Pilot plant scale extraction of alginates from Macrocystis pyrifera. 4. Conversion of alginic acid to sodium alginate, drying and milling

Abstract

The last three steps of the alginate production process were studied:conversion of alginic acid to sodium alginate, drying, and milling. Threemethods were used to follow the conversion reaction: measuring the pH (a) intheethanol-water liquid of the reaction mixture, (b) after dissolving a sample ofthe fiber taken from the reaction mixture, (c) after dissolving the driedsodiumalginate obtained from the reaction. To obtain a neutral dried sodium alginate,in the first method the pH should be adjusted to 9, and in the second the pHshould be adjusted to 8. The best method to control the reaction was todissolvea sample of the fiber and adjust the pH to 8. The best proportion to reach thecritical point, where pH just begins to rise, was 0.25 parts of sodiumcarbonateto 1 part of alginate in the initial dry algae. A pH above 7 may produce abreakdown of the molecule, reducing significantly the viscosity of the finalalginate. Four different temperatures were used to dry the alginate: 50, 60,70,and 80 °C. Drying time to reach 12% moisture ranged from 1.5h at 80 °C to 3 h at 50°C. The best drying temperature was 60 °C for2.5 h. The effect of drying temperature on alginate viscosity wasdependent on the alginate type. Low and medium viscosity alginates were notsignificantly affected, but alginate with high viscosity was reduced by 40 to54% using the temperature range of 60 to 80 °C. A fixed hammermill was used to reduce the particle size of the dried sodium alginate.Particlesize measurements showed that after a first milling the product contained 76%large particles (20–60 mesh) and 24% fine particles (80–120 mesh).After a third milling the product still contained 42.9% large particles. Nosignificant effect was found on alginate viscosity because of the millingsteps.