Pilot plant scale extraction of alginates from Macrocystis pyrifera. 2. Studies on extraction conditions and methods of separating the alkaline-insoluble residue

The effect of temperature (70, 80, 90 °C) and time (1-9 h) during the alkaline extraction step on alginate yield and quality were studied. The alginate yield increased with time and maximum yield was obtained after 3.5 h treatment, ranging from 19.4% at 70 °C to 21.9% at 90 °C. The viscosity of the alginate produced was inversely correlated with the temperature and time. At 70 °C the slope of the curve was almost zero (753 to 923 mPa s); at 90 °C the viscosity loss was 154 mPa s per hour during the first two hours, reducing from 523 to 86 mPa s after 5 h; 80 °C yielded values between those for 70 °C and 90 °C. The best conditions for alkaline extraction were using pH 10 at 80 °C for two hours. The curves obtained gave useful information for controlling the viscosity of the alginate during production. It was found that viscosity of the paste formed during alkaline extraction ('process viscosity') was the best parameter to determine the reaction rate during extraction. Alginate yield increased during filtration time from 17.6% to 23.7% after 55 min at 70 °C. In this step the viscosity of the alginate obtained remained almost constant (522-610 mPa s), indicating no degradation of the products during filtration. The best dilution to filter the alginate extract was obtained at 45 mPa s. Diatomaceous earth (Celite) and expanded lava (Perlite) were tested as filter aids. Expanded lava was the best filter aid, using 1 kg per kilogram of alginate produced. Three methods were studied to separate the alkaline-insoluble residues after extraction: filtration, centrifugation, flocculation, and combinations of them. The best system found was filtration with flocculant in a rotary vacuum filter, with a knife advance of 0.1 mm every 3.5 seconds and drum rotation of 2 rpm, yielding an average filtration flow rate of 10.5 L min^-1.