Speeding up enzymatic hydrolysis procedures for the multi-element determination in edible seaweed

A novel accelerated enzymatic hydrolysis by mean of ultrasonication has been developed and applied as sample pre-treatment for edible seaweed in order to determine total levels of As, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn. Three enzymes, α-amylase, pepsin and trypsin, have been evaluated and the released metals from seaweed have been measured by inductively coupled plasma-optical emission spectrometry (ICP-OES). Variables such as pH, sonication temperature, ultrasound frequency, ionic strength, hydrolysis time, extracting volume and enzyme mass were simultaneously studied by a Plackett-Burman design (PBD). Results have showed that there had not been any significant variable (confidence interval of 95%) affecting the conventional or the ultrasound-assisted enzymatic hydrolysis processes. This fact is attributed to the high salt content in seaweed, which generates a solution with a high ionic strength. The ultrasound-assisted enzymatic hydrolysis can be completed in 30 min when using an ultrasound frequency at 35 kHz. Quantitative recoveries were only reached when using pepsin, while recoveries close to 80% were obtained for the use of α-amylase and trypsin. The methods were validated by analyzing IAEA-140/TM, Fucus - Sea Plant Homogenate - and NIES-09, Sargasso, certified reference materials.