Agar is a strongly gelling polysaccharide extracted from gracilarioids, which is processed through conventional alkaline pre-treatment and chemical bleaching. However, the use of chemical bleaching agents has associated health risks to humans and the environment. The aim of the present study was to determine the effects of photo-bleaching in combination with NaOH pre-treatments at 5, 10 and 15% concentrations on the physico-chemical properties of agar extracted from Gracilariopsis heteroclada. The results highlighted that the gel strength of photo-bleached agar extracts PBA-15 (1822.00 g cm−2), PBA-10 (1465.60 g cm−2), and PBA-5 (1067.70 g cm−2) were significantly higher than in either a commercially sourced agar reference sample (756.20 g cm−2) or a chemically bleached agar (390.94 g cm−2). The physico-chemical properties of PBA extracts fell within the acceptable limits of food-grade agar. Photo-bleached agar extracts (PBA-5, PBA-10, PBA-15) also exhibited a higher lightness (L*) and whiteness index (WI) which makes these agars ideal for microbiological use. This study demonstrated that photo-bleaching by solar irradiation improved the physico-chemical properties of agar from G. heteroclada without the use of a chemical bleaching agent.
The current industrial practice of using chemical bleach to achieve the pure white colour of agar is deleterious to both human and environmental health. This study evaluates the potential of solar irradiation as an alternative bleaching process for agar extracted from Gracilariopsis heteroclada in Iloilo, Philippines. The physico-chemical properties of agar obtained from alkaline-treated seaweed after exposure to different bleaching conditions (e.g. solar irradiation, hypochlorite solution, and ultraviolet and fluorescent lights) were examined and compared with commercial bacteriological agar. Photobleaching through solar irradiation produced agar with superior gel strength (1038.61 g cm−2), high 3,6-anhydrogalactose content (41.44%) and low total inorganic sulphate content (1.87%) without compromising agar yield (19.37%). Solar irradiation offers very promising results as a simple, low-cost, environmentally friendly alternative to the chlorine bleaching process for agar extraction.
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