Effect of alkali treatment of red algae Gracilaria blodgettii and Gracilaria verrucosa (Rhodophyta) on agar quality

Abstract

The agarophytes Gracilaria blodgettii(Harvey) and G. verrucosa(Huds.)Papenfus from the Madras coast, were analyzed with and without pre-alkaline treatment(0.5, I, 2, 4, 6 and 8 N of NaOH) for the physical and chemical properties of their agar. Prealkaline treatment of both algae increased the gelling potential of the agar but reduced the yield. The yield of agar under various treatments ranged from 6.0-21% and 7.0-24% of dry weight in G. blodgettii and G. verrucosa respectively. The highest gel strength in 1.5% agar (414 g cm(-2)) was obtained when samples were treated with strong alkali [8 N] and the lowest (42 g cm(-2)) in untreated samples of G. blodgettii. Alkali treatment increased the melting and the gelling temperatures from 87 degrees C to 101 degrees C and 38 degrees C to 49 degrees C respectively. Of the two algae, G. blodgettii showed high values of 3,6-anhydrogalactose (49.0% dry wt) and galactose (34.7% dry wt) at 8 N and 0.5 N alkali treatment respectively. The amount of sulphate in the agar samples of both species showed an inverse relationship to alkali treatment. Shade dried samples showed a higher agar yield and gel quality compared with sun dried samples. The get strength of agar from G. blodgettii when treated with mild alkali (0.5 N NaOH) at 80 degrees C for 1 h was similar to that of bacteriological grade (Hi Media) agar. Proving that G. blodgettii and G. verrucosa could also be a good source material for commercial production of agar.