Agar extraction from marine algae: Influence of pre-extraction

CROCE, M.E.; PARODI, E.R.

Roma

Conferencia; International Conference on Industrial Biotechnology; 2014

Italian Association of Chemical Engineering

Agar is one of the most important soluble polysaccharides obtained and industrialized from marine algae. It has been considered as one of the major bio-products with large applicability from industry to medicine. The quality and properties of agar differs between algal species and depend on the molecular structure, therefore pre-extraction treatments have been developed in order to increase the quality of agar extracts. However, the results of pre-extraction methods are highly dependent on algal species. The objective of the present study was to evaluate four different pre-extraction treatments reported in the literature, on the yield and quality of polysaccharides extracted from the red algae Polysiphonia morrowii. Algal samples were collected in Anegada Bay (Argentina), cleaned with filtered seawater and later rinsed using tap water. Samples were oven dried at 60 °C and then grinded to obtain a powder. Pre-extraction treatments included NaOH 5 % at 90 °C, acetic acid 0.5 % at room temperature, pure acetone, and lemon citric acid. Treated samples were heated for 1 h in distilled water (500 mL) at 90 °C and polysaccharides were separated from the residue by centrifugation and filtration. Polysaccharides were obtained after 24 h. at room temperature by the traditional freeze-thaw procedure. For each treatment, 4.8 g of dried algae were used and the quality of crude extracts were evaluated using Infrared Spectroscopy (FTIR) and Thermogravimetry (TGA). Polysaccharide yield was calculated on the basis of algal dried weight and crude extract for each pre-extraction treatment. For each condition, 0.002 g of samples was dissolved into a matrix of 0.098 g of KBr and analyzed by FTIR. Results were compared with commercial dehydrated agar-agar and an agarose spectrum from the equipment library. The highest agar yield was obtained with NaOH 5 % (19.25 %), while the lower yield was obtained with acetic acid 0.5 % (0.21%). Pure acetone, acetic acid 0.5 % and lemon citric acid pre-extraction treatments resulted in a clearer extract, while NaOH 5 % originated an olive-dark extract. FTIR spectra were different according to each pre-extraction treatment. All spectra had a characteristic band at 930 cm-1 corresponding to 3,6-anhidrogalactose. A higher sulfation degree was observed in the spectra of samples extracted with pure acetone and lemon citric acid. Lower total sulfate was observed in samples obtained with NaOH and acetic acid. However, in NaOH samples two bands appear at 868 cm-1 and 881 cm-1, which suggested a higher degree of sulfation by substitution. TGA curves showed an initial weight loss for all samples, except for lemon citric acid samples, corresponding to water. The temperature for the start of degradation was approximately 230 °C for samples extracted by NaOH, acetic acid, and pure acetone. Degradation process for a reference commercial agar ended at 500 °C while all extracted samples show a continued degradation process above this temperature. The slowest weight loss was found for NaOH treated samples, while the fastest was for lemon citric acid.