supernatant solution was separated and neutralized with 1 M NaOH and referred to as fraction A. The insoluble material was dissolved in 1 M NaOH and the pH was decreased to 2.85 by the addition of 1 M HCl. The suspension was recentrifuged and the supernatant was separated and referred to as fraction B. The insoluble fraction was dissolved by neutralization with 1 M NaOH and referred to as fraction C. The fractions A, B, and C are enriched in MG, MM, and GG blocks