Processing-mediated reduction of elemental contaminants in seaweed food and feed products: Processing-mediated reduction of contaminant levels in seaweed products

Seaweed is now relatively commonly consumed across all population strata in the Netherlands and may provide a sustainable, plant-based protein source for humans and animals. Previous studies have revealed that seaweeds can accumulate high concentrations of iodine and heavy metals, although with high variation among seaweed species, geographic location, harvesting season, seaweed metabolic activity, and cultivation method. Food safety of seaweed is an important condition for bringing seaweed food products to the market. Finding ways to reduce the concentrations of iodine and other contaminants in seaweed containing products is therefore desirable. The aim of this study was to identify commonly used processing steps in the seaweed value chain and their potential to reduce concentrations of iodine, arsenic and heavy metals (nickel, cadmium, mercury, lead, and bromine) in the intermediate or final product. We first provide a short literature review of the current knowledge on iodine and heavy metal contaminations in seaweed and how the concentrations of these contaminants are affected by commonly used processing steps. We then report the results of a pilot study assessing changes in iodine and heavy metal concentrations along a series of processing steps in seven food or feed production chains, utilizing the green seaweed Ulva lactuca (in three production chains) or the brown seaweed Saccharina latissima (in four production chains) as starting products. The sequential processing steps we cover fall into roughly two categories: washing (washing, blanching, rehydrating dried seaweed) and structural disintegration (fermenting, drying). Our literature search highlighted that concentrations of iodine and heavy metals in seaweeds vary widely with seaweed species, but also with season, growth conditions and age of the plant tissue. Overall, washing and related processing steps (washing, soaking, blanching, rehydrating) are generally the most promising procedures to reduce the mobile fraction of iodine and heavy metals in the biomass. Combining these washing steps with additional heat treatment or destruction of cell walls may increase the efficiency of contaminant removal further. The results of our pilot support these findings as we find large differences in elemental concentrations among the three species tested (Ulva sp., Saccharina sp., and Palmaria sp.). Moreover, rehydration of dried seaweed (observed in four processing chains) generally resulted in reduced elemental concentrations (expressed as mg kg-1 dry weight) in the rehydrated biomass. Unexpectedly, we also observed increases in elemental concentrations that may suggest methodological artefacts (when measuring close to the limit of quantification), or contaminations by other, added ingredients (e.g., in dough mixtures), or remain unexplained. The results of this study can be used to determine the most processing steps with most potential for contaminant reduction and, moreover, will inform the choices of processing steps and treatment ranges of the intended follow-up study (PROCESS II).