Acid preservation of Laminaria hyperborea with formic and acetic acid and its effects on alginate

Abstract

Each year, 150,000 tons of Laminaria hyperborea is harvested commercially for alginate production in Norway. Traditionally, formalin has been an attractive preservative in the industry because it inhibits microbial growth and prevents alginate discolouration by cross-binding with polyphenols. However, due to potential health risks by handling formalin in large quantities, EU is restricting its use, and it is necessary to look for alternative preservation methods. Low molecular weight organic acids are used as food preservatives because they inhibit microbial growth when undissociated. The concentration of undissociated acid is a function pH, acid concentration and pK_a of the organic acid, hence different organic acids will have varied preservative efficiency at a given pH. This study investigated how preservation with formic acid (pK_a = 3.75) and acetic acid (pK_a = 4.76), affected the colour, yield, molecular weight, and composition of alginates from L. hyperborea stipe and lamina when stored at pH 3.7/4.7, 13/20 °C, and 2/8 weeks. The results were compared with formalin preservation (0.1 % formaldehyde). Microbial growth was prevented with both acids at pH 3.7, however, acid hydrolysis led to alginate depolymerisation. Acetic acid at pH 4.7 inhibited microbial growth in stipe biomass for 8 weeks, and when stored at 13 °C, the alginate molecular weight remained high (M_w = 426 kDa). Furthermore, acid preservation led to increased alginate yield and colour reduction compared with fresh and non-preserved biomass. Microbial growth in non-preserved samples led to enzymatic depolymerisation of alginates, resulting in a higher fraction and block length of guluronic acid.